For the first time, astronomers have found an object on a certain collision course with Earth. Fortunately, it is so small it is not expected to cause any damage, burning up in the atmosphere somewhere above northern Sudan in the wee hours of Tuesday morning. It may, however, produce a brilliant 'shooting star'.

The space rock, dubbed 2008 TC3, was first spotted on Monday in a survey by the Mt Lemmon Observatory near Tucson, Arizona.

Its brightness suggests it is no more than about 5 metres across - so small it will likely be destroyed in the atmosphere, says Andrea Milani Comparetti of the University of Pisa in Italy.

Rocks of such size are thought to hit the atmosphere every few months, says Steve Chesley, an astronomer at NASA's Jet Propulsion Laboratory.

"The event is not unusual - what is unique is that it's been predicted beforehand," Milani told New Scientist. "This is the first time we see something arriving, compute that it's going to impact, and announce it is going to impact before it happens."