99942 Apophis (pronounced /əˈpɒfɪs/, 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 small probability (up to 2.7%) 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, a possibility remains that during the 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole, a precise region in space no more than about 600 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. 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 April 16, 2008, the impact probability for April 13, 2036, is calculated as 1 in 45,000. An additional impact date in 2037 was also identified; the impact probability for that encounter was calculated as 1 in 12.3 million. Many scientists agree that Apophis warrants closer scrutiny and, to that end, in February 2008 the Planetary Society awarded $50,000 in prize money to companies and students who submitted designs for space probes that would put a tracking device on or near the asteroid.

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