Letter: Space invaders

Sir: The threat to Earth from a devastating asteroid impact has somehow been both overestimated and underestimated in Charles Arthur's article about the Chicxulub crater ("Asteroid that killed dinosaurs will strike again - in 35 million years", 4 December).

He states that the size of the object which excavated Chicxulub was 100km across. In fact, it is the new estimation of the crater's diameter which has been downsized to 100km, from 180km. Most authorities agree that the object which struck the Yucatan and probably wiped out the dinosaurs was less than 10km across.

Fortunately for us, 100km- diameter asteroids are very rare. They may have been present in large numbers during the Earth's formation, but there are probably only a few dozen in our Solar System at the present time.

On the other hand, just because an event may be expected to happen on average once every 100 million years, it does not mean that this is the most likely time interval. The arrival of two spectacular comets in our skies in the past two years is evidence that celestial objects have their own timetables, rather like London buses. No-one knows when the next alien exterminator will arrive. All we know is that, sooner or later, arrive it certainly will.

PETER BOND

Cranleigh, Surrey