Comets are dirty snowballs from outer space. From far beyond the outer planets Neptune and Pluto they come sweeping in towards the Sun, growing million-mile-long tails as they come. They swing around the sun and back out into space, not to be seen again for decades - or sometimes never.

Most comets have tails - the word comet means 'hairy star' - and for centuries they have been portents of disaster as they zoom across the sky. In fact the tails are made of particles of dust and ice, along with molecular fragments apparently from water, ammonia, methane, and other simple chemicals. These have been knocked off the snowballs by radiation from the sun. This is why the tails grow as the comets get nearer the sun, and why they point not back down the path of the comet but away from the sun; so after the comet has twirled round the sun the tails points forwards - in the direction they are going.

The most famous comet of all time was observed by Edmond Halley in 1682. He realised the same one had been seen in 1607 and 1531, and he predicted that it would return in 1759. He was right - Halley's Comet turned up on Christmas Day that year, although he had died in 1742.  That was one of the great predictions of science. We have plenty of this historical information, but little direct knowledge about these curious snowballs – but that is about to change.

A NASA probe called Deep Impact was launched six months ago, and has been zooming through space on a collision course with comet Tempel-1 – at least NASA hopes it is on a collision course. Tempel-1 is travelling at tens of thousands of miles per hour; so hitting it is a bit like trying to shoot a jet plane with a catapult. It’s a big target – about a quarter of the size of the Isle of Wight – but it’s dark and tricky to see clearly, and  250 million miles away. If all goes well Deep Impact will fire a 360-kg shell on 3 July, which on 4 July - my birthday – will smash into the comet.

The idea is to knock bits off, watch the explosion with dozens of telescopes, and see what the comet is made of. This is exciting for several reasons. Because comets come from outside the solar system they are truly alien, and may tell us something about how the solar system began to be formed, or even, possibly, about conditions elsewhere in the universe. Also some scientists believe that in addition to rock and ice, comets may contain organic molecules, including amino acids, and that these might have been the origin of life on earth. Perhaps in the distant past a comet loaded with amino acids crashed into the earth and the chemicals combined to form the first primitive life forms.

No one knows, but we may find out a little more from Operation Deep Impact.