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AUSTRALIA - Australian astronomer Rob McNaught was looking for something much darker and deadlier when he found the dazzling comet that now bears his name.
"I was looking for NEOs or near earth objects," he says, "asteroids and defunct comets that have orbits that cross ours and could
therefore one day collide with our planet and cause massive destruction."
Or "killer asteroids", such as the one that hit the Gulf of Mexico 65 million years ago and wiped out much of life on Earth - including, many scientists say, the dinosaurs. Even the small cometary fragment that exploded over Siberia, in the Tunguska Event of June 30, 1908, produced as much force as a 20-megaton hydrogen bomb.
Had it slammed into the atmosphere over a major city it would have burned millions of people to death. But Comet McNaught is neither killer asteroid nor rogue comet. In August, McNaught was scanning images taken by a telescope on Siding Spring Mountain, New South Wales, during an NEO-hunting sky patrol.
He found a point of light where none should have been. It was identified - from its motion against the stars, and by using spectroscopic analysis - as an incoming comet that had passed near the sun, deep inside the orbit of Mercury.
The patrol is regarded by McNaught and his colleague Gordon Garradd as far more important than a spectacle intriguing the millions of people who have seen Comet McNaught.
"We are tracking potentially dangerous objects, including some that will come so close to Earth or the other planets that their orbits may be speeded up or slowed down or bent in a way that puts them on a collision course with Earth with little warning," Garradd says. One such object is Apophis, a small asteroid about 320m wide, that was discovered in 2004.
On Friday, April 13, 2029, Apophis will pass Earth so low that it will plunge under the orbit of many dozens of communications satellites positioned 35,786km above the equator. It will appear as a steadily moving bright star crossing the skies of western Asia, the Middle East, Africa and Europe - all in a mere 30 minutes.
"Apophis is going to be a graphic example of how easily a near-Earth encounter could set up a subsequent collision," Garradd says.
"It only has to pass through a 'keyhole' 600m wide when it dives under or through the satellites in 2029 to have its orbit sufficiently bent to hit us on April 13, 2036.
"This object has been crossing Earth's orbit unseen around twice a year for centuries. But now that it has been found it is also entering a period where many of its crossings occur when Earth is in its vicinity, rather than on the other side of the sun."
The astronomers say that if it hits a satellite in 2029 it would be like an insect hitting a truck windscreen at a speed of 30km a second.
And while Apophis is well below the threshold diameters of one to two kilometres - where it could threaten mankind with similar global devastation that killed the dinosaurs - it is about 40 times larger than the calculated size of the Tunguska object, or the equivalent of more than 800 megatons of dynamite if its kinetic energy were released in a collision.
McNaught says that if Apophis proves to be a loose clump of boulders such as near-Earth asteroid Itokawa, examined close up by the Japanese probe Hayabusa in 2005, an impact with a large satellite might make it slowly expand into a cloud of objects, a potentially ugly scenario if it led to a later encounter with a sky full of Tunguska-sized fireballs.
However, if Aphophis proves to be a dense metallic asteroid, rich in nickel or iron, "there would be a very bright flash and Apophis would continue on as if nothing had happened".
Both astronomers emphasise that such scenarios are conjecture, but that their Siding Spring Survey work is part of a methodical and serious programme to identify potentially hazardous objects.
This is to give mankind as much warning as possible of a serious risk of collision - and perhaps to intercept and deflect them.
The simplest and increasingly most popular concept involves the space-tug technique - putting a heavy satellite in orbit around the dangerous object to modify its trajectory away from Earth.
The Siding Spring survey is part of a small network of sky patrol telescopes funded by by Nasa.
Last year, Siding Spring found 400 NEOs, or 60 per cent of all such objects discovered that year, plus Comet McNaught - the Great Comet of 2007.
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