So – You’re Still Interested in Astronomy? The Asteroid Belt6 min read

Hello again.

Last time we looked at the amazing dwarf planets in our Solar System, along with their moons. Today, we’ll take a quick skim through the asteroid belt and see what interesting things we can find.

1. So – what is the asteroid belt exactly?
The asteroid belt is a circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter, (That’s about two and a half times the distance of earth from the sun). It is occupied by billions of irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the Main Asteroid Belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and Trojan asteroids.


2. How did it get there?

Simply put, early in the life of the solar system, dust and rock circling the sun was pulled together by gravity into planets. But Jupiter, the largest planet, kept a number of the pieces from coalescing into another large body. Instead, its gravity disrupted the formation process, leaving an array of unattached asteroids.

3. Is there a lot of ‘stuff’ in the asteroid belt?

Not now. The Main Belt once contained enough material to form a planet nearly four times as large as Earth, but once again, Jupiter’s gravity not only stopped the creation of such a planet, it also swept most of the material clear, (about 99% of it) leaving far too little behind for a planet of any size to form. Indeed, if the entire mass of the Main Belt could somehow create a single entity, it would now weigh-in at less than half of the mass of the moon.

4. That doesn’t seem a lot?

It isn’t. What’s more, about half the mass of the belt is contained in the three largest asteroids: Vesta, Pallas, and Hygiea and the dwarf planet, Ceres.


At 945 km (587 miles) in diameter, Ceres is the largest object to be found within the main Asteroid Belt. In fact, Ceres possesses a third of the mass of the entire belt and is the largest of the minor planets within the orbit of Neptune.
The interesting thing about Ceres is that she has an icy mantle around a rocky core, and it is thought she may possess an internal ocean of liquid water.

5. Vesta

The brightest asteroid in the sky, Vesta is the second most massive body in the asteroid belt and is occasionally visible from Earth with the naked eye. It was the first asteroid to be visited by a spacecraft. The Dawn mission orbited Vesta in 2011, providing new insights into this rocky world.

When Vesta made a close approach to Earth in 1996, the Hubble Space Telescope mapped its topographic surface and features. This revealed a large crater at the south pole (Rheasilvia) that slices into its interior. That crater is 500 km in diameter (310 miles) — remember: Vesta itself is only 530 km (325 miles) across.


It cuts an average of 13 km into the crust, and most likely formed from an impact in the asteroid’s early life. The material ejected from this collision resulted in a number of smaller — Vestoid — asteroids that orbit near their parent, as well as some of the meteorites that have crashed into Earth.

Do we know anything else about Vesta?
Unlike most asteroids, the interior of Vesta is differentiated. Like the terrestrial planets, the asteroid has a crust of cooled lava covering a rocky mantle and an iron and nickel core. This lends credence to the argument for naming Vesta as a protoplanet, rather than as an asteroid.

6. Pallas

Pallas is the second asteroid to have been discovered and it is one of the largest asteroids in the Solar System. It is estimated to hold 7% of the mass of the asteroid belt, making it the third-most-massive asteroid. At 512 kilometers (318 miles) in diameter, Pallas is somewhat smaller than Vesta and is likely a remnant protoplanet.

Pallas’s surface appears to be comprised of silicate material. What’s more, at 34.8°, Pallas’ orbit is unusually highly inclined to the plane of the asteroid belt itself, and its orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.
(shame, because as we’re seeing from this brief showcase, the asteroid belt contains lots of interesting stuff).

7. Hygiea

Hygiea is the fourth largest asteroid in the Solar System by volume and mass, but not a great deal is known about her because of her carbon based composition. (It means she’s so dark she’s hard to see). However, we do know Hygeia possesses an elongated, bulbous shape, and her largest “bumps” are 350–500 km in diameter.

8. So, there’s not much to say about Hygiea then?

Well, actually there is. The thing that’s interesting about this cold little world is the fact that her orbit is eccentric. Sometimes she’s as far out as the edge of the asteroid field itself, where she approached the Hilda Family – (a small cluster of asteroids in resonance with Jupiter). At other times, Hygiea’s travelling brings her as close as Ceres.
Also, her day is 27 hours and 37 minutes long, giving her a rotational speed more than twice that of other asteroids.

9. And what about the rest?

The remaining bodies within the field range down to the size of a mere dust particle. What’s more, these flimsy bits of nothing are so thinly distributed that numerous unmanned spacecraft have traversed it without incident.
Nonetheless, this is space, and collisions between large asteroids do occur. When they do, such incidents can form an asteroid family whose members have similar orbital characteristics and compositions.

10. To summarize, what can we expect to find within the Main Asteroid Field?

In the first instance, individual asteroids within the belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).
As we’ve seen, these will range in size from dusty specs, asteroids, protoplanets and even dwarf planets.
And if this wasn’t enough, we have comets that pass through too, making the Main Asteroid Belt a particularly fascinating place to study.

So there we go. That’s our brief look at the wonderful world of the Main Asteroid Belt. Doesn’t it make you realize how lucky we are? All these incredible things, right on our doorstep and available to study.

Next time, what I thought I’d do is take a brief look at some of the star systems astronomers have found outside our own Solar System. It will really drive home how fortunate we are, and how much still remains to be found.

Until then…Keep looking up

Andrew Weston
Andrew Weston

Andrew P. Weston is Royal Marine and Police veteran from the UK who now lives on the beautiful Greek island of Kos with his wife, Annette, and their growing family of rescue cats. An astronomy and law graduate, he is the creator of the international number one bestseller, The IX, and also has the privilege of being a member of the Science Fiction and Fantasy Writers of America and the British Fantasy Society. When not writing, Andrew devotes some of his spare time to assisting NASA with one of their remote research projects, and writes educational articles for and Amazing Stories. He also enjoys Greek dancing and language lessons, being told what to do by his wife, and drinking Earl Grey Tea. If you would like to find out more, visit his blog or website at:

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