99942 Apophis (/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 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 0.8km[8][9] 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. By 2008, the keyhole had been determined to be less than 1 km wide.[8] 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.[10]
The diameter of Apophis is, as of the most recent 2014 observations, approximately 370 metres (1,210 ft).[2] Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036.[11] By May 6, 2013 (April 15, 2013 observation arc), the probability of an impact on April 13, 2036 had been eliminated.[2] 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.[2] As of December 2017, there were seven asteroids with a more notable cumulative Palermo Technical Impact Hazard Scale than Apophis.[12] On average, an asteroid the size of Apophis
The Sentry Risk Table estimates that Apophis would make atmospheric entry with 750 megatons of kinetic energy.[2] The impacts that created Meteor Crater or the Tunguska event are estimated to be in the 3–10 megaton range.[48] The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons and the biggest hydrogen bomb ever exploded, the Tsar Bomba, was around 57 megatons. In comparison, the Chicxulub impact has been estimated to have released about as much energy as 100,000,000 megatons (100 teratons).
The exact effects of any impact would vary based on the asteroid's composition, and the location and angle of impact. Any impact would be extremely detrimental to an area of thousands of square kilometres, but would be unlikely to have long-lasting global effects, such as the initiation of an impact winter.[citation needed] Assuming Apophis is a 325-metre-wide (1,066 ft) stony asteroid, if it were to impact into sedimentary rock, Apophis would create a 4.3-kilometre (14,000 ft) impact crater.[13]
In 2008, the B612 Foundation made estimates of Apophis's path if a 2036 Earth impact were to occur, as part of an effort to develop viable deflection strategies.[49] The result was a narrow corridor a few kilometres wide, called the "path of risk", extending across southern Russia, across the north Pacific (relatively close to the coastlines of California and Mexico), then right between Nicaragua and Costa Rica, crossing northern Colombia and Venezuela, ending in the Atlantic, just before reaching Africa.[50] Using the computer simulation tool NEOSim, it was estimated that the hypothetical impact of Apophis in countries such as Colombia and Venezuela, which were in the path of risk, could have more than 10 million casualties.[51] However, the exact location of the impact would be known weeks or even months in advance, allowing any nearby inhabited areas to be completely evacuated and significantly decreasing the potential loss of life and property. A deep-water impact in the Atlantic or Pacific oceans would produce an incoherent short-range tsunami with a potential destructive radius (inundation height of >2 m) of roughly 1,000 kilometres (620 mi) for the most of North America, Brazil and Africa, 3,000 kilometres (1,900 mi) for Japan and 4,500 kilometres (2,800 mi) for some areas in Hawaii.[52]