1.   Comets are composed of ice, perhaps with rocky nuclei, while asteroids are made of rocks and metals.

2.   The asteroid is made of material that is probably older than Earth and subject to some unknown forces that continue to change its surface.

3.   Yet scientists cannot agree on whether all S-type asteroids are made of primitive or melted material.

4.   In general, damage on the ground depends on what an asteroid is made of, varying from solid metal to a loosely bound aggregate.