The best-known episode is that in which the dinosaurs died 65 million years ago, but there have been many other catastrophic impacts both before then, and since.

These asteroid and comet impacts were not entirely a bad thing. If it had not been for those extinctions, then the age of the mammals and the eventual evolution of humans would not have occurred.

However, there is another way in which we should look favourably on objects from space that have hit the Earth. Initially our planet was hot and dry. The water and organic chemicals that made the initiation of primordial life feasible here seem to have been delivered to the early Earth by comets.

Both of these aspects of planetary science are addressed in two talks to be given at the UK National Astronomy Meeting in Cambridge by Dr. Duncan Steel of the University of Salford.

In a public lecture on the evening of Thursday 5 April, he will discuss the threat to civilisation posed by large near-Earth objects, under the title "The Spaceguard Project: Tackling the Asteroid Impact Hazard".

As Steel points out, the object that killed the dinosaurs was a big one (five to ten miles in size), but just 93 years ago a much smaller asteroid (just 60 or 70 yards across) blew up in the atmosphere above Siberia, producing a blast which would flatten all of London out to the M25, should the next such event have Marble Arch as ground zero.

The chance of that occurring is small, but the consequences are so phenomenal that it is a hazard we must take seriously. As a result, the UK Government is now considering what it could contribute to the international Spaceguard programme.

In a scientific paper to be presented on the morning of Wednesday 4 April, Steel will discuss the flip-side of the coin: how comets may have delivered the basic building blocks of life to the nascent Earth.

In a massive impact, the molecules of water and organic chemicals of which comets are largely composed would be pyrolysed (split into individual atoms). What Steel has proposed is that organic chemicals might have been delivered to the sterile early Earth through the tiny meteoroids released as comets come near the Sun.

With his co-worker Dr Christopher McKay of NASA-Ames Research Center in California, Steel has shown that heavy organic compounds similar to tar would survive heating by the Sun within these small meteoroids during the thousands of years between being spawned by a comet and eventually arriving in the terrestrial atmosphere.

Until now it has generally been presumed that such meteoroids - which produce the familiar meteors or shooting stars when they burn up on atmospheric entry - must be made of rock and metal, like most meteorites.

A prediction of the work by Steel and McKay is that such tarry meteoroids would burn up higher in the atmosphere than is feasible for rocky substances.

The tar would start to evaporate from a meteoroid at around 500 degrees Celsius - a temperature quickly attained due to frictional heating when it plummets into the upper atmosphere at hypervelocity. In contrast, it takes much longer for rock to reach its evaporation temperature of over 2000 degrees.

Using a radar located near Adelaide in Australia, Steel has shown that such tarry meteoroids are indeed continually entering the atmosphere now, representing a rain of organic chemicals onto the Earth.

The implication is that the basic building blocks of life were also supplied to our planet in this way around four billion years ago, as the Earth cooled from its original, intensely hot beginning.

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