Asteroid Asteroid 243 Ida, 1993. © NASA

By Dr Philip Bland
Reader in Meteorics and Planetary Science, Imperial College London

What is an asteroid?

A very good question! It’s actually pretty poorly defined. Basically, a lump of rock, mostly orbiting the Sun in the inner solar system (the majority are between Mars and Jupiter). Objects smaller than 10m across are called meteoroids. The largest asteroid, Ceres, was discovered in 1801 by Giuseppe Piazzi. It is 950km wide – that’s about one quarter the diameter of the moon. Objects smaller than 10m across are called meteoroids.

What are they made out of?

Some are solid rock, others are giant chunks of iron-nickel metal. In most cases nothing has happened to them since the earliest period of solar system history. Like rocks here on Earth, we can study them to work out how they were formed. The meteorites which fall to Earth (they’re actually tiny bits of asteroids) give us a unique window into the origins of our solar system.

Are they a threat to Earth?

Yes. The most famous is the one that wiped out the dinosaurs – an asteroid probably about 14km across, making a 200km-wide crater. London’s congestion zone is about 10km across and we know of roughly 70 impact craters on Earth larger than that, and there are certainly more to be discovered. Most small asteroids (10 - 100m) blow-up in the atmosphere, but the blast can cause significant local damage (e.g. city-wide). We get hit by those every few hundred years.

What are the most interesting asteroids?

You wouldn’t get a very good nights sleep on asteroid 2008HJ – it makes a full rotation every 42.67 seconds. And you’d get a bit warm on 3200 Phaethon – at its closest approach to the Sun, the surface of this object gets to 750°C (about 3 times as hot as a very hot oven).