1 in 45000

99942 Apophis (previously known by its provisional designation 2004 MN4) is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a relatively large probability that it would strike the Earth in 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029. However there remained a possibility that during the 2029 close encounter with Earth, Apophis would pass through a "gravitational keyhole", a precise region in space no more than about 400 meters across, that would set up a future impact on April 13, 2036. This possibility kept the asteroid at Level 1 on the Torino impact hazard scale until August 2006. It broke the record for the highest level on the Torino Scale, being, for only a short time, a level 4, before it was lowered.[1]

Additional observations of the trajectory of Apophis revealed the "keyhole" would likely be missed and on August 5, 2006, Apophis was lowered to a Level 0 on the Torino Scale. As of October 19, 2006, the impact probability for April 13, 2036, is estimated at 1 in 45,000. An additional impact date in 2037 has been identified; however, the impact probability for that encounter is 1 in 12.3 million.

Despite the fact that there is no longer any significant probability of an Earth impact, the Planetary Society is offering $50,000 in prize awards for a few of the best plans to put a tracking device on or near the asteroid.[2]