Astronomers came within minutes of alerting the world to a potential asteroid strike last month.
On 13 January a 30m object, 2004 AS1, was predicted to have 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 the President 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.
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.
At about 30m wide, the asteroid was big enough to cause considerable damage after exploding in the atmosphere.
As it was 2004 AS1 turned out to be bigger than anyone had thought - about 500m wide, and It sailed passed the Earth at a distance of about 12 million km (32 times the Earth-Moon distance).
http://neo.jpl.nasa.gov/cgi-bin/db?name=2004+AS1[Next!]
