A celestial close shave
14 February 2013 Last updated at 19:28 GMT
On Friday at about 2000 GMT an asteroid will pass very near the Earth.
At 50 metres across the mass is about the size of the object which hit Tunguska in 1908.
While it won't hit the planet, it will come within about 21,000 miles of the surface which is closer than many orbiting satellites and a close shave by astronomical standards.
Donna Traynor went to Armagh Observatory, one of the oldest in the world, where scientists are tracking the asteroid's progress.