30.4 h (1.27 d)[1][6] 30.55±0.12 h[7] 30.67±0.06 h[8] Tumbling:[9] 27.38±0.07 h (precession period),[9] 263±6 h (rotation period),[9] 30.56±0.01 h (twice the period of harmonic with strongest lightcurve amplitude)[9]
99942 Apophis (provisional designation2004 MN4) is anear-Earth asteroid and apotentially hazardous object, 450 metres (1,480 ft) by 170 metres (560 ft) in size.[3] Observations eliminated the possibility of animpact on Earth in 2029, when it will pass the Earth at a distance of about 31,600 kilometres (19,600 mi) above the surface.[12][13] It will also have a close encounter with the Moon, passing about 95,000 km from the lunar surface.[14]
There was a brief period of concern in December 2004 when initial observations indicated a probability of 0.027 (2.7%) that the asteroid would hitEarth on Friday, April 13, 2029.
A small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through agravitational keyhole estimated to be 800 metres in diameter,[15][16] which would have set up a future impact exactly seven years later on Easter Sunday, April 13, 2036.[17] This possibility kept it at Level 1 on the 0 to 10Torino 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.[15] 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.[18]
It is estimated that an asteroid as big or bigger coming so close to Earth happens only once in 800 years on average.[19][20] Such an asteroid is expected to actually hit Earth once in about 80,000 years.[21]
Preliminary observations byGoldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036 (probability less than one in a million).[22] In February 2013 the estimated probability of an impact in 2036 was reduced to7×10−9.[2][4] 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,[1] about the distance of the planetVenus when it overtakes Earth every 1.6 years. Simulations in 2013 showed that theYarkovsky 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 two in a million.[2][23] Radar observations in March 2021 helped to refine the orbit,[24] and in March 2021 theJet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years.[25][26] The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometres to now just a couple of kilometres,[27] greatly enhancing predictions of future approaches. Entering March 2021, six asteroids each had a more notable cumulativePalermo scale rating than Apophis, and none of those has a Torino level above 0.[28][b] 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.
When first discovered, the object received the provisional designation2004 MN4, 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.[34] Apophis is theGreek name ofApep, an enemy of theAncient Egyptian sun-godRa. He is the Uncreator, an evil serpent that dwells in the eternal darkness of theDuat and tries to swallow Ra during his nightly passage. Apep is held at bay bySet, the Ancient Egyptian god of storms and the desert.[35] Apophis will evolve from anAten- into anApollo-type asteroid as a result of the 2029 encounter. Apollo asteroids are usually named after Greek deities, Aten asteroids after Egyptian ones. The name Apophis is a nod to that: It is the Greek name of an Egyptian deity.[36][34]
Tholen and Tucker, two of the co-discoverers of the asteroid, are reportedly fans of the television seriesStargate SG-1. One of the show's persistent villains is an alien namedApophis. He is one of the principal threats to the existence of civilization on Earth through the first few seasons, and may have inspired the naming.[34] Tholen denied reports that the asteroid was named after the TV character, stating that the connection is coincidental.[36][37] 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.[34]
Proposed symbol for Apophis
The mythological creature Apophis is pronounced with the accent on the first syllable (/ˈæpəfɪs/).[c] In contrast, the asteroid's name is generally accented on the second syllable (/əˈpoʊfɪs/,[38] or/əˈpɒfɪs/ as the name was pronounced in the TV series).[39]
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 thedwarf planet symbols inUnicode, 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.[40][41]
Comparison between the best-fit convex and nonconvex shape models, and some of the available radar images of (99942) ApophisComparison of possible size of Apophis asteroid with theEiffel Tower andEmpire State Building
Based on the observed brightness, Apophis's diameter was initially estimated at 450 metres (1,480 ft); a more refined estimate based on spectroscopic observations atNASA'sInfrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is 350 metres (1,150 ft). As of 2013, NASA's impact risk page listed the diameter at 330 metres (1,080 ft), and an assumed mass of 4×1010 kg.[4] The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three.[4] Apophis's surface composition probably matches that ofLL chondrites.[42]
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 acontact binary) with a relatively bright surface albedo of0.35±0.10. The axis of its angular momentum points 59° south of theecliptic, which means that Apophis is aretrograde rotator.[3] Apophis is atumbler, 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 theprincipal axis of highestmoment 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 calledprecession). 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).[9]
The pre-2029 (red) and post-2029 (green) orbits of Apophis, and the orbit of Earth (yellow). The distance between the dotted curve and the solid curve shows how far the orbit is north or south of theecliptic plane. This gives the distance from Earth's orbit at the points where the solid curves cross the yellow curve.
Apophis has a low inclination orbit (3.3°) that varies from just outside the orbit ofVenus (0.746 AU, as compared to the aphelion of Venus, 0.728) to just outside the orbit ofEarth (1.099 AU).[1] 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.[20] 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.[43] At theascending 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.[20] After the 2029 Earth approach, the orbit will change dramatically. The period will change from around8⁄9 of a year to a bit under7⁄6. 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.[1]
The closest known approach of Apophis will occur on April 13, 2029, at 21:46UT, when Apophis will pass Earth at a distance of about 31,600kilometres (19,600 mi) above the surface.[12][13] Using the June 2024 orbit solution which includes theYarkovsky effect, the3-sigma uncertainty region in the 2029 approach distance is about ±3.3 km.[27][1] The distance, a hair's breadth in astronomical terms, is five times theradius of the Earth, one tenth the distance to theMoon,[48] and closer than the ring ofgeostationary satellites currently orbiting the Earth.[49][50] It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude 3.1[51] (visible to thenaked eye from rural as well as darker suburban areas, visible withbinoculars from most locations).[52] The close approach will be visible fromEurope,Africa, and westernAsia. Over the course of about a day, Apophis will move northwest fromCentaurus toPerseus and then southwest toPisces, an arc of 205°.[53] 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.[54] During the approach, Earth willperturb Apophis from anAten-class orbit with asemi-major axis of 0.92 AU to anApollo-class orbit with a semi-major axis of 1.1 AU.[55]Perihelion will lift from 0.746 AU to 0.895 AU and aphelion will lift from 1.10 AU to 1.31 AU.[55]
Orbital elements for 2029 (pre-flyby) and 2030 (post-flyby)[55]
Apophis also encounters the Moon at ~95,000 km from the lunar surface, ~17 hours after encounter with Earth.[14]
During the 2029 approach, Apophis's brightness will peak atmagnitude 3.1,[51] easily visible to the naked eye, with a maximum angular speed of 42° per hour. The maximum apparentangular diameter will be approximately 2 arcseconds. This is roughly equivalent to theangular diameter ofNeptune from earth. Therefore, the asteroid will be barelyresolved by ground-based telescopes not equipped withadaptive optics but very well resolved by those that are.[56] Because the approach will be so close,tidal forces are likely to alter Apophis's rotation axis, but Apophis will not approach within theRoche 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 aweathered Sq- to an unweatheredQ-type.[3][42]
Animation of 99942 Apophis orbit in 2028–2029
Around Sun
Around Earth
Sun· Earth· 99942 Apophis· Moon
History of close approaches of large near-Earth objects since 1908 (A)
(A) This list includes near-Earth approaches of less than 2lunar distances (LD) of objects withH brighter than 20. (B)Nominal geocentric distance from the center of Earth to the center of the object (Earth has a radius of approximately 6,400 km). (C) Diameter: estimated, theoretical mean-diameter based onH andalbedo range between X and Y. (D) Reference: data source from theJPL SBDB, withAU converted into LD (1 AU≈390 LD) (E) Color codes: unobserved at close approach observed during close approach upcoming approaches
In 2036, Apophis will pass the Earth at a third the distance of the Sun in both March and December.[1] Using the 2024 orbit solution, the Earth approach on March 27, 2036, will be no closer than 0.3089 AU (46.21 million km; 28.71 million mi; 120.2 LD), but more likely about 0.3097 AU (46.33 million km; 28.79 million mi).[1] For comparison, the planet Venus will be closer to Earth at 0.2883 AU (43.13 million km; 26.80 million mi; 112.2 LD) on May 30, 2036.[58][f] On December 31, 2036, Apophis will be a little bit further away than the March approach at about 0.33 AU (49 million km; 31 million mi).
Around April 19–20, 2051, Apophis will pass about 0.04 AU (6.0 million km; 3.7 million mi) from Earth and it will be the first time since 2029 that Apophis will pass within 10 million km of Earth.[1]
Although early simulations showed that there was a chance Apophis could hit the earth on April 12, 2068,[2] this was later excluded andJPL Horizons calculates that Apophis will be about 1.864 ± 0.0024 AU (278.85 ± 0.36 million km) from Earth,[59][60] making the asteroid much farther than the Sun.
By 2116, theJPL Small-Body Database andNEODyS close approach data start to becomedivergent.[1][61] In April 2116, Apophis is expected to pass about 0.02 AU (3 million km; 8 LD) from Earth, but could pass as close as 0.001 AU (150 thousand km; 0.39 LD) or as far as 0.1 AU (15 million km; 39 LD).[1]
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 automaticSentry system andNEODyS, a similar automatic program run by theUniversity of Pisa and theUniversity 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,[62][63] when the uncertainty region had shrunk to 83,000 km.[64] This probability, combined with its size, caused Apophis to be assessed at level 4 on theTorino scale[18] and 1.10 on thePalermo 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 aprecovery image that extended the observation arc back to March 2004.[31] The danger of a 2036 passage was lowered to level 0 on the Torino scale in August 2006.[65] With a cumulative Palermo scale rating of −3.22,[4] the risk of impact from Apophis is less than one thousandth the background hazard level.[4]
In July 2005, formerApollo astronautRusty Schweickart, as chairman of theB612 Foundation, formally asked NASA to investigate the possibility that the asteroid's post-2029 orbit could be inorbital resonance with Earth, which would increase the probability of future impacts. Schweickart also asked NASA to investigate whether atransponder should be placed on the asteroid to enable more accurate tracking of how its orbit is affected by theYarkovsky effect.[66]
On January 31, 2011, astronomers took the first new images of Apophis in more than three years.[67]
Illustration of a common trend where progressively reduced uncertainty regions result in an asteroid impact probability increasing followed by a sharp decrease
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 [the next good opportunity], we should be able to predict the location of2004 MN4 out to at least 2070."[68] Apophis passed within 0.0966 AU (14.45 million km; 8.98 million mi) of Earth in 2013, allowing astronomers to refine the trajectory for future close passes.[10][61][69] Just after the closest approach on January 9, 2013,[61] the asteroid peaked at anapparent magnitude of about 15.6.[70] TheGoldstone radar observed Apophis during that approach from January 3 through January 17.[71] TheArecibo Observatory observed Apophis once it entered Arecibo's field of view after February 13, 2013.[71] 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.[72] 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).[4]
Incorporating 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.[73] After 2015, its orbit kept it near the Sun from the perspective of Earth, precluding telescopic observations. It was not further than 60 degrees from the Sun between April 2014 and December 2019.
No observations of Apophis were made between January 2015 and February 2019 but observations began again in January 2020.[74] In March 2020, astronomersDavid Tholen and Davide Farnocchia measured the acceleration of Apophis due to theYarkovsky 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 thesemi-major axis to decrease by about 170 metres per year, causing an increase inecliptic longitude that isquadratic in time.[75] In late 2020 Apophis approached the Earth and passed 0.11265 AU (16.852 million km; 43.84 LD) from Earth on March 6, 2021, brightening to +15mag at that time. Radar observations of Apophis were carried out at Goldstone in March 2021.[76][24] The asteroid has been observed byNEOWISE (between December 2020 and April 2021)[77][78] and byNEOSSat (in January 2021).[7][79][8]
These observations showed that theimpact parameter 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,[76] less than the earlier nominal value of 47,659 km by 296 km because of theYarkovsky 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.[2]
Apophis was the target of an observing campaign by theInternational Asteroid Warning Network, resulting in the collection oflight curves,spectra, andastrometry.[7][79][8] 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.[80]
Hypothetical risk corridor for an impact on April 13, 2029, based on the 2020–21 planetary defense exercise
On February 21, 2021, Apophis was removed from theSentry Risk Table, as an impact in the next 100 years was finally ruled out.[81]
Severaloccultations of bright stars (apparent magnitude 8–11) by Apophis occurred in March and April 2021.[82][83][84][85] A total of five separate occultations were observed successfully, marking the first time that an asteroid as small as Apophis was observed using theoccultation method (beating the previous record set in 2019 by asteroid3200 Phaethon, which is more than ten times the size of Apophis).[83] The first event, on March 7, was successfully observed from the United States by multiple observers.[86][87][82] The next potential occultation, which occurred on March 11, was predicted to be visible from central Europe,[84] but was not observed, mainly because of bad weather (two negative observations were recorded fromGreece).[83] Another occultation occurred on March 22, but larger-than-expectedresiduals in the March 7 data had caused the majority of observers to be deployed outside of the actual path for the occultation,[82] and it was observed only by a single observer from the United States,amateur astronomer Roger Venable. 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 fromNew Mexico, again by Venable, alongside others.[85][82] Two more occultations, observable on April 10 and 11 fromJapan and New Mexico, respectively, were seen by several observers each.[82] The occultation measurements allowed refinement of the measurement of the asteroid size and orbit.
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,[88] Apophis became the asteroid with the most precisely measured Yarkovsky effect of all asteroids, at asignal-to-noise ratio (SNR) of 186.4,[89][g] surpassing101955 Bennu (SNR=181.6).[90]
The 2021 apparition was the last opportunity to observe Apophis before its 2029 flyby.[1]
Asteroid 99942 Apophis – radar observations March 8–10, 2021 (March 26, 2021)
The original NASA report mentioned impact chances of "around 1 in 300" in 2029, which was widely reported in the media.[18] 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.
Later that day, based on a total of 64 observations, the estimates were changed to 1 in 62 (1.6%), resulting in an update to the initial report and an upgrade to a Torino scale rating of 4.
2004-12-25
The chances were first reported as 1 in 42 (2.4%) and later that day (based on 101 observations) as 1 in 45 (2.2%). 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.
2004-12-26
Based on a total of 169 observations, the impact probability was still estimated as 1 in 45 (2.2%), the estimates for diameter and mass were lowered to 380 m and 7.5×1010 kg, respectively.
2004-12-27
Based on a total of 176 observations with anobservation arc of 190 days, the impact probability was raised to 1 in 37 (2.7%)[63] with a line of variation (LOV) of only 83,000 km;[64] diameter was increased to 390 m, and mass to 7.9×1010 kg.
Later that afternoon, aprecovery increased the span of observations to 287 days, which eliminated the 2029 impact threat.[31] The cumulative impact probability was estimated to be around 0.004%, a risk lower than that of asteroid2004 VD17, which once again became (temporarily) the greatest-risk object. A 2053 approach to Earth still posed a minor risk of impact, and Apophis was still rated at level one on the Torino scale for this orbit.
2004-12-28
12:23 GMT
Based 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.
2004-12-29
01:10 GMT
The only pass rated 1 on the Torino scale was for 2053-04-13.51 based on 139 observations spanning 287.71 days (2004-Mar-15.1104 to 2004-Dec-27.8243).
19:18 GMT
This was still the case based upon 147 observations spanning 288.92 days (2004-Mar-15.1104 to 2004-Dec-29.02821), though the close encounters were changed and reduced to 4 in total.
2004-12-30
13:46 GMT
No passes were rated above 0, based upon 157 observations spanning 289.33 days (2004-Mar-15.1104 to 2004-Dec-29.44434). The most dangerous pass was rated at 1 in 7,143,000.
22:34 GMT
157 observations spanning 289.33 days (2004-Mar-15.1104 to 2004-Dec-29.44434). One pass at 1 (Torino scale) 3 other passes.
2005-01-02
03:57 GMT
Observations spanning 290.97 days (2004-Mar-15.1104 to 2004-Dec-31.07992) One pass at 1 (Torino scale) 19 other passes.
2005-01-03
14:49 GMT
Observations spanning 292.72 days (2004-Mar-15.1104 to 2005-Jan-01.82787) One pass at 1 (Torino scale) 15 other passes.
2005–01
Extremely preciseradar observations atArecibo Observatory[32] refine the orbit further and show that the April 2029 close approach will occur at only 5.7 Earth radii,[33] approximately one-half the distance previously estimated.
2005-02-06
Apophis estimated to have a 1-in-13,000 chance of impacting in April 2036.[91]
2005-08-07
Radar observation[32] refines the orbit further and eliminates the possibility of an impact in 2035. Only the pass in 2036 remains at Torino scale 1 (with a 1-in-5,560 chance of impact).[92]
2005–10
It is predicted that Apophis will pass just below the altitude ofgeosynchronous satellites, which are at approximately 35,900 kilometres (22,300 mi).[93] Such a close approach by an asteroid of that size is estimated to occur every 800 years or so.[19]
2006-05-06
Radar observation at Arecibo Observatory[32] slightly lowered the Palermo scale rating, but the pass in 2036 remained at Torino scale 1[94] despite the impact probability dropping by a factor of four.
2006-08-05
Additional observations through 2006 resulted in Apophis being lowered to Torino scale 0.[65] (The impact probability was assessed as 1 in 45,000.)[65]
2008-04
News outlets carry the story that 13-year-old German studentNico Marquardt found a probability of 1 in 450 for a 2036 impact.[95] This estimate was allegedly acknowledged byESA and NASA[96][97][95] but in an official statement,[98] NASA denied they had made an error. 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 currentequatorial geosynchronous satellites, there is currently no risk of collision; and the effect on Apophis's orbit of any such impact would be insignificant.
2009-04-29
An animation is released[99] that shows how unmeasured physical parameters of Apophis bias the entire statistical uncertainty region. If Apophis is aretrograde rotator on the small, less-massive end of what is possible, it will be several hundred kilometres further ahead in 2029, resulting in a different change to its orbit, and then the measurement uncertainty region for 2036 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-massiveprograde rotator, it arrives a bit later on April 13, 2029, and the uncertainty region for 2036 is advanced along the orbit. Only the remote tails of the probability distribution could encounter Earth, producing a negligible impact probability for 2036.[19][20]
Criticism of older published impact probabilities rests on the fact that important physical parameters such as mass and spin that affect its precise trajectory had not yet been accurately measured and hence there were no associated probability distributions. The Standard Dynamical Model used for making predictions simplifies calculations by assuming Earth is a point mass. This could lead to a prediction error of up to 2.9 Earth radii for the 2036 approach, necessitating the consideration of Earth's oblateness during the 2029 passage for accurately forecasting the potential impact.[19] Additional factors that could greatly influence the predicted motion in ways that depend on unknown details, were the spin of the asteroid,[101] its precise mass, the way it reflects and absorbs sunlight, radiates heat, and the gravitational pull of other asteroids passing nearby.[19] Small uncertainties in the masses and positions of theplanets andSun could cause up to 23 Earth radii of prediction error for Apophis by 2036.[19]
2013-01
A 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.[2] First appearance of Sentry virtual impactors that also include mid-October dates.[102]
2013-01-09
TheEuropean Space Agency (ESA) announced that theHerschel 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.[10] 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.[10] Goldstone single-pixel observations of Apophis have ruled out the potential 2036 Earth impact.[22][103][104] Apophis will then come no closer than about 23 million kilometres (14×10^6 mi)—and more likely miss us by something closer to 56 million kilometres (35×10^6 mi).[103] The radar astrometry is more precise than was expected.[103]
2014-10-8
TheSentry Risk Table assessed Apophis as having a 6.7-in-a-million (1-in-150,000) chance of impacting Earth in 2068, and a 9-in-a-million (1-in-110,000) cumulative chance of impacting Earth by 2105.[105]
2020-03
By taking observations of Apophis with theSubaru Telescope in January and March 2020, as well as remeasuring older observations using the newGaia DR2 star catalog, astronomers positively detect theYarkovsky effect on Apophis. Thesemi-major axis thereby decreases by about 170 metres per year. The Yarkovsky effect is the main source of uncertainty in impact probability estimates for this asteroid.[75]
2021-02-21
Apophis was removed from theSentry Risk Table, as an impact in the next 100 years was finally ruled out.[81]
2021-03-15
10:44
JPL solution #207 using observations in 2020 and 2021 reduced the 3-sigma uncertainty region in the 2029 approach distance from ±700 km[106] to about ±3 km.[1] The June 2021 solution showed the Earth approach on March 27, 2036, will be no closer than 0.30889 AU (46.209 million km; 28.713 million mi; 120.21 LD).[1]
2024-06-25
JPL solution #220 includes observations through 2022-April-09.[1]
As of 2021, the Sentry Risk Table estimated that Apophis would impact Earth withkinetic energy equivalent to 1,200 MT ormegatons of TNT.[4] In comparison, the energy released by the eruption of Krakatoa was 200 MT, the total global nuclear arsenal has an energy equivalent to 1,460 MT, and theChicxulub impact and extinction event had an estimated energy of 100,000,000 MT (100 teratons). SeeTNT 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 animpact winter.[107] Assuming Apophis is a 370-metre-wide (1,210 ft) stony asteroid with a density of 3,000 kg/m3, if it were to impact intosedimentary rock, Apophis would create a 5.1-kilometre (17,000 ft)impact crater.[21][4]
In 2008, theB612 Foundation made estimates of Apophis's path if a 2036 Earth impact were to occur, as part of an effort to develop viabledeflection strategies.[108] The result was a narrow corridor a few kilometres wide, called the "path of risk", extending across southernRussia, across the northPacific (relatively close to the coastlines ofCalifornia andMexico), then right betweenNicaragua andCosta Rica, crossing northernColombia andVenezuela, ending in theAtlantic, just before reachingAfrica.[109] 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.[110] A deep-water impact in the Atlantic or Pacific oceans would produce an incoherent short-rangetsunami with a potential destructive radius (inundation height of >2 m) of roughly 1,000 kilometres (620 mi) for most of North America, Brazil and Africa, 3,000 km (1,900 mi) for Japan and 4,500 km (2,800 mi) for some areas in Hawaii.[111]
TheOSIRIS-REx spacecraft returned a sample ofBennu to Earth on September 24, 2023.[112] 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 briefflyby. In April 2022, the extension was approved, andOSIRIS-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.[113][114] For its Apophis mission after the sample return,OSIRIS-REx was renamedOSIRIS-APEX (short for OSIRIS-Apophis Explorer).[115]
In 2007, thePlanetary Society, aCalifornia-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.[116]
The commercial competition was won by a design calledForesight created bySpaceWorks Engineering.[117][116][clarification needed] SpaceWorks proposed a simple orbiter with only two instruments and a radio beacon at a cost of ~US$140 million, launched aboard aMinotaur 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 aDelta II 7925 rocket, to arrive at the asteroid after a cruise of 233 to 309 days. It would have carried four additionalBUOI probes that would have impacted the surface of Apophis over the course of two weeks.[116][118]
Apophis is the target of theEuropean Space Agency's proposedRamses (Rapid Apophis Mission for Security and Safety) mission, with a launch in 2026–2028[h] and rendezvous with the asteroid in 2029.[120][121]
China had planned an encounter with Apophis in 2022, several years prior to the close approach in 2029. This mission, now known asTianwen-2, would have included exploration and close study of three asteroids including an extended encounter with Apophis for close observation, and land on the asteroid1996 FG3 to conduct in situ sampling analysis on the surface.[123] The spacecraft launched on 28 May 2025, with a different set of targets.[124][125]
Studies by NASA, ESA,[126] and various research groups in addition to the Planetary Society contest teams,[127] have described a number of proposals for deflecting Apophis or similar objects, includinggravitational tractor,kinetic impact, andnuclear bomb methods.
On December 30, 2009,Anatoly Perminov, the director of the Russian Federal Space Agency, said in an interview thatRoscosmos will also study designs for a possible deflection mission to Apophis.[128]
On August 16, 2011, researchers at China'sTsinghua 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.[129]
On February 15, 2016, Sabit Saitgarayev, of theMakeyev Rocket Design Bureau, announced intentions to useRussianICBMs to target relatively smallnear-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.[130]
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.[131]
In the grand strategy video gameTerra Invicta, the player can mount a scientific expedition to Apophis when it flies close to Earth in 2029.[134]
In music, the asteroid Apophis is referred to in the song "The Profit of Doom" by gothic metal bandType O Negative on their 2007 albumDead 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.[133]
^Like all orbital elements, the E-MOID changes depending on the epoch it is defined at. At epoch May 2025, the E-MOID is 0.000038 AU, but in early 2029 it will be around 0.00006 AU (ca 9000 km).[2]
^Of the six asteroids with a higher Palermo scale rating than Apophis at the time:
^OnJanuary 8, 2022 Venus was even closer to Earth at 0.2658 AU (39.76 million km; 24.71 million mi; 103.4 LD).
^Using the March 9, 2021, solution, JPL gave the strength of the Yarkovsky effect as, with an uncertainty of. The SNR, defined as the size of the signal divided by the uncertainty, is. As of the latest orbit solution (June 29, 2021), the SNR is (again lower than Bennu's).
^Mannocchi et al. give possible launch dates of November–December 2026, April–May 2027, September–November 2027, or March–May 2028, with a launch in 2027 being the preferred option.
^abcYeomans, D.; Chesley, S.; Chodas, P. (December 23, 2004)."Near-Earth Asteroid 2004 MN4 Reaches Highest Score To Date On Hazard Scale".NASA/JPLCNEOS. RetrievedJanuary 31, 2024.Today's impact monitoring results indicate that the impact probability for April 13, 2029, has risen to about 1.6%, which for an object of this size corresponds to a rating of 4 on the ten-point Torino Scale.
^"Sentry: Earth Impact Monitoring". NASA/JPL Center for NEO Studies. Archived fromthe original on March 24, 2021. RetrievedMarch 3, 2021. (Use Unconstrained Settings to reveal 1979 XB with impact probability below 1e-6)
^Hill, J. (2010)."Apep (Apophis)". Ancient Egypt Online. RetrievedJuly 24, 2021.
^abPhait, Phil (April 19, 2021),"Apophis and Stargate",Bad Astronomy Newsletter, no. 315, retrievedAugust 11, 2025
^"Apophis Facts". NASA. March 28, 2025. RetrievedAugust 11, 2025.Tholen said he and co-discoverer, the late Roy Tucker, were already familiar with the name Apophis because of the evil and powerful character by that name in the television series Stargate SG-1. He said it was "icing on the cake" that the name they selected was also a character in a popular TV show. But Tholen said that, contrary to some reports, the asteroid was not named for the TV character.
^As an example, here is David Tholen, the discoverer of the asteroid, pronouncing the name:"DPS 52 Monday Press Conference". AAS Press Office. October 26, 2020 – via YouTube.
^"Update notes: Apophis (Mar 2015)".Sentry: Earth Impact Monitoring, Operational Notes.NASA/JPL Center for NEO Studies. March 2, 2015.Archived from the original on April 14, 2017. RetrievedMay 8, 2019.
^Range of Possible Impact Points on April 13, 2036 inScenarios for Dealing with Apophis, by Donald B. Gennery, presented at the Planetary Defense Conference. Washington, DC. March 5–8, 2007 (archived fromthe original on April 12, 2012).
^Paine, M. P. (January 1999)."The Threat is Out There"(PDF).Science of Tsunami Hazards.17 (3): 155.Archived(PDF) from the original on June 6, 2015. RetrievedMarch 14, 2021.
^OSIRIS-REx factsheet(PDF). Explorers and Heliophysics Projects Division.ehpd.gsfc.nasa.gov (Report).Goddard SFC:NASA. August 2011. This article incorporates text from this source, which is in thepublic domain.
^Sharma, Jonathan; Lafleur, Jarret; Shah, Nilesh; Apa, Jilian; Townley, Jonathan; Barron, Kreston (April 30, 2007)."PHAROS: Shedding Light on the Near-Earth Asteroid Apophis"(PDF).ESMD Space Grant Systems Engineering Paper Competition. Georgia Institute of Technology.Archived(PDF) from the original on November 15, 2024.
^Morelli, Andrea C.; Mannocchi, Alessandra; et al. (September 2023). "Initial Trajectory Assessment of the RAMSES Mission to (99942) Apophis".arXiv:2309.00435 [astro-ph.EP].
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