99942 Apophis

99942 Apophis is a near-Earth asteroid and potentially hazardous asteroid with a diameter of 370 metres (1,210 feet) that caused a brief period of concern in December 2004 when initial observations indicated a probability 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 in 2029. Until 2006, a small probability nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole of no more than about 800 metres (1⁄2 mi) in diameter,[11][12] which would have 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' rating on the Torino scale was lowered to zero. By 2008, the keyhole had been determined to be less than 1 km wide.[11] 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.[13]

99942 Apophis
Model of 99942 Apophis's shape, assuming the entire surface is of a similar composition.
Discovery[1]
Discovered by
  • Roy A. Tucker
  • David J. Tholen
  • Fabrizio Bernardi
Discovery siteKitt Peak[1]
Discovery dateJune 19, 2004
Designations
(99942) Apophis
Pronunciation/əˈpɒfəs/, (trad.) /ˈæpəfəs/
Named after
Ἄποφις Apophis
Alternative designations
2004 MN4
Minor planet category
AdjectivesApophidian /æpəˈfɪdiən/ (Latin Apŏpidis)
Orbital characteristics[1]
Epoch 21 January 2022 (JD 2459600.5)
Uncertainty parameter 0
Observation arc6267 days (17.16 yr)
Earliest precovery dateMarch 15, 2004
Aphelion1.0993 AU (164.45 Gm)
Perihelion0.7461 AU (111.61 Gm)
0.9227 AU (138.03 Gm)
Eccentricity0.1914
0.89 yr (323.7 d)
Average orbital speed
30.73 km/s
195.654°
Mean motion
1.11203°/day
Inclination3.339°
203.971°
Argument of perihelion
126.596°
Earth MOID0.0002442 AU (36,530 km; 22,700 mi)
Jupiter MOID4.1251 AU (617.11 Gm)
TJupiter6.465
Physical characteristics
Dimensions
  • 0.370 km (0.230 mi)
  • 0.45 × 0.17 km[2]
Mean radius
  • 0.185 km (0.115 mi)
  • 0.17±0.02 km[2]
Mass6.1×1010 kg (assumed)[3]
Mean density
  • ~3.2 g/cm3[4]
  • 2.6 g/cm3 (assumed)[3]
Synodic rotation period
30.4 h (1.27 d)[1][5]
30.55±0.12 h[6]
30.67±0.06 h[7]
Tumbling:[8]
27.38±0.07 h (precession period),[8] 263±6 h (rotation period),[8] 30.56±0.01 h (period of harmonic with strongest lightcurve amplitude)[8]
Geometric albedo
Temperature270 K
Sq[5]

    The diameter of Apophis is estimated to be approximately 370 metres (1,210 ft).[3] Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036.[14] By May 6, 2013 (April 15, 2013, observation arc), the possibility of an impact on April 13, 2036 had been eliminated altogether.[3] Apophis will make two modestly close approaches to Earth in 2036, but even the planet Venus will come closer to Earth in 2036. On April 12, 2068, the nominal trajectory has Apophis 1.87 AU (280 million km) from Earth.[15] Entering March 2021, six asteroids each had a more notable cumulative Palermo Technical Impact Hazard Scale than Apophis, and none of those has a Torino level above 0.[16][lower-alpha 1] On average, an asteroid the size of Apophis (370 metres) is expected to impact Earth once in about 80,000 years.[17] Observations in 2020 by the Subaru telescope confirmed David Vokrouhlický's 2015 Yarkovsky effect predictions.[18] The Goldstone radar observed Apophis March 3–11, 2021, helping to refine the orbit again,[19] and on March 25, 2021, the Jet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years.[20][21] The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometers to now just a couple of kilometers,[22] greatly enhancing predictions of future approaches. Apophis was removed altogether from the Sentry Risk Table the next day.[23]

    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.[1] On December 21, 2004, Apophis passed 0.0964 AU (14.42 million km; 8.96 million mi) from Earth.[1] Precovery observations from March 15, 2004, were identified on December 27, and an improved orbit solution was computed.[25][26] Radar astrometry in January 2005 further refined its orbit solution.[27][28] The discovery was notable in that it was at a very low solar elongation (56°) and at very long range (1.1 AU). See diagram below:

    Position of 99942 Apophis relative to the Earth and Sun, compared to the positions of other NEOs at the time of their discovery[29]

    When first discovered, the object received the provisional designation 2004 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.[30] 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.[31]

    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.[30]

    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 (/əˈpɒfɪs/) as the name was pronounced in the TV series.

    Physical characteristics
    Comparison between the best-fit convex and nonconvex shape models, and some of the available radar images of (99942) Apophis
    Comparison of possible size of Apophis asteroid to Eiffel Tower and Empire State Building

    Based upon the observed brightness, Apophis's diameter was initially estimated at 450 metres (1,480 ft); a more refined estimate based on spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is 350 metres (1,150 ft). NASA's impact risk page lists the diameter at 330 metres (1,080 ft) and lists a mass of 4×1010 kg based on an assumed density of 2.6 g/cm3.[3] The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three.[3] Apophis's surface composition probably matches that of LL chondrites.[32]

    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 0.35±0.10. Its rotation axis has an obliquity of −59° against the ecliptic, which means that Apophis is a retrograde rotator.[2]

    During the 2029 approach, Apophis's brightness will peak at magnitude 3.1,[33] 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. 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. [34] 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.[2][32]

    Orbit

    Apophis has a low inclination orbit (3.3°) that varies from just outside the orbit of Venus (0.746 AU) to just outside the orbit of Earth (1.099 AU).[1] After the 2029 Earth approach the orbit will vary from just inside of Earth's to just inside of Mars.

    Position uncertainty and increasing divergence[1]
    Date JPL SBDB
    nominal geocentric
    distance (AU)    		 uncertainty
    region
    (3-sigma)
    2004-12-210.09638 AU (14.418 million km)n/a
    2013-01-090.09666 AU (14.460 million km)n/a
    2029-04-130.000254128 AU (38.0170 thousand km)±3.4 km[22]
    2036-03-270.309756 AU (46.3388 million km)±130 thousand km
    2051-04-200.041455 AU (6.2016 million km)±250 thousand km
    2066-09-160.069562 AU (10.4063 million km)±910 thousand km
    2116-04-120.019462 AU (2.9115 million km)±13 million km[35][lower-alpha 2]

    2029 close approach

    The closest known approach of Apophis occurs at April 13, 2029 21:46 UT, when Apophis will pass Earth closer than geosynchronous communication satellites, but will come no closer than 31,600 kilometres (19,600 mi) above Earth's surface.[36] Using the June 2021 orbit solution which includes the Yarkovsky effect, the 3-sigma uncertainty region in the 2029 approach distance is about ±3.4 km.[22][1] The distance, a hair's breadth in astronomical terms, is five times the radius of the Earth, ten times closer than the Moon, and closer than more than 500 geostationary satellites currently orbiting the earth.[37][38] It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude 3.1[33] (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations).[39] 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.[40] Perihelion will lift from 0.746 AU to 0.894 AU and aphelion will lift from 1.099 AU to 1.31 AU.[40]

    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)
    PHA Date Approach distance in lunar distances Abs. mag
    (H)    		 Diameter (C)
    (m)    		 Ref (D)
    Nominal(B) Minimum Maximum
    (152680) 1998 KJ91914-12-310.6060.6040.60819.4279–900data
    (458732) 2011 MD51918-09-170.9110.9090.91317.9556–1795data
    (163132) 2002 CU111925-08-300.9030.9010.90518.5443–477data
    69230 Hermes1937-10-301.9261.9261.92717.5700-900[41]data
    69230 Hermes1942-04-261.6511.6511.65117.5700-900[41]data
    2017 NM61959-07-121.891.8461.93418.8580–1300data
    (27002) 1998 DV91975-01-311.7621.7611.76218.1507–1637data
    2002 NY402002-08-181.3711.3711.37119.0335–1082data
    2004 XP142006-07-031.1251.1251.12519.3292–942data
    2015 TB1452015-10-311.2661.2661.26620.0620-690data
    (137108) 1999 AN102027-08-071.0141.0101.01917.9556–1793data
    (153814) 2001 WN52028-06-260.6470.6470.64718.2921–943data
    99942 Apophis2029-04-130.09810.09630.100019.7310–340data
    2017 MB12072-07-261.2161.2152.75918.8367–1186data
    2011 SM682072-10-171.8751.8651.88619.6254–820data
    (163132) 2002 CU112080-08-311.6551.6541.65618.5443–477data
    (416801) 1998 MZ2116-11-261.0681.0681.06919.2305–986data
    (153201) 2000 WO1072140-12-010.6340.6310.63719.3427–593data
    (276033) 2002 AJ1292172-02-081.7831.7751.79218.7385–1242data
    (290772) 2005 VC2198-05-051.9511.7912.13417.6638–2061data
    (A) This list includes near-Earth approaches of less than 2 lunar distances (LD) of objects with H 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 on H and albedo range between X and Y.
    (D) Reference: data source from the JPL SBDB, with AU converted into LD (1 AU≈390 LD)
    (E) Color codes:   unobserved at close approach   observed during close approach   upcoming approaches

    2036 approaches

    In 2036 Apophis will approach the Earth at a third the distance of the Sun in both March and December.[1] Using the 2021 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] 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.[42][lower-alpha 3]

    2051 approach

    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 has passed within 10 million km of Earth.[1]

    2066/2068

    In the 2060s Apophis will approach Earth in September 2066,[1][43] and then from February 2067 to December 2071 Apophis will remain further from Earth than the Sun is.[44] On April 12, 2068, JPL Horizons calculates that Apophis will be about 1.864 ± 0.003 AU (278.85 ± 0.45 million km) from Earth,[45][15] making the asteroid much further than the Sun.

    By 2116 the JPL Small-Body Database and NEODyS close approach data start to become divergent.[1][43] 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 could pass as far as 0.1 AU (15 million km; 39 LD).[1]

    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% (1 in 37) on December 27, 2004,[46][47] when the uncertainty region had shrunk to 83,000 km.[48] This probability, combined with its size, caused Apophis to be assessed at level 4 on the Torino scale[13] 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.[26] The danger of a 2036 passage was lowered to level 0 on the Torino scale in August 2006.[49] With a cumulative Palermo Scale rating of −3.22,[3] the risk of impact from Apophis is less than one thousandth the background hazard level.[3]

    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.[50] On January 31, 2011, astronomers took the first new images of Apophis in more than 3 years.[51]

    Illustration of a common trend where progressively reduced uncertainty regions result in an asteroid impact probability increasing followed by a sharp decrease

    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 [the next good opportunity], we should be able to predict the location of 2004 MN4 out to at least 2070."[52] 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.[9][43][53] Just after the closest approach on January 9, 2013,[43] the asteroid peaked at an apparent magnitude of about 15.6.[54] The Goldstone radar observed Apophis during that approach from January 3 through January 17.[55] The Arecibo Observatory observed Apophis once it entered Arecibo's declination window after February 13, 2013.[55] 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.[56] 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).[3]

    2015 observations

    As of January 2019, Apophis had not been observed since 2015, mostly because its orbit has 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.[57]

    2020–21 observations

    No observations of Apophis were made between January 2015 and February 2019, and then observations started occurring regularly in January 2020.[58] 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 caused Apophis to drift by about 170 meters per year.[59] In late 2020, Apophis approached the Earth again. It passed 0.11265 AU (16.852 million km; 43.84 LD) from Earth on March 6, 2021, brightening to +15 mag at the time. Radar observations of Apophis were planned at Goldstone in March 2021.[19] The asteroid has been observed by NEOWISE (between December 2020 and April 2021)[60][61] and by NEOSSat (in January 2021).[6][62][7] Apophis was the target of an observing campaign by IAWN, resulting in the collection of light curves, spectra, and astrometry.[6][62][7] The observations were used to practice and coordinate the response to an actual impact threat.[63]

    Hypothetical risk corridor for an impact on 13 April 2029, based on the 2020–21 planetary defense exercise

    On February 21, 2021, Apophis was removed from the Sentry Risk Table, as an impact in the next 100 years was finally ruled out.[23]

    Several occultations of bright stars (apparent magnitude 8-11) by Apophis occurred in March and April 2021.[64][65][66][67] 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 Phaeton by more than a factor of ten).[65] The first event, on March 7, was successfully observed from the United States by multiple observers.[68][69][64] The next potential occultation, which occurred on March 11, was predicted to be visible from central Europe.[66] This event was missed, mainly due to bad weather (two negative observations were recorded from Greece).[65] On March 22, another occultation was observed only by a single observer from the United States, amateur astronomer Roger Venable. Due to larger-than-expected residuals in the March 7 data, the majority of observers had been deployed outside of the actual path for the March 22 occultation.[64] 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.[67][64] Two more occultations, observable on April 10 and April 11 from Japan and New Mexico, respectively, were seen by several observers each.[64]

    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,[70] Apophis became the asteroid with the most precisely measured Yarkovsky effect of all asteroids, at a signal-to-noise ratio (SNR) of 186.4,[71][lower-alpha 4] surpassing 101955 Bennu (SNR=181.6).[72]

    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)

    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.[13] 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.
    2004-12-25The 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.
    2004-12-26Based 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.
    2004-12-27Based on a total of 176 observations with an observation arc of 190 days, the impact probability was raised to 1 in 37 (2.7%)[47] with a line of variation (LOV) of only 83,000 km;[48] diameter was increased to 390 m, and mass to 7.9×1010 kg.
    Later that afternoon, a precovery increased the span of observations to 287 days, which eliminated the 2029 impact threat.[26] The cumulative impact probability was estimated to be around 0.004%, a risk lower than that of asteroid 2004 VD17, 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-Mar-15.1104 to 2004-Dec-27.8243). (As of February 2013 the 2053 risk is only 1 in 20 billion.)[3]
    19:18 GMTThis 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 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 (2004-Mar-15.1104 to 2004-Dec-29.44434). The most dangerous pass was rated at 1 in 7,143,000.
    22:34 GMT157 observations spanning 289.33 days (2004-Mar-15.1104 to 2004-Dec-29.44434). One pass at 1 (Torino scale) 3 other passes.
    2005-01-0203:57 GMTObservations spanning 290.97 days (2004-Mar-15.1104 to 2004-Dec-31.07992) One pass at 1 (Torino scale) 19 other passes.
    2005-01-0314:49 GMTObservations spanning 292.72 days (2004-Mar-15.1104 to 2005-Jan-01.82787) One pass at 1 (Torino scale) 15 other passes.
    2005–01Extremely precise radar observations at Arecibo Observatory[27] refine the orbit further and show that the April 2029 close approach will occur at only 5.7 Earth radii,[28] approximately one-half the distance previously estimated.
    2005-02-06Apophis (2004 MN4) had a 1-in-13,000 chance of impacting in April 2036.[73]
    2005-08-07Radar observation[27] 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).[74]
    2005–10It is predicted that Apophis will pass just below the altitude of geosynchronous satellites, which are at approximately 35,900 kilometres (22,300 mi).[75] Such a close approach by an asteroid of that size is estimated to occur every 800 years or so.[76]
    2006-05-06Radar observation at Arecibo Observatory[27] slightly lowered the Palermo scale rating, but the pass in 2036 remained at Torino scale 1[77] 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.[49] (The impact probability was 1 in 45,000.)[49]
    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.[78] Nearly all international press reported the news with false data caused by the review from Bild even though Marquardt denied.[79] This estimate was allegedly confirmed by ESA and NASA[80][78] but in an official statement,[81] 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.[81]
    2009-04-29An animation is released[82] 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.[83]
    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.[76] Additional factors that can greatly influence the predicted motion in ways that depend on unknown details, are the spin of the asteroid,[84] its precise mass, the way it reflects and absorbs sunlight, radiates heat, and the gravitational pull of other asteroids passing nearby.[76] 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.[76]
    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.[85] First appearance of Sentry virtual impactors that also include mid-October dates.[86]
    2013-01-09The European Space Agency (ESA) 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.[9] 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.[9] Goldstone single-pixel observations of Apophis have ruled out the potential 2036 Earth impact.[14][87][88] 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).[87] The radar astrometry is more precise than was expected.[87]
    2016-03-25The Sentry 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.[3]
    2020-03By taking observations of Apophis with the Subaru Telescope in January and March 2020, as well as remeasuring older observations using the new Gaia DR2 star catalog, astronomers positively detect the Yarkovsky effect on Apophis. The asteroid's position is found to shift by 170 meters per year. The Yarkovsky effect is the main source of uncertainty in impact probability estimates for this asteroid.[59]
    2021-01-20The Sentry Risk Table assessed Apophis as having a 2.6-in-a-million (1-in-380,000) chance of impacting Earth in 2068, and a 4.5-in-a-million (1-in-220,000) cumulative chance of impacting Earth by 2107.[3] The previous 2068 odds were 1-in-150,000.
    2021-03-1510:44JPL solution #207 using observations in 2020 and 2021 reduced the 3-sigma uncertainty region in the 2029 approach distance from ±700 km[89] to about ±2 km.[1]
    2021-03-2615:06Apophis was removed from the Sentry Risk Table and has no chance of impacting Earth in the next 100 years.[23] Apophis was the 2,656th object to be removed from the Sentry Risk Table.

    Possible impact effects

    The Sentry Risk Table estimates that Apophis would impact Earth with kinetic energy equivalent to 1,200 megatons of TNT. In comparison, the Chicxulub impact which caused the mass extinction event responsible for wiping out the dinosaurs has been estimated to have released about as much energy as 100,000,000 megatons (100 teratons). 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 370-metre-wide (1,210 ft) stony asteroid with a density of 3,000 kg/m3, if it were to impact into sedimentary rock, Apophis would create a 5.1-kilometre (17,000 ft) impact crater.[17][3]

    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.[90] 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.[91] 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.[92] 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 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.[93]

    Exploration

    OSIRIS-APEX post-Earth-encounter rendezvous

    The OSIRIS-REx spacecraft is expected to return a sample of Bennu to Earth in 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.[94][95] For its Apophis mission after the sample return, OSIRIS-REx will be renamed OSIRIS-APEX (short for OSIRIS-Apophis Explorer).[96]

    Other proposed space missions

    Planetary Society competition

    In 2007, The Planetary Society, a California-based space advocacy group, organized 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 Enterprises, Inc.[97] 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 two kilometres (1.2 miles). 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 (a multi-spectral imager, near-infrared spectrometer, laser rangefinder, and magnetometer) 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.

    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.[98]

    Cancelled 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 ZhengHe, 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.[99] The launch date is now scheduled for 2024, with a different set of targets.

    Proposed deflection strategies

    Studies by NASA, ESA,[100] and various research groups in addition to the Planetary Society contest teams,[101] 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.[102]

    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.[103]

    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.[104]

    In October 2022, a method of mapping the insides of a potentially problematic asteroid, such as 99942 Apophis, in order to determine the best area for impact was proposed.[105]

    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.[106]
    The Enter Shikari song, 'Zzzonked' makes reference to the asteroid in the second verse.

    The Type O Negative song "The Profit Of Doom" references the asteroid.

    See also
    • Asteroid capture
    • Asteroid impact prediction
    • Asteroid Redirect Mission
    • Earth-grazing fireball
    • List of impact craters on Earth
    • NEO Surveyor
    • Apophis

    Notes
    1. Of the six asteroids with a riskier palermo scale rating than Apophis:
      • (29075) 1950 DA and 101955 Bennu can not impact within the next 100 years.
      • 1979 XB and 2007 FT3 are lost short-arc objects that can not be recovered with targeted observations and await serendipitous survey observations.
      • 2000 SG344 and 2009 JF1 are less than 50 meters in diameter.
    2. The minimum possible Earth approach between 5–20 April 2116 is 0.001 AU (150 thousand km; 93 thousand mi; 0.39 LD).
    3. On January 8, 2022 Venus was even closer to Earth at 0.2658 AU (39.76 million km; 24.71 million mi; 103.4 LD).
    4. Using the 9 March 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 (29 June 2021), the SNR is (again lower than Bennu's).

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