What was believed to be a 30 metre asteroid, which had a one in four chance of impacting the northern hemisphere within a matter of days, was almost broadcast as a global impact alert by astronomers last month.

With limited observation points before a potential impact, it was an amateur who confirmed that the asteroid wasn't on a collision course, which was fortunate because the space rock turned out to be 500 metres in size, much larger than first thought.

Members of the International Astronomical Union's Working Group on Near Earth Objects had considered contacting President Bush to issue the alert, but others in the field believe that with the lack of evidence the warning would have been premature.