By Dr David Whitehouse
BBC News Online science editor
The US space agency Nasa is clarifying the procedure for telling the President if the Earth is in danger of being hit by a newly discovered asteroid.
There was uncertainty about where 2004 AS1 was headed
It follows the discovery on 13 January of a possibly dangerous object - 2004 AS1 - which for just a few hours had some observers worried it would hit us.
At the time some scientists were unsure at what stage to raise the alarm and who to call, but now the plan is clear.
If necessary, the President would start a Federal Emergency Response Plan.
When to call?
Although the near-Earth object (Neo) 2004 AS1 was not the type of thing to have wiped out the dinosaurs or threaten our species, it could still have caused considerable damage had it exploded in the atmosphere. Potentially, the loss of life could have been huge.
The first four observations of the object revealed it could be on a collision course with the Earth, but the uncertainties were large. There were many possible orbits the object could be on, and the vast majority of them did not threaten the Earth.
For astronomers, who find hundreds of small space rocks each day, this presented a problem: at what stage should they sound the alert?
Astronomer Clark Chapman, from the South Western Research Institute in Colorado, witnessed the events of 13 January and reported on them to a conference a few weeks later.
He said he was so alarmed by the possibility of an impact that he had considered calling the President, even if nobody else had been able to provide any harder information.
Lindley Johnson, Nasa's Near Earth Object Observation Program Scientist, is in no doubt what worried astronomers should do: "You call me," he said.
Johnson has access to Nasa's chief, Sean O'Keefe, who is a phone call from the President.
But do not make the call unless more than one set of preliminary observations are available, Johnson says, adding that no alarm should be raised until follow-up observations confirm the threat.
Lindley Johnson is keen to point out that Nasa is not operating a warning network for potentially impacting Neos. Its effort is to conduct a survey of large Neos to determine their orbits.
"However, the recent incident caused us at Nasa HQ to go ahead and do something we had been contemplating for a while - that is to formalise procedures in the unlikely event an object is discovered on an imminent impact trajectory," he told BBC News Online.
"By doing this we hope we will minimise the confusion which will undoubtedly occur should we ever have such an incident," he said.
"The Minor Planet Center (MPC) in Cambridge, Massachusetts, does the initial orbit determination for such objects, so that is where I would expect the first warning to come from.
Bush could start an emergency response plan
"The head of the MPC, Brian Marsden, would confer with Don Yeomans of Nasa's Neo Program Office. They know under what circumstances to call me at Nasa HQ."
Johnson stresses that in such circumstances the first priority is to get additional observations to confirm or modify the initial orbit determination.
"That's what Brian Marsden did," says Johnson, "and the threat went away."
2004 AS1 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.