What are asteroids made of?
Asteroids are made of rock, metal, or a mixture of both. Some dark ones contain water locked in clay and carbon-rich chemicals.
Asteroids are leftover building blocks from when the planets formed, about 4.5 billion years ago. They are not all the same: there are three main kinds.
Stony asteroids are made mostly of rock, rich in minerals such as olivine and pyroxene, with some sprinkles of metal. Scientists call many of these S-type asteroids. Most meteorites that fall to Earth come from asteroids like these. The Japanese spacecraft Hayabusa brought back grains from one, called Itokawa, in 2010.
Dark, carbon-rich asteroids, called C-type asteroids, are very dark, like charcoal. They contain rock, clay minerals with water locked inside, and carbon chemicals. Spacecraft have brought back samples from two of these, Ryugu and Bennu, and found they contain amino acids, the building blocks of proteins.
Metal-rich asteroids, such as Psyche, may be made largely of iron and nickel. Some scientists think they are the metal cores of small worlds that broke apart. Iron meteorites probably come from asteroids like these.
Many asteroids are not solid lumps but piles of rocks and gravel held together loosely by gravity, called rubble piles. They are like a bag of pebbles without the bag! Scientists learn this by measuring how heavy each asteroid is for its size.