99942 Apophis


99942 Apophis is a near-Earth asteroid with a diameter of 370 metres 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 in diameter, 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's 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.
As of 2014, the diameter of Apophis is estimated to be 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, the probability of an impact on April 13, 2036 had been eliminated. The next possible impact is on April 12, 2068 and the odds of an impact on that date, as calculated by the JPL Sentry risk table using observations through February 26, 2014, are 1 in 150,000., there were two 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, an asteroid the size of Apophis can be expected to impact Earth once in about 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 solution was computed. Radar astrometry in January 2005 further refined its orbit solution. The discovery was notable in that it was at a very low solar elongation and at very long range. See diagram below:
When first discovered, the object received the provisional designation, and early news and scientific articles naturally referred to it by that name. Once its orbit was sufficiently well calculated, it received the permanent number 99942. Receiving a permanent number made it eligible for naming by its discoverers, and they chose the name "Apophis" on July 19, 2005. Apophis is the Greek name of Apep, an enemy of the Ancient Egyptian sun-god Ra. He is 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.
Tholen and Tucker, two of the co-discoverers of the asteroid, are reportedly fans of the television series Stargate SG-1. One of the show's persistent villains is an alien named Apophis. He is one of the principal threats to the existence of civilization on Earth through the first few seasons, thus likely why the asteroid was named after him. 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.
The mythological creature Apophis is pronounced with the accent on the first syllable. In contrast, the asteroid's name is generally accented on the second syllable as the name was pronounced in the TV series.

Physical characteristics

Based upon the observed brightness, Apophis's diameter was initially estimated at ; a more refined estimate based on spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga is. NASA's impact risk page lists the diameter at and lists a mass of 4 kg based on an assumed density of 2.6 g/cm. The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three. Apophis's surface composition probably matches that of LL chondrites.
Based on Goldstone and Arecibo radar images taken in 2012–2013, Brozović et al. have estimated that Apophis is an elongated object 450 × 170 metres in size, and that it is bilobed with a relatively bright surface albedo of. Its rotation axis has an obliquity of −59° against the ecliptic, which means that Apophis is a retrograde rotator.
During the 2029 approach, Apophis's brightness will peak at magnitude 3.1, easily visible to the naked eye if one knows where to look, with a maximum angular speed of 42° per hour. The maximum apparent angular diameter will be ~2 arcseconds, so that it will be barely resolved by ground-based telescopes not equipped with adaptive optics. Due to the closeness of the approach, it is likely that tidal forces will alter Apophis's rotation axis. A partial resurfacing of the asteroid is possible, which might change its spectral class from a weathered Sq- to an unweathered Q-type.

Orbit

Close approaches

The closest known approach of Apophis comes on April 13, 2029, when the asteroid comes to within a distance of around 31,000 kilometres from Earth's surface. The distance, a hair's breadth in astronomical terms, is five times the radius of the Earth, ten times closer than the Moon, and even closer than some man-made satellites. It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude 3.1. The close approach will be visible from Europe, Africa, and western Asia. During the approach, Earth will perturb Apophis from an Aten-class orbit with a semi-major axis of 0.92 AU to an Apollo-class orbit with a semi-major axis of 1.1 AU.

2029/2036/2068 approaches

On April 13, 2029, Apophis will pass Earth closer than geosynchronous communication satellites, but will come no closer than above Earth's surface. Using the August 2017 orbit solution which includes the Yarkovsky effect, the 3-sigma uncertainty region in the 2029 approach distance is about ±700 km. The 2029 pass will be much closer than the center of the large uncertainty region predicted in 2004 when the observation arc was very short, but well within that region. Using the 2017 orbit solution, the approach in late March 2036 will be no closer than, but more likely about. In the 2060s Apophis is expected to again closely approach Earth in 2066. On the risk-listed date of April 12, 2068, Apophis could be more than from Earth, making the asteroid further than the Sun. On April 12, 2068, the odds of impact are 1 in 150,000 in large part because the line of variation extends over 840 million km.
EpochEarth MOID
2008
2012
2019

Refinement of close approach predictions

Six months after discovery, and shortly after a close approach to Earth on December 21, 2004, the improved orbital estimates led to the prediction of a very close approach on April 13, 2029, by both NASA's automatic Sentry system and NEODyS, a similar automatic program run by the University of Pisa and the University of Valladolid. Subsequent observations decreased the uncertainty in Apophis's trajectory. The probability of an impact event in 2029 temporarily climbed, peaking at 2.7% on December 27, 2004, when the uncertainty region had shrunk to 83,000 km. This probability, combined with its size, caused Apophis to be assessed at level 4 on the Torino scale and 1.10 on the Palermo Technical Impact Hazard Scale, scales scientists use to represent how dangerous a given asteroid is to Earth. These are the highest values for which any object has been rated on either scale. The chance that there would be an impact in 2029 was eliminated by late December 27, 2004, as a result of a precovery image that extended the observation arc back to March 2004. The danger of a 2036 passage was lowered to level 0 on the Torino scale in August 2006. With a cumulative Palermo Scale rating of −3.2, the risk of impact from Apophis is less than one thousandth the background hazard level.

2005 and 2011 observations

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 also asked NASA to investigate whether a transponder should be placed on the asteroid to enable more accurate tracking of how its orbit is affected by the Yarkovsky effect. On January 31, 2011, astronomers took the first new images of Apophis in more than 3 years.

2013 refinement

The close approach in 2029 will substantially alter the object's orbit, prompting Jon Giorgini of JPL to say in 2011: "If we get radar ranging in 2013 , we should be able to predict the location of out to at least 2070." Apophis passed within of Earth in 2013, allowing astronomers to refine the trajectory for future close passes. Just after the closest approach on January 9, 2013, the asteroid peaked at an apparent magnitude of about 15.7. The Goldstone radar observed Apophis during that approach from January 3 through January 17. The Arecibo Observatory observed Apophis once it entered Arecibo's declination window after February 13, 2013.
A NASA assessment as of February 21, 2013, that did not use the January and February 2013 radar measurements gave an impact probability of 2.3 in a million for 2068. As of May 6, 2013, using observations through April 15, 2013, the odds of an impact on April 12, 2068, as calculated by the JPL Sentry risk table had increased slightly to 3.9 in a million.

2015 observations

As of January 2019, Apophis has not been observed since 2015, mostly because its orbit has kept it very near the Sun from the perspective of Earth. It has never been further than 60 degrees from the Sun since April 2014, and will remain so until December 2019. With the most recent 2015 observations, the April 12, 2068, impact probability is now 6.7 in a million, and the asteroid has a cumulative 9-in-a-million chance of impacting Earth before 2106.

History of impact estimates

DateTimeStatus
2004-12-23The original NASA report mentioned impact chances of "around 1 in 300" in 2029, which was widely reported in the media. The actual NASA estimates at the time were 1 in 233; these resulted in a Torino scale rating of 2, the first time any asteroid had received a rating above 1.
2004-12-23Later that day, based on a total of 64 observations, the estimates were changed to 1 in 62, resulting in an update to the initial report and an upgrade to a Torino scale rating of 4.
2004-12-25The chances were first reported as 1 in 42 and later that day as 1 in 45. At the same time, the asteroid's estimated diameter was lowered from 440 m to 390 m and its mass from 1.2×1011 kg to 8.3×1010 kg.
2004-12-26Based on a total of 169 observations, the impact probability was still estimated as 1 in 45, the estimates for diameter and mass were lowered to 380 m and 7.5×1010 kg, respectively.
2004-12-27Based on a total of 176 observations with an observation arc of 190 days, the impact probability was raised to 1 in 37 with a line of variation of only 83,000 km; diameter was increased to 390 m, and mass to 7.9×1010 kg.
2004-12-27Later that afternoon, a precovery increased the span of observations to 287 days, which eliminated the 2029 impact threat. The cumulative impact probability was estimated to be around 0.004%, a risk lower than that of asteroid, which once again became the greatest-risk object. A 2053 approach to Earth still poses a minor risk of impact, and Apophis was still rated at level one on the Torino scale for this orbit.
2004-12-2812:23 GMTBased on a total of 139 observations, a value of one was given on the Torino scale for 2044-04-13.29 and 2053-04-13.51.
2004-12-2901:10 GMTThe only pass rated 1 on the Torino scale was for 2053-04-13.51 based on 139 observations spanning 287.71 days.
2004-12-2919:18 GMTThis was still the case based upon 147 observations spanning 288.92 days, though the close encounters have changed and been reduced to 4 in total.
2004-12-3013:46 GMTNo passes were rated above 0, based upon 157 observations spanning 289.33 days. The most dangerous pass was rated at 1 in 7,143,000.
2004-12-3022:34 GMT157 observations spanning 289.33 days. One pass at 1 3 other passes.
2005-01-0203:57 GMTObservations spanning 290.97 days One pass at 1 19 other passes.
2005-01-0314:49 GMTObservations spanning 292.72 days One pass at 1 15 other passes.
2005–01Extremely precise radar observations at Arecibo Observatory refine the orbit further and show that the April 2029 close approach will occur at only 5.7 Earth radii, approximately one-half the distance previously estimated.
2005-02-06Apophis had a 1-in-13,000 chance of impacting in April 2036.
2005-08-07Radar observation refines the orbit further and eliminates the possibility of an impact in 2035. Only the pass in 2036 remains at Torino scale 1.
2005–10It is predicted that the asteroid will pass just below the altitude of geosynchronous satellites, which are at approximately. Such a close approach by an asteroid is estimated to occur every 800 years or so.
2006-05-06Radar observation at Arecibo Observatory 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.
2006-08-05Additional observations through 2006 resulted in Apophis being lowered to Torino scale 0.
2008-04Nico Marquardt published a research paper in which he calculated the probability of Apophis to collide with a geosynchronous satellite during its flyby on April 13, 2029, and the consequences of this event to the likelihood of an Earth-collision 2036. Afterwards, the German newspaper Bild published an article stating a 100 times higher probability of an Earth-collision in the year 2036 than Marquardt calculated. Nearly all international press reported the news with false data caused by the review from Bild even though Marquardt denied. This estimate was allegedly confirmed by ESA and NASA but in an official statement, NASA denied the wrong statement. The release went on to explain that since the angle of Apophis's approach to the Earth's equator means the asteroid will not travel through the belt of current equatorial geosynchronous satellites, there is currently no risk of collision; and the effect on Apophis's orbit of any such impact would be insignificant.
2008-04-16NASA News Release 08-103 reaffirmed that its estimation of a 1-in-45,000 chance of impact in 2036 remained valid.
2009-04-29An animation is released that shows how unmeasured physical parameters of Apophis bias the entire statistical uncertainty region. If Apophis is a retrograde rotator on the small, less-massive end of what is possible, the measurement uncertainty region will get pushed back such that the center of the distribution encounters Earth's orbit. This would result in an impact probability much higher than computed with the Standard Dynamical Model. Conversely, if Apophis is a small, less-massive prograde rotator, the uncertainty region is advanced along the orbit. Only the remote tails of the probability distribution could encounter Earth, producing a negligible impact probability.
2009-10-07Refinements 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.
Criticism of older published impact probabilities rests on the fact that important physical parameters such as mass and spin that affect its precise trajectory have not yet been accurately measured and hence there are no associated probability distributions. The Standard Dynamical Model used for making predictions simplifies calculations by assuming Earth is a point mass; this can introduce up to 2.9 Earth radii of prediction error for the 2036 approach, and Earth's oblateness must be considered for the 2029 passage to predict a potential impact reliably. Additional factors that can greatly influence the predicted motion in ways that depend on unknown details, are the spin of the asteroid, its precise mass, the way it reflects and absorbs sunlight, radiates heat, and the gravitational pull of other asteroids passing nearby. Small uncertainties in the masses and positions of the planets and Sun can cause up to 23 Earth radii of prediction error for Apophis by 2036.
2013-01A statistical impact risk analysis of the data up to this point calculated that the odds of the 2036 impact at 7.07 in a billion, effectively ruling it out. The same study looked at the odds of an impact in 2068, which were calculated at 2.27 in a million.
2013-01-09The European Space Agency announced the Herschel Space Observatory made new thermal infrared observations of the asteroid as it approached Earth. The initial data shows the asteroid to be bigger than first estimated because it is now expected to be less reflective than originally thought. The Herschel Space Observatory observations increased the diameter estimate by 20% from 270 to 325 metres, which translates into a 75% increase in the estimates of the asteroid's volume or mass. Goldstone single-pixel observations of Apophis have ruled out the potential 2036 Earth impact. Apophis will then come no closer than about —and more likely miss us by something closer to. The radar astrometry is more precise than was expected.
2016-03-25The Sentry Risk Table assessed Apophis as having a 6.7-in-a-million chance of impacting Earth in 2068, and a 9-in-a-million cumulative chance of impacting Earth in 2082.

Possible impact effects

The Sentry Risk Table estimates that Apophis would impact Earth with kinetic energy equivalent to 1,200 megatons of TNT. The impacts that created Meteor Crater, Arizona about 50,000 years ago and the Tunguska event of 1908 are estimated to be between 3–10 megatons. The biggest hydrogen bomb ever exploded, the Tsar Bomba, was around 57 megatons while the 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons. In comparison, the Chicxulub impact has been estimated to have released about as much energy as 100,000,000 megatons.
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. Assuming Apophis is a stony asteroid with a density of 3,000 kg/m3, if it were to impact into sedimentary rock, Apophis would create a impact crater.

Expired 2036 path of risk

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. The result was a narrow corridor a few kilometres wide, called the "path of risk", extending across southern Russia, across the north Pacific, 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 were in the path of risk, could have more than 10 million casualties. 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 of roughly for most of North America, Brazil and Africa, for Japan and for some areas in Hawaii.

Potential space missions

Planetary Society competition

In 2007, The Planetary Society, a California-based space advocacy group, organized a $50,000 competition to design an unmanned space probe that would 'shadow' Apophis for almost a year, taking measurements that would "determine whether it will impact Earth, thus helping governments decide whether to mount a deflection mission to alter its orbit". The society received 37 entries from 20 countries on 6 continents.
The commercial competition was won by a design called 'Foresight' created by SpaceWorks Enterprises, Inc. SpaceWorks proposed a simple orbiter with only two instruments and a radio beacon at a cost of ~US$140 million, launched aboard a Minotaur IV between 2012 and 2014, to arrive at Apophis five to ten months later. It would then rendezvous with, observe, and track the asteroid. Foresight would orbit the asteroid to gather data with a multi-spectral imager for one month. It would then leave orbit and fly in formation with Apophis around the Sun at a range of. The spacecraft would use laser ranging to the asteroid and radio tracking from Earth for ten months to accurately determine the asteroid's orbit and how it might change.
Pharos, the winning student entry, would be an orbiter with four science instruments that would rendezvous with and track Apophis. Earth-based tracking of the spacecraft would then allow precise tracking of the asteroid. The Pharos spacecraft would also carry four instrumented probes that it would launch individually over the course of two weeks. Accelerometers and temperature sensors on the probes would measure the seismic effects of successive probe impacts, a creative way to explore the interior structure and dynamics of the asteroid.
Second place, for $10,000, went to a European team led by Deimos Space S.L. of Madrid, Spain, in cooperation with EADS Astrium, Friedrichshafen, Germany; University of Stuttgart, Germany; and University of Pisa, Italy. Juan L. Cano was principal investigator.
Another European team took home $5,000 for third place. Their team lead was EADS Astrium Ltd, United Kingdom, in conjunction with EADS Astrium SAS, France; IASF-Roma, INAF, Rome, Italy; Open University, UK; Rheinisches Institut für Umweltforschung, Germany; Royal Observatory of Belgium; and Telespazio, Italy. The principal investigator was Paolo D'Arrigo.
Two teams tied for second place in the Student Category: Monash University, Clayton Campus, Australia, with Dilani Kahawala as principal investigator; and University of Michigan, with Jeremy Hollander as principal investigator. Each second-place team won $2,000. A team from Hong Kong Polytechnic University and Hong Kong University of Science and Technology, under the leadership of Peter Weiss, received an honorable mention and $1,000 for the most innovative student proposal.

Planned Chinese mission

China plans an exploration fly-by mission to Apophis in 2022, several years prior to the close approach in 2029. This fly-by mission to Apophis is part of an asteroid exploration mission planned after China's Mars mission in 2022 currently in development, according to Ji Jianghui, a researcher at the Purple Mountain Observatory of the Chinese Academy of Sciences and a member of the expert committee for scientific goal argumentation of deep space exploration in China. The whole mission will include exploration and close study of three asteroids by sending a probe to fly side by side with Apophis for a period to conduct close observation, and land on the asteroid 1996 FG3 to conduct in situ sampling analysis on the surface. The probe is also expected to conduct a fly-by of a third asteroid to be determined at a later time. The whole mission would last around six years, said Ji.

Don Quijote mission

Apophis is one of two asteroids that were considered by the European Space Agency as the target of its Don Quijote mission concept to study the effects of impacting an asteroid.

Proposed deflection strategies

Studies by NASA, ESA, and various research groups in addition to the Planetary Society contest teams, have described a number of proposals for deflecting Apophis or similar objects, including gravitational tractor, kinetic impact, and nuclear bomb methods.
On December 30, 2009, Anatoly Perminov, the director of the Russian Federal Space Agency, said in an interview that Roscosmos will also study designs for a possible deflection mission to Apophis.
On August 16, 2011, researchers at China's Tsinghua University proposed launching a mission to knock Apophis onto a safer course using an impactor spacecraft in a retrograde orbit, steered and powered by a solar sail. Instead of moving the asteroid on its potential resonant return to Earth, Shengping Gong and his team believe the secret is shifting the asteroid away from entering the gravitational keyhole in the first place.
On February 15, 2016, Sabit Saitgarayev, of the Makeyev Rocket Design Bureau, announced intentions to use Russian ICBMs to target relatively small near-Earth objects. Although the report stated that likely targets would be between the 20 to 50 metres in size, it was also stated that 99942 Apophis would be an object subject to tests by the program.

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