A curious phenomenon of the early years of the Third Millennium is this:

- Day One, the media splash this news: an asteroid appears to be on a collision course with Earth;

- Day Two, there is global terror and a pervasive sense of doom;

- Day Three, fresh data show the rock will miss us by a comfortingly wide margin.

The phenomenon has happened so often and become so cringingly embarrassing that, for the guardians who scan the heavens for asteroid threats, the tale of the Little Boy Who Cried Wolf is now required reading.

Many blame the problem on an early-warning system based on the so-called Torino Scale.

This advises the public of the risk and estimated gravity of a collision, on an index ranking from zero (no risk) to 10 (a certain, massive impact likely to inflict global climate change of the kind that wiped out the dinosaurs).

"The problem is that information about these new discoveries is given to the public far too early. They are meaningless," says Benny Peiser, an astronomer at Liverpool Moores University in Britain who runs CCNet, an information network on potential asteroid threats.

"To give you an analogy: The Biblical Flood started with rain, so if it starts raining, there is a probability that this could be the start of the next inundation.

"So on that basis, you go to the press or the public to warn them. Then after a few days or a week, it stops raining and you realise that there is no flood."

The latest Apocalypse Not happened in early September.

It was 2003 QQ47, a behemoth 1.2 kilometers (two-thirds of a mile) across, which stood a remote risk of whacking into our home on March 21 2014, according to preliminary calculations made by Britain's Near-Earth Object Information Centre (NEOIC), set up by the government to track potentially dangerous asteroids.

The NEOIC rated the risk as grade one ("merits careful monitoring"), but then downgraded it to zero after a couple of days -- and after a predictable rash of Armageddon stories -- when further visual observations of QQ47 showed it would miss us.

Yet it was just the latest in a string of false alarms. Last year, there were 2002 NT7 and 2002 MN... in 2000, there was a conveniently millennium scare, 2000 SG344, which only lasted a few hours... and in 1999, there was 1999 AN10, which indicated our number would be up in 2027. None rated more than one on the Torino before being downgraded.

Defenders of the Torino say it is a useful tool for helping the public to gauge a very remote but still genuine risk, and blame sensationalist press coverage for the debacle.

Indeed, the inventor of the scale, Rick Binzel of the Massachusetts Institute of Technology (MIT) is revising the wording of his brainchild to try to avoid media hype. A grade one asteroid will now be described as "normal" instead of requiring "careful monitoring."

Others, though, say the Torino scale should be scrapped, mainly because it can be invoked from the early sighting of a new asteroid, with only scant data about the track the rock will take around the Sun.

Further observations, taken over days or weeks, give much more data about the orbital arc and, inevitably, the asteroid is found to show no risk of impact.

"It's time we got rid of it," says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Harvard, who says the public will not take the scale seriously if all it does is generate false alarms.

Even if the debate about the alarm system is heated, almost no-one favours hiding information from the public, says Peiser.

"There are one or two who say 'don't release that data to the public', but that doesn't work," he says.

"Firstly, we would get accused of a coverup and secondly we want other astronomers to use the information to do followup observations. So the information absolutely needs to be in the public domain."

All rights reserved. � 2004 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.

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