Discovery of Asteroids
On January 1,1801, a Sicilian astronomer, Giuseppe Piazzi (1746-1826), accidentally discovered a faint object whose orbital motion was that of a body 2.8 AU from the sun. Although Piazzi thought it was a comet, others noted that it was located about where a major planet would be expected according to Bode's law. The object was later named Ceres, after the Roman goddess of agriculture. Shortly afterward, three more objects were discovered with orbits near 2.8 A.U: Pallas in 1802, Juno in 1804, and Vesta in 1807. Since photographic techniques were introduced into astronomical research in the 1890s, nearly 3000 of these bodies have been discovered.
So instead of one planet in the slot at 2.8 AU, many small bodies orbit in the region between Mars and Jupiter. William Herschel called these objects asteraids because in a telescope they looked like stars. Almost 95 percent of them have orbits between 1.6 and 3.3 AU, with periods from 2 to 6 years. Their orbits are more elliptic than are those of the planets and more inclined to the ecliptic. They also move in the same direction as the planets around the sun.
For some time the asteroid Hidalgo was thought to have the largest orbit. Its orbital period is 14 years, with an aphelion just outside the orbit of Saturn. However, in October, 1977, a new asteroid, named Chi ron, was discovered; it travels in a highly eccentric orbit (eccentricity = 0.38) at an angle of 6.9° to the plane of the ecliptic. It ranges between 8.5 and 18.9 AU, or roughly between the orbits of Saturn and Uranus, with a period of 50.7 years. Because of its great distance from the asteroid belt, there is a question whether it might be the first discovery in an outer zone of asteroids. Or possibly it is not even an asteroid like those between Mars and Jupiter but is something related to a comet.
Asteroids vary in size from Ceres (1025 kilometers) down to an estimated 100,000 that are no more than 1 kilometer in diameter and countless numbers of even smaller ones. All the asteroids together may add up to no more than a few ten-thousandths of the mass of the earth. Ceres constitutes about 20 percent of the mass of all the asteroids.
Characteristics of Asteroids
Photometric studies of the asteroids have for some time been interpreted as showing that they differ in size, shape, and rotation. All but the largest of the asteroids are too small to show a measurable disk. From the variation in their brightness, it has been assumed that most have somewhat irregular shapes with periods of rotation measured in hours. Recent evidence suggests, however, that the brightness variations for some are the resu It of two or more asteroids in mutual orbit about each other; that is, some asteroids are binary systems.
Their colors put nearly all asteroids into two categories: Some are bright reddish, a sign of silicates and iron-nickel or other metallic grains, and they populate mostly the inner part of the asteroid belt; but most have the darker neutral color of material containing various carbon-rich or water-rich compounds (carbonaceous) and occupy the outer part of the belt.
Collisions between two asteroids may produce effects ranging from craters (if a small one collides with a large one) to fragmentation of the two asteroids (if they are of comparable size). For example, if the body producing crater Stickney on Mars's satellite Phobos had been a little larger, Phobos might have been broken into many small pieces. As it is, the grooves on Phobos may be large cracks produced by the impact.
