Mother Earth has a very close shave

IT DID not quite require the skills Sean Connery displayed in the film Meteor, nor yet was it a rerun of the mass extinction of the dinosaurs, but last week the Earth came the closest yet to an astronomical disaster.

An asteroid narrowly missed Earth on Tuesday, passing by at less than half the distance to the Moon, according to Duncan Steel of the Anglo-Australian Observatory, at Siding Spring, near Sydney.

In the film, Connery played a scientist who had to deal with a five-mile-wide asteroid plus sundry fragments which triggered avalanches and tidal waves. But last week's asteroid, some 20 metres across, was a less dramatic affair.

It travelled at 70,000 kilometres (43,000 miles) per hour, and went by at about 2100 GMT on 15 March between 160,000 and 180,000 kilometres (100,000 and 112,000 miles) away. 'In the cosmic scale of things that really is right in our backyard,' Dr Steel said.

Had it hit, the asteroid's immense kinetic energy would have been converted into a fireball of thermal energy, generating a shockwave and explosion equivalent to about 20 Hiroshima-size nuclear explosions.

There has been interest in such near-Earth asteroids over the past few years as scientists have come to accept the idea that it was an asteroid impact which drove the dinosaurs to extinction. And logic suggests that if it happened once, it could happen again.

The impact which did for the dinosaurs about 65 million years ago was of an asteroid some 10 kilometres (about six miles) across. The violence of its explosion raised huge clouds of dust into the air which shrouded the entire globe in a blanket of darkness for years, shutting out the sunlight and plunging the world into a 'nuclear winter'. Were such a cataclysm to happen again, humanity would be powerless to prevent its own extinction.

The impact of an asteroid just 350 metres across would cause an explosion equal to the all the world's nuclear arsenals. It is estimated that there are about 1,000 asteroids larger than one kilometre in diameter, whose orbit crosses that of the Earth. Most of them are candidates for eventual collision.

Dr Steel warned that while last week's asteroid was one of only three observed to pass the earth in a near-hit in the past three years, 'it's likely we're missing more than 99 percent of these things'.

According to Dr Jacqueline Mitton of the Royal Astronomical Society, the only systematic search for such near-Earth objects is being carried out by a husband and wife team of astronomers in the US. Eugene and Carolyn Shoemaker have been conducting a monthly survey using the Schmidt telescope at Mount Palomar. But recently they have had to curtail their work as a result of the intese demands for time on major astronomical telescopes.

The Shoemakers are also the scientific driving force behind the Clementine space mission, which should rendezvous with the minor planetoid Geographos later this year. Launched in January, and a rare collaboration between NASA and the Ballistic Missile Defense Organisation (the 'Star Wars' office), Clementine is currently surveying the Moon, after which it will deploy hardware developed by the Pentagon for the 'Star Wars' programme to track and analyse Geographos.

Interplanetary objects do collide with planets, as yet another connection of the Shoemakers will demonstrate in July this year. Carolyn is co-discoverer of Comet Shoemaker-Levy which has broken up into about 20 pieces, all of which will fall onto Jupiter around 25 July, with the explosive power of about 100 million megatons. Sadly this cosmic cataclysm will take place on the side of Jupiter not visible from Earth.

Along with the comets, asteroids represent a remnant left over from the primordial stuff from which the solar system was formed. Most orbit the Sun in a band beyond Mars, possibly because Jupiter's powerful gravitational field prevented the asteroidal material coming together there to form a major planet. Today, Jupiter's gravity still perturbs their orbit enough to send some of them into Earth-crossing paths.

The Earth sweeps up several tons of more fragmentary debris from space each year, with about 19,000 meteorites, usually weighing around 100g (3.5oz) landing annually. Although most meteorites are fragments broken off asteroids, some are bits of the Moon and a few, picked up in the Antarctic ice, have come from Mars.

But even these exotics do not exhaust the variety of objects raining down on us. On 30 June 1908 a mysterious explosion occurred near Tunguska in Siberia, flattening 2,000 square kilometres, and knocking down millions of trees.

No meteorite remains were found in the area, leading some scientists to suggest that the Siberian event could have been a fragment of anti-matter or the impact of a tiny black hole crashing on to the Earth. Less romantic souls however believe that the damage was done by a piece of the tail of a comet detonating in the upper atmosphere.

(Photograph omitted)