Fiction could become frightening reality

The prospect of the Earth eventually being hit by an asteroid large enough to wipe out civilisation is now widely believed to be inevitable among scientists.

Films such as Armageddon and Deep Impact raised awareness of such an event and the disastrous consequences that would ensue. Now, a Web-based system monitoring near-Earth objects has been designed to make sure that, if such a scenario did arise, we would have time to act.

Called Sentry, the online set-up is designed to improve communication between scientists about the discovery of new and potentially threatening asteroids. Developed over the past two years by Nasa, it is a database which uses information collected by astronomers to give a risk value to near-Earth objects. The most dangerous of these are put on the website's Risks Page, which currently has 36 listed on it.

Sentry draws data from asteroid monitoring centres every day and uses this information to discover the correct orbit of space objects. But asteroids move so slowly against the backdrop of stars that when one is discovered, astronomers have great difficulty predicting its exact path. As a result, a wide range of possibilities are generated as potential orbits, and Earth can often become a target in projected courses.

Asteroid detections have rapidly increased recently after Nasa was charged with finding 90 per cent of all near-Earth objects larger than 1km in diameter by 2008. This improved detection rate means a rise in false alarms is likely. A handful of these have already occurred, where scientists have caused a scare by predicting the possibility of an asteroid hitting Earth, the most famous of which occurred four years ago when an object called 1997 XF11 briefly appeared to be a threat.

By constantly monitoring the most up-to-date information on asteroids, it is hoped Sentry will help avoid needless anxiety by finding an asteroid's correct path at the earliest possible moment.

Donald Yeomans, manager of Nasa's Near-Earth Object Program Office, which over-sees Sentry, says: 'Objects normally appear on the Risks Page because their orbits can bring them close to the Earth and the limited number of available observations do not allow their trajectories to be well enough defined.

'The most likely outcome is the object will eventually be removed as new observations become available and the object's movement is more tightly constrained.' While the chance of a collision with an asteroid big enough to destroy mankind in our lifetime is very low, the probability of a collision in the next 50 years with an asteroid big enough to create a 5km crater is more likely than you'd think, with odds of 160 to 1. It is some comfort to know that most experts think, if observations did show a large object on its way, we would probably get at least ten years' warning.

Visit the Sentry homepage