Lots of rocks, lots more space

When we think about asteroid belts, we imagine scenes from movies like Star Wars, with ships flying around massive rocks at high speed, trying to avoid a collision. But the reality is rather different. Our asteroid belt is a region of space, roughly shaped like a massive doughnut, located between the orbits of Mars and Jupiter, 225 million km wide and around 140 million km thick. It contains hundreds of millions of rocky bodies known as asteroids, with a few very large objects above 10km. While hundreds of millions of large rocks floating around sounds scary, the volume of space they occupy is enormous, such that the average distance between asteroids (above 100m) is around 1 million km (or around 30cm in our scale model). That is 3 times the distance from the Earth to the moon, meaning if you flew through the asteroid belt in a spaceship, it's unlikely you would even see an asteroid, let alone hit one. There is no need to pull dangerous manoeuvres.

The biggest object in the asteroid belt is Ceres, a dwarf planet 952 km in diameter and roughly spherical like a planet. Ceres was the focus of the Dawn mission (launched in 2007), which imaged its surface in detail and analysed its composition. Lots of really interesting things were discovered, including large amounts of water ice below the surface and a set of 3 craters in the shape of a snowman. Purely a coincidence!

Justin Cowart, Public domain, via Wikimedia Commons

Asteroid belt history

The asteroid belt is full of interesting things, remnants from the early solar system, which offer valuable insights into its formation and evolution.

The many objects in the belt do not add up to a lot of mass. The asteroid belt today has a total mass of around 4% of our moon, so far less than the smallest planet, Mercury. But back in the early life of the solar system, the mass of objects in this space was much larger, perhaps 4 times the mass of the Earth. It's likely the current belt lost most of its mass through the influence of the other planets, particularly the nearby (astronomically speaking) Jupiter. Jupiter’s gravity caused orbital paths to change and asteroids to collide, sending material flying outwards and inwards in the solar system. Some of these fragments even hit the earth, becoming one of the many meteorites that have struck our planet over billions of years.

The belt asteroids are typically one of 3 types:

• C-type (Carbonaceous): These are the most common and contain a high percentage of carbon.

• S-type (Silicaceous): These contain silicate materials and nickel-iron.

• M-type (Metallic): These are made up mostly of metallic nickel and iron.

These are roughly the building blocks of the solar system planets. C-Type contains lots of carbon, the building blocks of life, while S-types are made of silicate materials that make up the rocks of the Earth’s crust and mantle. The M-Types are metallic, like the metallic cores of the planets. In some asteroids like Ceres there is also water.

Key Stats

NASA JPL CalTech, Public domain, via Wikimedia Commons