http://edition.cnn.com/2006/TECH/space/12/29/fs.asteroids/

Space's threat to civilization
POSTED: 1527 GMT (2327 HKT), December 29, 2006

The asteroid that has made the space community sit up and take notice is nicknamed Apophis, after the Egyptian god of destruction.

Over 300 meters in diameter and weighing 25 million tons, it is set to come within 20,000 miles of Earth in 2029. And that date for the near miss? April 13, 2029, which should get the doomsday soothsayers excited.

The threat is real, however. If Apophis passes the earth at a distance of exactly 18,893 miles it will open a "gravitational keyhole" causing it to enter a new orbit that would put it on a direct collision course with earth seven years later. Current predictions are that Apophis will pass between 18,880 and 20,880 miles from earth.