Ceres and asteroids

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Take a look at the map of the orbits of the planets on this board and you'll see that there's a big gap between Mars and Jupiter.

This struck 18th century astronomers as odd, and they decided to search for another planet in there, calling themselves the Celestial Police. They knew that it couldn't be bright, otherwise it would have been spotted already, so this meant checking every faint star in the zodiac, the path of the planets, to see if there were any extra ones that might be a moving planet.

But before they could even get going, an astronomer in Sicily, Giuseppe Piazzi, spotted something unusual while making his own star catalogue. On the very first evening of the 19th century, 1 January 1801, he noticed a star that was absent from previous catalogues. Over the next few nights he was able to plot its slow movement against the starry background. He had discovered a new member of the Solar System, which he named Ceres after the Roman goddess of agriculture, who was also the goddess of Sicily.

It moved fast enough that it had to be between Mars and Jupiter, but it was so faint that it couldn't be a full-sized planet. Then the Celestial Police got their act together and discovered another small body in the same part of the sky, Vesta. Within a few years more were discovered, and astronomers realised that they had not one single planet, but lots of minor planets on their hands. They became known as asteroids, meaning 'star-like'.

These bodies are now being discovered at a rate of over 2,000 a month, and they come in all sizes down to little fragments. Some of them are in orbits that come close to Earth, and frequently they actually hit Earth. But don't worry - the ones that hit us are not big ones, but tiny, and it's just possible that you might just have one in your back garden, because these are what we call meteorites.

You've heard of shooting stars, which are caused by grains of space dust burning up as they enter Earth's atmosphere. There's more about these when we get to the Comets. Some of these particles are actually small chunks of asteroids, and just occasionally bits of them survive their fiery plunge through the atmosphere and land on Earth. These bits are then called meteorites - just as other geological specimens are called ammonites or calcite and so on.

Most meteorites either weather away or get buried, but some remain to get picked up. They are quite rare, so if you do see an odd-looking stone in your garden, don't assume that it comes from space! But the genuine space rocks provide scientists with samples of the material that has been around since the birth of the Solar System, which have come all the way to Earth from the asteroid belt.

A remarkable NASA space probe named Dawn was launched in 2007 towards Vesta and Ceres. It went into orbit around Vesta in 2011. After 14 months it was then sent onwards to Ceres where it entered orbit in 2015, and it's still orbiting. The data from Dawn proved that some of the meteorites that land on Earth - about six in every hundred - have the same composition as Vesta, and are chunks of it that would have been knocked off it after a great collision with another asteroid way back. As a result we know now that we have more rocks from Vesta than the 382 kg the Apollo astronauts returned from the Moon! Very handy when you want to know about the early Solar System. NASA has plans for manned missions to these objects but not à la Bruce Willis in Armageddon - their plans are rather more realistic!

Credit: Ceres, photographed by the Dawn probe.
Photo: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.