What Are Asteroids Made Of?

The asteroid belt contains the majority of the solar system's asteroids.

An asteroid is a minor planet that revolves around the Sun, especially within the inner Solar System. Sometimes referred to as planetoids because they are too small to be planets, there are billions of asteroids in the Milky Way Galaxy, ranging in size from a few feet to hundreds of miles in diameter. However, despite the total number of asteroids that exist, the combined mass of all asteroids in the Solar System is less than the weight of Earth’s moon. Most asteroids are leftovers from the formation of the Solar System, which occurred 4.6 billion years ago, or debris from the collision of solar bodies, such as planets or larger asteroids. Ceres, which has a diameter of 625 miles, is the largest asteroid ever recorded, while 2015 TC25 is the smallest, with a diameter of only 6 feet. Most asteroids are irregularly shaped, but some, including Ceres, are spherical. 

Composition of Asteroids

The composition of an asteroid depends on its distance from the Sun. Those nearest to the Sun are made of carbon, with traces of oxygen, hydrogen, and nitrogen, while asteroids further from the Sun are composed of silicates. These silicates consist of silicon and oxygen, which are the two most abundant elements on Earth’s crust. Metallic asteroids are composed of nearly 80% iron and 20% nickel, as well as small amounts of gold, magnesium, iridium, platinum, and several other precious metals. Some asteroids are both silicate and metallic. In addition to metallic characteristics, some asteroids also contain elements necessary for the creation of water. For example, the study of Vesta, an asteroid with a diameter of 326 miles, discovered the presence of gullies created by flowing water.

Classification of Asteroids

Asteroids exist in the three regions of the Solar System, although the largest proportion have been identified in an area named the asteroid belt, which is located between Jupiter and Mars. The asteroid belt contains about 200 asteroids that are larger than 60 miles in diameter, plus millions of smaller asteroids. Additionally, certain bodies, such as Ceres, are classified as both an asteroid and a dwarf planet. Many asteroids are located outside the asteroid belt. For example, Trojan asteroids orbit large planets at Lagrange points, which are regions where the gravitational pull of the planets and Sun are balanced. Earth, Mars, and Neptune have Trojan asteroids. There are about 10,000 near-Earth asteroids (NEAs), and some cross Earth’s path, while others are contained within Earth's orbit. Fortunately, near-Earth asteroids are small and pose no immediate threat to the planet.

Asteroids can also be classified by composition. C-type asteroids are carbonaceous and grey, consisting of stone and clay silicate rocks, and account for 75% of asteroids in the Solar System. S-type asteroids are siliceous asteroids, are made of nickel-iron and silicate materials, vary in color from reddish to greenish, and account for 17% of asteroids. M-type asteroids are reddish, account for 9% of asteroids, and primarily exist in the asteroid belt between Mars and Jupiter.


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