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99942 apophis

99942 Apophis (, previously known by its provisional designation 2004 MN4) is a 370-meter diameter near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a probability of up to 2.7% that it would hit Earth on April 13, 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029. However, until 2006, a possibility remained that during the 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole, a small region no more than about 0.5 mile wide, or 0.8 km that would set up a future impact exactly seven years later on April 13, 2036. This possibility kept it at Level 1 on the Torino impact hazard scale until August 2006, when the probability that Apophis would pass through the keyhole was determined to be very small and Apophis' rating on the Torino scale was lowered to zero. By 2008, the keyhole had been determined to be less than 1 km wide. During the short time when it had been of greatest concern, Apophis set the record for highest rating on the Torino scale, reaching level 4 on December 27, 2004. In 2008, NASA reaffirmed the chance of Apophis impacting Earth in 2036 as being 1 in 45,000. As of 2014, the diameter of Apophis is estimated to be approximately 370 metres (1,210 ft). Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036. By May 6, 2013 (April 15, 2013 observation arc), the probability of an impact on April 13, 2036 had been eliminated. Using observations through February 26, 2014, the odds of an impact on April 12, 2068, as calculated by the JPL Sentry risk table are 1 in 150,000. As of July 2019, there were three asteroids with a more notable cumulative Palermo Technical Impact Hazard Scale than Apophis, though none of them have a Torino level greater than 0. On average, one asteroid the size of Apophis (370 metres) can be expected to impact Earth about every 80,000 years.

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