99942 Apophis (pronounced /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 significant 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 600 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 until August 2006. It broke the record for the highest level on the Torino Scale, being, for only a short time, a level 2, before it was lowered.[5]
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 on the Torino Scale. As of October 19, 2006, the impact probability for April 13, 2036, was calculated as 1 in 45,000. An additional impact date in 2037 was also identified; the impact probability for that encounter was calculated as 1 in 12.3 million.
Most experts agree that Apophis warrants closer scrutiny and, to that end, in February 2008 the Planetary Society awarded $50,000 in prize money to companies and students who submitted designs for space probes that would put a tracking device on or near the asteroid.[6]
Contents [hide]
1 Basic data
1.1 Naming
2 Close approaches
2.1 History of impact estimates
2.2 Possible impact effects
3 Amateur calculations
4 Potential space missions
5 Cultural reference
6 References
7 External links
7.1 Risk assessment
7.2 ESA
7.3 NASA
7.4 Older articles
7.5 More recent articles
[edit] Basic data
Based upon the observed brightness, Apophis's length was estimated at 450 metres (1,500 ft); a more refined estimate based on spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is 350 metres (1,100 ft).
In February 2005 it was 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. Apophis’s brightness will peak at magnitude 3.3, with a maximum angular speed of 42° per hour. The maximum apparent angular diameter will be ~2 arcseconds, which means it will be a starlike point of light in all but the very largest telescopes.
[edit] Naming
When first discovered, the object received the provisional designation 2004 MN4 (sometimes written 2004 MN4), 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), the first numbered asteroid with Earth-impact solutions (to its orbit determination from observations). Receiving a permanent number made it eligible for naming, and it promptly received the name "Apophis" as of July 19, 2005. Apophis is the Greek name of the Ancient Egyptian enemy of Ra: Apep, the Uncreator, a serpent that dwells in the eternal darkness of the Duat (underworld) and tries to swallow Ra during His nightly passage. Apep is held at bay by Set, the Ancient Egyptian god of Chaos.
Although the Greek name for the Egyptian god may be appropriate, Tholen and Tucker—two of the co-discovers of the asteroid—are reportedly fans of the TV series Stargate SG-1. The show's main antagonist in the first several seasons was an alien named Apophis who took the name for the Egyptian god and sought to destroy Earth.[7]
[edit] Close approaches
After the Minor Planet Center confirmed the June discovery of Apophis (then known only as 2004 MN4), the next close approach was computed to be April 13, 2029 by the automatic Sentry system of NASA's Near-Earth Object Program Office. NEODyS, a similar automatic system at the University of Pisa, Italy and the University of Valladolid, Spain also calculated this same approach date. On that date, it will become as bright as magnitude 3.3 (visible to the naked eye from rural and some darker suburban areas, visible with binoculars from most locations[8]). 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 the June discovery was confirmed, observatories throughout the world computed the probability of impact. Over the next several days, additional observations allowed for astronomers to narrow the cone of error. As they did, the probability of an impact event climbed, peaking at 2.7% (1 in 37). Combined with its size, this caused Apophis to be assessed at level four out of ten 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.
On Friday, April 13, 2029, Apophis will pass Earth within the orbits of geosynchronous communication satellites.[9] It will return for another close Earth approach in 2036.
Precovery observations from March 15, 2004 were identified on December 27, and an improved orbit was computed.[10] Radar astrometry further refined the orbit. The 2029 pass will actually be much closer than the first predictions, but the uncertainty is such that an impact is ruled out. The pass on April 13, 2036 was also determined to carry little risk.
The close approach in 2029 will substantially alter the object's orbit, making predictions uncertain without more data. "If we get radar ranging in 2013 [the next good opportunity], we should be able to predict the location of 2004 MN4 out to at least 2070." said Jon Giorgini of JPL.[11]
In July 2005, former Apollo astronaut Rusty Schweickart, as chairman of the B612 Foundation, formally asked NASA to investigate the possibility that the asteroid's post-2029 orbit could be in orbital resonance with Earth, which would increase the probability of future impacts. Schweickart asked for an investigation of the necessity of placing a transponder on the asteroid for more accurate tracking of how its orbit is affected by the Yarkovsky effect.[12]
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