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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. However, 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 through August 2006.

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 Torino impact hazard scale. As of August 5, 2006 the impact probability for April 13, 2036 is estimated at 1 in 43,000. Two additional impact dates, one in 2037 and the other in 2054, have been identified, however the impact probability for those is 1 in 11.8 million and 1 in 370 million respectively.

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