What Are Asteroids Made Of?

As close as many are to Earth, asteroids still hold a lot of questions for scientists. These relatively new discoveries seem to follow specific patterns and look as though they haven’t changed a bit in over four billion years. Asteroids could help unlock the secrets of the Solar System as we discover what asteroids are made of.

How Were Asteroids Made

What Is an Asteroid?

Asteroids are, by definition, small, rocky objects that orbit the Sun. They follow the same direction as the planets do but fail to maintain a spherical shape or have clear orbits, two characteristics that planets must possess.

These cosmic rocks range considerably in size from one to the other, with the largest asteroid, named Vesta, at 329 miles (530 kilometers) in diameter. The smallest known asteroids are less than 33 feet (10 meters) from side to side. Because they are so small, the very first asteroid wasn’t discovered until 1801.

There are over one million asteroids known to us, but there are likely millions more in our Solar System alone waiting to be discovered. Even for this impressive number of asteroids, all of them combined still don’t meet the mass of our Moon.

Where Are Asteroids Located?

Asteroids typically exist in one of five places in the Solar System.

The Asteroid Belt

Artist graphic of the Asteroid Belt
Artist graphic of the Asteroid Belt. (Image credit: NASA)

If you know anything about astronomy, you’ve likely heard of the Asteroid Belt. It’s here that a vast majority of the asteroids in our Solar System call home. This is also where the first asteroid was discovered over 200 years ago by accident while a Sicilian monk was studying the heavens.

The Asteroid Belt helps fill the sizable celestial gap between Mars and Jupiter. Because of the distance between these two planets (nearly 3.5 times the Earth’s distance from the Sun), early scientists believed these asteroids were once a planet that was ripped to shreds. However, there’s not enough mass there to constitute the remains of a planet.

Trans-Neptunian Objects

Beyond the realm of Neptune exists a part of the Solar System we are only beginning to understand. This section consists of the Kuiper Belt, the Scattered Disc, and the Oort Cloud. 

NASA’s New Horizons spacecraft met up with Pluto, the most well-known Kuiper Belt object, in 2015. It has since continued further into the Kuiper Belt and is still sending back regular data for scientists to see for the first time.

Transneptunian Object Sizes Artist Concept
These artist concepts depict some of the most well-known objects discovered outside of Neptune’s orbit. (Image credit: NASA)

The Kuiper Belt is home to asteroids of its own, and scientists speculate it could be upwards of 200 times the width of the Asteroid Belt. Among asteroids, three objects large enough to be classified as dwarf planets have also been discovered.

Trojan Asteroids

Trojan asteroids are leftovers that have been caught up in the orbit of a planet. These asteroids are far enough ahead or behind a planet’s orbit to not get turned into moons.

Artist's conception of Jupiter trojan asteroids.
Artist’s conception of Jupiter trojan asteroids. (Image credit:  Pablo Carlos Budassi  on Wikimedia Commons  CC BY-SA 4.0)

Perhaps due to its proximity to the Asteroid Belt, Jupiter has by far the most Trojan asteroids with an estimated one million. Neptune has 28 Trojans, likely from the Kuiper belt, and Mars has nine. Discovered ten years ago, Earth has a small, solitary Trojan asteroid.

Repurposed As Moons

The Moons of Mars
Mars is accompanied by two cratered moons: Phobos, an inner moon, and Deimos, an outer moon. (Image credit: NASA)

Some asteroids floated away from the Asteroid Belt and were likely caught by nearby planets. The two moons of Mars are tiny and have an irregular shape like asteroids do. Jupiter has probably stolen a few itself. It’s worth noting that some of Jupiter’s moons orbit in the wrong direction, indicating they did not form with the planet.

Near-Earth Asteroids

Several chunks of space debris have found themselves caught in orbits that come close to the Earth. Some of these near-Earth asteroids will never cross paths with our planet, but others do break through our orbit as they travel around the Sun.

What Are the Different Types of Asteroids?

Most of the asteroids discovered in the Solar System fall into one of three broad classes based on their composition.

C-Type Asteroids

C-type (also known as carbonaceous or chondrite) asteroids are by far the most common in the Solar System. These chunks of space debris make up over 75% of all asteroid types.

Carbonaceous asteroids contain a mix of dark organic carbons and silicate materials alongside small amounts of oxygen, nitrogen, and hydrogen. Amazingly, they have changed little since being formed over 4.5 billion years ago.

Most carbonaceous asteroids are found in the Asteroid Belt. Because of their carbonic structure, these asteroids are very dark.

Although many of the largest asteroids are C-types, they were hard to notice because they are so hard to see from Earth. In addition, they tend to populate the outer region of the Asteroid Belt. The largest known asteroid, Vesta, is a C-type.

Vesta Sizes Up
Vesta is the largest asteroid that has been visited by a spacecraft. The NASA Dawn spacecraft, which has entered orbit around Vesta, is currently observing it. (Image credit: NASA)

S-Type Asteroids

S-type asteroids are known as siliceous thanks to a very high concentration of silicon material alongside some nickel-iron. They are devoid of the carbons that make C-types so dark and instead have a much higher reflectivity. As a result, some of the brightest asteroids discovered fall into this category.

Silicate asteroids represent approximately 17% of all the discovered asteroids in the Solar System. They tend to reside in the inner part of the Asteroid Belt, closer to Mars. The largest, Eunomia, is big enough to see with just a pair of binoculars. 

M-Type Asteroids

M-type asteroids are metallic and are almost entirely made from nickel and iron. While some are pure, other rocks contain gold, iridium, platinum, palladium, and magnesium.

Although made of metal, these rocks are often reddish in color and not particularly vibrant. Only making up about 7% of asteroids, M-types are some of the least studied.

A Metal-Rich World
A Metal-Rich World, an artist’s concept of asteroid Psyche. (Image credit: NASA)

Of the known metallic asteroids, Psyche stands out as the largest. Its size, among other characteristics, has led scientists to wonder if it once was the metallic core of a planet. M-type asteroids are typically clustered towards the center of the Asteroid Belt.

How Were Asteroids Made?

As the Solar System formed, asteroids formed right alongside the planets we know today. As rocky objects run into each other at slow speeds, they tend to stick to each other instead of bounce off. In addition to a minimal amount of mass among the asteroids in the Asteroid Belt, Jupiter’s gravity likely caused them to move too quickly to form larger objects.

The other asteroids outside the Kuiper Belt likely originated within the Asteroid Belt before discovering their new homes. While the Kuiper Belt is still a mystery, it could be that Neptune caused a similar effect to Jupiter.

Asteroids, Meteoroids, and Comets

These three rocky objects are leftover pieces of the Solar System as it formed billions of years ago. They share many similarities, but there are some subtle differences between them.

Meteoroids are tiny pieces of space debris that range in size from a speck of dust to the size of the smallest of asteroids. They are often pieces of larger asteroids that have broken off due to a collision. Meteoroids then fly off into the confines of space, although some do find their way to Earth.

Rosetta at Comet
Artist’s impression of the Rosetta orbiter at comet 67P/Churyumov-Gerasimenko. (Image credit: NASA)

Comets, on the other hand, are frequently referred to as “dirty snowballs.” They’re made up of rock, dirt, and large portions of ice from trips far into the distant parts of the Solar System. As comets approach the Sun, that ice begins to melt, leaving a spectacular trail of melting ice in space.

Can We Mine Asteroids?

Mining out in the confines of space may seem like the plot for a science fiction movie. However, in 2010 the spacecraft Hayabusa landed on the surface of S-type asteroid Itokawa. Hayabusa spent three months on the surface collecting samples before returning to Earth.

Hayabusa 2 undertook a similar mission in just 2020. Landing on C-type asteroid Ryugu, the craft brought back its own samples. Psyche, the largest M-type asteroid, is estimated to have over $10 quintillion dollars worth of metals within! 

What if we began mining asteroids? Could we replenish all of the Earth’s natural resources that we’ve depleted over time?

Final Thoughts

The asteroids in our Solar System range considerably in size and composition but follow certain trends. The vast majority fall into one of three classes and appear relatively untouched since the dawn of time. Through past and future space missions, scientists are looking to understand how we can use asteroids to understand history and obtain new resources for the Earth. 

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.