From the August 2009 issue

What are asteroids made of?

James Vitello, Hamden, Connecticut
By | Published: August 21, 2009
Phobos, Deimos, Gaspra
This montage shows asteroid 951 Gaspra (top), Deimos (lower left), and Phobos (lower right) at the same scale and nearly the same lighting conditions. (For scale, Gaspra is about 10 miles, or 17 kilometers, long.) However, their surfaces appear remarkably different, possibly because of differences in composition or because of different impact histories.
NASA / JPL

Asteroids, leftover remnants from the formation of the solar system, are composed of rock, metal, or a combination of both.