What are asteroids made of?

The origins

Asteroids are the solid, rocky and irregular bodies remnants of the protoplanetary disk of dust and gas that formed around the Sun over 4.6 billion years ago. Much of the disk coalesced to form the planets, but some of the debris remained. During the early days of the Solar System, debris was constantly crashing together, and so small grains became small rocks, which crashed into other rocks to form bigger ones.

Since asteroids formed at the same time as other objects in our solar system, these space rocks can provide scientists with lots of information about the history of planets and the sun. By studying meteorites, which are tiny bits of asteroids that have flown through our atmosphere and landed on the Earth’s surface, they can learn more about the asteroids themselves.

The solar system is full of asteroids, most of which live in the main asteroid belt, a region between the orbits of Mars and Jupiter, along with comets and meteoroids. But asteroids can be found in other places too, like in the orbital path of planets. This means that the asteroid and the planet follow the same path around the sun. Earth and a few other planets have asteroids like these.

Asteroids are different from comets, which are mainly rock and ice. Comets usually have tails, which are made from ice and debris sublimating as the comet gets close to the sun. Asteroids typically don't have tails, even those near the sun. But recently, astronomers have seen some asteroids that have sprouted tails, such as asteroid P/2010 A2. This seems to happen when the asteroid has been hit or pummeled by other asteroids and dust or gas is ejected from their surfaces, creating a sporadic tail effect. These "active asteroids" are a newly recognized phenomenon, and still very rare.

What are asteroids made of? Composition

An asteroid’s composition is mainly determined by its proximity to the Sun. According to their composition, we can classify them in three main groups:

Dark C (carbonaceous) asteroids

Most asteroids (around a 75%) fall in this category. They are located in the outer belt and are believed to be close to the composition of the Sun, with little hydrogen or helium or other volatile elements. The asteroids that are nearest the Sun are mostly made of carbon, with smaller amounts of nitrogen, hydrogen and oxygen.

Bright S (silicaceous) asteroids

Located in the inner belt, closer to Mars. They are made up of oxygen and silicon, the number one and number two most abundant elements in the Earth’s crust. These asteroids tend to be metallic iron with some silicates of iron and magnesium.

Bright M (metallic) asteroids

These asteroids sit in the middle of the asteroid belt and are mostly made up of metallic iron. The metallic asteroids are composed of up to 80% iron and 20% a mixture of nickel, iridium, palladium, platinum, gold, magnesium and other precious metals such as osmium, ruthenium and rhodium. There are a few that are made up of half silicate and half metallic.
There are many other rare types based on composition as well, like for the V-type asteroids typified by Vesta that have a basaltic, volcanic crust.

Other minerals have been found on asteroids that have been visited by spacecrafts. For example, the Hayabusa spacecraft landed on Itokawa, a spud-shaped, near-Earth asteroid, and found out that it consisted mainly of olivine and pyroxene, a mineral composition similar to a class of stony meteorites that have pelted Earth in the past.

In addition to this, elements to create water are also present in asteroids. There are indications that asteroids contain water or ice in their interiors, and there’s even evidence that water may have flowed on the surface of at least one asteroid. According to this theory, when a smaller asteroid or comet slams into a bigger asteroid, the small asteroid or comet could release a layer of ice in the bigger asteroid. The force of the impact briefly turned the ice into water, which flowed across the surface, creating the gullies.

It's written in the stars

If you are reading this, you are probably an astronomy aficionado. Did you know that you can have your own customized star map of any significant date, like your birthday, ready to hang on your walls or even printed in a phone case? This is what Under Lucky Stars provides, based on the Bright Star Catalogue from Yale University. A truly unique gift, for yourself or for your loved ones!

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