No one was quite sure at the beginning where 2004 AS1 was head

Some scientists believed on 13 January that a 30m object, later designated 2004 AS1, had a one-in-four chance of hitting the planet within 36 hours.

It could have caused local devastation and the researchers contemplated a call to President Bush before new data finally showed there was no danger.

The procedures for raising the alarm in such circumstances are now being revised.

At the time, the president's team would have been putting the final touches to a speech he was due to make the following day at the headquarters of Nasa, the US space agency.

In it he planned to reset the course of manned spaceflight, sending it back to the Moon and on to Mars, but he could have had something very different to say.

If... the call had been made to the president it would have been disastrous Brian Marsden, Minor Planet Center

He could have begun by warning the world it was about to be hit by a space rock.

Bush would not have known where it would impact - only somewhere in the Northern Hemisphere. Experts would have been bouncing radar signals off the huge rock as he spoke in order to get more information about its trajectory.

At about 30m wide, the asteroid was cosmic small fry, not the type of thing to wipe out the dinosaurs or threaten our species, but still big enough to cause considerable damage after exploding in the atmosphere.

Potentially, the loss of life could have been much worse than 11 September.

In the end, Bush made no such announcement, but astronomers have admitted they were on the verge of making the call.

Shall we call the President?

In a paper presented at this week's Planetary Protection conference in California, veteran asteroid researcher Clark Chapman calls it a "nine-hour crisis".

He explains how word reached the astronomical community of an asteroid that had just been discovered by the twin optical telescopes of the Linear automated sky survey in New Mexico.

Bush's Nasa speech might have taken a different turn

The Minor Planet Center in Massachusetts - the clearing house for such observations - posted details on the internet requesting attention from astronomers, one of whom noticed something peculiar.

The object was expected to grow 40-times brighter in the next day - a possible sign that it was getting closer, very rapidly.

But with data from just four observations available, the uncertainties were large. There were many possible orbits the object could be on, and the majority of them did not threaten the Earth.

What to do? Tell the world about the uncertain situation or wait for more data?

For some astronomers, events reached a crescendo when Steven Chesley, a researcher at Nasa's Jet Propulsion Laboratory, looked at the available data and sent an e-mail saying the asteroid had a 25% chance of striking the Earth's Northern Hemisphere in a few days.

It was then that astronomers Clark Chapman and David Morrison, chair of the International Astronomical Union's Working Group on Near Earth Objects, contemplated picking up the telephone to the White House.

'Jumped the gun'

But many astronomers did not agree that waking up President Bush would have been wise.

"They completely misread the situation," said Benny Peiser of Liverpool John Moores University in the UK. "There was plenty of time to get other observers on the job."

Others also believe the call would have been premature.

"That would have jumped the gun before we knew much about the object," said Brian Marsden, of the Minor Planet Center.

Chapman was close to raising alarm

"I find it incredible that such action was contemplated on the basis of just four observations. That is just not enough to yield a sensible orbit.

"There was no need to panic as it was obvious that the situation would have been resolved, one way or another, in another hour or two," he told BBC News Online.

Fortunately for all concerned, shortly after the ominous Chesley e-mail, an amateur astronomer managed to dodge the clouds and take a picture of a blank patch of sky.

This was significant because if 2004 AS1 really was going to hit the Earth, it would have been in the amateur's sights. The fact that it was absent meant the rock would not strike us.

But Chapman says in his presentation that if it had been cloudy, and no more observations could have been obtained at the time, he would have raised the alarm.

Marsden disagrees. "If it had been cloudy and the call had been made to the President it would have been disastrous."

Many astronomers recognise that a false alarm could have brought ridicule on their profession. They are calling for more planning and less panic if it should happen for real next time.

And 2004 AS1? It turned out to be bigger than anyone had thought - about 500m wide. It eventually passed the Earth at a distance of about 12 million km - 32 times the Earth-Moon distance, posing no danger to us whatsoever.