A less than encouraging description of what precisely would happen if the Earth was struck by either an incoming comet or asteroid made for grim listening during a session at the British Association's Festival of Science meeting in Sheffield.
Scientists were interested in such things to provide an assessment of damage and the likelihood of post-impact recovery (if any), stated Dr Matthew Genge of the British Natural History Museum.
Governments were paying little attention to such events despite their proven frequency, he said, and for what there was, much of the focus was on the really big impacters. These included large asteroids one or two kilometres across, of the sort thought to have wiped out the dinosaurs 65 million years ago. Such an impact would kill most of the world's population and spark climate change lasting years. Happily, these events occur only every 10 million years or so.
Smaller impacters are much more common, however. The giant crater near Winslow, Arizona, was made by a nickel/iron meteorite just 150 feet across. It left a hole three-quarters of a mile across and 650 feet deep and happened only 49,000 years ago.
However, Dr Genge is more concerned about comets, low density combinations of dust and ice often described as dirty snowballs. "The most hazardous objects are comets," he says.
Asteroids have predictable orbits so an impact could be predicted thousands of years in advance - at least if you were watching for them. So-called "short period" comets, such as Halley, also have predictable orbits, so if one was hanging around we would have centuries to get ready for an impact.
Dr Genge fears the "long period" comets, which appear without warning with perhaps a year or two's notice, provided you see them coming at all.
We know what happens when small comets reach the Earth because this happened in 1908 over Tunguska, in Siberia, Dr Genge said. Small comets don't reach the ground: they are too unstable and usually explode violently high overhead. They still pack a tremendous punch, however. The 130 feet diameter comet blew apart 2.4 miles overhead in a 12 megaton explosion. It devastated 1,200 square miles of forest but caused few, if any, deaths because of the remoteness of the region.
The situation would be very different if a comet exploded over a city, however. A Tunguska event over London would kill at least 10 million people, Dr Genge said.
Dr Genge postulated an incoming comet moving at about 10 miles per second. First, a fireball would appear in the sky as the comet ploughed into the atmosphere. This would be followed by an intense flash as the comet exploded, but it would be all downhill from there. After the light comes the thermal flash which would burn up everything below, including spectators.
The flames would not last for long, however, because within a few seconds the shock wave would pulse by, blowing out the fires, as occurred at Tunguska, but also flattening everything within a radius of about 20 miles.
The final indignity occurs when anything not tied down is pulled back towards ground zero as the air rushes back immediately after the shockwave. That would knock down anything left standing.