`It is a dangerous place out there'

The Earth drifts through an enormous shooting gallery and we are the moving target, according to a new study of the plethora of nearby asteroids large enough to destroy our planet.

"It is a dangerous place out there," Dr William Bottke of Cornell University and colleagues conclude in the current issue of the journal Science. He leads a US-French team that has discovered the spatial and size distribution of a large group of asteroids called NEAs for near-Earth asteroids.

The study estimates that a veritable armada of asteroids, some 900 in number, pass uncomfortably close to us on a regular basis. Some fly by at no more than a few moon distances from Earth every year. "Sometime in the future, one of these objects could conceivably run into the Earth," Dr Bottke said.

All of the 900 referred to in the study are a kilometre in diameter or larger, a size that could spell extinction for 90 per cent or more of the species that currently occupy the Earth. "One kilometre in size is thought to be a magic number, because it has been estimated that these asteroids are capable of wreaking global devastation if they hit the Earth."

One of the more chilling findings of the study, which involved Cornell, the Spacewatch group at the University of Arizona and the Observatoire de la Cote d'Azur in Nice, is the fact that only about half of the expected number of NEAs have been found so far, Dr Bottke said. Equally disturbing is his admission that it is very difficult to decide which one might decide to drop by for a dramatic visit.

"Of those we have found, we can accurately predict whether they will strike the earth over the next hundred years or so, but we can't project out several thousands of years. So it is possible some of these asteroids eventually will move onto an earth-collision trajectory," Dr Bottke said.

Most asteroids drift along a safe distance away from us between the orbits followed by Mars and Jupiter. They come in all shapes and sizes and some of them are hundreds of kilometres across. Occasionally these bang into one another, breaking off chunks and bouncing the pieces into unstable orbits that can be perturbed by gravitational kicks produced by nearby planets.

This mechanism can encourage these wayward asteroids to go walkabout and some eventually find their way into our near neighbourhood. Visits by very large asteroids in our mature and stable solar system are rare nowadays.

The pock-marked lunar surface is ample evidence however that in eons gone by there were many more asteroids and comets doing the rounds and leaving behind their telltale craters.

The dinosaurs were most likely done in by a large impactor that would have caused wildfires all around the globe and brought on sudden climate change and species extinctions. There is also evidence of older major impacts, each of which would likely have left a significant punctuation mark on the progress of life on this planet.

The astronomers working on the study believe that their efforts will better quantify the likelihood of future catastrophic collisions with Earth. The work also hones their skill at finding these difficult to spot dark objects that could appear unannounced to destroy much of life as we know it.