Asteroids are usually thought of as coming from the main asteroid belt, the collection of celestial bodies with orbits lying between Mars and Jupiter. About 95 percent of asteroids reside in this main belt, which stretches from 1.7 to 4 AU from the Sun. Based on modern studies using infrared imaging, there seem to be between 1.1 million and 1.9 million main belt asteroids larger than a half mile (about 1 km) in diameter. Many millions or perhaps billions of smaller asteroids also orbit in the main belt. Though there are millions of individual bodies in the asteroid belt, the total mass of all the asteroids is less than one 10th of a percent of the mass of the Earth and was probably only a few times that when the asteroid belt was new. If all the bodies now in the asteroid belt were accumulated into one planetesimal, it would be only about 900 miles (1,400 km) in diameter, much smaller than the Moon.
Johannes Kepler, the prominent 17th-century German mathematician and astronomer, first noticed the large gap between Mars and Jupiter where the main belt resides, but the development of the telescope was required before the first bodies in this gap were discovered. Though it was thought that a planet had formed in the gap and was subsequently destroyed, it is now clear that Jupiter's disrupting gravity field is so strong that no planet could ever have formed in that orbit. The material of the asteroid belt must have formed near its current location in the solar nebula because its combination of rocky and metallic materials is consistent with an inner solar system origin. Some material from the asteroid belt (notably, the carbonaceous chondrites, discussed later) has clearly never been heated above 70°F (20°C) or so and therefore must never have been very close to the Sun.
Though some asteroids in the main belt seem to contain water (see the sidebar "Spectrophotometry and Mineral Absorption Bands" on page 118), others appear completely dry. Inside 2.5 AU from the Sun, all objects are dry. This distance, 2.5 AU, is sometimes called the "snow line." Inside 2.5 AU, temperatures in the solar nebula are too high to allow ices to condense. (The ice condensation temperature is reached at just about 2.5 AU.) Not coincidentally, the snow line also marks the boundary between the inner, rocky planets and the outer planets that are rich in ice and gas.
All asteroids orbiting between 1.7 and 4 AU are considered to be in the main belt, but asteroids are not distributed evenly over the width of the belt. Some regions of the belt have distinct populations with similar properties that are thought to be fragments of a common large parent body. Main belt families include the Hungaria, Flora, Cybele, and Hilda families (described below), as well as the Phocaea, Koronis, Eos, and Themis families. Studying these families gives scientists a chance to gather statistics on impact, collision, and disruption rates in the solar system. This information is the closest thing to knowing what happened in early asteroid history, when the larger, original bodies of the main belt were colliding and breaking up. By studying families, scientists can calibrate their models for how a body breaks up when it experiences a catastrophic collision.
Mars orbits at 1.52 AU. Shortly beyond Mars lies the Hungaria group, the innermost group of asteroids in the main belt. The Hungarias orbit between 1.78 and 2.0 AU, in orbits with eccentricities below about 0.2.The Flora family orbits from 2.1 to 2.3 AU, with eccentricities up to 0.9 and orbital inclinations up to 11°; beyond the Flora family lies the bulk of the main belt, between 2.3 and 3.25 AU.
The main belt asteroids Gaspra (right) and Ida (left) show craters and a regolith layer on their surfaces. (NASA/JPL)
(In general the eccentricities of the orbits go down with distance from the Sun.)
Gaspra is a Flora-family asteroid with a cratered body about 10 miles (17 km) long. In the image above, Gaspra (right) is shown with Ida (left), a main belt asteroid in the Koronis family (discussed below). Shown at the same scale, both objects are irregular in shape, suggesting that they are pieces derived from larger bodies by catastrophic collisions. Craters are more abundant on Ida, suggesting that it formed earlier than Gaspra. Both asteroids have linear depressions more than a thousand feet wide in places and a hundred feet or so deep, which may be where loose soil has partly fallen into fractures. These asteroids show evidence of having such a fragmental layer, which on Ida may be 165—330 feet (50—100 m) in depth.
The Koronis family orbits within the main belt. More than 200 asteroids are now identified as members of the Koronis family, orbiting at about 3 AU. 243 Ida, an asteroid famous for having its own orbiting moonlet, Dactyl, is a Koronis family member. Dactyl is the first confirmed and photographed natural satellite of an asteroid.The tiny moon is about 0.75 X 0.87 X 1 mile (1.2 X 1.4 X 1.6 km) across. Its name is derived from the Dactyli, a group of mythological beings who lived on Mount Ida.The Dactyli protected the infant Zeus after the nymph Ida hid and raised the god on the mountain.
The first full picture showing both asteroid 243 Ida and Dactyl, newly discovered at the time, is shown in the image below.The Galileo spacecraft took this image from a range of 6,755 miles (10,870 km). The moon is not identical in spectral properties to any area of Ida, though its overall similarity in reflectance and general spectral type suggests that it is made of the same basic rock types.
Ida itself is an irregular Type-S asteroid, with dimensions of 35 X 15 X 13 miles (56 X 24 X 21 km). Dactyl's orbit allowed scientists to
Asteroids in the main belt are divided into a series of families that may have originated as large bodies, later fragmented through impacts.
calculate Ida's mass, since given the dimensions of each of the objects and their orbital period and distance the density can be calculated.
After 3.25 AU there is a distinct gap before reaching the Cybele family of asteroids at 3.3 AU. The Cybele-family asteroids orbit between 3.3 and 3.5 AU. Like the Hungarias before them and the Hilda family after them, the Cybele asteroids have relatively low orbital eccentricities, below 0.3.The Cybeles appear to have formed from a large, common parent body that was broken up in the distant past.This family is named after 65 Cybele, the seventh-largest known asteroid, with a diameter of 192 miles (308 km). Another especially interesting Cybele-family asteroid is 121 Hermione. This asteroid was discovered by James C. Watson at Ann Arbor in 1872 and has a diameter of about 130 miles (209 km). 121 Hermione is notable because it has its own tiny moon, like the asteroid Ida and its tiny moon Dactyl. Hermione's tiny moon is about eight miles (13 km) in
Distribution of Asteroids in the Main Belt
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