What are asteroids really made of: Rocks, metals or something more?

Asteroids are small rocky bodies that travel around the Sun. They are much smaller than planets. They are very important because they can give us clues about the early composition of the Solar System. Unlike planets and moons which have changed a lot due to volcanic activity, erosion or earthquakes many asteroids have stayed the same for billions of years. By examining them scientists can learn about the original materials that formed planets, including Earth and how our Solar System has changed over time.

Some asteroids like Ceres, are so large that they are classified as dwarf planets. Asteroids are usually grouped into three main types based on what they are made of: C-type, S-type and M-type.

C-type Asteroids (Carbon-rich)

C-type asteroids are the most common asteriods and are dark in color and contain clay, silicate rocks and organic carbon while some also have water ice. These asteroids are mostly found in the outer part of the asteroid belt. A well-known C-type asteroid is Bennu, which was studied by NASA’s OSIRIS-REx mission.

S-type Asteroids (Stony)

S-type asteroids are the second most common type. They are made mostly of rock and metals like nickel and iron. These materials are similar to what is found in the crust and mantle of Earth and other rocky planets. Most S-types are found in the inner part of the asteroid belt. A famous example is Eros, one of the largest asteroid near earth.

M-type Asteroids (Metal-rich)

M-type asteroids are mostly made of iron and nickel. Some may be pieces of larger bodies that were broken apart in early space collisions. Others may have formed closer to the Sun where high heat caused metal to melt and settle in the center. These asteroids are believed to be the source of metallic meteorites. One important M-type asteroid is Psyche, which NASA is currently exploring with a space mission to learn more about its origin.