Sizes and Orbits

The largest representatives are Ceres, with a diameter of about 1030 km (about 640 mi), and Pallas and Vesta, with diameters of about 550 km (about 340 mi). About 200 asteroids have diameters of more than 97 km (more than 60 mi), and thousands of smaller ones exist. The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km (less than 100 mi). Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own.

Few scientists now believe that asteroids are the remnants of a former planet. It is more likely that asteroids occupy a place in the solar system where a sizable planet could have formed, but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter. Originally perhaps only a few dozen asteroids existed, which were subsequently fragmented by mutual collisions to produce the population now present.

The so-called Trojan asteroids lie in two clouds, one moving 60° ahead of Jupiter in its orbit and the other 60° behind. In 1977 the asteroid Chiron was discovered in an orbit between that of Saturn and Uranus. By the late 1980s, about 75 asteroids, the Amor asteroids, were known to intersect the orbit of Mars, about 50 Apollo asteroids to intersect the orbit of the earth, and less than 10 Aten asteroids to have orbits smaller than the earth's orbit. One of the largest inner asteroids is Eros, an elongated body measuring 14 by 37 km (9 by 23 mi). The peculiar Apollo asteroid Phaethon, about 5 km (about 3 mi) wide, approaches the sun more closely, at 20.9 million km (13.9 million mi), than any other known asteroid. It is also associated with the yearly return of the Geminid stream of meteors.