Origin of asteroids
Where do asteroids come from?
I have looked in several astronomy texts, and they pretty much agree that
the asteroids are material that never quite got together to form a planet.
Many astronomers believe that the early solar system was a swirling cloud of
dust and gas. Sometimes particles of dust would collide and stick together;
eventually, through many such chance collisions, some of these clumps would
be big enough to gravitationally attract other nearby bits of dust, getting
bigger yet. These clumps are called planetesimals, and the process I have
described is called accretion. You can imagine that eventually some of the
planetesimals would get big enough to absorb most of the other stuff in
their orbit, finally leaving perhaps only one (maybe some smaller ones
survive as moons). But in the case of the asteroids, Jupiter's gravity
prevented this accumulation from going to completion, leaving several small
bodies instead of one big one. That is the theory.
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Update: June 2012