Oliver Batchelor, of Ilam School, asks :-

Why do comets never hit the Earth?

Duncan Steel, an astrophysicist at the University of Adelaide in Australia, and a world authority on meteor impacts, responded.

I'm afraid, Oliver, that the answer is that they do.

Astronomers scour the skies for comets that could impact our planet, and in the history of mankind we have found a total of 855 comets (to the end of 1992), and of those 437 have Earth-crossing orbits, and thus might hit our planet. Many will not be back for thousands of years, but some return frequently. For example, comets Encke, Halley and Swift-Tuttle come back every three years and four months, 76 years, and 130 years, respectively.

In fact there is clear evidence that each of these could hit the Earth, since each gives rise to meteor showers due to small fragments burning up in the atmosphere (these are also called 'shooting stars'). These fragments, called meteoroids, are typically about the size of a marble, and do not reach the ground, but the comets themselves are about 5 to 15 km in size, and would cause a cataclysm should they happen to meet us. Their impact speeds would be 31, 66 and 60 kilometres per second respectively, and the energy released would be equivalent to millions of megatonnes of TNT: far greater than the world's nuclear arsenals.

However, as a rule of thumb there is only about a one in a thousand million chance of any particular comet hitting us in each orbit. Comets pose about 10 per cent of the impact hazard to mankind, the other 90 per cent being due to asteroids.

Comets are largely composed of ice whereas asteroids are rocky minerals which may in fact be comets which have lost all of their ice.

The Earth is struck by an asteroid or comet at least 1 km in size about once every 100,000 years. This size is the threshold at which occurs global effects which would cause 75-95 per cent of mankind to be killed either in the initial impact or in the aftermath. The last time that the Earth was hit by a big object was in 1908 when a fragment of Encke's Comet destructed in the atmosphere above Siberia, flattening some tens of thousands of (luckily) uninhabited forest.

Evidence for past impacts comes from fossil records which show mass extinctions of animal species many times in the past, the best know example being the death of the dinosaurs 65 million years ago.

There is also graphic evidence for impacts on the face of the Moon, which is pockmarked with craters which are evidence of past collisions by asteroids and comets.

The active geology (volcanoes, earthquakes, continental drift) and weathering (wind, rain, snow and ice) on the Earth wipe away craters quite quickly, in terms of geological and astronomical time scales. However, there are over a hundred known terrestrial impact craters, the best known of which is Meteor Crater in Arizona. In Australia there are 19 known craters, of which four are less than 6,000 years old; undoubtedly there are many more waiting to be recognized here.

There are no known impact craters in New Zealand, partly due to the very active geology and young terrain there, but it would be expected that there would be a few. Start looking.

Finally, your chance of dying due to an asteroid or comet hitting the Earth is about one in a thousand, maybe a little less. This is higher than your chance of dying in a severe storm, from a gun accident or of food poisoning, but only one tenth of the chance you will die in a car accident. Although I believe that mankind should take asteroid and comet impacts seriously, since it is a problem that we can now tackle, it is more important that you take care when cossing the road!