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Asteroids Where They Are The vast majority of asteroids are grouped in the asteroid belt, which is more like a loose grouping than a belt, and lies between 1.8 and 4.5 A.U. (1 A.U. is the average distance between Earth and the sun) from the sun - between the orbits of Mars and Jupiter. The asteroids are so small and far away that they appear as faint stars, if they even appear at all; no asteroid is bright enough to be seen without some optical aid, except Ceres (see below) on its closest approach to Earth. The asteroid belt is usually thought of as a defined region where asteroids abound (reference many science fiction movies, with spaceships flying in and out, dodging debris). This is actually very unlike the asteroid belt -- the region is so vast that asteroids are usually hundreds of thousands of kilometers from their closest neighbor. If the asteroid belt were taken to be the width of its densest region (3.2-1.8 A.U.), and it were considered flat, it would have an area of about 6·1017 km2. Even if there are 1000 times more asteroids than we know of today, then on average each asteroid would have over one million km2 to itself.
Other than belt asteroids, there are several other classifications of asteroids, based upon their location and orbit in the solar system:
There are also many asteroids that gallivant around the solar system on highly elliptical orbits like comets. Of note, the asteroid Icarus, when closest to the sun, lies within Mercury's orbit - it comes close enough to the sun that Relativity must be taken into account in order to accurately predict its orbit. Another example is Hildago, whose closest approach is between the orbit of Mars and Earth, and its farthest from the sun between Saturn and Uranus. It is estimated that the main belt alone contains well over 1 million asteroids. The total number in the solar system is estimated as much higher, especially once TNOs are considered. What They Are The first asteroid was discovered in 1801 by Giuseppe Piazzi of Italy. He named it "Ceres" after the Roman goddess of grain. Ceres is the largest known asteroid at approximately 950 km (590 miles) in diameter, and it lies in the belt of asteroids between Mars and Jupiter (see the above section) at an average distance from the sun of 2.6 A.U. Ever since, asteroids have received an official designation of a number (starting with Ceres of number "1"), and most larger ones have received a name based in Roman mythology. If they have a name, then they are usually referred to with the number then the name, such as 951 Gaspra. Currently, asteroids are also referred to by the International Astronomical Union (the only official body that can name astronomical objects) as minor planets. Asteroids range in size from dust particles to many miles across. Most current theories hold that asteroids are bits and pieces left over from the formation of the solar system. They are also formed from other asteroids as they collide and break apart, as comets disintegrate, or even when the outer moons of the larger planets collide. Past theories have suggested that the asteroids are remnants of a planet that was destroyed early in the solar system's history. However, that theory is no longer held in much regard, for if all of the asteroids in the belt were combined, they would form a body less than 1500 km (932 miles) in diameter -- less than half the size of Earth's moon, and so there is not enough material to make a planet. Asteroids are made of rock and metal. They are mainly grouped into three categories: Stony, Iron-Nickel, and a mixture of the two. Most asteroids that we know about (92.8%) fall into the first category, and are made of Silicates. 5.7% are Iron-Nickel. The balance form the third type. Despite their relative abundance, stony asteroids that have fallen to Earth are the hardest to find because they look like terrestrial rocks and they weather much faster than the metallic ones. Asteroids have a confusing system of nomenclature, especially when they are on Earth. While still in orbit, they are asteroids. Once they enter the atmosphere, they are called meteors, and once they land, they are termed meteorites. Interesting Facts and Features Asteroids are too small to be spherical in shape. Instead, they are usually ellipsoids, but some are dumbbell-shaped, and others form even stranger ones. Asteroids bare a tale of the violence of the solar system; the larger ones have many sizeable craters pockmarking their surface.
As previously stated, if all the asteroids in the belt were combined into one, it would form a body less than 1500 km in diameter. Noting the immense size of Ceres, it comprises over 1/3 the total suspected mass of the belt (2.3 x 1021 kg). 26 known asteroids are larger than 200 km (124 miles). We probably know 99% of the asteroids that are greater than 100 km (62 miles), and there are probably literally millions of asteroids that are greater than 1 km (0.62 miles) in diameter. Over 300,000 asteroids have been found. When asteroids break apart, the pieces don't always fly off in random directions. Sometimes, they will continue in the same orbit as the original asteroid. When several asteroids are seen in relatively the same place and traveling along similar orbits, they are called orbital families. Some Famous Asteroids
Data and Graphs The following plots were made from data available at ftp://ftp.lowell.edu/pub/elgb/astorb.html. The data for these plots was acquired on June 7, 2006, and comprise of information for 336,341 asteroids. This is a data set of all numbered and most unnumbered asteroids. Absolute Magnitude The absolute magnitude of an object is a measure of how bright it is relative to the star Vega. Vega is defined to have an absolute magnitude of 0; anything brighter than Vega is a negative absolute magnitude, and anything fainter is a positive magnitude. Magnitudes are on a logarithmic scale such that for every increase in 2.5 magnitudes, the object is 10 times fainter. So, a star that is 10th magnitude means that it is 10,000 times fainter than Vega. The absolute magnitude of asteroids is very faint, which is a result of them being very small and not reflecting a lot of sunlight. The absolute magnitude histogram on the right is more a reflection of how good our detection technology is than what the actual distribution of the brightness of asteroids is. It has been binned by 0.1 magnitudes. The current distribution is an almost perfect Gaussian, which is indicated by the blue line. The mean of the fit is 15.751±0.004 and the width is 1.642±0.005. The actual distribution of magnitudes is probably more of an exponential, considering that there are many more smaller asteroids than larger ones. There is also a slightly larger tail towards the bright end (lower numbers) of magnitudes. This is an artifact again of our detection methods: It is much easier to spot a bright object than a faint one, and so we know of many more bright asteroids than faint ones. Semi-Major Axis Asteroids are found throughout the solar system. However, in the interest of readability, I have isolated in the graph on the right just the solar system out to Jupiter. The data have been binned by 0.01 A.U. This allows a certain structure to the location distribution of the asteroids to be discerned. At about 5.2 A.U., there is a marked increase in the number of asteroids. These asteroids are the Trojans, the asteroids that orbit the Sun with Jupiter. They are not moons of Jupiter, they just share its orbit around the Sun. From about 3.2 A.U. in to about 1.8 A.U. is the realm of the main asteroid belt. Almost all known asteroids orbit within this region between Mars and Jupiter. The next graph shows an expanded view of the main asteroid belt, from 1.5 to 3.5 A.U. The data have been re-binned with intervals of 0.005 A.U. It is clear from this chart that there are many gaps in the distribution of the asteroids, and in some cases almost no asteroids are found in the locations. These are called Kirkwood Gaps, and they arise because of resonances with Jupiter. (Martian resonances are inconsequential compared with those from Jupiter.) At a distance of 2.49 A.U. from the Sun, Jupiter's gravitational
pull on an object is only about 0.3% of the Sun's. Over time, though,
this pull adds up and serves to nudge an object away from that region,
but this will really only work if the pull is repeated at a regular interval.
2.49 A.U. happens to lie at a distance from the Sun such that an object's
year there will be exactly 1/3 of Jupiter's. There are many other resonances that happen, and based upon how often an asteroid feels Jupiter's tug, the relative strength of the resonance is established. The resonance is especially effective at nudging an asteroid if the tug occurs in the same location in the asteroid's orbit and only in that location, so the 2, 3, 4, and 5 to 1 resonances are the strongest. The major resonances are:
The asteroid belt is bounded by the 5:1 and 2:1 resonance. Asteroids that are pushed outside of these resonances will generally become planet-crossing asteroids. Eccentricity The eccentricity of an orbit is how much it varies from a perfect circle. A stable orbit can have an eccentricity anywhere from a perfect circle with an eccentricity of 0, up to a highly elliptical orbit with an eccentricity up to (but not including) 1. If an orbit had an eccentricity of 1, it would be parabolic and escape from the system. If it were larger than 1, it would be hyperbolic and also escape from the system. Earth's eccentricity is 0.017, while Jupiter's is 0.094. In our solar system, the planet with the largest eccentricity is Pluto at 0.244, and Mercury with 0.205. The planet with the lowest eccentricity is Venus with 0.007. Unless there is some gravitational tugging (such as with the Galilean Satellites) that keeps an orbit eccentric, orbits will usually circularize with time. The eccentricities of the asteroids have been binned for this histogram by 0.0025. As shown in this diagram, the average eccentricity is about 0.17. In general, asteroids' orbits are far from perfect circles. This is probably due to perturbations from the planets - especially Jupiter - and collisions. A collision between asteroids can dramatically alter their orbits. The eccentricity can also be an effect of sampling. As discussed in the sub-section below on eccentricity vs. semi-major axis, objects that are farther away from the Sun are more likely to be found by Earth-based searches if they have a large eccentricity. Orbital Inclination When the solar system formed, most of the solar nebula evolved into a disk, and from this the planets, moons, asteroids, comets, and Sun evolved. Because of this, the planets all generally orbit in the same plane. The largest difference is Pluto, which orbits about 17° off from Earth's orbit. The next largest is Mercury which orbits 7° from Earth. The rest of the planets orbit within 2.5° of Earth's orbital plane. The orbital inclination histogram on the right, which is binned in intervals of 0.1°, shows that asteroids are another exception to this rule. The average orbital inclination is about 8.2°. A probable mechanism for creating the - sometimes very large - orbital inclinations are collisions. When two asteroids hit each other, their orbits can be changed dramatically. And, since the inclination of orbits is much less subject to gravitational perturbations than other orbital elements, they really don't have any driving force to put them back into an orbit with the rest of the solar system. Another factor is that when a rather large asteroid is broke up, the pieces tend to stay close to the original orbit, which creates a "family" of asteroids. This increases the raw number of asteroids with a given orbital inclination. Semi-Major Axis vs. Eccentricity This semi-logarithmic scatter plot of eccentricity as a function of the semi-major axis shows mostly what was discussed above. Visible are the general asteroid trends, such as the large number of main belt asteroids, trojans, a sprinkling of Centaurs, and the TNOs. Most asteroids have an eccentricity relatively low, with higher eccentricities tapered off. Two important observations, however, can be made. The first is made by looking at the asteroids that orbit within Mars' orbit, about 1.5 A.U. from the Sun. There are many fewer asteroids in this region than in the main belt, but on average, their eccentricities are much higher. This is a strong indication that they did not form in these locations, but that they were pushed or pulled into this region. When that happens, the asteroid is likely to return to its original orbit distance for at least part of its orbit, until it evolves more. This will cause it to have a high eccentricity, for during part of its orbit it will be much closer to the Sun than during another part of it. For the Kuiper Belt Objects beyond the orbit of Neptune, there is also a large increase in eccentricity. This is probably an artifact of how we detect them. Since we are limited by how much light we can detect from an asteroid, the closer it is to us, the more likely we are to detect it. So, if an object were to have a semi-major axis of, for example, 100 A.U., then it might be too faint to detect. But if it were on an elliptical orbit and this brought it within 35 A.U. of the Sun, then we would have about 8 times more light from it, and so we would be more likely to detect it. So, objects that lie from the Sun are much more likely to be detected today if they have highly eccentric orbits. Semi-Major Axis vs. Orbital Inclination Here is an identical semi-logarithmic scatter plot, except that it is the orbital inclination that is plotted against the semi-major axis. This is a much truer picture of what the asteroid distribution actually looks like in space than the eccentricity vs. semi-major axis. Probably the most important feature of this plot is that the Hirayama Families become apparent. This is the name given to "families" of asteroids, which are asteroids that tend to share the same orbital characteristics, and they probably had a common parent body that was destroyed during a collision. Hirayama was the first astronomer to suggest that they existed. The next graph is a blown-up section of the orbital inclination vs. semi-major axis. It has been made into a number density image by creating a 2-D histogram. The 2 A.U. range has been divided into 0.005 A.U. increments, and the inclination of 0°-50° has been divided into 0.25° increments. The number of asteroids per bin is based upon the color displayed in the color bar in the upper left corner. The reason the data are displayed this way is that, even with dots only one pixel large, over 300,000 asteroids would make the graph look like a lot of red. With this chart, it now becomes clear that there are
some obvious clusters of asteroids, as indicated by the larger number
density in certain areas. These groupings are the Hirayama Families. The Hirayama families are named for the main asteroid in the group. Some of the major ones are:
It is also fairly apparent from this image that there is a resonance at 2 A.U., 2.5 A.U., 2.8 A.U., 2.9 A.U., and 3.3 A.U., since these are distances at which the number density of asteroids is almost 0. 
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But,
there are asteroids all over the solar system -- they are not just confined
to the belt. Their location only indicates what kind of orbit they have,
as most asteroids look pretty much the same, such as the image of Gaspra
on the right.
One
of the most surprising features of asteroids is that several have been
observed to have moons of their own. The first asteroid to be observed
with a moon was 243 Ida (58 x 23 km); it's moon is called Dactyl, and
measures approximately 0.75 x 0.87 x 1.0 miles. It is now estimated that
between 10-30% of asteroids have moons.
This
means that once every three years, an asteroid in that location will
feel a tug from Jupiter, and over time, this will remove it from that
location. This is known as a 3:1 resonance.
