Astrophysicists' calculations have led to a consensus that over the next 100 million years, most near-Earth asteroids will have been thrown back out by close gravitational encounters with the inner planets or will collide with the inner planets. Conversely, a new supply will be constantly generated from the Main Belt and incoming comets which will replace these losses. Astrogeology theory and observations This section is for those who wish understand why asteroids are made up of what they are. It's more technical than most, and can be skipped. Both occurred early in the solar system 4.6 billion years ago when most large bodies were molten or very hot, due to such sources as early radioactivity, the heat of impacts and accretion, and a unique electromagnetic field which existed early in solar system formation.
Gravitational differentiation occurs only in large bodies with a significant gravitational field.
Simply put, the heavier elements sink to the center core, whereas the lightweight elements rise to form the crust. The core is made up of free metal, predominantly iron and nickel. The crust consists of lightweight metal oxide silicates. In between these two extremes is the mantle, composed of heavier metal oxide silicates than crustal material. Thus, the core is made up of much different material than the crust, and the mantle consists of significantly different material than either.