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Asteroid Ida. Image: NASA
If you put all the asteroids together to form one object, it would have a mass of three billion trillion kilograms. That sounds like a lot, but it is actually just four percent of the mass of the Moon!
In between Mars and Jupiter is a ring of small, rocky objects that we call asteroids. None of them are big enough to be proper planets – the largest asteroid is called Ceres, which is about 950 kilometres across, and is described as a dwarf planet like Pluto. Most asteroids, to be honest, are pretty dull to look at, with dark, grey surfaces filled with craters. Most are not even round, but potato shaped! Ceres is certainly one of the more interesting asteroids. It was discovered on New Year’s Day in the year 1801 by an Italian called Giuseppe Piazzi, and was the first asteroid to ever be found. Ceres is interesting because its surface is probably made from ice mixed in with dirt and rock, and underneath the surface could possibly be an ocean of water!
Another three asteroids quickly followed – Vesta, Juno and Pallas. At the time, astronomers thought they were planets, and almost 40 years passed before another asteroid was discovered in 1845. After that, astronomers began to discover asteroids by the bucket-load, and they quickly worked out that these weren’t planets at all, but small pieces of rock left over from the formation of our Solar System and the planets.
The first time a spacecraft made a close flyby of an asteroid was in 1991 when Galileo flew past Gaspra which lives very close to the inner edge of the Asteroid Belt. Gaspra shows just how oddly shaped an asteroid can be – it measures 19 x 12 x 11 kilometres. The Dawn spacecraft is currently on its way to visit the two largest asteroids in the Asteroid Belt: Vesta, which it passed in July 2011, and Ceres.
Not all the asteroids are found in the Asteroid Belt. Some orbit the Sun closer than the Earth, or between Earth and Mars. Some asteroids, called Centaurs, have even been found as far away as the planet Uranus. But it is the asteroids that cross Earth’s path, called Near Earth Objects, or NEOs, that cause scientists to worry. You’ve probably seen films like Armageddon and Deep Impact, and although they were fiction, scientists are worried that one day an asteroid will collide with Earth. It’s happened before – 65 million years ago an asteroid collision wiped out the dinosaurs. And just 100 years ago a very small asteroid, maybe just ten metres across, exploded over an area of Russia called Tunguska, and the force of the explosion flattened tress for miles and miles. But don’t worry too much – the odds of an asteroid hitting the Earth during our lifetimes are very small.
Artist's impression of the Dawn spacecraft