Astronomers at the Finnish Meteorological Institute reported in the British weekly journal Nature Thursday that the comet was one of a group of five that raced through the solar system between May and July 1997.

The four others had already been observed, but no one detected the fifth, even though it was bright enough to be observable by amateur astronomers in their back gardens, according to the team.

It was only when the Finns belatedly assessed data recorded by an instrument aboard a US-European Sun-monitoring satellite, SOHO, that they made the discovery. The fifth comet has been dubbed K2.

The instrument, SWAN, is designed to study the solar wind -- the streams of hot gas emitted by the Sun -- by sensing hydrogen atoms emitted at ultraviolet wavelengths.

"Because the comet was almost constant in brightness over several months, it should have been easily observable from the ground," the authors say, adding that this "underlines the need for full-sky surveillance of comets."

In an adjoining commentary, University of Maryland astronomer Michael A'Hearn said the belated discovery showed the holes in a US-led programme to detect space objects that could pose a potential threat to the Earth.

The scheme is skewed in favour of detecting large asteroids -- those which are bigger than one kilometer (five-eighths of a mile) across -- rather than comets, he said.

This is because asteroids do not move very far from Earth and return close to our planet every few years, making their path easy to calculate.

Comets, on the other hand, have a much wider variety of orbits and make a flyby at much longer periods, making it far more difficult to calculate any change in their trajectories.

A'Hearn said that since the start of 1997, 22 newly discovered comets had come as close to the Sun, or closer, than K2 -- and 11 of them have passed inside Earth's orbit.

Such findings raised questions about the priority given to comet detection and the viability of conventional spotting methods, he said.

"For our peace of mind, it is important to know whether comets represent 10 percent of the potential large impacts on Earth (as is commonly thought) or a much larger fraction," he said.

Asteroids or comets can wreak astonishing destruction if they penetrate the Earth's protective atmosphere.

A large object, believed to be up to 10 kms (six miles) long, smashed into Mexico's Yucatan peninsula 65 million years ago, triggering a firestorm and a dust cloud that wiped out the dinosaurs, scientists believe.

In 1908, an asteroid or comet about 60 metres (200 feet) long exploded over Siberia with the force of 600 times the Hiroshima bomb, reducing a 40-km (25-mile) wide patch of forest to matchwood.

In January, US astrophysicists put the number of major asteroids at around 700, with a margin of error of plus or minus 230.

These are classified as bodies ranging in size from one to 10 kilometers (half a mile to six miles) whose swing around the Sun takes them relatively close to the Earth's orbit.

However, the vast majority of dangerous objects between 100 metres (yards) and one km (five-eights of a mile) have still to be detected, although the risk of any collision is considered extremely remote.

Copyright 2000 AFP. All rights reserved. The material on this page is provided by AFP and may not be published, broadcast, rewritten or redistributed.

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