(99942) Apophis

99942 Apophis (provisional designation ) is a near-Earth asteroid and a potentially hazardous object, 450 metres (1,480 ft) by 170 metres (560 ft) in size, that caused a brief period of concern in December 2004 when initial observations indicated a probability of 0.027 (2.7%) that it would hit Earth on Friday, April 13, 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth in 2029. A small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole estimated to be 800 metres in diameter, which would have set up a future impact exactly seven years later on Easter Sunday, April 13, 2036. This possibility kept it at Level 1 on the 0 to 10 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 Level 0. 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 ever on the Torino scale, reaching Level 4 on December 27, 2004.

The discovery of Apophis in 2004 is rather surprising, because it is estimated that an asteroid as big or bigger coming so close to Earth happens only once in 800 years on average. Such an asteroid is expected to actually hit Earth once in about 80,000 years.

Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036 (probability less than 1 in a million). In February 2013 the estimated probability of an impact in 2036 was reduced to . It is now known that in 2036, Apophis will approach the Earth at a third the distance of the Sun in both March and December, about the distance of the planet Venus when it overtakes Earth every 1.6 years. Simulations in 2013 showed that the Yarkovsky effect might cause Apophis to hit a "keyhole" in 2029 so that it will come close to Earth in 2051, and then could hit another keyhole and hit Earth in 2068. But the chance of the Yarkovsky effect having exactly the right value for this was estimated as 2 in a million. Radar observations in March 2021 helped to refine the orbit, and in March 2021 the Jet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years. The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometres to now just a couple of kilometres, greatly enhancing predictions of future approaches. Entering March 2021, six asteroids each had a more notable cumulative Palermo scale rating than Apophis, and none of those has a Torino level above 0. However, Apophis will continue to be a threat possibly for thousands of years until it is removed from being a potentially hazardous object, for instance by passing close to Venus or Mars.

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 (56°) and at very long range (1.1 AU).

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 (on June 24, 2005). 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 (/ˈæpəfɪs/). In contrast, the asteroid's name is generally accented on the second syllable (, or as the name was pronounced in the TV series).As an example, here is David Tholen, the discoverer of the asteroid, pronouncing the name:

Symbols were given to the first few asteroids in the 19th century, though this practice faded when it became clear that there were a great number of them: such symbols are now extremely rarely used by astronomers. In 2008, Denis Moskowitz, a software engineer who devised most of the dwarf planet symbols in Unicode, proposed a symbol for Apophis. His symbol is based on ancient Egyptian depictions of Apep. The added star is similar to many of the 19th-century asteroid symbols.

Physical characteristics and rotation

Based on 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 (2005) is . As of 2013, NASA's impact risk page listed the diameter at , and an assumed mass of 4 kg. 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 (possibly a contact binary) with a relatively bright surface albedo of . The axis of its angular momentum points 59° south of the ecliptic, which means that Apophis is a retrograde rotator. Apophis is a tumbler, which means that it does not rotate around a fixed axis. Rather, the axis of rotation moves in the frame of reference of the asteroid with a period of around 263 hours (called the rotation period). The angle between it and the principal axis of highest moment of inertia varies, as does the angle between that principal axis and the vector of angular momentum (from around 12° to around 55° twice every period). During this period, the angle between the long axis of Apophis and the angular momentum vector swings between around 78° and 102° (90°±12°). But the principal axis of highest moment and the rotation axis both move around the constant axis of angular momentum much faster, with a time-averaged period of 27.38 hours (this is called precession). The result is that Apophis appears to be flipping, making a revolution on average every 30.56 hours. Every 263 hours, the principal axis with highest moment goes around 263/27.28 times (ca 9.6), whereas the long axis goes around 263/30.56 times (ca 8.6).

Orbit

Apophis has a low inclination orbit (3.3°) that varies from just outside the orbit of Venus (0.746 AU, as compared to the aphelion of Venus, 0.728) to just outside the orbit of Earth (1.099 AU). Although its orbit changes slightly each time it comes close to Earth, at present it comes near Earth once in 7.75 years on average (four times between April 14, 1998, and April 13, 2029). Because of its eccentric orbit, these moments are not evenly spaced and tend to occur between December and April, when Apophis is in the outer portions of its orbit. In fact, the eccentricity and semi-major axis are such that (before 2029) Apophis is always receding from Earth around May 1 and is always approaching around December 2. At the ascending node (where Apophis crosses the plane of Earth's orbit from south to north) Apophis is very close to where Earth is around April 13 of any year, and this is what gives rise to close encounters such as the one on April 13, 2029. The orbit also passes south of where the earth is in mid December, producing for example the close approaches of December 16, 1889, and December 18, 1939. After the 2029 Earth approach, the orbit will change dramatically. The period will change from around of a year to a bit under . It will still come very close to Earth's yearly April 13 location. It will no longer pass close to Earth's yearly mid-December location, but will then pass close to Earth's mid-September location. This will cause a close encounter on September 11, 2102, after which the uncertainty in the location of Apophis will increase rapidly with time.

2029 close approach

The closest known approach of Apophis will occur on April 13, 2029, at 21:46 UT, when Apophis will pass Earth at a distance of about above the surface.2029-Apr-13 approach:
38012km "geocentric distance"
– 6378km "Earth radius" = 31634km Using the June 2024 orbit solution which includes the Yarkovsky effect, the 3-sigma uncertainty region in the 2029 approach distance is about ±3.3 km. The distance, a hair's breadth in astronomical terms, is five times the radius of the Earth, one tenth the distance to the Moon, and closer than the ring of geostationary satellites currently orbiting the Earth. It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude 3.1 (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations). The close approach will be visible from Europe, Africa, and western Asia. Over the course of about a day, Apophis will move northwest from Centaurus to Perseus and then southwest to Pisces, an arc of 205°. Approaching Earth its speed relative to Earth will be 6.0 km/s. Earth's gravity will accelerate it to 7.4 km/s at the time of closest approach, and then slow it back down to 6 as it departs. 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. Perihelion will lift from 0.746 AU to 0.895 AU and aphelion will lift from 1.10 AU to 1.31 AU.

During the 2029 approach, Apophis's brightness will peak at magnitude 3.1, easily visible to the naked eye, with a maximum angular speed of 42° per hour. The maximum apparent angular diameter will be approximately 2 arcseconds. This is roughly equivalent to the angular diameter of Neptune from earth. Therefore, the asteroid will be barely resolved by ground-based telescopes not equipped with adaptive optics but very well resolved by those that are. Because the approach will be so close, tidal forces are likely to alter Apophis's rotation axis, but Apophis will not approach within the Roche limit where it would be broken up by tidal forces. A partial resurfacing of the asteroid is possible, which might change its spectral class from a weathered Sq- to an unweathered Q-type.

2036 approaches

In 2036, Apophis will pass the Earth at a third the distance of the Sun in both March and December. Using the 2024 orbit solution, the Earth approach on March 27, 2036, will be no closer than , but more likely about . For comparison, the planet Venus will be closer to Earth at on May 30, 2036. On December 31, 2036, Apophis will be a little bit further away than the March approach at about .

2051 approach

Around April 19–20, 2051, Apophis will pass about from Earth and it will be the first time since 2029 that Apophis will pass within 10 million km of Earth.

2066 and 2068

Although early simulations showed that there was a chance Apophis could hit the earth on April 12, 2068, this was later excluded and JPL Horizons calculates that Apophis will be about from Earth, making the asteroid much farther than the Sun.

By 2116, the JPL Small-Body Database and NEODyS close approach data start to become divergent. In April 2116, Apophis is expected to pass about from Earth, but could pass as close as or as far as .

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 and the probability of an impact event in 2029 temporarily climbed, peaking at 2.7% (1 in 37) 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 scale (corresponding to an impact hazard over 12 times the background level), scales scientists use to represent how dangerous a given asteroid is to Earth. These are the highest values at which any object has been rated on either scale. The chance that there would be an impact in 2029 was eliminated later in the day of 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 −3.22, the risk of impact from Apophis is less than one thousandth the background hazard level.

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.

2011 observations

On January 31, 2011, astronomers took the first new images of Apophis in more than three 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 he next good opportunity 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 15.6. 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. The 2013 observations basically ruled out any chance of a 2036 impact.

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 (1 in 256,000).

2015 observations

As of January 2019, Apophis had not been observed since 2015, mostly because its orbit kept it very near the Sun from the perspective of Earth. It was not further than 60 degrees from the Sun between April 2014 and December 2019. With the early 2015 observations, the April 12, 2068, impact probability was 6.7 in a million (1 in 150,000), and the asteroid had a cumulative 9 in a million (1 in 110,000) chance of impacting Earth before 2106.

2020–2021 observations

No observations of Apophis were made between January 2015 and February 2019 but observations began again in January 2020. In March 2020, astronomers David Tholen and Davide Farnocchia measured the acceleration of Apophis due to the Yarkovsky effect for the first time, significantly improving the prediction of its orbit past the 2029 flyby. Tholen and Farnocchia found that the Yarkovsky effect causes the semi-major axis to decrease by about 170 metres per year, causing an increase in ecliptic longitude that is quadratic in time. In late 2020 Apophis approached the Earth and passed from Earth on March 6, 2021, brightening to +15 mag at that time. Radar observations of Apophis were carried out at Goldstone in March 2021. The asteroid has been observed by NEOWISE (between December 2020 and April 2021) and by NEOSSat (in January 2021).

These observations showed that a number called ζ (basically how far behind Earth Apophis would pass if it were not deflected by the gravitational pull of Earth) in 2029 will be about 47,363 km, less than the earlier nominal value of 47,659 km by 296 km because of the Yarkovsky effect. This means that Apophis will not hit Earth in the coming century, in particular avoiding the keyhole 212.14 km below nominal that would have led to a collision in 2068.

Apophis was the target of an observing campaign by the International Asteroid Warning Network, resulting in the collection of light curves, spectra, and astrometry. The observations were used to practice and coordinate the response to an actual impact threat. Ignoring all earlier observations, the estimated probability of an impact in 2029 reached 16 percent before going down to zero.

On February 21, 2021, Apophis was removed from the Sentry Risk Table, as an impact in the next 100 years was finally ruled out.Removed Objects
from Sentry Risk Table

Several occultations of bright stars (apparent magnitude 8–11) by Apophis occurred in March and April 2021. A total of five separate occultations were observed successfully, marking the first time that an asteroid as small as Apophis was observed using the occultation method (beating the previous record set in 2019 by asteroid 3200 Phaethon, which is more than ten times the size of Apophis). The first event, on March 7, was successfully observed from the United States by multiple observers. The next potential occultation, which occurred on March 11, was predicted to be visible from central Europe. This event was missed, mainly because of bad weather (two negative observations were recorded from Greece). On March 22, another occultation was observed only by a single observer from the United States, amateur astronomer Roger Venable. Larger-than-expected residuals in the March 7 data had caused the majority of observers to be deployed outside of the actual path for the March 22 occultation. This single detection then allowed the prediction of several more events that would have been unobservable otherwise, including an occultation on April 4, which was observed from New Mexico, again by Venable, alongside others. Two more occultations, observable on April 10 and 11 from Japan and New Mexico, respectively, were seen by several observers each.

On March 9, 2021, using radar observations from Goldstone taken on March 3–8 and three positive detections of the stellar occultation on March 7, 2021, Apophis became the asteroid with the most precisely measured Yarkovsky effect of all asteroids, at a signal-to-noise ratio (SNR) of 186.4, surpassing 101955 Bennu (SNR=181.6).

The 2021 apparition was the last opportunity to observe Apophis before its 2029 flyby.

History of impact estimates

Possible impact effects

As of 2021, the Sentry Risk Table estimated that Apophis would impact Earth with kinetic energy equivalent to 1,200 MT or megatons of TNT. In comparison, the energy released by the eruption of Krakatoa was 200 MT, total global nuclear arsenal has an energy equivalent to 1,460 MT, and the Chicxulub impact and extinction event had an estimated energy of 100,000,000 MT (100 teratons). See TNT equivalent examples for an extended table of comparable energies.

The exact effects of an impact would vary based on the asteroid's composition, and the location and angle of impact. Any impact of Apophis 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 (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 were in the path of risk, could have more than 10 million casualties. 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 for most of North America, Brazil and Africa, for Japan and for some areas in Hawaii.

Exploration

OSIRIS-APEX post-Earth-encounter rendezvous

The ''OSIRIS-REx'' spacecraft returned a sample of Bennu to Earth on September 24, 2023. After ejecting the sample canister, the spacecraft can use its remaining fuel to target another body during an extended mission. Apophis is the only asteroid which the spacecraft could reach for a long-duration rendezvous, rather than a brief flyby. In April 2022, the extension was approved, and ''OSIRIS-REx'' will perform a rendezvous with Apophis in April 2029, a few days after the close approach to Earth. It will study the asteroid for 18 months and perform a maneuver similar to the one it made during sample collection at Bennu, by approaching the surface and firing its thrusters. This will expose the asteroid's subsurface and allow mission scientists to learn more about the asteroid's material properties. For its Apophis mission after the sample return, ''OSIRIS-REx'' was renamed ''OSIRIS-APEX'' (short for OSIRIS-Apophis Explorer).

Other proposed space missions

Planetary Society competition

In 2007, the Planetary Society, a California-based space advocacy group, organised a $50,000 competition to design an uncrewed 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 Engineering. 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. ''Pharos'', the winning student entry, would be an orbiter with four science instruments that would rendezvous with and track Apophis. The spacecraft would have been launched in April or May 2013 aboard a Delta II 7925 rocket, to arrive at the asteroid after a cruise of 233 to 309 days. It would have carried four additional ''BUOI'' probes that would have impacted the surface of Apophis over the course of two weeks.

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.

Chinese mission

China had planned an encounter with Apophis in 2022, several years prior to the close approach in 2029. This mission, now known as ''Tianwen-2'', would have included exploration and close study of three asteroids including an extended encounter with Apophis for close observation, and land on the asteroid 1996 FG₃ to conduct in situ sampling analysis on the surface. The launch date is now scheduled for May 2025, with a different set of targets.

RAMSES

Apophis is the target of the European Space Agency's proposed RAMSES (Rapid Apophis Mission for Security and Safety) mission, with a launch in 2026–2028 and rendezvous with the asteroid in 2029.

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.

In October 2022, a method of mapping the inside of a potentially problematic asteroid, such as 99942 Apophis, in order to determine the best area for impact was proposed.

Popular culture

In Id Software's video game ''Rage'', the backstory involves the asteroid colliding with Earth on August 23, 2029. The asteroid almost wipes out the human race and ushers in a post-apocalyptic age.

In music, the asteroid Apophis was referenced in the song "The Profit of Doom" by gothic metal band Type O Negative on their 2007 album ''Dead Again''. The lyrics refer to the asteroid 99942 Apophis, which at that time was considered to have a possibility of hitting Earth on Friday, April 13, 2029.

See also

* Asteroid capture
* Asteroid impact prediction
* Asteroid Redirect Mission
* Earth-grazing fireball
* List of impact structures on Earth
* NEO Surveyor
* List of predicted asteroid impacts on Earth

Notes

References

External links

*

Apophis Asteroid

Asteroid Apophis orbit from recent observations
EPSC Abstracts Vol. 6, EPSC-DPS2011-1212, 2011, EPSC-DPS Joint Meeting 2011


(Sormano Astronomical Observatory)

Interactive 3D gravity simulation of Apophis's 2029 Earth flyby

Risk assessment

Apophis Orbital Prediction Page at NASA JPL

99942 Apophis page
from NEODyS

MBPL – Minor Body Priority List (technical List) at Sormano Observatory



NASA
* ttps://web.archive.org/web/20041228072731/http://neo.jpl.nasa.gov/news/news148.html Possibility of an Earth Impact in 2029 Ruled Out for Asteroid (JPL)

Radar Observations Refine the Future Motion of Asteroid
(JPL)

Animation explaining how impact risk is determined
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Impact Probability
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Discoveries by Roy A. Tucker
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Minor planets to be visited by spacecraft
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