Asteroid impact could signal end of world as we know it

A dinosaur - if you could find one - would have plenty to say about what happens when a big asteroid or comet comes smashing into the Earth. There is extensive evidence to show that a colliding asteroid brought an end to the reign of Tyrannosaurus Rex 65 million years ago.

Scientists estimate that a six to eight-mile-wide body hit the Earth at Chicxulub, Mexico, causing a crater 120 miles across. Only two days ago the discovery of additional Mexican craters associated with Chicxulub were announced. These, researchers say, were blasted into the surface by material thrown up by the initial impact.

We can only guess at what happened immediately after this impact, but we know one thing for sure - it was big. The successful dinosaur populations which had evolved and dominated the Earth for millions of years were wiped out. A thin black band of material - spiked with extra-terrestrial iridium - can be found today embedded in the sedimentary geological record right around the world. It is found between the Cretaceous and Tertiary sediments, the so called K/T boundary, and it bears silent testimony to the global consequences of the impact.

Late on Wednesday the International Astronomical Union, which keeps track of "near-Earth objects", the kinds of things that did for the dinosaurs, announced that an asteroid would pass very close to the Earth in 2028 and might hit it.

The IAU asked astronomers to have a look at the body, dubbed 1997 XF11, as it was unsure whether it would hit us or shave by at an uncomfortably close 26,000 miles out. Researchers were estimating that if worst came to worst, it would arrive at 6.30 p.m. GMT on October 26th.

So much for concerns about funding that pension. All is changed today, however, with yesterday's reassuring calculations from the Jet Propulsion Laboratory in California. Senior JPL scientist Dr Don Yeomans and colleague Dr Paul Chodas took a second look at the data on 1997 XF11. "We re-did the analysis and the close approach distance moved way out 600,000 miles, which is two-and-a-half times the lunar orbit," Dr Yeomans told Reuters. "The probability of impact, we don't want to say that it's absolutely zero," Dr Chodas said. "But it is very, very tiny and the impact is extremely unlikely." Before you get too comfortable about that pension, however, consider this. The Earth orbits through a swarm of near-Earth objects which ply their own circuits around the sun. The science of detecting and tracking them is still relatively young, however.

The University of Arizona, which found the asteroid, runs a Space Watch programme for near-Earth objects and there are similar efforts dotted around the world. They suffer from lack of numbers and lack of funds.

We don't need an asteroid to visit to be able to calculate the massive discharge of energy that would occur with such an impact, however. The physics of fast-moving objects is well understood and we do have the results of Chicxulub and calculations relating to the asteroid that exploded over Tunguska, Siberia, in 1908, which flattened hundreds of square miles of forests.

The first consideration is the size and composition of the body. Asteroids are stony hard, and thought to contain metals which make them heavy. A comet, on the other hand, is much lighter, a loose combination of ice and space dust. Either way, the bigger and heavier the body, the bigger the bang.

The second point is the speed of the body and its angle of impact. Comets can move at 40 miles per second or more, so free bodies in space have pace. The faster the body, the greater the energy delivered on impact. Equally, if it hits head on, there will be much more energy than if the body strikes a low-angled glancing blow.

A mile-wide body, such as 1997 XF11 and striking a near direct hit in the Pacific, for example, could be expected to throw up a tidal wave of more than biblical proportions, according to current theories.

Coastal cities would be inundated by a shoreline wave front that could exceed 300 feet high and that volume of water could wash inland 10 or 15 miles, depending on local topography. If it hit the Irish Sea or the Atlantic, you might stay dry in the Dublin mountains, but forget the Cork hinterland or the Shannon estuary.

There is no reason for those living on the other side of the Earth to feel safe, however. The Chicxulub impact is thought to have ejected cubic miles of material that would have become molten on impact, been thrown up in all directions only to return to Earth over the next few hours. This material, akin to lava, would rain down everywhere, setting everything aflame. If the resultant firestorm didn't get you, then over time you would either starve or freeze. Smoke, high-altitude dust and chemical gases would blot the sky, destroying agriculture and bringing years of winter because sunlight couldn't reach us. Something always survives, as the gradual dominance of mammal populations showed after the death of the dinosaurs. Some estimates for a 1.5 mile-wide asteroid impact suggest that we would lose at least 75 per cent of the world population, depending on how it hit. It becomes too difficult to estimate what fraction of the remainder would get through battles over food, years of freezing temperatures and disease.

Can we do anything about an incoming asteroid? Dr Yeomans, in common with many JPL and NASA scientists, supports the "intercepting rocket carrying a nuclear warhead" approach. The object is not to destroy the body - near impossible given it is a mile across - but to deflect it. Slow it by a centimetre per second while still millions of miles away and it would miss us. Deflect it sideways by even a fraction of a degree and it skews away harmlessly on a non-impacting trajectory.

Should we worry about such matters? The dinosaurs were busy munching vegetation or one another as the asteroid approached, blissfully unaware of the doom that was upon them. It would be different for us. Astronomers spotted 1997 XF11 30 years out, giving us plenty of time to ponder our collective futures had it been on an impacting path.