Planets

BLASTING OFF TO OTHER WORLDS

For over fifty years, a number of nations have been involved in the exploration of outer space. This research is very costly. Has this money been well-spent or wasted?

Some people believe that most space research should be eliminated because of its expense. These people point out the fact that it costs billions of dollars to send astronauts to the moon, but all they bring are some worthless rocks. These people say that the money wasted in oouter space could be spent on more important projects on earth, such as providing housing for homeless people, improving the education system, saving the environment, finding cures for diseases. However, other people believe that space research has provided many benefits to mankind. They point out that hundreds of useful products, from personal computers to foods, are the direct or indirect results of space research. They say that weather are communication satellites have benefited people. Supporters of the space program point tto the scientific knowledge, acquired about the sun, the moon, the planets.

I agree with those people who support space research and want it to continue. Space research will bring even more benefits in the future. Moreover, just as individual people nneed challenges to make their lives more interesting, I believe the human race itself needs a challenge. I think that the peaceful exploration of outer space provides just such a challenge.

Mars or ,,The Red Planet“ is looked upon as the next frontier to space researches. Mars has several enormous canyons. Mars has one mountain which is twice as tall as Everest. It could cost as much as 700 billion dollars to put an astronaut on Mars. That makes it the most expensive journey in history. Even so, it’s a journey which scientists are planning, and they hope that it will happen in the next 25 years in five stages.

There are long-term prospects for space travel, too. After we have explored oour own solar system we predict to travel to other galaxies in huge ,,star ships“. These will wander through space for thousands of years. Each will contain a large human population. When the ship discovers a suitable new planet, some of these people will colonise it. This way, the human race will gradually colonise the whole universe.

In fact, one day, life on Earth could be just distant memory. The TV series ,,Star Trek“ could hold the answer.

PLANETS

The planets divide nneatly into two broad categories: terrestrial and jovian. The terrestrial are basically small, rocky worlds and include Mercury, Venus, Earth, Mars; the jovian planets are gas giants and consist of Jupiter, Saturn, Uranus, Pluto, the outermost and smallest planet (although some scientists argue that it shouldn’t be considered in this privileged class), is an oddball that doesn’t fit easily into either category.

The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto. Jupiter is so big that all the other planets could fit inside it.

In general, planetary scientists have scrutinized the terrestrial planets in far greater detail than the jovian planets. Earth, of course, has been an object of scientific inquiry ever since people first started to ponder their place in the universe. Sophisticated spacecraft have examined both Venus and Mars from orbit and from the surface. (NASA’s most recent success placed two rovers on the martian surface during 2004.)

Mercury remains the most enigmatic of the terrestrial worlds. It lies so close to the Sun that observations from Earth reveal preciuos little. In the mid-1970s, NASA sent the Mariner 10 spacecraft on three separate flybys of the innermost planet. TThe spacecraft revealed a startling fact: Mercury has such I high density that more than half of it must be made out of iron and nickel. The planet’s surface shows lots of craters, most dating from the age of heavy bombardment that characterized the solar system about 4 billion years ago. During this period, errant comets and debris left over the solar system’s formation pummeled most planets and moons.

Of all the planets, Venus most resembles Earth. The two have nearly the same size and density, yet the moniker ,,Earth’s twin“ fails miserably for Venus. Earth has a relatively benign climate conducive to the presence of liquid water (and thus life). The surface of Venus, however, bakes at a temperature of 8000 Fahrenheit. Its massive atmosphere of carbon dioxide traps solar radiation and creates a runaway greenhouse effect. Atmospheric pressure at the planet’s surface is nearly 100 times greater than that on Earth’s. Craters on the surface of Venus show that volcanic activity resurfaced the entire planet about 600 million years ago. This contrasts with Earth, where steady volcanism and erosion gradually covered up signs of ancient impacts.

Mars has long fascinated humans, in no small part because its surface is tthe only one that can be seen clearly from Earth. Changes in its appearance led some imaginative scientists to believe that a dying civilization was tapping into dwindling water supplies. Those hopes were dashed when the first spacecraft images revealed a crater and apparently barren surface. Yet subsequent missions revealed a more nuanced world, where craters share space with massive (albeit extinct) volcanoes, giant canyons, and dry channels. The most recent rovers have left little doubt that liquid water once existed on the martian surface. So the question remains: Could life

ever have started on so Red Planet? A famous meteorite from Mars known as ALH84001 contains tantalizing evidence of possible microfossils. And, if water once flowed on the surface, life might have followed.

The jovian planets seem to have less diversity than terrestrial counterparts because all we see are the tops of their cloud layers. Jupiter, Saturn, Uranus and Neptune all have thick atmospheres consisting largely of hydrogen and helium. Various minor constituents create the subtle colors that cause them to look different through a telescope. The term ,,gas giant“ fits these planets perfectly – even the smallest, Uranus, weights in at 15 times the mass of Earth. All the

jovian planets have ring systems as well, although only Saturn’s shines bright enough to be seen easily from Earth.

Prior to Charon’s discovery, astronomers believed that Pluto was much larger. Because Pluto is so distant, the images of Charon and Pluto were blurred together making the planet appear much larger. Pluto stands apart from the other planets because it is much smaller and less massive than others and has the most elongated orbit. It also consists of a mixture of iice and rock, which puts it more in line with some of the moons of the outer planets. Most scientists now consider it to be the largest Kuiper Belt object, a group of objects now numbering more than 700 that orbit beyond Neptune. Even so, it also remains officially a planet.

Moons in the solar system run the gamut from small objects that likely were captured by their parent planets – think of the martian satellites Phobos and Deimos, as well aas most of the dozens of small, irregular satellites orbiting the gas-giant planets – to big objects that rival the planets themselves in size. Jupiter tows its own miniature solar system with it as it orbits is the Sun. Its ffour large moons – Io, Europa, Ganymede, and Callisto (in order from Jupiter)- were discovered by Galileo when he first pointed this telescope at Jupiter in 1610. The largest, Ganymede, has a diameter of 3.270 miles, making it larger than Mercury. Tidal forces from Jupiter heat Io’s interior so intensely that this moon is the most volcanically active body in the solar system. The same tidal forces heat Europa’s interior, melting the moon’s subsurface ice and creating perhaps the largest ocean of liquid water in the solar system.

Saturn also hosts several moons, including the mysterious Titan. This moon, second in size to Ganymede, possesses a hazy, nitrogen-rich atmosphere thicker than Earth’s atmosphere that hides its surface from view. The equally eenigmastic Iapetus features one hemisphere that appears ten times brighter than the opposite one. Both will be prime targets for NASA’s Cassini spacecraft, which went into orbit around Saturn in early July 2004.

Of all the moons in the solar system, none has been studied more thoroughly than Earth’s. Even from Earth, the Moon appears big enough to show detail through a telescope. Its highly crater highlands stand in stark contrast to the darker, lightly crater Maria, crater by giant impacts tthat took place during the era of heavy bombardment and subsequently filled with lava. The Moon ranks as the fifth largest satellite in the solar system and was born in what seems to be a unique process. Most of the large moons in the solar system were created in protoplanetary disks, dusty disks that surrounded the planets during their formation. The moons condensed out of these disks in much the same way as the planets condensed out of the solar nebula. But our Moon appears to have formed when an object the size of Mars gave a glancing blow to the protoEarth, ejecting debris into orbit that eventually coalesced into the Moon.

The Inner Planets vs. the Outer Planets

The inner planets (those planets that orbit close to the Sun) are quite different from the outer planets (those planets that orbit far from the Sun).

The inner planets are: Mercury, Venus, Earth, Mars. They are relatively small, composed mostly of rock, and have few or no moons.

The outer planets include: Jupiter, Saturn, Uranus, Neptune and Pluto. They are mostly huge, mostly gaseous, ringed and have many moons (again, the exception is Pluto, which is small, rocky and has only one moon).

Temperatures on the PPlanets

Generally, the farther from the Sun, the cooler planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere.

Density of the Planets

The outer, gaseous planets are much less dense than the inner, rocky planets.

The Earth is the densest planet. Saturn is the least dense planet; it would float on water.

The Mass of the Planets

Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, Pluto are orders of magnitude less massive.

Gravitational Forces on the Planets

The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet’s surface is proportional to its mass and to the inverse of the planet’s radius squared.

A Day on Earth of the Planets

A day is the length of times that it takes a planet to rotate on its axis (3600 ). A day on Earth takes almost 24 hours.

The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than iits year; a year on Venus takes only 224.7 Earth days).

The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours. When you observe Jupiter from Earth, you can see some of its features change.

The Average Orbital Speed of the Planets

As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun.

A Tenth Planet?

No tenth planet beyond Pluto has been directly observed. A few astronomers think that there might be a tenth planet (or companion star) orbiting the Sun far beyond the orbit of Pluto. This distant planet/companion star may or may not exist. The hypothesized origin of this hypothetical object is that a celestial object, perhaps a hard-to-detect cool, brown dwarf star (called Nemesis), was captured by the Sun’s gravitational field. This tenth planet is hypothesized to exist because of the unexplained clumping of some long-period comet’s orbits. The orbits of these far-reaching comets seem to be affected by the gravitational pull of a distant.

GENERAL INFORMATION ON MERCURY

Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly

smaller in diameter than the moons Ganymede and Titan but more than twice as massive:

orbit: 57.910.000 km (0.38 AU) from the Sun

diameter: 4.880 km

mass: 3.30e23kg

An account of the non-discovery of a planet inside Mercury’s orbit. A much more interesting tale than you might imagine. In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it mmoves so quickly across the sky.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit is the Sun, not the Earth.

Mercury has been visited by only one spacecraft, Mariner 10. It flew bby three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safety imaged by HST). A few discovery-class mission to Mercury, MESSENGER was launched in 2004 aand will orbit Mercury starting in 2011 after several flybys.

Mercury’s orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million. The perihelion of its orbit processes around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury’s orbital parameters but could not adequately explain them using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades. It was thought that another planet (sometimes called Vulcan) might exist in an orbit near Mercury’s to account for the discrepancy. But despite much effort, no such planet was found. The

real answer turned out to bbe much more dramatic: Einstein’s General Theory of Relativity. Its correct prediction of the motions of Mercury was an important factor in the early acceptance of the theory. Until 1962 it was thought that Mercury’s ,,day“ was the same length as its ,,year“ so as to keep that same face to the Sun much as the Moon does to the Earth. But this way shown to be false in 1965 by doppler radar observations. It is now known that Mercury rrotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonance at all).

This fact and the high eccentricity of Mercury’s orbit would produce very strange effects for an observer on Mercury’s surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury’s surface would see different but equally bizarre motions.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable.

Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no plate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the second ddensest major body in the solar system, after Earth. Actually Earth’s density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury’s dense iron core is relatively larger than Earth’s, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.

Mercury’s interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth’s mantle and crust) is only 500 to 600 km thick. At least some of the core is probably molten.

Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury’s atmosphere is constantly being replenished.

The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut through the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank bby about 0.1% (or a decrease of about 1 km in the planet’s radius).

One of the largest features on Mercury’s surface is the Caloris Basin; it is about 1300 km in diameter. It is thought to be similar it the large basins (maria) on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system. The impact was probably also responsible for the odd terrain on the exact opposite side of the planet.

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity but some may be the result of the deposition of ejecta from cratering impacts.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury. But more data will be needed for confirmation.

Amazingly, radar observations of Mercury’s north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.

Mercury has a small magnetic field whose strength is about 1% of Earth’s.

Mercury has no known satellites.

Mercury is often visible with binoculars or even the unaided eye, but it is always very

near the Sun and difficult to see in the twilight sky. There are several Web sites that show the current position of Mercury (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Mercury is so close to the Sun that you can see it near sunrise or sunset.

The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person on Mercury would weight 38 pounds. To calculate your wweight on Mercury, just multiply your weight by 0.38 (or go to the planetary weight calculator).

Mercury’s thin atmosphere consist of trace amounts of hydrogen and helium. The atmospheric pressure is only about 1×10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.

Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no ,,greenhouse effect“ on MMercury. When the Sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.

Mercury is just over a third as far from the Sun as the Earth is; it is 0.387 A.U. from the Sun ((on average). Mercury’s orbit is very eccentric; at aphelion (the point in the orbit farthest from the Sun) Mercury is 70 million km from the Sun, at perihelion Mercury is 46 million km from the Sun. There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet’s orbit. Since Mercury’s axis is directly perpendicular to its motion (not tilted), it has no seasons.

If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth.

Mercury has no moons.

So, Mercury was named after Mercury, the mythical Roman winged messenger and escort of dead souls to the underworld. It was named for the speedy Mercury bbecause it is the fastest-moving planet.

GENERAL INFORMATION ON VENUS

Venus is the second planet from the Sun and the sixth largest. Venus’ orbit is the most nearly circular of that of any planet, with an eccentricity of less than 1%.

Orbit: 108.200.000 km (0.72 AU) from the Sun

Diameter: 12.103.6 km

Mass: 4.869e24 kg

The latest results from Magellan in an accessible and easygoing book. Covers mythology and history of our ,,sister planet“ as well as up to date science and a hhistory of the Magellan project.

Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and beauty. The planet is so named probably because it is the brightest of the planets known to the ancients. (With a few exceptions, the surface features on Venus are named for female figures).

Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. Like Mercury, it was probably thought to be two separate bodies: Eosphorus as the morning star and Hesperus as the evening star, but the Greek astronomers knew better.

Since Venus is an inferior planet, it shows phases when viewed with a telescope from the perspective of Earth. Galileo’s observation of this phenomenon was important evidence in favor of Copernicus’s heliocentric theory of the solar system.

The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. Most recently, the orbiting US spacecraft Magellan produced detailed maps of Venus’ surface using radar.

Venus’ rotation iis somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus’ year) and retrograde. In addition, the periods of Venus’ rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known. Venus is sometimes regarded as Earth’s sister planet. In some ways they are very similar:

• Venus is only slightly smaller than Earth (95% of Earth’s diameter, 80% of Earth’s mass).

• Both have few craters indicating relatively young surfaces.

• Their densities and chemical compositions are similar.

Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth.

The pressure of Venus’ atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth’s oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our vview of the surface. This dense atmosphere produces a run-away greenhouse effect that raises Venus’s surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus’ surface is actually hotter than Mercury’s despite being nearly twice as far from the Sun.

There are strong (350kph) winds at the cloud tops but winds at the surface are very slow, no more than a few kilometers per hour. Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. Earth would have suffered the same fate had it been just a little closer to the Sun. We may learn a lot about Earth by learning why the basically similar Venus turned out so differently.

Most of Venus’ surface consists og gently rolling plains with little relief. There are also several broad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two large highland areas: Ishtar Terra in the northern hemisphere (about the size of Australia) and Aphrodite Terra along the equator (about the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is surrounded by the highest mountains on Venus including the enormous

Maxwell Montes.

Data from Magellan’s imaging radar shows that much of the surface of Venus is covered by lava flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons) such as Sif Mons. Recently

announced findings indicate that Venus is still volcanically active, but only in a few hot spots; for the most part it has been geologically rather quiet for the past few hundred million years.

There are no small craters on Venus. It seems that small meteoroids bburn up in Venus’ dense atmosphere before reaching the surface. Craters on Venus seem to come in bunches indicating that large meteoroids that do reach the surface usually break up in the atmosphere.

The oldest terrains on Venus seem to be about 800 million years old. Extensive volcanism at that time wiped out the earliest surface including any large craters from early in Venus’ history.

Magellan’s images show a wide variety of interesting and unique features including pancake volcanoes which seem tto be eruptions of very thick lava and coronae which seem to be collapsed domes over large magma chambers. The interior of Venus is probably very similar to that of Earth: an iron core about 3000km in radius, a molten rrocky mantle comprising the majority of the planet. Recent results from the Magellan gravity data indicate that Venus’ crust is stronger and thicker than had previously been assumed. Like Earth, convection in the mantle produces stress on the surface which is relieved in many relatively small regions instead of being concentrated at plate boundaries as is the case on Earth.

Venus has no magnetic field, perhaps because of its slow rotation.

Venus has no satellites, and thereby hangs a tale.

Venus is usually visible with the unaided eye. Sometimes (inaccurately) referred to as the „morning star“ or the „evening star“, it is by far the brightest „star“ in the sky. There are several Web sites that show the current position of Venus (and tthe other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

On June 8 2004, Venus will pass directly between the Earth and the Sun, appearing as a large black dot travelling across the Sun’s disk. This event is known as a „transit of Venus“ and is very rare: the last one was in 1882, the next one in 2012 but after than you’ll have to wait until 2117. While no longer of great sscientific importance as it was in the past, this event will be the impetus for a major journey for many amateur astronomers. For all the details see Fred Espenak’s site.

This is a planet on which a person would asphyxiate in the poisonous atmosphere, be cooked in the extremely high heat, and be crushed by the enormous atmospheric pressure.

Venus’ mass is about 4,87 x 1024 kg. The gravity on Venus is 91% of the gravity on Earth. A 100 pound person would weight 91 pounds on Venus.

The density of Venus is 5.240 kg/m 3 , slightly less dense than the Earth and the third densest planet in our Solar System (after the Earth and Mercury).

Venus is 67.230.000 miles (108.200.000 km) from the Sun. Venus has an almost circular orbit. On average, Venus is 0.72 AU, 67.230.000 miles = 108.200.000 km from the Sun.

Venera 3 (from the U.S.S.R.) was the first manmade abject to reach Venus. This Soviet spacecraft was launched on November 16, 1965. On March 1. 1966, the spacecraft arrived at Venus and the capsule parachuted down to the planet, but contact was lost just before entry into the atmosphere.

Venus was named after the Roman goddess of love.

GENERAL INFORMATION ON EEARTH

Earth is the third planet from the Sun and the fifth largest:

Orbit: 149.600.000 km (1.00 AU) from the Sun

Diameter: 12.756.3 km

Mass: 5.972e24 kg

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

0 – 40 Crust

40 – 400 Upper mantle

400 – 650 Transition region

650 – 2700 Lower mantle

2700 – 2890 D ‘ ‘ layer

2890 – 5150 Outer core

5150 – 6378 Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The

inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth iis in mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms);

Atmosphere = 0.0000051

Oceans = 0.0014

Crust = 0.026

Mantle = 4.043

Outer core = 1.835

Inner core = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500K, hotter than the surface of the Sun. the lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth in inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth’s chemical composition (by mass) is:

34.6% Iron

29.5% Oxygen

15.2% Silicon

12.7% Magnesium

2.4% Nickel

1.9% Sulfur

0.05% Titanium

The Earth is the densest major body in the solar system.

The other terrestrial planets probably have similar

structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth’s crust is divided into several sseparate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse mmotion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:

• North American Plate – North America, western North Atlantic and Greenland

• South American Plate – South AAmerica and western South Atlantic

• Antarctic Plate – Antarctica and the ,,Southern Ocean“

• Eurasian Plate – eastern North Atlantic, Europe and Asia except for India

• African Plate – Africa, eastern South Atlantic and western India Ocean

• Indian – Australian Plate – India, Australia, New Zealand and most of Indian Ocean

• Nazca Plate – eastern Pacific Ocean adjacent to South America

• Pacific Plate – most of the Pacific Ocean (and the southern coast of California)

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.

The Earth’s surface is very young. In the relatively short (by astronomical standards) period of 500.000.000 years or sso erosion and tectonic processes destroy and recreate most of the Earth’s surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion yyears old. There is no record of the critical period when life was first getting started.

71% of the Earth’s surface is covered with water. Earth is the only planet on which water can exist in liquid from on the surface (though there may be liquid ethane or methane on Titan’s surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth’s temperatures relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth’s continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

The Earth’s atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth’s atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to tthese various ,,sinks“ and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth’s surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid – 21 C to a comfortable +14C); without it the oceans would freeze and life as we know it would be impossible.

The presence or free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under ,,normal“ circumstances would quickly combine with other elements. The oxygen in Earth’s atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

The interaction of the Earth and the Moon slows the Earth’s rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth’s magnetic field and the Earth’s upper atmosphere causes the auroras. Irregularities in these factors cause the magnetic poles tto move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The ,,geomagnetic north pole“ is the position on the Earth’s surface directly above the south pole of the Earth’s field).

The Earth’s magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. the outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

Earth has only one satellite, the Moon. But:

• Thousands of small artificial satellites have also been placed in orbit around the Earth.

• Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they are not really moons, the term ,,companion“ is being used. It is somewhat similar to the situation with Saturn’s moons Janus and Epimetheus.

• Lilith doesn’t exist but it’s an interesting story.

The Moon is the only natural satellite of Earth:

Orbit: 384,400 km from Earth

Diameter: 3476 km

Mass: 7,35e22 kg

The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the

Sun. as the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon’s phases. The time between successive new moons is 29,5 days (709 hours), slightly different from the Moon’s orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

Due to its size and composition, the Moon is sometimes classified aas a terrestrial ,,planet“ along with Mercury, Venus, Earth and Mars.

The Moon was first visited by the Soviet spacecraft Luna2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969; the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and aagain in 1999 by Lunar Prospector. The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon’s gravitational attraction is stronger on the side of the Earth nearest to the MMoon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth’s surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so

the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day.

But the Earth is not completely fluid, either. The Earth’s rotation carries the Earth’s bulges slightly ahead of the point directly bbeneath the Moon. this means that the force between the Earth and the Moon is not exactly along the line between their centers producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of traditional energy from the Earth and the Moon, slowing down the Earth’s rotation by about 1,5 milliseconds/century and rising the Moon into a higher orbit by about 3,8 centimeters per year. (the opposite effect happens to satellites with uunusual orbits such as Phobos and Triton). The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth’s rotation is now being slowed by the Moon’s influence so in the distant past the Moon’s rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon’s rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earth’s rotation will be slowed to match the Moon’s period, too, as is the case with Pluto and Charon.

Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side was completely unknown until the Soviet spacecraft Luna3 pphotographed it in 1959.

The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon’s south pole which are permanently shaded. This has now been confirmed by Lunar Prospector. There is apparently ice at the north pole as well. The cost of future lunar exploration just got a lot cheaper.

The Moon’s crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon’s mass). Unlike the Earth, however, the Moon’s interior is no longer active. Curiously, the Moon’s center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side.

There are two primary types of terrain on the Moon: the heavily crater and very old highlands and the relatively smooth and younger maria. The maria (which comprise about 16% of the Moon’s surface) are huge impact craters that were later flooded by molten lava. Most of the surface is ccovered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknown reason, the maria are concentrated on the near side.

Most of the craters on the near side are named for famous figures in the history og science such as Tycho, Copernicus and Ptolemaeus. Features on the far side have more modern references such as Apollo, Gagarin and Korolev (with a distinctly Russian bias since the first images were obtained by Luna3.

In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the largest impact basin in the solar system and Orientale on the western limb (as seem from Earth; in the center of the image at left) which is a splendid example of a multi-ring crater.

A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these

precious samples.

Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the solar system not available on the Earth.

Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion wwhich asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large object (as big as Mars or more) aand that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.

The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism iindicating that there may have been a global magnetic field early in the Moon’s history.

With no atmosphere and no magnetic field, the Moon’s surface is exposed directly to the solar wind. Over its 4 billion year lifetime many irons from the solar wind have become embedded in the Moon’s regolith.

Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind.

The Earth’s mass is about 5.98 x 1024 kg.

The Earth’s axis is tilted from perpendicular to the plane of the ecliptic by 23.450 . this tilting is what gives us the four seasons of the year: Summer, Spring, Winter and Autumn. Since the axis is tilted, different parts of the globe are oriented ttowards the Sun at different times of the year. This affects the amount of sunlight each receives.

STORY ABOUT THE MOON

American astronaut Neil Armstrong became the first man to set foot on the Moon in July 1969.

Human footprints on the lunar surface won’t disappear for millions of years. That’s because there’s no rain or wind to erode them.

On average, Earth’s nearest neighbour is 384.00 miles away. A train travelling at 161 kilometers per hour would take just under 100 days tto travel that distance.

So far, astronauts have brought back 382 kilos of rock and dust from the Moon.

The lunar surface area is 25% larger than Africa. It takes the Moon 27.3 days to travel around the Earth. As it does so, we see different amounts of its sunlit side. That’s why it seems to get larger and then smaller. It’s important to build lunar bases as a starting point for longer journeys into the solar system. The first base should be completed by 2010. Only 20 or 30 scientists will live in it. The base will have its own oxygen and water under a large roof or ,,dome“. This will make it possible for the astronauts to live and work without spacesuits. It also means that they’ll be able to grow food.

If bases like this first one are a success, lunar cities will quickly follow. These will have schools, cinemas, roads, offices and universities. Thousands of people will travel from Earth and live on them. Some 21st century citizens may even be born, live and die on the Moon.

GENERAL INFORMATION ON MARS

Mars is the fourth planet from the Sun and the seventh largest:

Orbit: 227,940,000 km (1,52 AU) ffrom the Sun

Diameter: 6,794 km

Mass: 6,4219e23 kg

Mars (Greek: Ares) is the god of War. The planet probably got this name due to its red color, Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of

agriculture before becoming associated with the Greek Ares, those in favor of colonizing and tcrraforming

Mars may prefer this symbolism.) The name of the month March derives from Mars.

Mars has been known since prehistoric times. It is still a favorite of science Fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous „canals“ „seen“ by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses.

The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Maps 2. The

first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars

Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers „Spirit“ and“Opportunity“ landed on Mars sending back geologic data and many pictures. Mars’ orbit is significantly elliptical. One result of this is a temperature variation of about 330 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars’ climate. While the

average temperature on Mars is about 21 8 K (-55 C, -67 F), Martian surface temperatures range widely

from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer. Though Mars is much smaller than Earth, its surface area is about the same as the land

surface area of Earth. Except for Earth, Mars has the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular:

• Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.

• Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 1 0 km high.

• Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep;

• Hellas Plaitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter.

Much of the Martian surface is very old

and cratered, but there are also much younger rift valleys, ridges, hills and plains.

The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that tthey are due to a very large impact shortly after Mars’ accretion). Mars Global Surveyor.has produced a nice 3D map ot’Mars that clearly shows these features.

The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth’s and a thin crust. Data from Mars Global Surveyor indicates that MMars’ crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. Mars’ relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction oof sulfur in addition to iron (iron and iron sulfide).

Like Mercury and the Moon, Mars appears to lack active plate tectonics at present; there is no evidence of recent horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity, however.

There is very clear evidence of erosion in many places on Mars including large floods and small river systems. At some time in the past there was clearly some sort of fluid oon the surface. Liquid water is the obvious fluid but other possibilities exist. There may have been large lakes or even oceans; the evidence for which was strenghtened by some very nice images of layered terrain taken by Mars Global Surveyor and the mineralology results from MER Opportunity. But it seems that this occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. (Valles Marineris was NOT created by rrunning water. It was formed by the stretching and cracking of the crust associated with die creation of the Tharsis bulge.)

Early in its history, Mars was much more like Earth. As with Earth almost aii of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth’s plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun. Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth’s) but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for-months. Mars’ thin atmosphere produces a greenhouse effect but it is only enough tto raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.

Mars has permanent ice caps at both poles composed of water ice and solid carbon dioxide („dry ice“). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the norther summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. ESA’s Mars Express has shown that a similar layer of water ice exists below the southern cap as well. The mechanism responsible for the layering is unknown but may be due to climatic change related to long-term changes in the inclination of Mars’ equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).

Rccent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars’ atmosphere now seems to be both colder and dryer than measured by the Viking landers.

The Viking landers pperformed experiments to determine the existence of life in Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the

most favorable locations. More experiments will be done by future missions to Mars. A small number of meteorites the SNC meteorites) are believed to have originated on Mars.

(On 1996 Aug 6, David McKay et al announced the first identification of organic compounds in a Martian meteorite. The authors further suggest that these compounds, in conjunction with a number of other mineralogical features observed in the rock, may be evidence of ancient Martian microorganisms.

Exciting as this is, it is important to note while this evidence is strong it by no means establishes the fact of extraterrestrial life. There have also been several contradictory studies published since the McKay paper. Remember, „extraordinary claims require extraordinary evidence.“ Much work remains to be done before we can be confident of this most extraordinary claim.

Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding made by Mars

Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars’ interior and for the past history of its atmosphere and hence for the possibility of ancient life.

When it is in the nighttime sky, Mars is easily visible with the unaided eye. Its apparent brightness varies greatly according to its relative position to the Earth. There are several WWeb sites that show the current position of Mars (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Mars’ Satellites:

Mars has two tiny satellites which orbit very close to the martian surface:

Satellite Distance (000 km) Mass (kg) Radius (km)

Phobos 9 11 1,08e16

Hall 1877

Deimos 23 6 1,80e15

Phobos („FOH bus“) is tlie larger and innermost of Mars’ two moons. Phobos is closer to its primary than any other moon in the solar system, less than 6000 km above the surface of Mars. IIt is also one of the smallest moons in the solar system.

orbit: 9378 km from the center of Mars

diameter: 22.2 km (27 x 21.6 x 18.8)

mass: I,08el6 kg

In Greek mythology, Phobos is one of the sons of AAres (Mars) and Aphrodite (Venus), „phobos“ is Greek for „fear“ (the root of „phobia“). Discovered J 877 August 18 by Hall; photographed by Mariner 9 in 1971, Viking I in 1977. and Phobos in 1988.

Phobos orbits Mars below the synchronous orbit radius. Thus it rises in the west, moves very rapidly across the sky and sets in the east, usually twice a day. It is so close to the surface that it cannot be seen above the horizon from all points on the surface of Mars.

And Phobos is doomed: because its orbit is below synchronous altitude tidal forces are lowering its orbit (current rate: about 1.8 meters per century). In about 50 million years it will either crash oonto the surface of Mars or (more likely) break up into a ring. (This is the opposite effect to that operating to raise the orbit of the Moon.)

Phobos and Deimos may be composed of carbon-rich rock like C-type asteroids. But their densities are so low that they cannot be pure rock. They are more likely composed of a mixture of rock and ice. Both are heavily cratered. New images from Mars Global Surveyor indicate that Phobos is covered with aa layer of fine dust about a meter thick, similar to the regolith on the Earth’s Moon.

The Soviet spacecraft Phobos2 detected a faint but steady outgassing from Phobos. Unfortunately, Phobos 2 died before it could determine the nature of the material; water is the best bet. Phobos 2 also returned a few images.

The most prominent feature on Phobos is the large crater named Stickney, the maiden name of Hall’s wife. Like Mimas’ crater Herschel (on a smaller scale) the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on me surface were probably also caused by the Stickney impact.

Phobos and Deimos are widely believed to be captured asteroids. There is some speculation that they originated in the outer solar system rather than in the main asteroid belt.

Phobos and Deimos may someday be useful as „space stations“ from which to study Mars or as intermediate slops to and from the Martian surface; especially if the presence of ice is confirmed.

Deimos („DEE mos“) is the smaller and outermost of Mars’ two moons. It is one of the smallest known moons in the solar system

orbit: 23,459 km from Mars

Diameter: 12,6 km (15 x 12.2 x 11)

mass: 1.8el5 kg

In Greek mythology, Deimos is one of the sons of Ares (Mars) and Aphrodite (Venus); „deimos“ is Greek for „panic“. Discovered 1877 August 12 by Hall, photographed by Viking1 in 1977.

Deimos and Phobos are composed of carbon-rich rock like C-type asteroids and ice. Both are heavily cratered. Deimos and Phobias are probably asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars.

MARS

„The Red Planet“

Mars, the red planet, is the fourth planet from the sun and the most Earth-like planet in our solar system. It is about half the size of Earth and has a dry, rocky surface and a very thin atmosphere.

MARS’ SURFACE

The surface of Mars is dry, rocky, and mostly covered with iron-rich dust. There are low-lying plains in the northern hemisphere, but the southern hemisphere is dotted with impact craters. The ground is frozen; this permafrost extends for several kilometers. The north and south poles of Mars are covered by ice caps composed of frozen carbon dioxide and water.

Scientists have long thought that there is no liquid water on the surface of Mars now, but recent photos from Mars indicate that there might be ssome liquid water near the surface. The surface of Mars shows much evidence of the effects to ancient waterways upon the landscape; there are ancient, dry rivers and lakes complete with huge inflow and outflow channels. These channels were probably caused by catastrophic flooding that quickly eroded the landscape.

Scientists think that most of the water on Mars is frozen in the land and frozen in the polar ice caps.

Mantle: Silicate rock, probably hotter than the Earth’s mantle at corresponding depths.

Core: The core is probably iron and sulphides and may have a radius of 800-1,500 miles (1,300-2,400 km). More will be known when data from future Mars missions arrives and is analyzed.

MASS AND GRAVITY

Mars’ mass is about 6.42 x IOA23 kg. This is I/9th of the mass of the Earth. A 100-pound person on Mars would weigh 38 pounds.

LENGTH OF A DAY AND YEAR ON MARS

Each day on Mars takes 1.03 Earth days (24.6 hours). A year on Mars takes 687 Earth days; it takes this long for Mars to orbit the sun once.

MARS’ ORBIT

Mars is 1.524 times farther from than the sun than the Earth is. It averages 141.6 million miles (227.9 million km) from the sun. Its orbit

is very

elliptical; Mars has the highest orbital eccentricity of any planet in our Solar System except Pluto.

ATMOSPHERE

Mars has a very thin atmosphere. It consists of 95% carbon dioxide (CO2), 3% nitrogen, and 1.6% argon (there is no oxygen). The atmospheric pressure is only a fraction of that on Earth (about 1% of Earth’s atmospheric pressure at sea ievel), and it varies greatly throughout the year. There are large stores of frozen carbon dioxide at the north and south poles. DDuring the warm season in each hemisphere, the polar cap partly melts, releasing carbon dioxide. During the cold season in each hemisphere, the polar cap partly freezes, capturing atmospheric carbon dioxide. The atmospheric pressure varies widely from season to season; the global atmospheric pressure on Mars is 25% different (there is less air, mostly carbon dioxide) during the (northern hemisphere) winter than during the summer. This is mostly due to Mars’ highly eccentric orbit; Mars is about 20% closer to tthe Sun during the winter than during the summer. Because of this, the northern polar cap absorbs more carbon dioxide than the southern polar cap absorbs half a Martian year later.

Occasionally, there are clouds in Mars’ atmosphere. Most of tthese clouds are composed of carbon dioxide ice crystals or, less frequently, of frozen water crystals.

There are a lot of fine dust particles suspended in Mars’ atmosphere. These particles (which contain a lot of iron oxide) absorb blue light, so the sky appears to have little blue in it and is pink/yellow to butterscotch in color.

TEMPERATURE RANGE

Mars’ surface temperature averages -81 °F (-63 °C). The temperature ranges from a high of 68° F(20° C) to a low of-220° F(-140° C). Mars is much colder than the Earth.

MARS’ MOONS

Mars has 2 tiny moons, Phobos and Deimos. They were probably asteroids that were pulled into orbit around Mars.

SPACECRAFT VISITS

Mariner 4 was the first spacecraft to visit Mars (in 1965). Two Viking sspacecraft landed in 1976. Mars Pathfinder landed on Mars on July 4, 1997, broadcasting photos. For more on the Mars missions, click here.

THE FACE ON MARS

This photograph of the Cydonia Mense region of Mars was taken by NASA’s Mars Global Surveyor in 1998. It is a coincidental alignment of rocks and other geologic formations that happens to look like a human face from this angle.

DISCOVERY OF MARS

Mars has been known since ancient times.

MARS’ NAME AND SYMBOL

Mars was named after the RRoman god of war. Jupiter is die fifth and largest planet in our solar system. This gas giant has a thick atmosphere, 39 known moons, and a dark, barely-visible ring. Its most prominent features are bands across its latitudes and a great red spot (which is a storm).

GENERAL INFORMATION ON JUPITER

Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (318 times Earth):

orbit: 778,330,000 km {5.20 AU) from Sun

diameter: 142,984 km (equatorial)

Jupiter is the fourth brightest objcct in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright „wandering star“. But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter’s four large moons Io, Europa. Ganymede and Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter. This was the first discovery of a center

of motion not apparently centered on the Earth. lt was a major point in favor of Copgniicus’s heliocentric thcory of the motions of the planets (along wilh olher new evidence from his telescope: Ihe phases of Venus aand the mounlains on the Moon). Galileo’s outspoken support of the Copernican theory got him in trouble with the Inguisition. Today anyone can repcal Galileo’s observations (vvithout fear of retribution
using binoculars or an inexpensive telescope.

Jupiter was first visited by Pioneer J O in 1973 and later by Pioneer 11. Voyager l. Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiler for eighl years. It iš štili regularly observed by the Hubbie Space Telescope.

The gas planets do not have solid surfaces, their gaseous material simply gets denser

with depth (Ihe radii and diameters quoted for the planets are for levels corresponding to a pressure of l atmosphere). What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the I atmosphere level).

Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and „rock“. This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neplune have much

less hydrogen and helium.

Our knowledge of the interior of Jupiter (and the other gas pllanets) is highly indirect and likely to remain so for some time. (The data from Galileo’s atmospheric probe goes down only about 150 km below the cloud tops.)

Jupiter probably has a core of rocky material amounting to somelhing like 10 to 15 Earth-masses.

Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter’s interior hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter’s magnetic field. This

layer probably also contains some helium and traces of various „ices“.

The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.

Recent experiments

have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers.

Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide

and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected tlie topmost layer while another may have seen the second). But the probe’s entry ppoint was unusual – Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe cntry site may well have been one of the warmest and least cloudy areas on Jupiter at that time.

Data from the Galileo atmospheric probe also indicate that there is much less water than expected. The

expectation was that Jupiter’s almosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun. But it nnow appears that the

actual concentration much less than the Sun’s. Also surprising was the high temperature and density of the uppermost parts of the almosphere.

Jupiter and the other gas planets have high velocity winds which are confined in wide bbands of latitude.

The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences

between these bands are responsible for the colored bands that dominate the planel’s appearance. The light colored bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data

from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter’s atmosphere was also found to be quite turbulent. This indicates tthat Jupiter’s winds are driven in large part by its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter’s clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter’s atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colors, but the details are unknown. The colors correlate with the cloud’s altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the llower layers through holes in the upper ones.

The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is

usually attributed to Cassini. or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen

on Saturn and Neptune. It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter’s liquid layers and is probably responsible for the complex motions we see in tthe cloud tops. Satum and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not.

Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at leasl 80 times more massive to become a star).

Jupiter has a huge magnetic field, much stronger than Earth’s. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter’s magnetosphere is far from spherical – it extends „only“ a few million kilometers in the direction toward the Sun.) Jupiter’s moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travelers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter’s magnetic field. This „radiation“ is similar to, but much more intense than, that found within Earth’s Van Allen belts. It would be iimmediately fatal to an unprotected human being.

The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter’s ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth’s Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin.

Jupiter has rings like Saturn’s, but much fainter and smaller. They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after traveling 1 billion km it was at least worth a quick look to see if any rings might be present. Everyone else thought that the chance of finding anything was nil, but there they were. It was a majorcoup. They have since been imaged in the infra-red from ground-based telescopes and by Galileo.Unlike Saturn’s, Jupiter’s rings are dark (albedo about .05). They’re probably composed of very small grains of rocky material. Unlike Saturn’s rings, they seem to contain no ice.

Particles in Jupiter’s rings probably don’t stay there for long (due to atmospheric and magnetic drag). The Galileo spacecraft found clear evidence that the rings are continuously resuppl led by dust formed by micrometeor impacts on the

four inner moons, which are very energetic because ofJupiter’s large gravitational field. The inner halo ring is broadened by interactions with Jupiter’s magnetic field.

July 1 994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results. The

effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST.

When it is in the nighttime sky, Jupiter is often the brightest „star“ in the sky (it is second only to Venus, which is sseldom visible in a dark sky).

The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope.

There are several Web sites that show the current position of Jupiter (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Jupiter’s Satellites

Jupiter has 63 known satellites (as of Feb 2004): the four large GaliJean moons, 34 smaller named ones, plus many mmore small ones discovered recently but not yet named:

Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them ffarther from Jupiter.

Io, Europa and Ganymede are locked together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is almost part of this as well. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede (eight times the period of Io).

Jupiter’s satellites are named for other figures in the life of Zeus (mostly his numerous lovers).

Many more small moons have been discovered recently but have not as yet been officially confirmed or named. The most up to date info on them can be found at Scott Sheppard’s site.

Jupiter’s Rings

Metis (^MEEtis1′ sav/metis.au) is the innermost of Jupiter’s known satellites:

orbit: 128,000 km from Jupiter

diameter: 40 km

mass: 9.56el6 kg

Metis wwas a Titaness who was the first wife of Zeus (Jupiter).

Discovered by Synnott in 1979 (Voyager 1).

Metis and Adrastea lie within Jupiter’s main ring. They may be the source of the material comprising the rmf

Small satellites within a planet’s rings are sometimes called „mooms“.

Adrastea, the distributor of rewards and punishments, was the daughter of Jupiter and Ananke. Discovered by graduate student David Jewitt (working under Danielson) in 1979 (Voyager1).

Metis and Adrastea orbit inside the synchronous orbit radius and inside the RRoche limit. They may be small enough to avoid tidal disruption but their orbits will eventually decay.Adrastea is one of the smallest moons in the solar system.

Amallhea („am al THEE uh“) is the third of Jupiter’s known satellites:

orbi t : 181,300 km from Jupiter

Diameter: 189 km (270 x 166 x 150)

Mass: 3,5e18 kg

Amalthea was the nymph who nursed the infant Jupiter with goat’s milk.

Discovered by Barnard 1892 September 9 using the 36 inch (91 cm) refractor at Lick Observatory. Amalthea was the last moon to be discovered by direct visual observation.

.Amalthea and Himalia are Jupiter’s fifth and sixth largest moons; they are about the same size but only 1/15 the size of next larger one, Europa.

Like most of Jupiter’s moons, Amalthea rotates synchronously; its long axis is pointed toward Jupiter. Amalthea is the reddest object in the solar system. The reddish color is apparently due to sulfur originating from lo.

Earlier it was thought that its size and irregular shape should imply that Amalthea is a fairly strong, rigid body. But measurements of it’s mass made during Galileo’s last orbit indicate otherwise. It now appears that Amalthea’s density is only about the same as water and since iit is unlikely to be composed of ice it is most likely a loose „rubble pile“ with a lot of empty spaces.

Like lo, Amalthea radiates more heat than it receives from the Sun (probably due to the electrical currents induced by Jupiter’s magnetic field).

Thebe („THEE bee“) is the fourth of Jupiter’s known satellites:

orbit: 222,000 km from Jupiter

Diameter: 100 km (100 x 90)

Mass: 7,77e17 kg

Thebe was a nymph, daughter of the river god Asopus.

Discovered by Synnott in 1979 (Voyager1).

The image above shows Thebe’s leading side which has three or four large (compared to Thebe’s size) craters. The image at left shows the trailing

side.

Io ( „EYE oh“ sav/io.au) is the fifth of Jupiter’s known satellites and the third largest; it is the innermost of the Galilean moons. lo is slightly larger than Earth’s Moon.

orbit : 422,000 km from Jupiter

diameter: 3630 km

mass: 8,93e22 kg

The pronunciation „EE oh“ is also acceptable.

Io was a maiden who was loved by Zeus (Jupiter) and transformed into a heifer in a vain attempt to hide her from the jealous Hera.

Discovered by Galileo and Marius in 1610.

In contrast to most of the moons in the outer solar system, lo and Europa may be somewhat ssimilar in bulk composition to the terrestrial planets, primarily composed of molten sih’cate rock. Recent data from Galileo indicates mat lo has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.

Io’s surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and from their number per unit area to estimate the age of lo’s surface. But there are very few, if any; impact craters on Io. Therefore, the surface is very young.

Instead of craters, Voyager 1 found hundreds of volcanic caideras. Some of the volcanoes are active! Striking photos of actual eruptions with plumes 300 km high were sent back by both Voyagers and by Galileo

This may have been the most important single discovery of the Voyager missions; it was the first real proof that the interiors of other „terrestrial“ bodies are actually hot and active. The material erupting from lo’s vents appears to be some form of sulfur or sulfur dioxide.

The volcanic eruptions change rapidly. In just

four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.

Recent images taken with NASA’s Infrared Telescope Facility on Mauna Kea, Hawaii show a new and very large eruption. A large new feature near Ra Patera has also been seen by HST. Images from Galileo also show many changes from the time of Voyager’s encounter. These observations confirm that lo’s surface is very active iindeed.

So has an amazing variety of terrains: caideras up to several kilometers deep, lakes of molten sulfur, mountains which are apparently NOT volcanoes, extensive flows hundreds of kilometers long of some low viscosity fluid (some form of sulfur?), and volcanic vents. Sulfur and its compounds take on a wide range of colors which are responsible for lo’s variegated appearance.

Analysis of the Voyager images led scientists to believe that the lava flows on lo’s surface were composed mostly of various compounds oof molten sulfur. However, subsequent ground-based infra-red studies indicate that they are too hot for liquid sulfur. One current idea is that lo’s lavas are molten silicate rock. Recent HST observations indicate that the material may be rich in sodium. OOr there may be a variety of different materials in different locations.

Some of the hottest spots on lo may reach temperatures as high as 2000 K though the average is

much lower, about 130 K. These hot spots are the principal mechanism by which lo loses its heat.

The energy for all this activity probably derives from tidal interactions between lo, Europa, Ganymede and Jupiter. These three moons are locked into resonant orbits such that lo orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though lo, like Earth’s Moon always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bbends lo by as much as 100 meters (a 100 meter tide!) and generates heat the same way a coat hanger heats up when bent back and forth. (Lacking another body to perturb it, the Moon is not heated by Earth in this way.)

Io also cuts across Jupiter’s magnetic field lines, generating an electric current. Though small compared to the tidal heating, this current may carry more than 1 trillion watts. It also strips some material away from lo which fforms a torus of intense radiation around Jupiter. Particles escaping from this torus are partially responsible for Jupiter’s unusually large magnetosphere.

Recent data from Galileo indicate that lo may have its own magnetic field as does Ganymede.

lo has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.

Unlike the other Galilean satellites, lo has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive oft“ the volatile elements in the vicinity of lo but not so hot to do so farther out.

Europa („yoo ROH puh“) is the sixth of Jupiter’s known satellites and the fourth largest; it is the second of the Galilean moons. Europa is slightly smaller than the Earth’s Moon.

orbit: 670,900 km from Jupiter

diameter: 3138 km

mass: 4.80e22 kg

Europa was a Phoenician princess abducted to Crete by Zeus, who had assumed the form of a white bull, and by him the mother of Minos.

Discovered by Galileo and Marius in 1610.

Europa and Io are somewhat similar in bulk composition to the terrestrial planets: primarily composed of silicate rock. Unlike lo, however, Europa has a thin outer layer of ice. Recent data from Galileo indicate tthat Europa has a layered internal structure perhaps with a small metallic core.

But Europa’s surface is not at all like anything in the inner solar system. It is exceedingly smooth: few features more than a few hundred meters high have been seen. The prominent markings seem to be only albedo features with very low relief.

There are very few craters on Europa; only three craters larger than 5 km in diameter have been found. This would seem to indicate a young and active surface. However, the Voyagers mapped only a fraction of the surface at high resolution. The precise age of Europa’s surface is an open question.

The images of Europa’s surface strongly resemble images of sea ice on Earth. It is possible that beneath Europa’s surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities.

Europa’s most striking aspect is a series of dark streaks crisscrossing the entire globe. The larger ones are roughly 20 km across with diffuse outer edges and a central band of lighter mmaterial. The latest theory of their origin is that they are produced by a series of volcanic eruptions or geysers.

Recent observations with HST reveal that Europa has a very tenuous atmosphere (le-11 bar) composed of oxygen. Of the many moons in the solar system only five others (lo, Ganymede. Callisto, Titan and Triton) are known to have atmospheres. Unlike the oxygen in Earth’s atmosphere, Europa’s is almost certainly not of biologic origin. It is most likely generated by sunlight and charged particles hitting Europa’s icy surface producing water vapor which is subsequently split into hydrogen and oxygen. The hydrogen escapes leaving the oxygen.

The Voyagers didn’t get a very good look at Europa. But it is a principal focus of the Galileo mission. Images from Galileo’s first two close encounters with Europa seem to confirm earlier theories that Europa’s surface is very young: very few craters are seen, some sort of activity is obviously occurring. There are regions that look

very much like pack-ice on polar seas during spring thaws on Earth. The exact nature of Europa’s surface and interior is not yet clear but the evidence is now strong for a subsurface ‘ocean’. Galileo has found that

Europa has a weak magnetic field (perhaps 1/4 of the strength of Ganymede’s). And most interestingly, it varies periodically as it passes thru Jupiter’s massive magnetic field. This is very strong evidence that there is a conducting material beneath Europa’s surface, most likely a sally ocean Ganymede („GAN uh meed“) is the seventh and largest of Jupiter’s known satellites. Ganymede is the third of the Galilean moons.

orbit: 1,070,000 km from Jupiter

diameter: 5262 km

mass: 1.48e23 kg

Ganymede was a Trojan bboy of great beauty whom Zeus carried away to be cup bearer to the gods. Discovered by GaHlco and Marius in 1610.

Ganymede is the largest satellite in the solar system. It is larger in diameter than Mercury but only about half its mass. Ganymede is much larger than Pluto.

Before the Galileo encounters with Ganymede it was thought that Ganymede and Callisto were composed of a rocky core surrounded by a large mantle of water or water ice with an ice ssurface (and that Titan and Triton were similar). Preliminary indications from the Galileo data now suggest that Callisto has a uniform composition while Ganymede is differentiated into a three layer structure: a small molten iron or iron/sulfur core surrounded by aa rocky silicate mantle with a icy shell on top. In fact, Ganymede may be similar to lo with an additional outer layer of ice.

Ganymede’s surface is a roughly equal mix of two types of terrain: very old, highly cratered dark regions, and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (ngni). i neir origin is cieany or a rectomc nature, but tne details arc unknown. In this respect, Ganymede may be more similar to the Earth than either Venus or Mars (though there is no evidence of recent tectonic activity).

Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on F.uropa. has been found recently by fftST (note that this is definitely NOT evidence of life).

Similar ridge and groove terrain is seen on Hnceladus. Miranda and Ariel. The dark regions are similar to the surface of Callisto.

Extensive cratering is seen on both types of terrain. The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon, Craters both overlay and are cross cut by the groove systems indicating the the grooves are quite ancient, too. Relatively young craters with rays oof ejecla are also visible.

Unlike the Moon, however, the craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede’s icy crust which can flow over geologic time and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a „ghost“ of a crater are known as palimpsests.

Galileo’s first flyby of Ganymede discovered that Ganymede has its own magnetosphere field embedded inside Jupiter’s huge one. This is probably generated in a similar fashion to the Earth’s: as a result of motion of conducting material in the interior.

Callisto („ka LIS loh“) is the eighth of Jupiter’s known satellites and the second largest. It is the outermost of the Galilean moons.

orbit: 1,883,000 km from Jupiter

diameter: 4800 km

mass: 1.08e23 kg

Calisto was a nymph, beloved of Zeus and hated by Hera. Hera changed her into a bear and Zeus then placed her in the sky as the constellation Ursa Major.

Discovered by Galileo and Martus in 1610.

Calisto is only slightly smaller than Mercury but only a third of its mass.

Unlike Ganymede, Callisto seems to have little internal structure; however there arc ssigns from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron. Titan and Triton are probably similar.

Callisto’s surface is covered entirely with craters. The surface is very old, like the highlands of the Moon and Mars. Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.

The largest craters are surrounded by a series of concentric rings which iook like huge cracks but which have been smoothed out by eons of slow movement of the ice. The largest of these has been named Valhalla. Nearly 3000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result of a massive impact. Other examples arc Caiiisto’s Asgard (left). Mare Orientale on the Moon and Caloris Basin on Mercury.

Like Ganymede, Callisto’s ancient craters have collapsed. They lack the high ring mountains, radial rays and central depressions common to craters on the Moon and Mercury. Detailed images from Galileo show that, in some areas at least, small craters have mostly been obliterated. TThis suggests that some processes have been at work more recently, even if its just slumping.

Another interesting feature is Gipul Catena, a long series of impact craters lined up in a straight Sine. This was probably caused by an object that was lidally disrupted as it passed close to Jupiter (much like Comet SL 9) and then impacted on Callisto.

Callislo has a very tenuous atmosphere composed of carbon dioxide.

Galileo has detected evidence of aweak magnetic field which may indicate some sort of salty fluid below the surface. Unlike Ganymede, with its complex terrains, there is little evidence of tectonic activity on Callisto. While Cailisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history. The different geologic histories of the two has been an important problem for planetary scientists; (it may be related to the orbital and tidal evolution of Ganymede). „Simple“ Callisto is a good reference for comparison with other more complex worlds and it may represent what the other Galilean moons were like early in their history.

Leda

Jupiter XIII

Leda („LEE duh“) is the ninth of Jupiter’s known satellites and the smallest:

orbit: 11,094,000 km from Jupiter

diameter: 16 km

mass: 5.68el5 kg

Leda was queen

of Sparta and the mother, by Zeus in the form of a swan, of Pollux and Helen of Troy. Discovered by Kowal in 1974. Leda, Ananke, and Sinope are among the smallest moons in the solar system.

Himalia

Jupiter VI

Himalia („hih MAL yuh“) is the tenth of Jupiter’s known satellites:

orbit: 11,480,000 km from Jupiter

diameter: 186 km

mass: 9.56el8 kg

Himalia was a nymph who bore three sons of Zeus (Jupiter). Discovered by Pcrrinc in 1904. Unlike the inner satellites, the orbits oof Leda, Himalia, Lysithea and Elara are significantly inclined to Jupiter’s equator (about 28 degrees). ‘

Lysithea

Lysilhea („ly SITI1 ee uh“) is the eleventh of Jupiter’s known satellites:

orbit: 11,720,000 km from Jupiter

diameter: 36 km

mass: 7.77el6 kg

Lysilhea was a daughter of Oceanus and one of Zeus’ lovers. Discovered bv Nicholson in 1938.

Klara

Jupiter VII

Elara („EE !ai uh“) is the twelfth of Jupiter’s known satellites:

Orbit: 11,737,000 km from Jupiter

diameter: 76 km

mass: 7.77el7 kg

Blara was the mother by Zeus of the ggiant Tityus. Discovered by Perrinc in 1905. 1 ,eda, Himalia, Lysithea and Elara may be remnants of a single asteroid that was captured by Jupiter and broken up.

Ananke

Jupiter XII

Ananke („a NANG kee“) is the thirteenth of Jupiter’s known satellites:

orbit: 221,200,000 km from Jupiter

Diameter:30km

mass: 3.82el6 kg

Ananke was the mother of Adrastea, by Jupiter.

Discovered by Nicholson in 1951.

Ananke, Carme, Pasiphae and Sinope have unusual but similar orbits.

Carme

Jupiter XI

Carme („KAR mee“) is the fourteenth of Jupiter’s known satellites:

orbit: 22,600,000 km from Jupiter

diameter: 40 km

mass: 9-56el6 kg

CaniiE was the mother, by Zeus of Britomartis, a Cretan goddess. Discovered by Nicholson in 1938. Ananke, Carme, Pasiphae and Sinope are especially unusual in that their orbits are retrograde.

Pasiphae

Jupiter VIII

Pasiphae („pah SIF ah ee“) is the fifteenth of Jupiter’s known satellites:

orbit: 23,500,000 km from Jupiter

diameter: 50 km

mass: 1.91el7 kg

Pasiphae w;is the wile of Minos and mother by a white bull, of the Minotaur. Discovered by P. Melotte in 1908. Ananke, CCarme, Pasiphae and Sinope have orbits highly inclined to Jupiter’s equator (about 150 degrees).

Sinope

Jupiter IX

Sinope („sah NOH pee“) is me outermost of Jupiter’s known confirmed satellites:

orbit: 23,700,000 km from Jupiter

diamet e r : 3 6 km

mass: 7.77el6 kg

Sinope was a woman said to have been unsuccessfully (!) courted by Zeus. Discovered by Nicholson in 1914. Anankc, Carme, Pasiphae and Sinope may be remnants of a single asteroid that was captured by Jupiter and broken up.

GENERAL INFORMATION ON SATURN

Saturn iis the sixth planet from the Sun and the second largest:

Orbit: 1.429.400.000 km (9.54 AU) from the Sun

Diameter: 120.536 km (equatorial)

Speed: 5.68e26 kg

In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and father of Zeus (Jupiter). Saturn is the root of the English word ,,Saturday“.

Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes though the plane of Saturn’s rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn’s rings remained unique in the known solar system until 1977 when

very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).

Saturn was first visited by Pioneer11 in 1979 and later by Voyager1 and Voyager2. Cassini arrived on July 1, 2004 and will orbit Saturn for at least four years.

Saturn is visibly flattened (oblate) when viewed tthrough a small telescope; its equatorial and polar diameters vary by almost 10% (120.536 km vs. 108.728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so. Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and ,,rock“, similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Saturn’s interior is similar to Jupiter’s consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

Saturn’s interior is hot (12000K at the core) and Saturn radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not sufficient to explain Saturn’s luminosity; some additional mechanism may be at work, perhaps the ,,raining out“ of helium deep in Saturn’s interior.

The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the ccloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of Saturn’s atmospheric circulation could be studied. Saturn also exhibits long-lived ovals and other features common on Jupiter. In 1990, HST observed enormous white cloud near Saturn’s equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed.

Two prominent rings (A and B) and one faint ring (C) can seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Divission (but this is somewhat of a misnomer since it was very likely never seen by Encke). Saturn’s rings, unlike the rings of the other planets, are very bright.

Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely.

Saturn’s rings are extraordinarily thin: though they’re 250.000 km or more in diameter they’re less than one kilometer thick. Despite their impressive appearance, there’s really very

little material in the rings – if the rings were compressed into a single body it would be no more than 100 km across.

The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called ,,spokes“ which were first reported by amateur astronomers. Their nature remains a mystery, but may have something to do with Saturn’s magnetic field.

Saturn’s outermost rring, the F-ring, is a complex structure made up of several smaller rings along which ,,knots“ are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager1 images is not seen in the Voyager2 images perhaps because Voyager2 imaged regions where the component rings are roughly parallel. They are prominent in the Cassini images which also show some as yet unexplained wispy spiral structures.

There are complex tidal rresonances between some of Saturn’s moons and the ring system: some of the moons, the so-called ,,shepherding satellites“ (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping the rings in place; Mimas seems to be responsible for the paucity oof material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is located inside the Encke Division. The whole system is very complex and as yet poorly understood.

The origin of the rings of Saturn (and the other jovian planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable and must be regenerated by ongoing processes, perhaps the breakup of larger satellites. The current set of rings may be only a few hundred million years old.

Like the other jovian planets, Saturn has a significant magnetic field.

When it is in the nighttime sky, Saturn is easily visible to the unaided eye. Though it is nnot nearly as bright as Jupiter, it is easy to identify as a planet because it doesn’t ,,twinkle“ like the stars do. The rings and the larger satellites are visible with a small astronomical telescope. There are several Web sites that show the current position of Saturn (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Saturn’s satellites

Saturn has 30 named satellites plus one discovered in 2003 and two in 22004 that are as yet named:

• Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously.

• The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact gravitationally in such a way as to maintain stable relationships between their orbits: the period of Mimas’ orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance.

Satellite Distance

(*1000 km) Radius

(km) Mass

(kg) Discoverer Data

Pan 134 10 ? Showalter 1990

Atlas 138 14 ? Terrile 1980

Prometheus 139 46 2,70e17 Collins 1980

Pandora 142 46 2,20e17 Collins 1980

Epimetheus 151 57 5,60e17 Walker 1980

Janus 151 89 2,01e18 Dollfus 1966

Mimas 186 196 3,80e19 Herschel 1789

Enceladus 238 260 8,40e19 Herschel 1789

Tethys 295 530 7,55e20 Cassini 1684

Telesto 295 15 ? Reitsema 1980

Calypso 295 13 ? Pascu 1980

Dione 377 560 1,05e21 Cassini 1684

Helene 377 16 ? Laques 1980

Rhea 527 765 2,49e21 Cassini 1672

Titan 1222 2575 1,35e23 Huygens 1655

Hyperion 1481 143 1,77e19 Bond 1848

Iapetus 3561 730 1,88e21 Cassini 1671

Phoebe 12952 110 4,00e18 Pickering 1898

Saturn’s Rings

NAME RADIUS

INNER RADIUS

OUTER WIDTH APPROX.

POSITION APPROX.

MASS (KG)

D-Ring 67,000 74,500 7,500 (ring)

C-Ring 74,500 92,000 17,500 (ring) 1.1e18

Maxwell Divission 87,500 88,000 500 (divide)

B-Ring 92,000 117,500 25,500 (ring) 2.8e19

Cassini Divission 115,800 120,600 4,800 (divide)

Huygens Gap 117,680 (n/a) 285-440 (subdiv)

A-Ring 122,200 136,800 14,600 (ring) 6.2e18

Encke

Minima 126,430 129,940 3,500 29%-53%

Enckle Division 133,410 133,740

Keeler Gap 136,510 136,550

F-Ring 140,210 30-500 (ring)

G-Ring 165,800 173,800 8,000 (ring) 1e7?

E-Ring

180,000

480,000

300,000

(ring)

This categorization is actually somewhat misleading as the density of particles varies in a complex way not indicated by a division into neat regions: there are variations within the rings; the gaps are not entirely empty; the rings are not perfectly circular. Pan is the innermost of Saturn’s known satellites:

Orbit: 133.583 km from Saturn

Diameter: 20 km

Mass: ?

Pan was the god of woods, fields, and flocks, having a human torso and head with a goat’s legs, horn’s, and ears. Pan is within the Encke Division in Saturn’s A ring.

Small moons near the rings produce wave patterns in the rings. Prior to the discovery of Pan, an analysis of the patterns in the edge oof Saturn’s A ring predicted the size and location of a small moon. Pan was discovered by reexamining the 10 year old Voyager photos at the predicted spot. It is possible that there are more moons within Saturn’s rings yet to be discovered.

Atlas is the second of Saturn’s known satellites:

Orbit: 137.670 km from Saturn

Diameter: 30 km (40 x 20)

Mass: ?

Atlas was a Titan condemned by Zeus to support the heavens upon his shoulders; son of Iapetus and the nymph Clymene; brother of Prometheus and Epimetheus.

Prometheus („pra MEE thee us“) is the third of Saturn’s known satellites:

Orbit: 139.350 km from Saturn

Diameter: 91 km (145 x 85 x 62)

Mass: 2.7e17 kg

Prometheus was a Titan who stole fire from Olympus and gave it to humankind, for which Zeus punished him horribly; son of Iapetus; brother of Atlas and Epimetheus. „Prometheus“ is Greek for „foresight“.

Prometheus is the inner shepherd satellite of the F ring.

Prometheus has a number of ridges and valleys and several craters about 20 km in diameter but appears to be less cratered than the neighboring moons Pandora, Janus and Epimetheus.

From their very low densities and relatively high albedos, it seems likely that Prometheus, Pandora, Janus aand Epimetheus are very porous icy bodies.

The 1995/6 Saturn Ring Plane Crossing observations found that Prometheus was lagging by 20 degrees from where it should have been based on Voyager 1981 data. This is much more than can be explained by observational error. It is possible that Prometheus’s orbit was changed by a recent encounter with the F ring, or it may have a small companion moon sharing its orbit.

Epimetheus („ep eh MEE thee us“ say/epimetheus.au) is the fifth of Saturn’s known satellites:

Orbit: 151.422 km from Saturn

Diameter: 115 km (144 x 108 x 98)

Mass: 5.6e17 kg

Epimetheus was the son of Iapetus and brother of Prometheus and Atlas of Pandora. „Epimetheus“ is Greek for „hindsight“. Epimetheus was first observed by Walker in 1966. But the situation was confused since Janus is in a very similar orbit. So Walker officially shares the discovery of Epimetheus with Fountain and Larson who showed in 1977 that there were two satellites involved. The situation was clarified in 1980 by Voyager1.

Epimetheus and Janus are „co-orbital“.

There are several craters larger than 30 km in diameter as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus must be quite

old.

The dark line across the surface in the image above is actually the shadow of the Saturn’s F-ring.

Saturn’s beautiful rings are only visible from Earth using a telescope. Saturn’s bright rings are made of ice chunks (and some rocks) that range in size from the size of a fingernail to the size of a car. Although the rings are extremely wide (almost 185.000 miles = 300.000 km in diameter), they are very thin (about 0.6 miles = 1 km thick).

Saturn’s mmass is about 5.69 x 1026 kg. Although this is 95 times the mass of the Earth, the gravity on Saturn is only 1.08 times the gravity on Earth. This is because Saturn is such a large planet (and the gravitational force a planet exerts upon an object at the planet’s surface is proportional to its mass and to the inverse of its radius squared).

A 100 pound person would only weigh 108 pounds on Saturn.

Saturn is the only planet in oour Solar System that is less dense than water. Saturn would float if there were a body of water large enough.

Each day on Saturn takes 10.2 Earth hours. A year on Saturn takes 29.46 Earth years; it takes 19.46 Earth yyears for Saturn to orbit the Sun once.

At aphelion (the place in its orbit where Saturn is farthest from the Sun), Saturn is 1.503.000.000 km from the Sun. at perihelion (the place in its orbit where Jupiter is closest to the Sun), Saturn is 1.248.000.000 km from the Sun.

The mean temperature on Saturn (at the cloud tops) is 88 K (-1850 C; -2900 F).

Saturn was named for the Roman god of agriculture.

GENERAL INFORMATION ON URANUS

Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.

Orbit: 2,870,990,000 km (19,218 AU) from the Sun

Diameter: 51,118 km (equatorial)

Mass: 8,683e25 kg

Careful pronunciation may be necessary to aavoid embarrassment; say „YOOR a nus“ say/uranus.au, not „your anus“ or „urine us“.

Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods).

Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically searching the sky with his telescope on March 13, 1781. It has actually been seen many times bbefore but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it „the Georgium Sidus“ (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it „Herschel“. The name „Uranus“ was first proposed by Bode in conformity with the other planetary names from classical mythology but didn’t come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager2 on Jan 24, 1986.

Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus’ axis is almost parallel to the ecliptic. At the time of Voyager2’s passage, Uranus’ south pole was pointed almost directly at the Sun. this results in the odd fact that Uranus’ polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter as its equator than at its poles. The mechanism underlying this is unknown.

Actually, there’s an ongoing battle over which of Uranus’ poles is its north pole. Either its axis inclination is a bit over 90 degrees and its rotation is direct, or it’s a bit less than 990 degrees and the rotation is retrograde. The problem is that you need to draw a diving line *somewhere* , because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180). Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed.

Uranus’ atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager2. Recent observations with HST show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw. It now seems clear that the differences are due to seasonal effects since tthe Sun is now at a lower Uranian latitude which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus’s equator.

Uranus’ blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter’s but they are hidden from view by the overlaying methane layer.

Like the other gas planets, Uranus has rings. Like Jupiter’s, they are very dark but like Saturn’s they are composed of fairly large particles ranging up to 10 meters in diameter is addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn’s to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone.

Voyager2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings.

Uranu’s magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to

the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus.

Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars. A small astronomical telescope will show a small disk. There are some several Web sites that show the current position of Uranus (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such ccharts can be created with a planetarium program.

Uranus’ Satellites:

Uranus has 21 named moons and six unnamed ones:

• Unlike the other bodies in the solar system which have names from classical mythology, Uranus’ moons take thei names from the writings of Shakespeare and Pope.

• They from three distinct classes: the 11 small very dark inner ones discovered by Voyager2, the 5 large ones and the newly discovered much more distant ones.

• Most have nearly circular orbits in the plane of Uranus’ equator; the outer 44 are much more elliptical.

Satellite Distance (*1000km) Radius(km) Mass(kg) Discovered Date

Cordelia 50 13 ? Voyager2 1986

Ophelia 54 16 ? Voyager2 1986

Bianca 59 22 ? Voyager2 1986

Cressida 62 33 ? Voyager2 1986

Desdemona 63 29 ? Voyager2 1986

Juliet 64 42 ? Voyager2 1986

Portia 66 55 ? Voyager2 1986

Rosalind 70 27 ? Voyager2 1986

2003U2 75 6 ? Showalter 2003

Belinda 75 34 ? Voyager2 1986

986U10 76 40 ? Voyager2 1986

Puck 86 77 ? Voyager2 1985

2003U1 98 8 ? Showalter 2003

Miranda 130 236 6,30e19 Kuiper 1948

Ariel 191 579 1,27e21 Lassell 1851

Umbriel 266 585 1,27e21 Lassell 1851

Titania 436 789 3,49e21 Herschel 1787

Oberon 583 761 3,03e21 Herschel 1787

2001U3 4281 6 ? Sheppard 2003

Caliban 7169 40 ? Gladman 1997

Stephano 7948 15 ? Gladman 1999

Trinculo 8578 5

Sycorax 12213 80 ? Nicholson 1997

2003U3 14689 6 ? Sheppard 2003

Prospero 16568 20 ? Holman 1999

Setebos 17681 20 ? Kavalaars 1999

2002U2 21000 6 Sheppard 2003

Uranus’ Rings:

Ring Distance (km) Width (km)

1986U2R 38000 2,500

6 41840 1-3

5 42230 2-3

4 42580 2-3

Alpha 44720 7-12

Beta 45670 7-12

Eta 47190 0-2

Gamma 47630 1-4

Delta 48290 3-9

1986U1R 50020 1-2

Epsilon 51140 20-100

GENERAL INFORMATION ON NEPTUNE

Neptune is the eighth planet from the sun in our solar system. This giant, frigid planet has a hazy atmosphere and strong winds. This gas giant is orbited bby eight moons and narrow, faint rings arranged in clumps. Neptune’s blue color is caused by the methane (CH4) in its atmosphere; this molecule absorbs red light.

Neptune cannot be seen using the eyes alone. Neptune was the first planet whose existence was predicted mathematically (the planet Uranus’s orbit was perturbed by an unknown object which turned our to be another gas giant, Neptune).

SIZE

Neptune is about 30,775 miles (49,528 km) in diameter. This is 3.88 times the diameter of the Earth. If Neptune were hollow, it could hold aimosl 60 Earths.

Neptune is the fourth largest planet in our Solar System (after Jupiter, Saturn, and Uranus).

MASS AND GRAVITY

Neptune’s mass is about 1.02 x 1026 kg. This is over 17 times the mmass of the Earth, but the gravity on Neptune is only 1.19 times of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet’s surface is proportional to its mass and to the inverse of its radius squared).

A 100-pound person would weigh 119 pounds on Neptune.

LENGTH OF A DAY AND YEAR ON NEPTUNE

Each day on Neptune takes 19.1 Earth hours. A year on Neptune ttakes 164.8 Earth years; it takes almost 165 Earth years for Neptune to orbit the sun once. Since Neptune was discovered in 1846, it has not yet completed a single revolution around the sun.

NEPTUNE’S ORBIT AND DISTANCE FROM THE SUN

Neptune is about 30 times farther from the sun than the Earth is; it averages 30.06 A.U. from the sun. Occasionally, Neptune’s orbit is actually outside that of Pluto; this is because of Pluto’s highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21,1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.

At aphelion (the point in Neptune’s orbit farthest from the sun) Neptune is 4,546,000,000 km from the sun, at perihelion (the point in Neptune’s orbit closest from the sun) Neptune is 4,456,000,000 km from the sun.

Neptune’s rotational axis is tilted 30 degrees to the plane of its orbit around the Sun (this is few degrees more than the Earth). This gives Neptune seasons. Each season lasts 40 years; the poles are in constant darkness or ssunlight for 40 years at a time.

TEMPERATURE

The mean temperature is 48 K.

DISCOVERY OF NEPTUNE

Neptune’s existence was predicted in 1846, after calculations showed perturbations in the orbit of Uranus. The calculations were done independently by both J.C. Adams and Le_yeirier. Neptune was then observed by J.G. Galle and d’Arrest on September 23, 1846.

SPACECRAFT VISITS

Neptune was visited by NASA’s Voyager 2 in August, 1989. Before this visit, virtually nothing was known about Neptune.

NEPTUNE’S NAME AND SYMBOL

Neptune was named after the mythical Roman god of the seas. Neptune’s symbol is the fishing spear.

Neptune is the eighteen planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus.

Orbit: 4,504,000,000 km (30,60 AU) from Sun

Diameter:49,532km(equatorial)

mass: 1.0247e26 kg

In Roman mythology Neptune {Greek: Poseidon) was the god of the Sea.

After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton’s laws. It was therefore predicted that another more distant planet must be perturbing Uranus’ orbit.

Neptune was first observed by Galle and d’Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on tthe observed positions of Jupiter. Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune’s discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune’s actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.

More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune’s motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.

Neptune has been visited by only one spacecraft. VoyagerJZ on Aug 25 1989. Much of we know about Neptune comes from this single encounter.

But fortunately,

recent ground-based and HST observations have added a great deal, too. Because Pluto’s orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few

years.

Neptune’s composition is probably similar to Uranus’: various „ices“ and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. BBut there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.

Neptune’s blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.

Like a typical gas planet. Neptune has rapid winds confined to bands of latitude and large storms or vvortices. Neptune’s winds are the fastest in the solar system, reaching 2000 km/hour.

Like Jupiter and Saturn, Neptune has an internal heat source — it radiates more than twice as much energy as it receives from the Sun.

At the time oof the Voyager encounter, Neptune’s most prominent feature was the Great Dark Spot in the southern hemisphere. It was about half the size as Jupiter’s Great Red Spot (about the same diameter as Earth). Neptune’s winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as „The Scooter“. It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.

However, HST observations of Nepttine in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being mmasked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune’s northern hemisphere. This indicates that Neptune’s atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.

Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager^’s images showed them to be complete rings with bright clumps. One of the rings appears to have a curious ttwisted Structure. Like Uranus and Jupiter, Neptune’s rings are very dark but their composition is unknown. Neptune’s rings have been given names: the outermost is Adams (which contains three prominent ares now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.

Neptune’s magnetic field is, like Uranus’, oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.

Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.

Neptune’s Satellites

Neptune has 13 known moons; 7 small named ones and Triton plus four discovered in 2002 and one discovered in 2003 which have yet to be named.

GENERAL INFORMATION ON PLUTO

Pluto is the farthest planet from the Sun (usually) and by far the smallest. Pluto is smaller than seven oof the solar system’s moons (the Moon, io, Europa. Ganymede, Callisto. Titan and Triton).

orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)

diameter: 2274 km

mass: 1.27e22 kg

In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name perhaps because it’s so far from the Sun that it is in perpetual darkness and perhaps because „PL“ are the initials of Percival Lowell. Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde .W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no tenth planet.

Pluto is the only planet tthat has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface. There is a planned mission called New Horizons that will launch in 2006 if it gets funded.

Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.

Pluto’s radius is not well known.

Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon’s orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around die center of mass of the system which requires much finer measurements — they’re so small and far away that even I1ST has difficulty. The ratio of their

masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won’t get really accurate data until a spacecraft is sent.

Pluto is the second most contrasty body in the Solar System (after larretus).

There are some who think Pluto would be better classified as a large asteroid or comet rather than as a planet. Some consider it to be the largest ol the Kuipcr Belt objects (also known as Trans-Neptunian Objects). There is considerable merit to tthe latter position, but historically Pluto has been classified as a planet and it is very likely to remain so.

Pluto’s orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January’ 1979 thru February 1! 1999). Pluto rotates in the opposite direction from most of the other planets.

Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto’s orbital period is exactly 1.5 times longer than Neptune’s. Its orbital inclination is aalso much higher than the other planets’. Thus though it appears mat Pluto’s orbit crosses Neptune’s, it really doesn’t and they will never collide.

Like Uranus, the plane of Pluto’s equator is at almost right angles to the plane oof its orbit. The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The „warmer“ regions roughly correspond to the regions that appear darker in optical wavelengths.

Pluto’s composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto’s surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.

Little is known about Pluto’s atmosphere, but it probably consists pprimarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto’s atmosphere may exist as a gas only when Pluto is near its perihelion: for the majority of Pluto’s long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. NASA mission planners want to arrive at Pluto while the atmosphere is still uunfrozen. The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune’s, but this now seems unlikely. A more popular idea is mat Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Qorlcjoud. Like the Earth’s Moon, Charon may be the result of a collision between Pluto and another body.

Pluto can he seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several days will be required to reliably find it. Suitable charts can be created with many planetarium programs.

Charon

Charon („KAlRen“ sav/charon.au) is Pluto’s only known satellite:

orbit: 19,640 km from Pluto

diameter: 1172 km

mass: 1.90e21 kg

Charon is named for the mythological figure who ferried the ddead across the F-Jver Acheron into Hades (the underworld).

(Though officially named for the mythological figure, Charon’s discoverer was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllabic sounding like ‘shard’ („SHAHR en“).

Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.

Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth’s Moon).

Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.

Charon’s radius is not well known. JPL’s value of 586 has an error margin of+/-I3, more than two percent. Its mass and density are also poorly known.

Pluto and Charon are also unique in that not only does Charon rotate synchronoosly but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.)

Charon’s composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn’s icy moons (i.e. Rhea). Its ssurface seems to be covered with water ice. Interestingly, this is quite different from Pluto.

Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.

It has been proposed that Charon was formed by a giant impact similar to the one mat formed Earth’s Moon.

It is doubtful that Charon has a significant atmosphere.

GENERAL INFORMATION

Pluto is the ninth and usually the farthest planet from the sun in our solar system. It is also the smallest planet in our solar system and the last to be discovered. It is smaller than a lot of the other planets’ moons, including our moon. Pluto is the only planet in our solar system that has not been visited by our spacecraft yet. We only have blurry pictures of its surface; even the Hubble Space, Telescope orbiting the Earth can only get grainy photos because Pluto is so far from us.

SIZE

Pluto is about 1,413 miles (2274 km) in diameter. This is about 1/5 the diameter of the Earth. Pluto is the smallest planet in our Solar System.

MASS AND GRAVITY

Pluto’s mass is about 1.29 x 1022 kg. This is about l/500th of the mass of the Earth. The

gravity on Pluto is 8% of the gravity on Earth.

Pluto is the least massive planet in our Solar System

A 100 pound person on Pluto would weigh only 8 pounds.

LENGTH OF A DAY AND YEAR ON PLUTO

Each day on Pluto takes 6.39 Earth days. Each year on Pluto takes 247.7 Earth years (that is, it takes 247.7 Earth years for Pluto to orbit the Sun once).

PLUTO’S ORBIT

Pluto is 39 times farther from than the sun than the Earth is. Pluto ranges ffrom 2.8 to 4.6 biilion miles (4.447 billion to 7.38 billion km) from the Sun. From Pluto, the sun would look like a tiny dot in the sky.

Occasionally, Neptune’s orbit is actually outside that of Pluto; this is because of Pluto’s highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the oorbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.

Orbital Eccentricity

Pluto has a very eccentric orbit; that means that its distance from the sun varies a lot during its orbit around the sun. Sometimes it iis even closer to the Sun than the planet Neptune (it was that way from January 1979 to February 11, 1999)! Pluto also rotates about its axis in the opposite direction from most of the other planets.

Orbital Inclination

Pluto’s orbit is tilted from the plane of the ecliptic. This angle, its orbital inclination, is 17.15°. This is the largest inclination of any of the planets.

TEMPERATURE ON PLUTO

Pluto is VERY, VERY cold. Its temperature may range from between -396°F to -378°F (-238°C to -228°C, or 35 K to 45 K). The average temperature is -393°F (-236°C = 37 K).

PLANETARY COMPOSITION

Pluto’s composition is unknown. It is probably made up of about 70% rock and 30% water. This is determined from density calculations; Pluto’s ddensity is about 2,000 kg/m3. There may be methane ice together with frozen nitrogen and carbon dioxide on the cold, rocky surface.

ATMOSPHERE

Not much is known about Pluto’s atmosphere. It is probably mostly nitrogen with a little carbon monoxide and methane – definitely not brealheable by humans. The atmospheric pressure is probably very low. The atmosphere forms when Pluto is closest to the Sun and the frozen methane is vaporized by the solar heat. When it is farther from the SSun, the methane freezes again. From Pluto, the sky would appear black, even when the Sun (the size of a star) is up.

PLUTO’S MOON

Pluto has one moon, Charon, that is almost as big as Pluto itself. Although Charon is small, about 1,172 km (728 miles) in diameter, it about half of the size of Pluto itself. Charon orbits about 19,640 Ian from Pluto on average, It may be covered by water ice and probably has no atmosphere. Charon is in a synchronous orbit around Pluto. That is, Charon is always over the same spot on Pluto; Charon’s orbit takes exactly one Pluto day.

Charon was discovered by Jim Christy in 1978. Charon was named after the mythological demon who ferried people across the mythological river Styx into Hades.

DISCOVERY OF PLUTO

Pluto was the last planet to be discovered. Planet „X“ was the temporary name given to the then-unknown planet beyond Neptune that disturbed the orbits of Uranus and Neptune. Percival Lowell calculated the rough location of Planet „X’s“ orbit, but died in 1916 before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh in 1930 and named Pluto. He did his observations at the LLowell Observatory in Arizona.

PLUTO’S NAME

Pluto was named after the Roman god of the underworld, Pluto.

Its symbol is the combined letters „P“ and „L,“ either for Percival Lowell or for Pluto.

The name Pluto was suggested by Venetia Bumey of England, who was 11 years old at the time. She suggested the name to her grandfather, who was Librarian at Oxford. He passed her idea to the astronomers who were trying to name the newly-discovered planet.

PLUTO: PLANET, ASTEROID, OR TNO

Pluto’s unusual orbit makes some scientists think that Pluto is not a regular planet, but a „minor planet“ or a Trans Neptunian Obiect (TNO) [Kuiper Belt objects left over from the formation of the solar system]. In the future, Pluto may be listed as an asteroid (it will probably be given the asteroid number 10,000) and also as the first TNO – it will also still be considered a planet, albeit an unusual one.

OUR SUN

Billions of years ago, our galaxy gave birth to an unassuming star. Even today, that star is the only one astronomers can study in detail.

In this ultraviolet image of the Sun, a prominence arcs thousands of miles above the solar surface and white patches reveal mmagneticaHly acive regions. The Solar and Hcliospheric Observatory, SOHO, returned this image in 1997. It’s one of many spacecraft dedicated to monitoring our star.

Approximately 4.6 billion years ago, a cold cloud of gas and dust buried deep in one of the Milky Way galaxy’s spiral arms started to collapse. Perhaps strong winds from a massive star or a shock wave from a nearby supernova explosion triggered the collapse — from our distance in time, we’ll never know for sure.

Whatever the cause, the force of gravity then started to work its magic: The cloud began to contract and fragment. One of those fragments was destined to become our Sun and the rest of the solar system. The other fragments also spawned stars that have long since moved away from their birthplace -— there’s no way to determine which ones might have been our siblings. But while the star-formation process was going full bore, our small part of the galaxy probably looked like the Orion Nebula (M42) or one of the other similar star-forming regions we see around us today.

Let’s head back to our budding solar system. As gravity continued to compress the solar nebula, the central

region that would become the Sun drew in the vast majority of material. Because the nebula was rotating, however, not all of the gas and dust could fall into the proto-Sun being forged at the center. Instead, some of it formed a disk that ultimately would condense into the planets and other, smaller members of the solar system.

The proto-Sun continued to contract and, as it did so, grew hotter. This persisted until its centra! temperature rose high enough to ignite tthe fires of nuclear fusion. The heat created by these nuclear reactions produced a pressure that counteracted the inward pull of gravity, and the object became the stable star we call the Sun.

The solar corona bursts into view at totality during the December 4, 2002, total solar eclipse. The Sun continues lo produce energy in the same way. In the core, where temperatures reach 15 million keivins (about 27 million degrees Fahrenheit), positively charged protons (the nuclei of hydrogen aatoms) can overcome their mutual repulsion and fuse together. In essence, four hydrogen nuclei combine into one helium nucleus in a process called the proton-proton chain. Because the helium nucleus weighs a little less than the four hydrogen nuclei combined, tthe reactions create energy according to Einstein’s famous equation E^mc2. To keep the Sun shining, about 600 million tons of hydrogen must be converted to helium every second. Despite this prodigious consumption, the Sun has enough hydrogen to keep shining for another 5 to 6 billion years.

It can take a million years or more for the energy created at the Sun’s center to fight its way to the surface, where it gets radiated into space. Despite being a huge ball of gas, the Sun appears to have a sharp edge because the energy radiates from a thin layer only a couple hundred miles thick, compared with the Sun’s overall radius of 432,000 miles (695,000 kilometers).

Astronomers call this thin llayer the photosphere, and it has an average temperature of about 6,000 keivins (10,000° F). The photosphere represents the lowest level of the Sun’s atmosphere. Above it lies the slightly hotter chromosphere, another thin layer that measures between 1,000 and 2,000 miles thick. Above the chromosphere lies the corona, a superheated region where temperatures rise to millions of degrees. Despite this great temperature, the corona has such a low density that we normally don’t see it when looking in visible llight. Only when the Moon blocks the much brighter photosphere from view during a total solar eclipse does the corona emerge into view. Because the Sun’s gravity isn’t strong enough to hold onto such hot gas, the outer atmosphere essentially boils off into space. This „solar wind“ permeates the solar system and, among other things, causes the ionized gas tails of comets to point away from the Sun.

In late October 2003, this giant sunspot sunspot group, designated 10488, grew lo 12 times Earth’s surface area.

The most conspicuous features on the Sun are aptly called sunspots. These dark splotches belong to the photosphere and occasionally grow large enough to be visible to the naked eye from Earth. (Remember, never look directly at the Sun without using a safe solar filter.) Sunspots appear dark only in contrast to the surrounding photosphere. They glow at a temperature some 1,500 keivins (2,700° F) cooler than the photosphere and thus don’t emit as much light. However, if you could somehow remove a sunspot and place it in the night sky, it would appear quite bright. The biggest sunspots have diameters of 25,000 to 30,000 miles, dwarfing the size of Earth. They can llast anywhere from a few hours to a few months. Because the Sun rotates in slightly less than a month, some sunspots cross the solar disk more than once. Sunspots also tend to cluster, with some sunspot groups containing a hundred or more individual spots. These large groups possess strong magnetic fields and often give rise to flares, the largest explosions in the solar system. A typical flare lasts for 5 to 10 minutes and releases as much energy as a million hydrogen bombs. The biggest flares last for several hours and emit enough energy to power the United States (at its current rate of electric consumption) for 100,000 years.

Observations of sunspots over the past couple centuries show that the number of spots varies with time. This solar cycle averages about 11 years from sunspot maximum to minimum and back again. The last solar maximum occurred in 2000, and the next is predicted around 2011. Interestingly enough, the soiar cycle apparently hasn’t always been so. Sunspot numbers were much lower between 1645 and 1715 than now, and scientists have deduced other periods of lesser and greater activity.

Our Sun is a normal main-sequence G2 star, one of more than 100 bbillion stars in our galaxy.

Diameter: 1,390,000 km

Mass: 1,98e30 kg

Temperature: 5800 K (surface)

15,600,00 K (core)

The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an „ordinary“ star. That’s true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than hall“ the mass of the Sun. The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else („metals“) amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it’s as much as 36 days. This odd behavior is due to

the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.

Conditions at the Sun’s core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun’s density iis more than 150 times that of water.

The Sun’s energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nucjear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the ssurface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

The surface of the Sun, called the photosphere, is at a temperature of about 55800 K. Sunspots are „cool“ regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun’s magnetic field. A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse. Temperatures In the corona are over 1,000,000 K. It just happens that the Moon and (he Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in aapproximately the same plane as the Earth’s orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun’s disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide aarea of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you’re likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel halfway around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it’s time to sleep. And you can see the solar corona. It is well worth a major journey.

The Sun’s magnetic field is very strong (by terrestrial standards) and very complicated. Its magnctosphere (also known as the heliosphere) extends well beyond Pluto.

In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the ssolar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth

ranging from power line surges to radio interference to the beautiful aurora borealis.

Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, that it does at lower latitudes. The composition of the solar wind aiso appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.

Further study of the solar wind will be done by me recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.

The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun’s limb.

The Sun’s output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter hhalf of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun’s output has increased by about 40%.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate „peacefully“ for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).

The Sun’s satellites

There are nine planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as „smaller objects“ has been the source of some controversy, but in the end it is really only a matter of definition.)

Planet Distance

(000 km) Radius (km) Mass (kg) Discovered Date

Mercury 57,910 2439 3,30e23

Venus 108,200 6052 4,87e24

Earth 149,600 6378 5,98e24

Mars 227,940 3397 6,24e23

Jupiter 778,330 71492 1,90e27

Saturn 1,426,940 60268 5,69e26

Uranus 2,870,990 25559 8,69e25 Herschel 1781

Neptune 4,497,070 24764 1,02e26 Galle 1846

pluto 5,913,520 1160 1,31e22 Tombaugh 1930

COMETS

Comets are dark, solid bodies a few kilometers across that orbit the Sun in eccentric paths. Comets can be described as „dirty snowballs“

containing a mixture of dust and frozen gases. Some of the icy material – perhaps less than 1 percent – evaporates as the comet nears the Sun, creating an envelope of gas and dust that enshrouds the solid body. This envelope, called the coma, may be up to 620,000 miles (1,000,000 kilometers) across. Swept back by the solar wind and the radiation pressure of sunlight, this material forms the comet’s tail. Comet tails can spat a distance greater than that sseparating the Earth from the Sun. That such a small amount of material could create visible features so large has led some to describe comets as „the closest thing to nothing anything can be and still be something.“

To the naked eye, the coma of a bright comet looks star-like, a tiny ball of light set within a milky glow. The comet’s tail or tails fan out from th coma. If present, a broad dust tail may be the most sstriking visual feature, arcing across ten degrees of sky or more. The glowing gas tail is straighler, narrower and often fainter than the dust tail. Within the coma, and invisible to both the naked eye and the most powerful telescopes, llies the small icy body responsible for this grand apparition – the comet’s nucleus.

Bushy stars

The ancient Chinese names for comets reflect their visual appearance. A comet with a prominent tail was called a „broom star“ (huixing}, while one with no obvious tail was a „bushy star“ (poxing). Until the mid-1400s, the Chinese made the most detailed and complete observations of comets. As early as 200 b.c., they employed official skywatchers to record and interpret any new omens in the heavens. These officials recognized, some nine centuries before their European counterparts, that eomet tails always point away from the Sun. The Chinese interest in comets, however, was for their astrological importance as signs of coming change.

The Greeks likewise recognized aa comet with an extended tail as a „bearded star“ (aster pogonias) and one without a tail was a „long-haired stai (aster kometes), from which our modern word derives. Aristotle regarded them as a fiery atmospheric phenomenon, to be lumped together with meteors and the aurora. They could not be planets, he reasoned, because comets can appear far from the ecliptic. He thought of comets as being whipped up by the motion of the Sun and stars around the Earth. TTheir appearance was a warning of coming droughts and high winds. As these ideas were extended in the Middle Ages, comets became viewed less as a portent of disaster than as a cause. They were viewed as a fiery corruption of the air, pockets of hot contaminated vapor that could bring earthquakes, disease, and famine.

This 1910 photo of Comet Haiiey shows its head, or coma, and the beginning of its long tail. Some of these ideas were being questioned seriously when the great comet of 1577 attracted the attention of Danish observer Tycho Brahe. He could see no reason why comet tails should always point away from the Sun if they were products of the weather. He measured the position of the comet with respect to the stars at different times during the night in an effort to find its parallax — a clue to the object’s true distance from Earth. His observations, which indicated that the comet lay beyond the Moon but not as far off as Venus, helped invigorate the scientific study comets. More than a century later, Isaac Newton showed that comets obeyed Johannes Kepler’s laws of planetary motion and concluded „comet are a sort of pplanet revolved in very eccentric orbits around the Sun.“

Future observations of the comet of 1682 would eventually remove any lingering doubts. Newton’s friend Edmond Halley began collecting accurate cometary observations in 1695 to compare the orbits of many comets. Halley noticed that several comet orbits seemed similar and shared roughly the same period, between 75 and 76 years. „Many considerations incline rnc to believe the comet of 1531 observed by Apianus t have been the same as that described by Kepler . in 1607 and which I again observed in 1682,“ Halley wrote. „Whence I would venture confidently to predict its return, namely in the year 1758. And if this occurs, there will be no further cause for doubt that the other comets ought to return also.“ Halley’s confidence proved well founded — the first comet ever predicted to return was again spotted on December 25, 1758, It has been known as Halley’s Comet ever since.

Naming comets

Comets are more commonly named for their discoverers; up to three independent co-discoverers may share the credit. Increasingly, those discoverers are not individuals, but dedicated small-body discovery programs or solar-observing satellites. Numerous comets have been named for the Lincoln Near Earth Asteroid RResearch (LINEAR) project of the Massachusetts Institute of Technology in Boston, the Near Earth Astero. Tracking (NEAT) program operated by the Jet Propulsion Laboratory in Pasadena, California, and the Lowell Observatory Near-Earth Object Search (LONEOS) run by Loweli Observatory in Flagstaff, Arizona. The pace of comet discovery has more than doubled in recent decades, up from an average of about a dozen per year in the late 1980s to about 30 per year in this century’s opening years. The Sun-monitoring Solar and Heliospheric Observatory (SOHO) satellite has found 850 comets so far. This tally increases by an average of 80 per year, making SOHO history’s most prolific, if unintended, comet discoverer.

Comet Hyakutake, which was discovered visually, turned out to be one of the great comets of recent years. David Healy Uarge_nmage] Because the names of discoverers don’t allow for a unique identification, comets receive a more prosaic official name. This consists of a one-letter prefix, usually a C for „comet“ or a P for „periodic,“ followed by the year of discovery and an uppercase letter that indicates the half-monl in which the discovery occurred. For example, an A represents January 1 though 15, B is January 16 through 31,

and so on. (The letter I isn’t us to avoid confusion with earlier nomenclature that used Roman numerals, and the letter Z isn’t necessary.) After this letter comes a number that represents the order of discovery during the half-month. Halley’s Comet, which was the first comet discovered or recovered in the second half o October 1982, therefore receives the designation P/1982U1. When the return of a comet is well established, either through a recovery or by observing a second passage tthrough perihelion, astronomers add a number to the prefix. Since Halley was the first comet whose return was identified, its full designation becomes 1P/1982UI.

Astronomers have accumulated detailed orbital information on more than 1,500 individual comets. Of those, only about 10 percent complete an orbit around the Sun in less than 200 years. A typical „short-period“ comet travels once around the Sun every 7 years in an orbit inclined to Earth’s by some 13 degrees, passing no closer to the SSun than about 1.5 AU, or just within the mean distance of Mars. Halley’s Comet is the brightest and most active member of this group. The remaining population consists of long-period comets, those that take at least 200 years to rreturn to the inner solar system. So comet aficionados pin their hopes to the unanticipated arrival of an as-yet-unknown long-period comet.

How bright will it be?

The two most important considerations in assessing the visibility of a comet are its distance from the Sun at closest approach, which controls the comet’s activity, and its distance from Earth, preferably after the intense heating of it closest approach to the Sun. Halley, for example, was an impressive sight in 1910, but anemic in 1986 — a disappointment even to those who traveled far from city lights. The main difference between the two apparitions was the comet’s distance from Earth. Haiiey reached perihelion at a time when Earth was on the opposite side of the SSun, and the comet never came closer to Earth than 0.417 AU (38.7 million miles or 62.4 million km), which is about three times the distance of its 1910 approach.

Another example of the importance of proximity was the 1983 display of comet IRAS-Araki-AIcock (C/1983 HI). A small and relatively inactive comet, it was discovered first by the Infrared Astronomical Satellite (IRAS) in late April and originally identified as an asteroid. In early May, amateurs Genichi Araki of Japan and GGeorge Alcock of England independently discovered the object. It soon became an obvious sight to the unaided eye high in the northern sky, and on May 12 the comet brushed past Earth at 0.0312 AU (2.9 million miles or 4.7 million km) — closer than any comet since 1770. A typical comet might move across the sky by a degree or so a day, too slowly for the eye to notice. IRAS-Araki-Alcock was so close that its motion was clearly evident to observers, who compared its movement to that of the minute hand on a clock. At its best, the comet was about twice the apparent diameter of the Moon and looked like a star nestled within a puff of smoke. It showed no evidence of a tail — a fine example of a „bushy star“ — and faded from view by the third week of May.

Intrinsically larger or more active comets can produce a spectacle without getting quite so close to us. Comet West (C/1975 VI) improved dramatically within a week of its very close approach to the Sun, aided in large part by the break-up of its nucleus into four fragments. West dominated the morning sky of eearly March 1976 with complex gas and dust tails extending 25 degrees or more. A decade earlier an even more spectacular comet, Ikeya-Seki (C/1965 SI), could be seen even during the daylight as it raced past the Sun, skimming its surface by less than one solar diameter. This intense heating led to the break-up of the nucleus into at least two fragments and a corresponding increase in brightness. During the days around perihelion Ikeya-Seki could be seen as a star-like object in broad daylight just by blocking the Sun with a hand — the brightest comet of the 20lh century. It emerged from the Sun’s glare in the last week of October 1965 sporting a bright tail about 25 degrees long. Any list of „great comets“ must include both West and Ikeya-Seki.

Sungrazers

Ikeya-Seki’s punishing orbit places it into a category of comets known as the „sungrazers.“ Heinrich Kreutz extensively examined the orbits of sungrazing comets and suggested that they shared a common ancestry. Kreutz argued that the comets he studied were possibly fragments of some much larger comet that fell apart at a close approach to the Sun. Sungrazers have perihelion distances less than 0.02 AU, orbital periods of aa few centuries, and other distinguishing orbital characteristics, but they were also apparently rare. Brian Marsden of the Harvard -Smithsonian Center for Astrophysics identified eight members, and suspected three others, in his 1 965 and 1 989 studies of the Kreutz group. By his second study, 1 5 apparent sungrazing comets had been discovered by the SOL WIND and Solar Maximum Mission satellites, and Marsden noted these „discoveries suggest that members may in fact be coming back to the Sun more or less continuously.“ Like these fragments, most of the comets so far discovered by comet-champion SOHO also do not survive their passage. Marsden believes that nearly all of them belong to the Kreulz group, although mere are too few observations to uniquely determine their orbits. The SOHO sungrazers are probably just a few meters across. Marsden speculates that a historical sungrazer, one the Greek Ephorus reported to have split in two pieces in the winter of 372 b.c. might even be the granddaddy of them all.

Comet duds

Even when orbital geometry promises a good display, the comet itself may simply fail to cooperate. Comet Kohoutek (C/1973 El), which was widely predicted to be the „comet of the century“ in

1973, did manage to become a naked-eye object but never lived up to its publicity. Another example is Comet Austin (C/1989 XI), discovered in December 1989 by New Zealand amateur Rodney Austin. The comet’s orbit was favorable, but as Austin closed on the Sun it failed to maintain its rapid brightening and, in the end, proved a bigger dud than Kohoutek.

Both Austin and Kohoutek appear to have been new comets, those making their first close pass by the Sun. AAstronomers believe that comets originate from two „cold storage“ zones that surround the planetary system. The inner portion of this comet cloud is a thick disk centered on the ecliptic that begins near the orbit of Neptune (about 30 AU) and extends beyond the orbit of Pluto to 50 AU. Often called the Kuiper Belt, it contains a few tens of thousands of icy objects larger than about a half-mile across; at least 800 are currently known. A much larger aand more diffuse component, called the Oort cloud and containing perhaps a trillion comets, forms a Sun-centered spherical shell extending from the outer Kuiper Belt to about one-third of a light-year or more into space. Many astronomers believe that the KKuiper Belt is the source for the short-period comets and that the Oort cloud, from which comets are more easily dislodged, is the source for the long-period comets. Feeble gravitational disturbances from passing stars and interstellar gas clouds remove enough orbital energy from Oort cloud comets that they begin their million-year-long fall toward the Sun. Long-period comets may arrive from any direction, their elongated orbits randomly oriented to the orbits of the planets, while the short-period comets are confined closer to the ecliptic. New arrivals from the comet cloud probably retain a coating of highly volatile ices, such as frozen carbon dioxide, that begins to evaporate at much lower temperatures than frozen water. Such comets „turn on“ at relatively large ddistances from the Sun, but brighten only until the coating evaporates.

Recent great comets

Comet Hyakutake {C/1996 B2) was, in the words of Brooks Observatory comet expert John Bortle, „one of the grandest of the millennium.“ It was discovered visually by Japanese amateur Yuji Hyakutake when at a distance of 2.0 AU — and only 55 days before its closest approach to Earth (March 25, 1996, 0.102 AU). By late March, midnorthem observers could see it directly overhead before dawn with aa tail at least 30 degrees long. In the days around closest approach it was an easy object even from cities and its motion against the stars, like that of IRAS-Araki-AIcock, was evident in minutes. On March 27, as it moved near Polaris, Hyakutake was visible all night long and could easily be seen from the suburbs. From a reasonably dark sky the comet was truly something special, showing a tail that spanned some 70 degrees or longer – all the more impressive because it seemed lo contain relatively little dust. Hyakutakc took us by complete surprise, upstaging the appearance of another comet that was already widely anticipated.

Although Hale-Bopp did not pass particularly close to Earth, it’s tremendous activity made it an unforgettable sight.

That comet was Hale-Bopp (C/1995 01). What made Hyakutake a great comet was its unusually close pass, which turned a faint and relatively inactive comet into an apparently bright one. But Hale-Bopp was another matter. It was the brightest and most active comet to pass inside Earth’s orbit since the one Tycho Brahe examined in 1577. Hale-Bopp showed unusually high activity even at great distance from the Sun and was widely expected to be the oone that would end the bright comet drought. It was discovered July 23, 1995, by Alan Hale in New Mexico and Thomas Bopp in Arizona within minutes of one another. After perihelion on April I, 1997, Hale-Bopp became a striking object in the northwestern sky, cruising through Cassiopeia and Perseus with a pair of tails. The straight, faint gaseous tail was easy to see from a moderately dark site, but the comet’s most striking aspect was its dramatically curved 25-degree-long dust tail. Observers in the Northern Hemisphere could see Hale-Bopp with the naked eye, even from urban sites, and it remained well placed for viewing throughout April and into May. As an indication of the comet’s unusual activity, consider that it was never closer to Earth than 122 million miles (197 million km) and passed no closer to the Sun than 91 percent of Earth’s distance.

Stadust flew within 149 miles (240 kilometers) of Comet Wild 2’s nucleus on January 2, 2004.

Astronomers believe comets may be the best-preserved remnants of the cloud of dust and gas in which the Sun and planets formed. In the deepfreeze of the outermost solar system, they have remained largely unchanged during the 4 bbillion years the solar system has existed. Planetary scientists study comets for the same reason paleontologists study fossils: to catch a glimpse of the most ancient past. And what better way to scrutinize comets than by visiting them directly? Japan, the European Space Agency (ESA), and the Soviet Union began

the direct exploration of comets in 1985 by sending separate missions past Halley’s Comet. The ESA probe, Giotto, returned the first detailed images of a comet’s nucleus, revealing a dark, peanut-shaped body, a hint of hills and craters, and several bright jets spewing streams of gas and dust. Another burst of comet exploration is now under way:

• In 2001 the experimental Deep Space 1 probe made a distant pass by the nucleus of Comet 19P/Borrelly and returned images of its pockmarked and relatively inactive surface.

• In 2004, the Stardust mission passed through the coma of 8IP/Wild — and in 2006 will return to Earth with the first direct samples of cometary material.

• On July 4, 2005, a projectile released from Deep Impact wili strike the sunlit side of Comet 9P/Tempel’s nucleus, an event expected to create a blast that leaves a football-field sized crater -— and expose fresh ice to the Sun —

while ground-based observatories and the Deep Impact spacecraft watch.

• The ESA has launched its ambitious mission for Rosetta, which will rendezvous with and orbit the inbound Comet 67P/Churyumov-Gerasimenko in 2014. It will also place a small lander on the comet’s surface.

There are thousands of known asteroids and comets and undoubtedly many more unknown ones. Most asteroids orbit between Mars and Jupiter. A few (e.g. 2060 Chiron) are farther out. There are also some asteroids whose orbits carry them closer to tthe Sun than the Earth (Aten, Icarus, Hephaistos). Most comets have highly elliptical orbits which spend most of their time in the outer reaches of the solar system with only brief passages close to the Sun.

The distinction between comets and asteroids is somewhat controversial. The main distinction seems to be that comets have more volatilcs and more elliptical orbits. But there are interesting ambiguous cases such as 2060 Chiron (aka 95 P/Chiron) and 3200 Phaethon and the Kuiper Belt oobjects which seem to share some aspects of both categories.

Asteroids are sometimes also referred to as minor planets or planetoids (not lo be confused with „lesser planets“ which refers to Mercury and Pluto).

Unlike me otner small bodies iin the solar system, comets have been known since antiquity. There are Chinese records of Comet Halley going back to at least 240 BC. The famous Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of Comet Halley.

As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated. Of these 184 are periodic comets (orbital periods less than 200 years); some of the remainder are no doubt periodic as well, but their orbits have not been determined wilh sufficient accuracy to teli for sure.

Comets are sometimes called dirty snowballs or „icy rnudballs“. They are a mixture of ices (both water and frozen gases) and dust that for some reason ddidn’t get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.

When (hey are near the Sun and active, comets have several distinct parts:

• nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;

• coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;

• hydrogen cloud, huge (millions of km in diameter) but very sparse envelope oof neutral hydrogen;

• dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye;

• ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.

Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which lake them far beyond the orbit of Pluto: these are seen once and then disappear for millennia. Only the short- and intermediate-period comets (like Comet Hailcy), stay within the orbit of Pluto for a significant fraction of their orbits.

After 500 or so passes near the Sun off most of a comet’s ice and gas is lost leaving a rocky object very much like an asteroid in appearance. (Perhaps half of the near-Earth asteroids may be „dead“ comets.) A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Son or to be ejected out of the solar system by a close encounter (esp. with Jupiter). By far the most famous comet is CComet Halley but SJL 9 was a „big hit“ for a week in the summer of 1994.

Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity, the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tut fie Comet Halley is the source of the Orionid shower in October.

Many comets are first discovered by amateur astronomers. Since comets are brightest when near the Sun, they are usually visible only at sunrise or sunset. Charts showing the positions in the sky of some comets can be created wilh a planetarium program. 1705 Edmond Halley predicted, using Newton’s newly formulated laws of

motion, that the comet seen in 1531, 1607, and 1682 would return in 1758 (which was, alas, after his death). The comet did indeed return as predicted and was later named in his honor The average period of Halley’s orbit is 76 years but you cannot calculate the dates of its reappearances by simply subtracting multiples of 76 years from 1986. The gravitational pull of the major planets alters the orbital period from revolution to revolution Nongravitation effects (such as the reaction from gasses boiled off during its passage near the Sun) also play an important, but smaller, role in altering the orbit Between the years 239 BC and 1986 AD me orbital period has varied from 76.0 years (in 1986) to 79.3 years (in 451 and 1066). The closest perihelion passage to the time of Jesus are 11 BC and 66 AD; neither event look place in Jesus’ lifetime. Its most famous appearance was in 1066 when it was seen at the Battle of Hastings, an event commemorated in the Bayeux Tapestry.

Comet Halley was visible in 1910 and again in 1986. its next perihelion passage will be in 2061. Halley’s orbit is retrograde and inclined 18 degrees to the ecliptic. And, like all comets, highly eccentric.

Only four comets have been visited by spacecraft. NASA’s ICE passed through the tail of Comet Giacobini-Zinner in 1985; Comet Gngg Skjellerup was visited by Giotto in 1989. In 1986, five spacecraft from the USSR. Japan, and the European Community visited Comet Halley; ESA’s Giotto obtained close-up photos of Halley’s nucleus (above and right). NASA’s technology demonstration spacecraft DSJ. imaged the nucleus of Comet Borrelly in 2001. The

nucleus of Comet Halley is approximately 16x8x8 kilometers.

Contrary to prior expectations, Halley’s nucleus is very dark: its albedo is only about 0.03 making it darker than coal and one of the darkest objects in the solar system.

The density of Halley’s nucleus is very low: about 0.1 gm/cm3 indicating that it is probably porous, perhaps because it is largely dust remaining after the ices have sublimed away. Halley is almost unique among comets in that it is both large aand active and has a well defined, regular orbit. This made it a relatively easy target for Giotto et al, but may not be representative of comets in general.

Comet Halley will return to the inner solar system in the year 2061.

Officially known as 2003 VB12, this object is the most distant body known that orbits our Sun, [t is at present over 90 AUs away, 3 times as far as Pluto.

Sedna is about 1800 km in ddiameter, slightly smaller than Pluto.

Perhaps the most interesting aspect of Sedna is its orbit. Though it is not yet known to high precision it is clear that Sedna’s orbit is highly elliptical with a perihelion of about 75 AU aand an orbital period of about 10500 years. This puts it well beyond the Kuipcr Belt and yet well inside what was thought to be the inner edge of the Oort Cloud.

Sedna’s physical composition is a bit of a mystery. You would expect it to be mostly ices but apparently that’s not the case. About all that’s known at this time is that it is very red.

Sedna is definitely not the „Planet X" that many astronomers anticipated before the discovery of Pluto. Planet X was supposed to be a much larger object.

Sedna is not even officially a planet at all. That classification decision is up to the IAU and they are not likely to decide to do sso. On the first day of January 1801, Giuseppe Piazzi discovered an object which he first thought was anew comet. But after its orbit was better determined it was clear that it was not a comet but more like a small planet. Piazzi named it Ceres, after the Sicilian goddess of grain. Three other small bodies were discovered in the next few years (Pallas, Vesta, and Juno). By the end of the 19th century there were several hundred. Several hundred tthousand asteroids have been discovered and given provisional designations so far. Thousands more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth. There are 26 known asteroids larger than 200 km in diameter. Our census of the largest ones is now fairly complete: we probably know 99% of the asteroids larger than 100 km in diameter. Of

those in the 10 to 100 km range we have cataloged about half But we know very few of the smaller ones; there are probably considerably more than a million asteroids in the 1 km range.

The total mass of all the asteroids is less than that of the Moon.

11 comets and asteroids have been explored by spacecraft so far, as follows: ICE flyby of Comet Giacobini-Zinner. Multiple flyby missions to Comet Halley. Giotto (retarget) to Comet Grigg-Skellerup. Galileo flybys of asteroids Gaspra and Ida (and Ida satellite Dactyl). NEAR- Shoemaker flyby of asteroid Mathilde on the way to orbit and land on Eros. DS-1 flybys of asteroid Braille and Comet Borreliy. Stardust flyby asteroid Annefrank and recent sample collection from Comet Wild 2. For future we can expect: Hayabusa (MUSES-C) tto asteroid Itokawa, Rosettato Comet Churyumov-Gerasmenko, Deep Impact to Comet Tempel 1, and Dawn to orbit asteroids Vesta and Ceres.

243 Ida and 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. The NEAR mission flew by 253 Mathildc (lefti on 1997 June 27 returning many images.

NEAR (now renamed „NEAR-Shocmaker") entered orbit around 433 Eros (right) in January 1999 and returned a wealth of images and data.. At the end of its mission it actually landed on Eros.

The largest asteroid by far is 1 Ceres. It is 933 km in diameter and contains about 25% of the mass of all the asteroids combined. The next largi are 2 Pallas, 4 Vesta and 10 Hygiea which arc between 400 and 525 km in diameter. All other known asteroids are less than 340 km across. There is some debate as to the classification of asteroids, comets and moons. There are many planetary satellites that are probably better though of as captured asteroids. Mars’s tiny moons Dcimos and Phobos. Jupiter’s outer eight moons. Saturn’s outermost moon, Phoebe, and perhaps soi of the newly discovered moons of Saturn. Uranus and Neptune are all more similar to asteroids than to the llarger moons. (The composite image the top of this page shows Ida, Gaspra, Deimos and Phobos approximately to scale.)

Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo:

• C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles:

• S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;

• M-type, most of the rest: bright (albedo .10-. 18); pure nickel-iron.

• There are also a dozen or so other rare types.

Because of biases involved in the observations (e.g. the dark C-types are harder to see), the percentages above may not be representative of the true distribution of asteroids. (There are actually several classification schemes in use today.)

There is little data about the densities of asteroids. But by sensing the Doppler effect on radio waves returning to Earth from NEAR owing to the (very slight) gravitational tug between asteroid and spacecraft, Mathilde’s mass could be estimated. Surprisingly, its density turns out to be not much greater than that of water, suggesting that it is not a solid object but rather a

compacted pile of debris.

Asteroids are also categorized by their position in the solar system:

• Main Belt: located between Mars and Jupiter roughly 2 – 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).

• Near-Earth Asteroids (NEAs): ones that closely approach the Earth

• Atcns: semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU;

• Apollos: semimajor axes greater than 1.0 AU aand perihelion distances less than 1.017 AU

• Amors: perihelion distances between 1.017 and 1.3 AU;

• Trojans: located near Jupiter’s Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5). (There may also be a few small asteroids in the Lagrange points of Venus and Earth ((see Earth’s Second Moon) that are also sometimes known as Trojans; 5261 Eureka is a „Mars Trojan".)

Between the main concentrations of asteroids in the Main Belt are relatively empty regions known as the Kirkwood gaps. These are regions where aan object’s orbital period would be a simple fraction of that of Jurjiter. An object in such an orbit is very likely to be accelerated by Jup into a different orbit.

There also a few „asteroids" (designated as „Centaurs") in the outer solar system: 2060 Chiron (aka95 P/Chiron) orbits between Saturn and Uranus; the orbit of 5335 Damocles ranges from near Mars to beyond Uranus; 5145 Pholus orbits from Saturn to past Neptune. There are probably many more, but such planet-crossing orbits are unstable and they are likely to be perturbed in the future. The composition of these objects is probably more like that of comets or the Kuiper Belt objects than that of ordinary asteroids. In particular, Chiron iis now classified as a comet.

4Vesta has been studied recently with HST. It is a particularly interesting asteroid in that it seems to have been differentiated into layers like the terrestrial planets. This implies some internal heat source in addition to the heat released by long – lived radio-isotopes which alone would be insufficient to melt such a small object. There is also a gigantic impact basin so deep that it exposes the mantle beneath Vesta’s outer crust.

Though tthey are never visible with the unaided eye, many asteroids are visible with binoculars or small telescopes.

Asteroid table

A few asteroids and comets are listed below for comparison, (distance is the mean distance to the Sun in thousands of kilometers; masses in kilograms).

N0 Name Distance Radius Mass Discovered Date

2062 Aten 144514 0,5 ? Helin 1976

3554 Amun 145710 ? ? Shoemaker 1986

1566 Icarus 161269 0,7 ? Baade 1949

433 Eros 172800 33x13x13 3 Witt 1989

1862 Apollo 220061 0,7 ? Reinmuth 1932

2212 Hephaistos 328884 4,4 ? Chernykh 1978

951 Gaspra 330000 8 ? Neujmin 1916

4 Vesta 353400 265 3,0e20 Olbers 1807

3 Juno 399400 123 ? Harding 1804

15 Eunomia 395500 136 8,3e18 De Gasparis 1851

1 Ceres 413900 466 8,7e20 Piazzi 1801

2 Pallas 414500 261 3,18e20 Olbers 1802

243 Ida 428000 35 ? ? 1880?

52 Europa 463000 156 ? Goldschmidt 1858

10 Hygiea 470300 215 9,3e19 De Gasparis 1849

511 Davida 475400 168 ? Dugan 1903

911 Agamemnon 778100 88 ? Reinmuth 1919

2060 Chiron 2051900 85 ? Kowal 1977

Very small rocks orbiting the Sun are sometimes called incteoroids to distinguish them from the larger asteroids. When such a body enters the Earth’s atmosphere it is healed to incandescence and the visible streak in the sky is known as a meteor. If a piece of it survives to i Ihe Earth’s surface it is known as a meteorite.

Millions of meteors bright enough to see strike the Earth every day (amounting to hundreds of tons of material). All but a tiny fraction burn i the atmosphere before reaching the ground. The few that don’t are our major source of physical information about the rest of the solar systen Finally, the space between the pianets is not empty at all. It contains a great deal of microscopic dust and gas as well as radiation and magne fields.

Meteors and meteor showers

You can see a „shooting star" on any dark night — but some nights of me year are mmuch better than others.

Those spending enough time under the night sky eventually will see a „shooting star," a streak of light that flashes across the sky in less than a second. This is a meteor, a glowing trail caused by the incineration of a piece of celestial debris entering our atmosphere. Many meteors are quick flashes, but some last long enough for us to track their brief course across the sky. Now and then, a meteor truly will light up the night, blazing brighter than Venus — and although rarely, even brighter than the Moon — leaving in its wake a dimly glowing trail that may persist for minutes. Under a dark sky, any observer can expect to see between two and seven meteors each hour any night of the year. These are sporadic meteors, their source bodies — meteoroids — are part of me dusty background of the inner solar system.

Several times during the year, Earth encounters swarms of small particles that greatly enhance the number of meteors. The result is a meteor shower, during which observers may see dozens of meteors every hour. Concentrations of material within the swarms may produce better-than-averagc displays in some years, wwith rates of hundreds per hour. And every now and then, we’re treated to a truly spectacular display in which thousands of visible meteors can be seen for a brief period. These are referred to as meteor storms.

The meteors that appear during a meteor shower seem to radiate from one point in the sky. This illusion is an effect of perspective, just as a roadway seems to converge in the distance. Usually, meteor showers take the name of the constellation from which the meteors appear to radiate. For example, during the Perseid shower in August, meteors seem to streak from a point in the constellation Perseus. If a bright star happens to lie near the radiant, the shower may take the star’s name — for instance: the Eta (q) Aquarids.

The science of meteor astronomy began in 1833, when a storm of 60,000 meteors an hour shocked the world. By the 1860s, it had become clear that many meteor showers were annual — including the normally placid Leonids, which produced the big storm — and that they were somehow related to comets. Astronomers now consider comets to be dirty snowballs consisting of a mixture of dust and frozen gases.

A comet becomes visible only during its closest approach to the Sun, when areas on the comet’s icy surface become warm enough to evaporate. The resulting jets of evaporating gases carry with them any solid matter mixed with the original ice.

At each pass near the Sun, the comet ejects a stream of material. The particles comprising the stream orbit the Sun in slightly different paths than the source comet. Each particle receives small accelerations from forces other than gravity, aand these orbits become increasingly modified over time. The ejected streams become more diffuse with age and lose their individual identities. Concentrated initially near the comet, the debris diffuses along each stream’s orbit and eventually forms a thin band of material (hat Earth encounters every year. A meteor shower occurs on the date in the year when Earth passes nearest to the band of material associated with a comet’s orbit.

Most of the meteors seen during one of the annual sshowers arise from fluffy particles not much larger than sand grains. As a particle enters Earth’s atmosphere, it collides with gas atoms and molecules. The particle becomes wrapped in a glowing sheath of heated air and vaporized material boiled off iits own surface. Meteors become visible at altitudes between 50 and 75 miles (80 and 120 kilometers), with faster particles typically shining at greater heights. Many of the faster, brighter meteors may leave behind a train — a dimly glowing trail that persists for many seconds or, more rarely, minutes. Larger debris may create a fireball — a spectacular meteor bright enough to outshine even Venus. Occasionally, a fireball will fragment; this event is accompanied by bright flares and even „sparks" thrown a short distance from the meteor’s main trail. Such a fireball is called a bolide.

The best way to enjoy a meteor shower is to dress warmly, set down a blanket or lawn chair at a dark site, gget comfortable, and watch the stars. On any night of the year, meteors appear faster, brighter, and more numerous after midnight. That’s when your location has turned into Earth’s direction of motion around the Sun and plows into meteor particles nearly head-on, rather than having them catch up from behind. The peak activity of a meteor shower occurs in the hours when Earth passes closest to the orbit of the shower particles. The ideal circumstance for any observer is for tthe shower to peak at a time when its radiant is highest in the sky during the morning hours; most of the year’s best showers have the potential to meet these criteria.

Quadrantids. Generally visible between December 28 and January 6, the Quadrantids have a sharp activity peak around January 3. Typical rates vary between 40 and 100 per hour; about 5 percent leave trains. When the shower was first recognized as annual in 1839, the radiant occurred in a constellation no longer recognized — Quadrans Muralis (Wall Quadrant). It’s now divided between Hercules, Bodies, and Draco. The cold nights of northern winters and typically faint meteors keep this shower from being truly popular.

Until late 2003, this was the only major meteor shower whose parent body remained unknown. But that year astronomers found a near-Earth asteroid named 2003 EH1. When astronomers estimated the theoretical speed and radiant for a hypothetical meteor shower caused by particles from 2003 EH1, the results fell sciuarelv in the middle of those measured for the Ouadrantids bv meteor observers. Astronomers susnect the object is a fragment from the breakup of a comet — and perhaps the event that gave birth to the Quadrantids.

Lyrids. The Lyrids aappear from April 16 to 25 and peak (at 10 to 15 per hour) around April 21; the radiant lies between Hercules and Lyra. Chinese observations of this display date back to 687 b.c., making the Lyrids the earliest recorded meteor shower. Astronomers recognized the Lyrids as an annual shower in 1839 and connected it it to its parent comet, CM861 Gl, in 1867. Lyrid meteors are bright and rather last (30 miles [48 km] per second), and about 15 percent leave persistent trains.

Eta Aquarids. The first of the year’s two showers that derive from Halley’s Comet, the Eta Aquarids occur from April 19 to May 28, with a peak (10 to 20 per hour) around May 6. This shower is best for observers in the Southern Hemisphere, where the hourly rate climbs to about 50. The radiant is located near the Y-shaped asterism in Aquarius and named for one of those stars. The shower was discovered in 1870 and linked to Halley in 1876. The meteors are among the fastest (42 miles [67 km] per seco[67 km] are faint on average, but the brighter ones have a yellowish color; about 30 percent leave trains.

Southern Delta Aquarids. This is the mmost active of a diffuse group of streams and, as the name suggests, is best seen in the Southern Hemisphere. These meteors may be seen between July 12 and August 19 and peak (15 to 20 per hour) near July 28. The meteors are medium speed (27 miles [43 km] per seco[43 km]ey tend to be faint, and few leave trains.

Perseid meteors appear to stream from a point — called the radiant — in the constellation Perseus. The shower is best after midnight local time, when the radiant rises in the northeast.

Perseids. The best known of all meteor showers, the Perscids never fail to put on a good show and —thanks to the shower’s late-summer peak— are usually widely observed. The earliest record of this event comes from China in a.d. 36. Generally visible from July 17 to August 24, meteor speed (37 miles [60 km] per seco[60 km]ightness, and a high proportion of trains (45 percent) distinguish the Perseids from other showers active at this time. It became the first meteor shower linked to a comet (109P/Swift-Tuttle) in 1865. Models of the Perseids predict a gradual decline in activity from a peak in 2004.

Draconids. The Draconids are sometimes called

the Giacobinids; in a break with convention, this name honors the shower’s parent comet, 21P/Giacobini-Zinner. Draconid activity occurs between October 6 and 10, with apeak on October 8 (if it occurs at all). In 1933 and 1946, the shower produced brief but intense meteor storms (more than 5,000 per hour); in 1998, it reached a rate of about 500 meteors an hour over eastern Europe. The occurrence of the shower is intimately tied to the proximity of its parent comet. AAccording to Donald Yeomans, a comet expert at the Jet Propulsion Laboratory in Pasadena, California, the most intense showers occur when Earth grazes the comet’s orbit within a few months of its passage. When Comet Giacobini-Zinner returns in 2005, astronomers expect a short-lived outburst of hundreds of meteors an hour. Most researchers agree that a full-fledged meteor storm — defined as 1,000 meteors an hour or more — will occur in 2018. Draconids are slow-moving meteors, encountering Earth at less tthan 12 miles (20 km) per second, and they typically are faint.

Orionids. This is the sister stream of the Eta Aquarids, also arising from the debris of Halley’s Comet. Discovered in 1864, the Orionids were nol linked to Halley until 11911. Orionid meteors can be found between October 2 and November 7, with apeak of about 25 per hour around October 21. Orionid meteors are among the fastest (42 miles [67 km] per seco[67 km]ey generally are faint, and about 20 percent leave trains that persist one or two seconds.

Southern Taurids. Visible between October 1 and November 25, this is the strongest of several streams originating from Comet Encke. A broad maximum occurs between November 3 and 5, but this shower usually brings an hourly rate of less than 15 meteors. The shower was first recognized in 1869 and was associated with Comet Encke in 1940. Its meteors generally are faint and quite slow (19 miles [30 km] a second[30 km]use they approach Earth from behind and must catch up.

Leonids. Leonid meteors generally arrive between November 14 and 21, with apeak hourly rate on November 17 of between 10 and 15 meteors per hour; about half of these meteors leave trains that can persist for several minutes. Because Earth runs into the orbiting particles almost directly head-on, Leonid meteors travel faster than those of any other shower — 45 miles (71 km) per second. The shower’s most notable feature is iits habit of producing periodic, dramatic meteor storms as Earth intercepts streams of dense material ejected at previous returns of Comet Tcmpel-Tultle, Our planet passed through such streams annually from 1998 to 2003. Although small outbursts of up to 200 meteors per hour have been forecast for 2006 and 2007, computer models show that Jupiter’s tug on the dense Leonid streams causes them to miss Earth until at least 2098. Because the stream responsible for the predicted outbursts was ejected in 1933, only its smallest particles have been able to drift into i path that Earth will intersect. This means any outburst, if one occurs at all, will be rich in faint meteors.

Geminids. The Geminids are active between December 7 and 17 and peak near December 13, with typical hourly meteor rates around 80 but occasionally more than 100. Because the Geminids intersect Earth’s orbit near the side directly opposite the Sun, this shower is one of the few that are good before midnight. The parent body of the Geminids is a curious object designated 3200 Phaethon. What makes Phaethon interesting is that it appears to be an asteroid instead of a comet. Planetary scientists suggest that many of the aasteroids whose orbits cross Earth’s may be, in fact, worn-out comets.

How big are the meteors that we see as shooting stars? What factors affect their color, length, and intensity?

A bright Geminid fireball flashes through the winter sky. Frank Zullo

The vast majority of the meteors you see at night arc actually smaller than a grain of sand. However, several hundred tons of meteors burn up in he atmosphere every day.

Most meteors are yellowish or white, with the color depending on how bright they are (your eyes have a hard time discerning color in faint objects) and how hot they get. Some extremely bright meteors can appear blue or white.

The intensity of the meteor depends on how hot it gets, how big it is (a bigger meteor can generate more energy because there is more material to heat up), and how fast it is coming.

The length of the trail the meteor leaves on the sky depends mostly on viewing angle: If the meteor is coming across your line of sight you might see a long trail, but if it is coming straight at you, you won’t see any trail at all (and you’d better hope the meteor burns up!). — PPHIL PLAIT,

A meteor is a bright streak of light in the sky {a „shooting star“ or a „falling star“) produced hy the entry of a small meteoroid into the Eartr atmosphere. if you have a dark clear sky you will probably see a few per hour on an average night; during one of the annual meteor shower may see as many as 100/hour. Very bright meteors are known as fireballs; if you see one please report it.

The upper air burst into life!

And a hundred fire-flags sheen,

To and fro they were hurried about!

And to and fro, and in and out,

The wan stars danced between

And the coming wind did roar more loud,

And the sails did sigh like sedge;

And the rain poured down from one black cloud;

The Moon was at its edge

May have been inspired by the Leonid meteor shower that he witnessed in 1797.

Meteorites are bits of the solar system that have fallen to the Earth. Most come from asteroids, including few are believed to have come specifically from 4 Vesta; a few probably come from comets. A small number of meteorites have been shown to be of Lunar (23 finds) or Martian (22)

origin.

One of the Martian meteorites, known as ALH84001 (left), is believed to show evidence of early life on Mars.

Though meteorites may appear to be just boring rocks, they are extremely important in that we can analyze them carefully in our labs. Aside from the few kilos of moon rocks brought back by the Apollo and Luna missions, meteorites are our only material evidence of the universe beyond the Earth.

Meteorite Types

Iron Primarily iron and nickel; similar to type M asteroids.

Stony Iron Mixtures oof iron and stony material like type S asteroids.

Chondrite By far the largest number of meteorites fall into this class; similar in composition to the mantles and crust of the terrestrial planets.

Carbonaceous Chondrite Very similar in composition to the Sun less volatiles; similar to type Casteroids.

Achondrite Similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites.

A „fall“ means the meteorite was witnessed by someone as it fell from the sky. A „find“ means the meteorite was nnot witnessed and the meteorite was found after the fact. About 33% of the meteorites are witnessed falls. The following table is from a book by Vagn F. Buchwald. Included are all known meteorites (4660 in all, weighing a total oof 494625 kg) in the period 1740-1990 (excluding meteorites found in Antarctica).

A very large number of mcteoroids enter the Earth’s atmosphere each day amounting to more than a hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites. The largest found meteorite (Hoba, in Namibia) weighs 60 tons.

The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth’s surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rrare) ones make craters.

A good example of what happens when a small asteroid hits the Earth is Barringer Crater (a.k.a. Meteor Crater) near Winslow, Arizona, It was formed about 50,000 years ago by an iron meteor about 30-50 meters in diameter. The crater is 1200 meters in diameter and 200 meters deep. About 120 impact craters have been identified on the Earth, so far (see below). A more recent impact occurred in 1908 in a remote uninhabited region oof western Siberia known as Timguska. The impactor was about 60 meters in diameter and probably consisting of many loosely bound pieces. In contrast to the Barringer Crater event, the Tunguska object completely disintegrated before hitting the ground and so no crater was formed. Nevertheless, all the trees were flattened in an area 50 kilometers across. The sound of the explosion was heard half-way around the world in London.

There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in a million years or so on the average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disastrous consequences.

The impact of a comet or asteroid about the size of Hephaistos or SL9 hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago. It left a 180 km crater now buried below the jungle near Chicxulub in the Yucatan Peninsula.

Calculations based on the observed number of asteroids suggest that we should expect about 3 craters 10 km or more across to be formed on the Earth every million yyears. This is in good agreement with the geologic record. It is more difficult to compute the frequency of larger impacts like Chicxulub but once per 100 million years seems like a reasonable guess. Here are educated guesses about the consequences of impacts of various sizes:

Impactor Diameter (meters) Yield (megatons) Interval (years) Consequences

<50 <10 <1 Meteors in upper atmosphere most don’t reach surface

75 10-100 1000 Irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area size of city.

160 100-1000 5000 Irons, stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo)

350 1000-10,000 15,000 Land impacts destroy area size of small state; ocean impact produces mild tsunamis.

700 10,000-100,000 63,000 Land impacts destroy area size of moderate state (Vitginia); ocean impact makes big tsunamis.

1700 100,000-1,000,000, 250,000 Land impact raises dust with global implication; destroys area size of large state (California, France).

More recent studies indicate a slightly lower frequency.

Our solar system consists of the sun, nine planets (and their moons), an asteroid belt, and many comets and meteors. The sun is the center of our solar system; the planets, their moons, the asteroids, comets, and other rocks and gas all orbit the sun.

The nine planets that orbit the sun are (in order from the sun): Mercury. Venus. Earth. MMars, Jupiter, Saturn. Uranus, Neptune, and Pluto. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic).

Easy ways to remember the order of the planets are the mnemonics: „My Very Excellent Mother Just Sent Us Nine Pizzas“ and „My Very Easy Method Just Simplifies Us Naming Planets“ The first letter of each of these words represents a planet – in the correct order.