From Wikipedia: 99942 Apophis 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 remained that during the 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole, a precise region in space no more than about a half-mile wide, 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, when the probability that Apophis will pass through the keyhole was determined to be very small. Apophis broke the record for the highest level on the Torino Scale, being, for only a short time, a level 4, before it was lowered. Its diameter is approximately 270 meters (885 ft). As of October 7, 2009, the odds of an April 13, 2036 impact are considered to be 1 in 250,000.
After the Minor Planet Center confirmed the June discovery of Apophis, an April 13, 2029 close approach was flagged by NASA’s automatic Sentry system and NEODyS, a similar automatic program run by the University of Pisa and the University of Valladolid. On that date, it will become as bright as magnitude 3.4 (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations).
This close approach will be visible from Europe, Africa, and western Asia. As a result of its close passage, it will move from the Aten to the Apollo class.
After Sentry and NEODyS announced the possible impact, additional observations decreased the uncertainty in Apophis’ trajectory. As they did, the probability of an impact event temporarily climbed, peaking at 2.7% (1 in 37). Combined with its size, this caused Apophis to be assessed at level 4 on the Torino Scale and 1.10 on the Palermo scale, scales scientists use to represent the danger of an asteroid hitting Earth. These are the highest values for which any object has been rated on either scale.
The 2029 pass will actually be much closer than the first predictions, but the uncertainty is such that an impact is ruled out. Similarly, the pass on April 13, 2036, carries little risk of an impact.
Possible impact effects:
NASA initially estimated the energy that Apophis would have released if it struck Earth as the equivalent of 1480 megatons of TNT. A later, more refined NASA estimate was 880 megatons, then revised to 510 megatons. The impacts which created the Barringer Crater or the Tunguska event are estimated to be in the 3–10 megaton range.
The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons and the biggest hydrogen bomb ever exploded, the Tsar Bomba, was around 50 megatons. In comparison, the Chicxulub impact, believed by many to be a significant factor in the extinction of the dinosaurs, has been estimated to have released about as much energy as 100,000,000 megatons.
Path of risk where 99942 Apophis may impact Earth in 2036.
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.
The B612 Foundation made estimates of Apophis’ path if a 2036 Earth impact were to occur, as part of an effort to develop viable deflection strategies. The result is 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.
Using the computer simulation tool NEOSim, it was estimated that the hypothetical impact of Apophis in countries such as Colombia and Venezuela, which are in the path of risk, could have more than 10 million casualties. An impact several thousand kilometres off the West Coast of the US would produce a devastating tsunami.
- A radar observation on August 7, 2005, refines the orbit further and eliminates the possibility of an impact in 2035. Only the pass in 2036 remains at Torino Scale 1.
- In October 2005 it is predicted that the asteroid will pass just below the altitude of geosynchronous satellites, which are at 35,786 kilometres (22,236 mi). Such a close approach by an asteroid of this size is expected to occur only every 1,300 years or so.
- A new radar observation at Arecibo Observatory on May 6, 2006, slightly lowered the Palermo scale rating, but the pass in 2036 remained at Torino Scale 1 despite the impact probability dropping by a factor of four.
- Additional observations through 2006 resulted in Apophis being lowered to Torino Scale 0 on August 6, 2006. Around this time, the impact probability was lowered to 1 in 45,000.
- As of October 7, 2009, refinements to the precovery images of Apophis by the University of Hawaii’s Institute for Astronomy, the 90-inch Bok Telescope, and the Arecibo Observatory have generated a refined path that reduces the odds of an April 13, 2036 impact to about 1 in 250,000.
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