What are asteroids made of?
Studies of an asteroid’s reflected light as well as analyses of meteorites have provided information about the composition of asteroids. Astronomers classify asteroids into two broad groups based on their composition. One group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Their composition has not changed much since the solar system formed. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials.
There are three main kinds of asteroids, made up of rock, or metal, or a mixture of the two. The rocky ones, known as carbonaceous (C-types), are usually dark and difficult to spot. The lighter-colored ones, known as silicaceous (S-types), contain some metal. The pure metal M-types are the brightest and rarest.
There are several different types of asteroid, and they tend to be sorted by distance from the Sun, although there are plenty of exceptions. Generally the nearest to the Sun tend to be mostly carbon, much like the two moons of Mars. These are called carbonaceous. Further out tend to be primarily silicate rock. Beyond these we find the metallic asteroids, made of about 70-80% iron, and the balance mostly nickel, but with many other metals mixed in, including some that are rare on Earth, such as iridium. There are also some that are about half silicate and half metallic.
