It is now well-established that various mass extinction events identified in the paleontological record were triggered by the cataclysmic explosions produced when large asteroids or comets happened to collide with Earth.

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: Had it not been for those extinctions, the age of the mammals and the eventual evolution of humans would not have occurred.

And there is another way in which we should look favorably 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 Thursday evening, he will discuss the threat to civilization 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 (only 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 program.

In a scientific paper presented this morning, Steel discussed 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 pyrolyzed (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 pass 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, as are 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 2,000 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.

Dr. Duncan Steel is Reader in Space Technology at the University of Salford. In 1990 he began the first Southern Hemisphere search program for near-Earth objects (NEOs), based at the Anglo-Australian Observatory in New South Wales. That program terminated in 1996, leaving only the American NEO search projects operating in the Northern Hemisphere. The southern sky is still uncovered.

Steel was one of the six foreign members of NASA's Spaceguard Committee, which made recommendations to the US Congress on how NEOs might be searched out and tracked. He was the only non-US member of the corresponding NASA Interception and Deflection Committee, which addressed the vexing question of how we might tackle an NEO found to be due to collide with the Earth.

In 1996, he was elected the initial Vice-President of the international Spaceguard Foundation, which has its HQ in Rome. Over the past year he was one of the main advisers to the UK NEO Task Force, whose report to the Government was published last September. (Editor's Note: See this website).

Steel has published several books, including the first popular account of the NEO impact hazard (Rogue Asteroids and Doomsday Comets, Wiley, 1995), and most recently a heavily-illustrated book on the same topic (Target Earth, Time-Life, 2001). He was science adviser for the Discovery Channel's "Three Minutes to Impact," which won an Emmy in 1998.

Related websites:

Images accompanying this story

Royal Astronomy Society

UK National Astronomy Meeting Website

[Contact: Dr. Duncan Steel]

04-Apr-2001