Asteroids are leftover planetesimals that date from the early period of formation of the Solar System. They formed in and reside in the inner Solar System, between the orbits of Mars and Jupiter, 2 to 4 AU from the Sun. Planetary physics suggests that conditions might have been right for a planet to form between Mars and Jupiter, but that the intense gravitational pull of the giant jovian planet prevented sufficient aggegation of planetesimals to constitute a planetary body there. Instead, we have a Main Belt of asteroids ranging in size from small to large. At least 6000 asteroids have been discovered to date, with several hundred being discovered each year. There may be hundreds of thousands of asteroids in the Main Belt, too small to be seen from Earth. Kirkwood gaps exist in the Main Belt as a result of disturbance to these regions by gravitational forces of Jupiter.
The combined mass of the asteroids is less than that of Earth’s Moon. The largest of the asteroids is Ceres, with a diameter of 914 kilometers. Ceres accounts for approximately 25% of the mass of all the asteroids. The next largest asteroids are Pallas (588 km), Vesta (576 km and possibly possessing an internal heat source), and Hygiea (430 km). Some 26 asteroids are known with diameters greater than 200 kilometers. It is estimated that we have identified 99% of the asteroids of this size. In comparison, we know of just 50% of those asteroids in the range of 10-100 kilometers diameter, and we have located only a small percentage of perhaps a million smaller asteroids with 1 kilometer diameters.
The only asteroids which have been studied at close range are Ida and Gaspara. Their surface appearances have been recorded by photographs taken by the Galileo probe. As mentioned above, the two moons of Mars, Deimos and Phobos, have chemical composition suggesting that they are captured asteroids originally from the Main Belt. Saturn’s moon Phoebe and some newly discovered moons of Uranus and Neptune also may be captured asteroids.
Asteroids are classified on the basis of their brightness or albedo, and by spectral evidence of their composition. They contain silicaceous minerals, carbon compounds, and iron and nickel metals. Some asteroids also contain small quantities of trapped crystalline water. There are C-type, S-type, and M-type asteroids identified in these classification schemes. Asteroids also are classified according to location, and not all have orbits within the Main Belt. The five basic groupings of asteroids are: (1) Main Belt asteroids, those found between Mars and Jupiter at 2-4 AU; (2) Atens asteroids, with semimajor axes greater than 1.0 AU and aphelion distances (the point of farthest orbit from the Sun) of more than 0.983 AU; (3) Apollo asteroids, which cross the radius of Earth’s orbit, and semimajor axes greater than 1.0 AU and perihelion (closest approach to the Sun) distances less than 1.017 AU; (4) Amor asteroids, lying just outside Earth’s orbit, with perihelion distances between 1.017 and 1.3 AU; (5) Trojan asteroids having resonant motion, as they are locked in orbit with Jupiter and located approximately 60 degrees in front of and 60 degrees behind the planet, at its “Lagrange” or “trojan” points. Jupiter has at least several hundred trojan satellites, and there may be a thousand or more. Earth and Venus also may have small Trojan asteroids orbiting in synchrony with them; evidence of the existence of Earth’s “other moons” dates to the mid-1840s and has been the subject of popular science fiction stories.
Those asteroids which are in near-Earth orbits are on highly chaotic, unstable orbits about the Sun. One of these near-Earth asteroids, 1995CR, traveled within 4.5 million miles of Earth in February 1995. It was detected by the 0.9 meter telescope at Kitt Peak National Observatory as it made this close passage by Earth. An upcoming space mission will fly by the near-Earth asteroid Eros (discovered in 1898) in the months ahead.