SOME ASTRONOMERS worry about "the big one", a civilisation-destroying impact between the earth and a kilometre-wide asteroid. However being hit by a "little one" would still be devastating, an astronomer has warned.
This year marks the 100th anniversary of the 1908 Tunguska event when an incoming asteroid no more than 60 metres across blew up over Siberia. This puny object managed to flatten 2,000 sq km (772 sq miles) of forest, more than twice the area of Co Dublin.
If a similar asteroid "air burst" happened over Dublin today it would completely destroy the city, stated Prof Mark Bailey, director of Armagh Observatory.
Last night he delivered a talk on Tunguska and the risk of asteroid impacts as part of the Royal Astronomy Society's national annual meeting, which started yesterday at Queen's University Belfast.
The subject gives a whole new meaning to the question, "Did the earth move for you?" If we are talking asteroids, it most certainly did.
Tunguska in fact was not even that bad as things go. It was a rocky asteroid and so blew up eight kilometres overhead after slamming into the upper atmosphere. The energy released was equivalent to the bang given off by up to 10 million tonnes of high explosives.
A similar-sized asteroid made mostly of nickel-iron had an even bigger impact when it slammed into what is now Arizona about 50,000 years ago. Travelling at several kilometres a second, it punched a huge hole 1.2km across and 170m deep to deliver the famed Meteor Crater.
Armagh Observatory has long conducted research on the "near Earth objects" that pose a threat to earth.
There are an estimated 1,100 "big ones" out in space and only some of these have so far been located, Prof Bailey said.
While they have the potential on impact to plunge the earth into years of winter and wipe out a quarter of the world's population, such impacts might only occur once in 100,000 years, he suggested.
These big objects are much easier to find compared to the "little ones" however, and the small-object impacts typically occur once every few hundred years.
While they won't wipe out the planet, Tunguska and Meteor Crater show that these impacts could cause "local devastation", Prof Bailey added. Little is being done or can be done about them.
"In a way we live with that risk," Prof Bailey said.