III.

Origin of Asteroids

Asteroids are ancient objects. According to current theories, the Sun formed about 4.6 billion years ago and was surrounded by a disk of dust, ice, and gas. Planets are thought to have formed within this disk by the clumping together (accretion) of particles of dust and ice into larger and larger objects, producing small bodies called planetesimals. Gravitation drew groups of planetesimals together to form protoplanets, which in turn built up full-size planets. Asteroids and comets are left over from the planetesimal-forming stage and can preserve clues about the earliest material that formed the solar system.

The asteroid belt likely represents a region in which a large rocky planet could not form because the gravitation of the giant planet Jupiter disrupted the accretion process. Scientists estimate that the ancient asteroid belt may have held enough material to equal twice the mass of Earth. However, gravitational effects of Jupiter and orbiting protoplanets in the early solar system cleared out most of the objects in the asteroid belt. The planetestimals were either thrown out of the solar system or were sent crashing into planets and moons, or into the Sun. The mass of all the material now left in the asteroid belt is estimated to be only about 0.001 the mass of Earth. If all the objects in the asteroid belt were combined into a single body, it would be much smaller than Earth’s Moon.

The early asteroid belt evidently contained a number of objects large enough to heat up inside from radioactive minerals and form metallic cores and lava. Mutual collisions of asteroids later broke up or reshaped these larger bodies, accounting for unusual types of metallic and igneous asteroids and meteorites.

Collisions that broke up larger asteroids may also explain asteroid families—groups of asteroids that have the same chemical makeup and have closely related orbits within the asteroid belt. The 4 Vesta and the 8 Flora families are among the best known of such asteroid families. In some cases asteroid debris has reformed into new asteroids that appear to be piles of rubble loosely held together by gravity. Debris from collisions in the asteroid belt also includes dust particles and small rocky meteoroids that can burn up as meteors in Earth’s atmosphere or fall as meteorites if they reach the ground.