On Wednesday 29 September the Earth will nearly be clobbered by a big asteroid - nearly, but luckily not quite (Stardate, BBC2 1.30 pm). Toutatis's path has been carefully calculated for some years into the future, and it will miss us by nearly a million miles. At its closest approach, it will be four times as far away as the moon. However, that is scarily close, and the question arises: Are we in danger of being wiped out?

Asteroids are chunks of rock that failed to stick together to make a planet when the solar system was formed. Most of them started in the gap between Mars and Jupiter, but have been caught by the gravitational pull of either Jupiter or the sun, or both, and are now in huge orbits around the sun. Comets are similar, but composed mainly of ice. Meteors are the flaming streaks we see in the sky when things from space burn up in our atmosphere, and meteorites are lumps of metallic stone that fall to earth.

Toutatis looks rather like a three-mile-long potato. If it were to hit the earth it could wipe out a big city, and perhaps cause as much global destruction as the impact that obliterated the dinosaurs 65 million years ago. What are the chances that you will get flattened one day? Much less than being killed in a car accident. At present there are 626 'potentially hazardous asteroids' or PHAs, but all their paths have been calculated, and none is expected to hit the earth. Astronomers are continuously watching for new ones, and tracking them through space. If they spot a really dangerous one far enough away, they may be able to prevent disaster - watch the programme for more details.

What intrigues me is the amount of stuff that actually falls to earth - every day 100 tonnes of space dust, hundreds of pebbles, and dozens of chunkier rocks. 22,000 meteorites have been picked up, most of them in Antarctica, but only 20 have ever been found in Britain. Scientists reckon there should be thousands more just waiting to be found - black, heavy, and magnetic, because they are made mostly of iron. I'm off to look, but I have decided it's not worth worrying about the big one. A PHA as big as Toutatis does hit the earth occasionally, but only about once in a million years.