Watching out for the end of the world

Atop a sacred mountain in the Tohono O'Odham reservation in Arizona, a small band of wise men is undertaking a regular ritual that will - they hope - save this planet from destruction. They are not, however, priests of the native North American tribes but astronomers from the University of Arizona, just down the road in Tucson. They are seeking out asteroids that may crash into our planet and wipe out most forms of life. This is Project Spacewatch.

The leader of the team is Tom Gehrels, a lifelong devotee of asteroids. In 1980, the subject came into sharper focus when geologists produced evidence that the dinosaurs had been wiped out by an asteroid 10 kilometres across - a huge impact by terrestrial standards but only a small rock on the cosmic scale. Mr Gehrels set out to find as many asteroids that cross the Earth's orbit as he could. Although most miss our planet on any given orbit, one day one could be on a collision course.

The team's telescope was the first large telescope built entirely in America, in 1919, and was installed at the University of Arizona campus. But astronomers were casting covetous eyes at the high summit of Kitt Peak, in the Tohono O'Odham reservation. In 1958, they invited elders of the tribe to peer through this telescope at the moon. The astonished Indians dubbed the astronomers "the men with the long eyes", and gave them permission to bring their strong magic to Kitt Peak.

Now this venerable telescope has been equipped with a state-of-the-art electronic detector to detect faint objects, and a computer programme that can pick out the images of speeding asteroids from a background of millions of fixed stars.

Over the past decade, the telescope has detected tens of thousands of celestial rocks. Most reside in their home territory, the asteroid belt beyond Mars. But Spacewatch has also hunted down its intended quarry - dozens of asteroids zooming across Earth's orbit. The team has even picked out asteroids between Earth and the Moon, but these have been so small that they pose no great threat to life on Earth.

The versatile Spacewatch telescope has enjoyed other adventures. In 1983, the team received a call from the Mount Palomar Observatory, in California, where Gene and Carolyn Shoemaker, with David Levy, had found a strange celestial object. The detailed pictures from the Spacewatch telescope revealed an astonishing sight - a comet with several heads and the same number of tails.

This comet, Shoemaker-Levy 9, was on a collision course not with Earth but with Jupiter. The fireworks from last year's encounter have made the efforts of the Spacewatch team even more urgent. The comet fragments were small, but they wrought havoc on the mighty planet. Space debris of any size is more dangerous than previously imagined.

There are probably something like 2,000 asteroids crossing Earth's orbit that could damage life here. So far, only 5 per cent have been discovered. According to a recent report, detection of the other 95 per cent is critical for science and the future of life on our planet. Fittingly, the report's author is Gene Shoemaker.

Planets and stars

Expect a shower of cosmic debris to strike the Earth on the night of 12-13 August, but don't worry - it poses no threat to life. These are fragments of dust from a comet's tail, each lighter than a grain of sand and as insubstantial as fluff. As each particle burns up in the atmosphere, it leaves a streak of light - a shooting star, or meteor.

We observe this shower of meteors in August every year. It's called the Perseids because the shooting stars seem to spread out from the constellation Perseus. This year, however, the Moon will be well up when the Perseids reach their maximum, so you will see only the most brilliant meteors.

High in the south, three bright stars called the Summer Triangle form a huge V shape. At the lower point of the V is Altair, the brightest star in Aquila (the eagle). To the top right is Vega, in Lyra (the lyre), and to the top left Deneb, the tail of Cygnus (the swan).

Low in the south-west, look out for Jupiter, the most brilliant object at night after the Moon. Matching it in the south-east, but noticeably fainter, is Saturn. Saturn looks odd at the moment - its famous rings are missing. We are currently viewing the planet from directly above its equator, so the rings are presented edge-on. They are so thin that you cannot see them.

August diary (all times BST)

4 Moon at first quarter (4.16am)

10 Full moon (7.16am)

12-13 Maximum Perseid meteors

18 Moon at last quarter (4.04am)

21 Venus at superior conjunction (behind sun)

26 New moon (5.32am)