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99942 Apophis (, previously known by its provisional designation ) is a 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, 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 800 m (~0.5 mi) wide,〔 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. During the short time when it had been most concerning, Apophis set the record for highest rating on the Torino scale, reaching level 4.〔 The diameter of Apophis is, as of the most recent 2013 observations, approximately .〔 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.〔 As of October 8, 2014, using observations through February 26, 2014, the odds of an impact on April 12, 2068, as calculated by the JPL Sentry risk table is (1 in 149,000).〔 Of objects not recently observed, there are about ten asteroids with a more notable Palermo Technical Impact Hazard Scale than Apophis.〔 On average, an asteroid the size of Apophis (325 meters) can be expected to impact Earth about every 80,000 years.〔 ==Discovery and naming== Apophis was discovered on June 19, 2004, by Roy A. Tucker, David J. Tholen and Fabrizio Bernardi at the Kitt Peak National Observatory.〔 On December 21, 2004, Apophis passed from Earth.〔 Precovery observations from March 15, 2004, were identified on December 27, and an improved orbit was computed.〔〔 Radar astrometry in January 2005 further refined the orbit.〔〔 When first discovered, the object received the provisional designation , and news and scientific articles about it referred to it by that name. When its orbit was sufficiently well calculated, it received the permanent number 99942 (on June 24, 2005). Receiving a permanent number made it eligible for naming, and it received the name "Apophis" on July 19, 2005.〔 Apophis is the Greek name of an enemy of the Ancient Egyptian sun-god Ra: Apep, the Uncreator, an evil serpent that dwells in the eternal darkness of the Duat and tries to swallow Ra during his nightly passage. Apep is held at bay by Set, the Ancient Egyptian god of storms and the desert. David J. Tholen and Tucker—two of the co-discoverers of the asteroid—are reportedly fans of the TV series ''Stargate SG-1''. One of the show's persistent villains is an alien named Apophis. In the fictional world of the show, the alien's backstory was that he had lived on Earth during ancient times and had posed as a god, thereby giving rise to the myth of the Egyptian god of the same name. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「99942 Apophis」の詳細全文を読む スポンサード リンク
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