Minor Planets

Main-belt asteroids revolve around the Sun on nearly circular orbits. It takes a main-belt asteroid 2.8–8.0 years to complete one circuit around the Sun. Images obtained by NASA's space probe Galileo, presently in orbit around Jupiter, have confirmed the long-held belief that asteroids are irregularly shaped objects. Galileo imaged the asteroid Gaspra in October 1991 and found it to be a highly cratered, 11.2 × 5.6 mi (18 × 9 km) rocky body. When Galileo flew past the asteroid Ida in August 1993 it imaged an elongated, 31.2 × 9.3 mi (55 × 15 km) object sporting many scars and impact craters. The irregular shapes of these asteroids reflect an extensive history of collisions (and insufficient mass to shape themselves into spheres by gravitational force). Perhaps the most surprising of Galileo's discoveries during the Ida encounter, however, was the detection of a small moonlet, 0.62 mi (1 km) in diameter, orbiting the asteroid.

The main-belt region is not evenly populated with asteroids, and several zones have been found in which virtually no asteroids reside (Fig. 1). The American astronomer Daniel Kirkwood (1814–1895) first noticed these empty regions, or gaps, in 1866. Now called Kirkwood gaps, these asteroid-devoid zones are located near orbits for which the time to complete one circuit around the Sun is a simple fraction (e.g., 1/2, 2/3, 3/4) of Jupiter's orbital period. For example, given that Jupiter orbits the Sun once every 11.86 years, an asteroid belt gap is expected at a distance of 3.3 astronomical units (AU) from the Sun, where any orbiting body would have a period of 5.93 years, one-half that of Jupiter. Such a gap does indeed exist. These Kirkwood gaps are produced by

orbital resonance with Jupiter. When the orbital period of an asteroid is a simple fraction of Jupiter's, it will experience strong, evenly-spaced, frequently repeated tugs from Jupiter's gravitational field, like a child being pushed on a swing. Over time these periodic tugs will alter the asteroid's orbit, and the orbits of other, similarly placed asteroids, ultimately clearing out a gap at that particular distance from the sun. When no gravitational resonance exists, an asteroid's orbit tends to be stable.

Asteroids are classified according to their color and reflection spectra. An asteroid's color is determined by measuring how bright it appears through several specially constructed filters which pass only well-defined wavelengths of light. The color, indicated by a quantity called the color index, is essentially a measure of how well the asteroid reflects sunlight at different wavelengths. An indication of an asteroid's surface composition can be gleaned by measuring its reflection spectrum. Most asteroids are classified as C-type or S-type.

The C-type asteroids have a bluish color, and their reflection spectra indicate the presence of carbonaceous material at their surfaces. The S-type asteroids, on the other hand, are more reddish in color and their reflection spectra indicate that presence of surface silicate material. Other classifications include M-type, indicating the presence of surface metals, and R-type, indicating a deep, dark red color. Observations have revealed that the S-type asteroids tend to reside in the inner main belt, near the orbit of Mars. The C-type asteroids, in contrast, tend to reside toward the outer edge of the main belt nearer to Jupiter' s orbit. The S-type asteroids are thought to be the primary source of stony and stony-iron meteorites, while the M-type asteroids are the most likely source of the iron meteorites.