The Sun is a micro variable star. We used to think that the Sun was a constant star, but we have learned that every star is variable to some degree.

The Sun has an absolute magnitude of 4.8 at the standard distance of 10 parsecs (2062648 AUs). This compares to an apparent magnitude -26.7 at 1 AU (8.3 light minutes). Of the hundred nearest stars only three Sirius, Alpha Centauri 1&2 are larger. Alpha Eridani is almost as large and then stars get small fast. Most are faint red dwarfs. The Sun has many features which have no counterpart in the planets. Here are some of these features:

The Sun emits a thin solar wind of particles driven from the Sun's surface by light pressure, electrical charges or magnetic fluxes. This solar wind can be detected throughout the solar system. Where these solar winds encounter magnetic fields in some of the large planets, these particles stream down along the lines of force causing auroras.

The Sun's corona is its very tenuous outermost atmosphere, an extremely hot gas which is in the millions of degrees. We normally do not see this gas, but during eclipses these pale outer atmosphere creates strange flame like images many times the size of the normal Sun's disk.

The chromosphere is the "atmosphere" of the Sun. It is much more dense than the corona (and much cooler) but it still is relatively thin when compared to the photosphere. The chromosphere absorbs certain frequencies of the Sun's light. Each absorbed frequency is specific to a particular type of atom. The pattern of alternating bright and dark bands (the Sun's spectra) gives us a great deal of information about the Sun's composition and chemistry.

The photosphere is what we think of as the Sun's "surface". When you look at the Sun through filters, it is the photosphere which displays texture. The photosphere emits the large majority of the Sun's light. This photosphere is not uniform. The Sun's surface has a mottled texture which reminds many people of the surface of a pot of cooking oatmeal. This granulation is the top of "bubbles" percolating up from lower levels in the Sun.

Sunspots are blotches which are slightly cooler than the surface as a whole. They would be brilliant if they weren't against the even more brilliant general surface. They increase and decrease in a pair of 11 years cycles (north and south hemispheres). During periods of intense sunspots, long range communications on the Earth may be disrupted. The area surrounding a sunspot is called an active region. Active regions are intense areas of magnetic flux.

Some solar storms create huge loops of gas along lines of magnetic force forming a prominence. These eruptions can be hundreds of times the diameter of the Earth. Solar flares occur when a granulation bubble breaks through the surface before it cools to the surface temperature. Material from the much hotter interior is exposed. Not only does visible light increase but so does the Sun's ultraviolet and x-ray radiation. Solar flares can be extremely disruptive. Sometimes the Sun's belches out a huge puff of electrically charged gas plasma. If this coronal mass(or discharge) happens to hit the Earth, power lines can be damaged, astronauts must seek shelter in the deepest parts of their spacecraft and aurora are intense.

The Sun generates its power in a central fusion core where the temperature is 15 million degrees Kelvin. Hydrogen gas is transmuted (changed) into helium with a great release of energy similar to the process in a hydrogen bomb. The Sun does not explode because its huge gravity hold the nuclear explosion in check. The radiation zone lies above the core. It is electrically conductive gas (properly called a plasma) transmits the electromagnetic radiation by direct radiation. As the radiation works its way outwards, it is progressively reduced in frequency from very short wavelength gamma radiation to x-rays and ultraviolet frequencies. The convective zonelies above the radiative zone and below the photosphere. This layer transmits energy by rotating vortices (bubbles). This "boiling" occurs in gas which is no longer so hot that it is a charged plasma.

Rocky Planets

The innermost planets in the solar system are formed with a central core surmounted by a rocky mantle and a thin crust (and a very thin ocean on Earth). Although we do not have a great deal of direct evidence, we believe that these worlds formed while the newly ignited Sun was propelling a titanic solar wind. Although the rocky materials and heavy metallic cores could form, a dense hydrogen and helium atmosphere similar to the outer gas giants was not possible.

All these worlds bear scars from the period roughly 4.5 billion years ago when they were formed out of the collisions of countless smaller bodies. Mercury presents a visual surface that is easy to confuse with the Moon. It is heavily crated. Earth bears definite crater marks, although the forces of weather and plate tectonics have erased many of these scars. We can see traces of craters on Mars and radar images of Venus reveal similar terrain. All this confirms that the early solar system abounded with small proto-planets that criss-crossed the more circular orbits of what became the major planets.

Some of these criss-crossing worlds hit the planets and merged with them. We recently say Shoemaker-Levy 9 revisit this process on Jupiter and we know that 65 million years ago, the dinosaurs died when a relatively small remaining proto-planetesimal struck the Yucatan Peninsula. Eventually only relatively large bodies in relatively circular orbits survived. Today, only Pluto is in a criss-crossing orbit and it survives simply because it is in a strange 2 to 3 synchronous orbit with Neptune which always keeps them at least 1/3 of Neptune's orbit apart.

Mercury

Mercury, the innermost of the planets, shows the effects of it close proximity to the Sun in many ways. While it looks like the Moon as far as its topology goes, it differs from the Moon drastically. Its iron nickel core is a much larger percentage of its total volume. Mercury would also be the densest planet if it was as large as the Earth. Its gravity is too little to significantly compress its core.

It  was once though that Mercury was tidally locked to the Sun, much the same way as the Moon is tidally locked to the Earth or the Galilean Moons are tidally locked to Jupiter. Tidally locked bodies rotate on their axis such that their "day" and their "year" are identical. Mercury revolves in synchrony with it orbit about the Sun in a ration of three Mercurian days in a Mercurian year.

The combined effects of Mercury's high eccentricity orbit and the planet's proximity to the Sun make it impossible to successfully use Newtonian physics to predict its position over long periods of time. The orientation of the orbit revolves slowly in accordance with Einstein's General Theory of Relativity.

Venus

The planet Venus has long been imagined as a paradise or at least an Eden. It great brilliance and its lunar like phases (suspected although not viewed since antiquity) made it a natural connection to the Moon. It is not coincidence alone that both bodies where associated with Goddesses - Venus/Aphrodite and Diana/Selene. From the middle of the 19th century until about 2/3rds of the way through the 20th century, Venus was believed to be a damp, watery world, somewhat warmer than Earth but still a very likely abode for life. Numerous stories were written about this cloud covered world with people slogging through swamps or being besieged by rains as amphibian wildlife provided the local monsters.

The first signs that Venus might not be quite such a rainy swampland were the microwave signals from large radio telescopes. If the curves were to be believed, the temperatures weren't merely very warm but positively blast furnace like in intensity. This was confirmed when the first probes parachuted into a hellish world where the surface was hot enough to melt lead or tin, the air was as dense as water thirty feet deep , the clouds were boiling sulfuric (battery) acid, and water was no where to be found.

Venus had other surprises. It rotates backwards. Its north pole points in the same direction as the other planet's South Pole. It has an extremely long day (18 of our days longer than its year). Weirdly, three Venerean days is just about exactly two Earth years.

The air is so dense that light is refracted completely around the planet. The cloud layer is so dense that daytime is only somewhat brighter than nighttime. If we could see much of anything on the surface, everything in the distance would seem red or orange because other frequencies of light are absorbed far above the surface.

Venus is just close enough to the Sun, so that a runaway greenhouse effect took place. Electromagnetic radiation is trapped beneath the cloud deck. When volcanoes and crevasses open, any sulfurous gasses remain gas rather than cooling to a solid as they would on Earth.

Venus' apparent diameter changes radically from its farthest position to nearest point. At its most distant, it is about 1.8 AUs from the Earth. At its nearest it is about 0.2 AUs from the Earth. If we could see Venus at its very nearest, it would be 9 times as large as when it is at its most distant. Unfortunately, both these extremes occur when it aligns with the Sun.

You might think that Venus would be brightest when it was closest, but this is incorrect. As Venus approaches, it becomes an ever thinner crescent. As Venus approaches "full Venus" (ala full Moon), it also reaches its smallest disk. Venus is most brilliant at the point where it displays the greatest illuminated surface, a balancing act between its phases and its proximity to Earth.

Earth

When we are asked what planets are visible, we almost always forget to mention the single planet which is always visible, day or night. It is the one underfoot, the Earth. The planet Earth (called Terra by the Romans and Gaia by the Greeks) really should probably be called planet Water. About 7/10ths of it surface is washed by oceans, seas, lakes and rivers. Earth is a very high contrast planet, with brilliant white clouds against large blue areas and smaller orange brown areas. It has two very prominent polar ice caps.

Earth is not only the only known abode of life in the solar system, but a planet which has been radically altered by life. Our very atmosphere was manufactured by a mutant strain of bacteria. These strange bluish green bacteria started to break down compounds and emit a ferociously caustic gas oxygen. Most of the life on Earth died when in came in contact with this deadly poison. Today only in a few places where oxygen cannot reach do we find the survivors of this first and most dramatic case of air pollution. The mutant bacteria live on everywhere as blue/green algae. The scant survivors are the anaerobic bacteria found in hot springs and badly sterilized cans of food - botulism. You might think that life was basically a surface feature but you would be wrong. Our deepest wells and mines encounter bacteria many miles inside the Earth, living on whatever chemicals can sustain life.

Moon

Isn't it a bit out of place to call the Moon a rocky planet? Isn't the definition of a planet a large world which revolves around the Sun? Aren't things which revolve around planets called satellites? The answer to all of these questions simply points out how very odd the Moon really is.

• The Moon is a large world, bigger than Pluto and not much smaller than Mercury.
• While the Moon circles the Earth, it always moves forward relative to the Sun. In fact the Sun's gravity controls the Moon's orbit about 3 times as strongly as the Earth's gravity. This does not happen with any other natural satellite. It is not adequate to compute the position of the Moon simply by treating the problem as finding where the Earth is and then computing an elliptical sub orbit for the Moon. All other natural satellites can be calculated this way for almost all purposes.
• Add to these two conditions the effect of tidal drag and you get a world which is more difficult to compute than any other object in the solar system except for comets and asteroids which happen to come very close to large worlds.

The Moon is composed of a rocky mantle which is extremely similar to its co-planet Earth, but it lacks a metal core. [We can tell this through various tests such as a lack of a magnetic field and "moon quakes" which allow seismometers placed on the surface to allow examing the interior as we do on Earth. Only one theory has survived rigorous computational modeling. The Moon must have been formed when the Earth was struck a glancing blow by an early proto-planet in an elliptical orbit. This proto-planet must have been the size of Mars approximately. Some of the material from the proto-planet and a great deal of the material from the Earth's mantle collected in a dense ring quite close to the surface. Eventually this coalesced into the Moon.

Tides

Throughout most of the world, there are two high tides and two low tides every lunar day (just under 25 hours). If gravity alone were the cause of the tides, there would be just one on the side of the Earth facing the Moon. In any case, the Sun would raise a much greater tide because the gravity of the Sun on the Earth is 832 times the force of the Moon on the Earth.

Tides are caused by an imbalance between orbital speeds at the center of a body and orbital speeds at the surface. Consider our diagram. Assume the body is following the gray line. The yellow dot at the center of gravity follows this orbit with neither an excess no a deficit of speed. However, the red outer particle is actually in a slightly larger orbit but still traveling at the same speed as the yellow dot. This means that the red particle is traveling slightly too fast for the orbit it is in. Left to itself the red particle would change its orbit's shape slightly creating a larger elliptical orbit. The situation for the blue dot is very similar, except that it is going just a bit too slow and would like to drop into a smaller orbit.

If the body were perfectly rigid, all that would happen is a tension at right angles to the orbit. However if the body is either completely fluid or covered with a fluid, a bulge is created on both the inward and outward sides of the body - a tide. On Earth, additional complications occur. First the Earth is spinning once in 24 hours (rather than just under 25 hours for the Moon). This means Earth is trying to accelerate the tidal bulge. This in turn exerts a breaking action on the Earth (the Earth's rotation is slowing a small amount every day and has been for billions of years). The loss of angular momentum must be matched by the laws of physics and the result is the Moon slowly recedes from us. A second complication is that the Sun also raises a smaller tide which is in a 24 hour cycle. This means that throughout the lunar month the tides sometimes reinforce each other and sometimes counter each other. This causes flood and neap tides.

The tides are not only raised on the Earth, but on the Moon. The Moon's tidal bulge is cast in concrete or more precisely in granite. The face we see is raised. If the Moon drifts to one side or another (a motion called libration), the Earth exerts a torque on the bulge and returns the Moon to face us.

The closer you get to a massive body the more severe the tidal forces on an extended body. Inside a certain distance (Roche's Limit), solid bodies cannot form. Would be satellites which venture too close to large planets become rings.

Extremely massive bodies with extremely small diameters (white dwarfs, neutron stars, pulsars and black holes) can have tides so strong that nothing can withstand them. An astronaut venturing too close to one of these monsters would feel his feet pulled into the black hole (or whatever) while his head was being wrenched off into outer space.

Mars

No planet has held a greater fascination for us than Mars. It is a place we might be able to live on if we provided ourselves with breathable air and some warmth. For those of us born before the space age, Mars was almost magical. It was the only planet with a surface that could be seen. At closest oppositions, it is quite possible to pick out polar caps, mountainous areas and plains.

However, Mars' image plays tricks on our eyes. The edges of two types of terrain seemed to be marked by long "canali". When Schaparelli used this term, it only meant "channel" in Italian but it wasn't long before the word was being called "canals". In turn, canals implied canal builders, and this in turn became ever more fanciful stories of dying races and desperate attempts to eke out meager water reserves from the polar caps for farms along the canals. Mars had other tricks to play on our eyes. Sometimes canals changed. We now know that these changes if there were seen at all were simply the result of dust storms covering and revealing the land below.

Mars is currently the target of a quixotic mission to send men to the planet. Whatever the social forces that prompt this, relatively little scientific information will be gained that could not be gained remotely.

Gas Giants

During the formation of the solar system, many planetesimals formed as whirls in a disk that spun around the proto-sun. Heavy elements sank towards the center of these bodies while the outer portions were wrapped in gasses - primarily hydrogen and helium. Once the Sun reached a point where its internal temperatures allowed it to ignite nuclear fusion, everything changed. The innermost planetesimals were lashed by extremely powerful solar winds which stripped away most of the hydrogen and helium. All the while all of the large planetesimals were accumulating smaller planetesimals eventually forming the eight large planets.

By now the innermost worlds were scoured clear of most of their atmospheres. Only denser and heavier gasses remained. On Earth, much of the hydrogen combined with oxygen to form water.

However, farther out the solar wind abated and the growing planetesimals could gather huge reserves of hydrogen and helium. So much hydrogen was gathered that it began to compress into unusual forms such as metallic hydrogen.

Jupiter

Jupiter is a planet of superlatives. Only Earth rivals it for markings. It outweighs all the other planets put together by at least a factor of 2. It spins on its axis faster than any planet. A jovian day takes less than 10 hours. Its "surface" gravity is more than twice that of the Earth. It has vast storms, larger in diameter than the Earth that swirl madly for hundreds (perhaps thousands?) of years. One such storm called the Great Red Spot has been continuously viewed since Galileo's time. Moving pictures from space craft dramatically show the clouds racing around the center at speeds of several hundred miles per hour.

Jupiter has a huge core of metallic hydrogen. Metallic hydrogen is hydrogen gas compressed so densely that it begins to behave like metals on Earth. In particular, it carries electrical current. This in turn creates a magnetic field which acts as a vast buffer between Jupiter and the Sun's solar wind. If we could see magnetic lines of force the magnetosphere would be larger in our sky than either the Moon or the Sun. Jupiter has huge lightning storms with bolts so powerful that its innermost satellites, Amalthea and Io are sometimes hit by them.

Jupiter volume is just about as large as it is possible for a body to be without becoming a star. If you dumped more material into Jupiter, its diameter would begin to actually shrink as gravity increased the density faster than material could be added.

It is a mistake to think that Jupiter is nearly big enough to make a star. It is nowhere close to that mass. It would have to be between 15 times its current mass to be a brown dwarf and 80 times its current mass to be the smallest main sequence red dwarf. In spite of not being able to sustain nuclear reactions in its core, Jupiter generates more than twice as much light (in the infrared spectrum) as falls on it from the Sun.

Saturn

When you think of Saturn, the words "the ringed planet" almost certainly jump to mind. While all the other gas giants have rings, none were discovered before the advent of space craft and some of them can only be detected when they blot out a background star as the planets passes in front. With Saturn there is no such problem. Saturn's Rings can be seen with the most modest tools.

The rings are swarms of tiny rock sized pieces of icy material. Long before the 20th century, astronomers and physicists knew that the rings where formed of countless particles. No rigid ring could have survived the tidal stresses on it.

Uranus

Uranus has to be the most featureless planet in the solar system. It is called green, but it is not the beautiful green of an emerald or an aquamarine but a pot of day old pea soup. You do not see the banding or cloud structures you see on Jupiter, Saturn or Neptune.

Uranus does have four substantial Moons, though none of them as large as Pluto. Perhaps the most interesting thing about Uranus is that its axis is tilted an extraordinary 98 degrees. It keeps its north pole pointing towards the Sun. Effectively, it has a warm pole and a cold pole, but differences in temperature are minimized by surface air flow.

Uranus was the first planet found by telescopes. This is a little odd because this planet is visible to the unaided eye under favorable conditions. However, it is so dim and unremarkable, that no one ever noticed it was moving slowly.

Neptune

Neptune is a deep blue with white wispy clouds and a dark blue spot similar to the Great Red Spot on Jupiter. Its orbit is nearly circular, with only Venus slightly more so. Neptune is the first planet which cannot be seen with the unaided eye. However, even the most modest binoculars can make it out as a faint bluish star, if you know where to look. At Neptune's distance, sunlight is almost a thousand times dimmer than on Earth. The Sun appears as a small disk, but could be easily mistaken for an extremely bright star.

Once we past Neptune, the solar system changes radically. The hydrogen gas required to form a gas giant thinned too much to form a ninth large planet. Only icy clumps of water, methane and ammonia which chunks of rocky planetesimals and dust were left to form bodies. Just outside Neptune's orbit various small icy worldlets formed. Much farther out where the Sun's gravity feebly contends with passing stars, the final layer of the solar system the cometary Oort cloud lies.

Ice Worlds

Pluto and Charon

              AUs             39.529      Period        248.6
           Orbital Vel.  4.7        Eccentricity   24.8%
           Inclination    17.15       Diameter     2200
           Mass             0.17         Density       2.0
           Rotation       -6.387     Obliquity      118.0
           Gravity         9%          Escape Vel.   1

Beyond the orbit of Neptune lies a variety of icy worlds and planetesimals collectively called transneptunian objects. If these objects happen to approach the sun we would call them by a more familiar name - a comet. The structure of both Pluto and its satellite Charon are very similar in nature to the icy satellite of Neptune (Triton) and the transneptunian objects. In the late 1990s, the IAU made a move to reclassify Pluto as a transneptunian object rather than a planet. There were many reasons. Pluto is very much smaller than the other planets. It had a highly elliptical cometary orbit, actually coming inside of Neptune's orbit for a while. The plane of its orbit was well outside the plane of the ecliptic. It lacked either the rocky structure of the inner planets or the gaseous structure of the outer planets. The attempt to reclassify it failed when the wife of its Pluto's discover (Clyde Tombaugh) made an appeal.

When Pluto was first discovered, it was assumed to be a very large world. This was necessary if it was the cause of "perturbations" of Neptune's orbit that had been measured. In fact the "perturbations" turned out to be measurement and round off errors. A second reason that Pluto was thought to be large was its high relative brightness. This turned out to be a byproduct of its snowy icy surface (rather than duller rock or gas).

Satellites

Satellites in the solar system range from rocky planetesimals only a handful of miles across to great worlds larger than Pluto or Mercury. Some of these satellites had atmospheres and another seems to have a watery ocean. One satellite is white on one side and coal dark on the other. One has a huge crater that makes the satellite look like an eyeball.

Some small rocky and icy planetesimals of irregular dimensions end up as captured moons of the outer planets. Capturing an asteroid requires something to slow asteroid down such as a brush with the planet's atmosphere or a close pass by a large moon. Unless the excess speed of an asteroid can be discarded by brushing the atmosphere or by slingshot orbit changes around a large Moon, any incoming asteroid will simply shoot by a large planet.

When our first robot spacecraft reach the Jovian System we could not believe what we were seeing. We expected that the four Galilean Moons would be more or less alike. Astronomers had predicted they would be a rocky center with an icy covering. We will look at some of the large satellites, but the smaller satellites are worthy of study as well.

Io is a festival world of brilliant reds, and yellows, black smudges and white streaks. You say you don't like the way it looks, well wait a few weeks and Io will change - dramatically. It is by far and away the most tectonically active world in the solar system. Io turns itself inside out ever million years or so. It has active volcanoes which spew sulfurous compounds onto the surface at a pace which far exceeds anything on Earth. It is as if the whole surface was like Volcano National Park on Hawaii.

If the surface wasn't wild enough, Io goes around Jupiter in a doughnut shaped cloud tube of ionized material (probably sulfur). It is as if Io has a strange atmosphere going all away around Jupiter.

Io has a really rough time being so close to Jupiter. Jupiter raises tremendous tidal forces in Io bending the rock and minerals back and forth. All this causes a great deal of heat from friction as materials rubs each other. Jupiter periodically blasts the surface of Io with great lightning bolts. The static from these flashes create the loudest radio noise in the 10 meter wavelengths.

Europa is the next major satellite out from Jupiter. It takes Europa twice as long to circle Jupiter as Io and half as long as Ganymede. Look at the surface of Europa. Those long lines are actually cracks, not in rock but ice. Mounting evidence suggests that this ice floats on a world wide watery ocean that in turn lies on a rocky core. If this sounds like our polar seas, it is hardly surprising.

While astronomers are fairly certain that Europa has some sort of an ocean, there are questions whether it is water, slush or some more exotic mixture with water and other materials acting as an antifreeze.

Ganymede is a very respectable world in its own right. While not as dense as Mercury it has a diameter which is greater. It has a surface which is three quarters of the Earth's land area. Ganymede is more like the Moon than any of the other Galilean Moons. It shows the type of cratering we see on both the Moon and Mercury.

Callisto is more like Pluto than any of the other Moons. Far enough from Jupiter so that tidal forces do not create frictional heat in huge amounts, it is the coldest of the Galilean Moons. Much of Callisto is icy material.

Titan, seconding size to Ganymede by a scant few miles, is another very substantial world. This moon is an orange brown color, but not because we see some tan colored soil by the only substantial atmosphere retained by any natural satellite. Its composition appears to be primarily methane with perhaps some ammonia. The molecular weight of methane is high enough that unlike hydrogen or helium, a small world can retain large quantities of it. It is through that this atmosphere is very similar to the original atmosphere of Earth before blue/green algae changed the atmosphere to a nitrogen and oxygen rich air.

Companions

Companions are bodies which orbit a third body yet are linked together in various odd ways.

Saturn has two moons Epimetheus and Janus which both orbit Saturn on opposite sides of a thin ring. Because one of the moons is an orbit which is closer to Saturn, it travels faster and eventually overtakes the outer moon. As it gets close the outer moon decelerates the inner moon while the inner moon accelerates the outer moon. The two moons exchange orbits with the prior outer moon now on the inside and vice versa.

Earth has a companion as well, a small asteroid called Cruithne (pronounced croo-en-ya - a Celtic hero). Although its orbit is tipped steeply to the Earth's orbit, it has a semi-major axis that AVERAGES about 0.9999 AU. The Earth is so large that it scarcely is affected by Cruithne's minuscule gravity. What happens to Cruithne is quite something else. When Cruithne is slightly inside the Earth's orbit it catches up and the Earth slingshots to a slightly outer orbit which causes it to start to lose ground to the Earth. Eventually it loses enough ground that Cruithne is overtaken by the Earth at which time the slingshot works in reverse pulling Cruithne into an orbit closer to the Sun and speeding up the asteroid. If you counted the Earth and the Sun as a fixed line, Cruithne would trace a horseshoe pattern with respect to the Earth. It travels on the inside of the shoe when it is catching up and the outside of the shoe when it slows down. The whole cycle takes about 4 centuries.

Co-planets

When a satellite is extremely massive with respect to its planet, it really forms a binary system with both bodies revolving around a common center of gravity (the barycenter). Most satellites are very small with respect to the planet. Ganymede is the largest satellite in the solar system, but Jupiter mass is 12,817 times that of Ganymede. The pull of Ganymede is trivial compared to the pull of Jupiter on Ganymede. Two planets have satellites which are very large relative to the primaries Earth and Pluto. The Earth is only 81 times as massive as the Moon. Pluto is about 8 times as massive as Charon.

Pluto and Charon rotate about their barycenter which is 1/8 the distance from Pluto to Charon. The barycenter lies outside of Pluto.

The barycenter of the Earth and the Moon lies about 4740 kilometers from the center of the Earth in the direction of the Moon. If you didn't see the Moon from Mars, you could easily deduce it was there because the Earth moves about 1/3 of it diameter side to side every 29.5 days. This also greatly complicates calculating where the planets are because we cannot ignore our movement around the barycenter for precise calculations.

Asteroids

      

Vesta is one of the largest asteroids but as you can see from this photograph, it is not spherical like planets or larger satellites. Residing as it does inside the asteroid belt, even if it had original cooled as a sphere, collisions with countless smaller asteroids would have ensured that it was pockmarked and gouged.

Dactyl is a satellite of Ida. They were the first such pair discovered. Since their discovery other pairs have been discovered. At least a few of the asteroid pairs are contact pairs. They rotate very slowly because their gravitic pull on each other is very small.

In the year 2004 Toutatis will come within 6 Moon distances from Earth. Currently there is a lot of activity trying to establish the orbits of asteroids which cross the orbit of Earth. Unlike long term comets which we may not see until it is too late, it is entirely possible to identify any potentially dangerous Earth crossing asteroids early enough to do something meaningful to pull the asteroid out of a dangerous collision.

It is rather frightening to realize how small a civilization ending asteroid can be. The asteroid which killed the dinosaurs 65 million years ago was not some huge near planet sized body, but something on the order of the size of a mountain. The effects of such a collision are far ranging. There is no place on earth that would escape if a ten mile diameter asteroid hit us. It would compress a shaft of air ten miles across until all the air became nitric oxide - the basis of nitric acid. If the asteroid hit land a huge crater would be dug and ejecta thrown up in the air blotting out the Sun for several years. During this time nothing would grow. Hitting an ocean would have even more terrible effects. The fact we haven't been hit in 65 million years is not too much comfort to those of us who watched Shoemaker Levy 9 crash into Jupiter.

      

Comets are planetesimals composed largely of volatile materials. When a comet comes close enough to the Sun it often out gasses a long and brilliant tail. Comets travel in extremely elongated elliptical orbits. At the most distant point in the orbit, the comet moves so slowly that it may stay out there thousands or even millions of years. When they move into the solar system, the relentless gravity of the Sun accelerates day after day for decade upon decade. They can be traveling at speeds up to 70,000 miles per hours when they round the Sun.

Sometimes long term comets happen to pass near a gas giant - particularly Jupiter. Either the comet will be speeded up or slowed by the encounter. If it is speeded up it will pass the Sun with excess velocity that will cause the comet to leave the solar system forever. If it slows down, the comet may become an inner comet like Halley's which has a period under one hundred years. There are two principle reservoirs of comets, the Kuiper Belt (home of the transneptunian objects) just outside the orbit of Neptune and the Oort Cloud about 50,000 AUs (about 8/10 of a light-year) from the Sun. The Kuiper belt is a disc similar to the asteroid belt but filled with comets. The Oort cloud is spherical. The Oort belt is so far from the Sun that occasionally the Sun and another star pass close enough that the other star pulls comets of the Oort belt. Some are lost to the solar system forever but other are started on their slow way into the inner system.
 

          

Debris from comets which have melted and released their stony components, as well as broken pieces of asteroids and occasionally, pieces of planets ejected when comets crash into the planets are scattered all over the solar system. Most of then are dust particles or sand like specks. However, larger pieces exist.

If a meteor hits the Earth's atmosphere it burns up from friction. If the particle is large enough to hit the ground it is called a meteorite. Roughly 100 tons of meteors fall on the Earth each and every day.

Meteorites come in three basic forms, ones which are largely iron come from the core of some proto-planet, ones which are are stony or a mixture of stone and iron which come from the collision of asteroids, and carbonaceous chondrites which are composed of carbon compounds. The meteorite ALH84001 is believed to have been chipped off Mars and eventually hit the Earth. ALH84001 caused quite a stir when scientist thought they detected fossil Martian bacteria on it. Such claims are very muted now. The bacteria fossils could have been from Earth or simply have been crystalline deposits.

Following the paths of melting comets, the density of particles can become unusually high. These create the so called meteor storms when the Earth orbit interests the orbit of such a comet. For example, Comet Tempel-Tuttle is the source of the swarm of meteors we call the Leonids. Swarms seem to come from a single point in the sky called the radiant. This radiant stays located' basically in a constellation and gives the swarm its name.