Model of 99942 Apophis's shape, assuming the entire surface is of a similar composition.
Roy A. Tucker|
David J. Tholen
|Discovery site||Kitt Peak|
|Discovery date||19 June 2004|
|MPC designation||(99942) Apophis|
|Epoch 7 December 2007 (JD 2454441.5)|
|Uncertainty parameter 0|
|Observation arc||3635 days (9.95 yr)|
|Aphelion||1.09851 AU (164.335 Gm)|
|Perihelion||0.74605 AU (111.607 Gm)|
|0.92228 AU (137.971 Gm)|
|0.89 yr (323.5 d)|
Average orbital speed
|Earth MOID||0.000659446 AU (98,651.7 km; 61,299.3 mi)|
|Jupiter MOID||4.12656 AU (617.325 Gm)|
|Dimensions||0.370 km (0.230 mi)|
|0.185 km (0.115 mi)|
|Mass||6.1×1010 kg (assumed)|
Equatorial surface gravity
|27 m/s2 (average) 0.000|
Equatorial escape velocity
30.4 h (1.27 d)|
period of harmonic with strongest lightcurve amplitude:
|19.7 ± 0.4|
99942 Apophis (//, previously known by its provisional designation 2004 MN4) is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a probability of up to 2.7% that it would hit Earth on April 13, 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029. However, until 2006, a possibility remained that during the 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole, a small region no more than about 0.5 mile wide, or 0.8 km that would set up a future impact exactly seven years later on April 13, 2036. This possibility kept it at Level 1 on the Torino impact hazard scale until August 2006, when the probability that Apophis would pass through the keyhole was determined to be very small. By 2008, the keyhole had been determined to be less than 1 km wide. During the short time when it had been of greatest concern, Apophis set the record for highest rating on the Torino scale, reaching level 4 on 27 December 2004.
The diameter of Apophis is, as of the most recent 2014 observations, approximately 370 metres (1,210 ft). Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036. By May 6, 2013 (April 15, 2013 observation arc), the probability of an impact on April 13, 2036 had been eliminated. Using observations through February 26, 2014, the odds of an impact on April 12, 2068, as calculated by the JPL Sentry risk table are 1 in 150,000. As of March 2018[update], there were seven asteroids with a more notable cumulative Palermo Technical Impact Hazard Scale than Apophis. On average, one asteroid the size of Apophis (370 metres) can be expected to impact Earth about every 80,000 years.
- 1 Discovery and naming
- 2 Physical characteristics
- 3 Orbit
- 4 Possible impact effects
- 5 Potential space missions
- 6 Popular culture
- 7 References
- 8 External links
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 0.0963 AU (14,410,000 km; 8,950,000 mi) 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.
When first discovered, the object received the provisional designation 2004 MN4, and news and scientific articles about it referred to it by that name. When its orbit was sufficiently well calculated, it received the permanent number 99942 (on June 24, 2005). Receiving a permanent number made it eligible for naming by its discoverers, and it received the name "Apophis" on July 19, 2005. Apophis is the Greek name of an enemy of the Ancient Egyptian sun-god Ra: Apep, 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. David J. Tholen and Tucker—two of the co-discoverers of the asteroid—are reportedly fans of the TV 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.
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. The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three.
During the 2029 approach, Apophis's brightness will peak at magnitude 3.4, with a maximum angular speed of 42° per hour. The maximum apparent angular diameter will be ~2 arcseconds, so that it will be barely resolved by ground-based telescopes not equipped with adaptive optics.
After the Minor Planet Center confirmed the June discovery of Apophis, an April 13, 2029 close approach was flagged by NASA's automatic Sentry system and NEODyS, a similar automatic program run by the University of Pisa and the University of Valladolid. On that date, it will become as bright as magnitude 3.4 (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. During the close approach in 2029 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.
After Sentry and NEODyS announced the possible impact, additional observations decreased the uncertainty in Apophis's trajectory. As they did, the probability of an impact event in 2029 temporarily climbed, peaking at 2.7% (1 in 37) on 27 December 2004. This probability, combined with its size, caused Apophis to be assessed at level 4 on the Torino Scale and 1.10 on the Palermo Technical Impact Hazard Scale, scales scientists use to represent how dangerous a given asteroid is to Earth. These are the highest values for which any object has been rated on either scale. The chance that there would be an impact in 2029 was eliminated by late December 27, 2004 as a result of a precovery image that extended the observation arc back to March 2004. The danger of a 2036 passage was lowered to level 0 on the Torino Scale in August 2006. With a cumulative Palermo Scale rating of −3.2, the risk of impact from Apophis is less than one thousandth the background hazard level.
|PHA||Date||Approach distance in lunar distances||Abs. mag
|(152680) 1998 KJ9||1914-12-31||0.606||0.604||0.608||19.4||279–900||data|
|(458732) 2011 MD5||1918-09-17||0.911||0.909||0.913||17.9||556–1795||data|
|(163132) 2002 CU11||1925-08-30||0.903||0.901||0.905||18.5||443–477||data|
|(27002) 1998 DV9||1975-01-31||1.762||1.761||1.762||18.1||507–1637||data|
|(137108) 1999 AN10||2027-08-07||1.014||1.010||1.019||17.9||556–1793||data|
|(153814) 2001 WN5||2028-06-26||0.647||0.647||0.647||18.2||921–943||data|
|(163132) 2002 CU11||2080-08-31||1.655||1.654||1.656||18.5||443–477||data|
|(416801) 1998 MZ||2116-11-26||1.068||1.068||1.069||19.2||305–986||data|
|(153201) 2000 WO107||2140-12-01||0.634||0.631||0.637||19.3||427–593||data|
|(276033) 2002 AJ129||2172-02-08||1.783||1.775||1.792||18.7||385–1242||data|
|(290772) 2005 VC||2198-05-05||1.951||1.791||2.134||17.6||638–2061||data|
|(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
On April 13, 2029, Apophis will pass Earth within the orbits of geosynchronous communication satellites, but will come no closer than 19,400 miles (31,200 km) above Earth's surface. The 2029 pass will be much closer than had first been predicted. The pass in late March 2036 will be no closer than about 23 million kilometres (14×106 mi)—and will most likely miss Earth by about 56 million kilometres (35×106 mi).
2005 and 2011 observations
In July 2005, former Apollo astronaut Rusty Schweickart, as chairman of the B612 Foundation, formally asked NASA to investigate the possibility that the asteroid's post-2029 orbit could be in orbital resonance with Earth, which would increase the probability of future impacts. Schweickart also asked NASA to investigate whether a transponder should be placed on the asteroid to enable more accurate tracking of how its orbit is affected by the Yarkovsky effect. On January 31, 2011, astronomers took the first new images of Apophis in more than 3 years.
The close approach in 2029 will substantially alter the object's orbit, prompting Jon Giorgini of JPL to say: "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." Apophis passed within 0.0966 AU (14,450,000 km; 8,980,000 mi) of Earth in 2013, allowing astronomers to refine the trajectory for future close passes. Just after the closest approach on 9 January 2013, the asteroid peaked at an apparent magnitude of about 15.7. Goldstone observed Apophis during that approach from January 3 through January 17. The Arecibo Observatory observed Apophis once it entered Arecibo's declination window after February 13, 2013.
A NASA assessment as of 21 February 2013 that does not use the 2013 radar measurements gave an impact probability of 2.3 in a million for 2068. As of 6 May 2013, using observations through April 15, 2013, the odds of an impact on 12 April 2068 as calculated by the JPL Sentry risk table had increased to 3.9 in a million (1 in 256,000).
As of April 2018, Apophis has not been observed since 2015, mostly because its orbit has put it very near the Sun from the perspective of Earth. It has not been further than 60 degrees from the Sun at any point since April 2014, and will remain so until December 2019. With the most recent 2015 observations, the 12 April 2068 impact is now 6.7 in a million (1 in 150,000), and the asteroid has a cumulative 9 in a million (1 in 110,000) chance of impacting Earth before 2106.
History of impact estimates
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|2004-12-23||The original NASA report mentioned impact chances of "around 1 in 300" in 2029, which was widely reported in the media. The actual NASA estimates at the time were 1 in 233; these resulted in a Torino scale rating of 2, the first time any asteroid had received a rating above 1.|
|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 an observation arc of 190 days, the impact probability was raised to 1 in 37 (2.7%); 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. 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.|
|2004-12-28||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). (As of February 2013[update] the 2053 risk is only 1 in 20 billion.)|
|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 have changed and been 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 precise radar observations at Arecibo Observatory refine the orbit further and show that the April 2029 close approach will occur at only 5.7 Earth radii, approximately one-half the distance previously estimated.|
|2005-02-06||Apophis (2004 MN4) had a 1 in 13,000 chance of impacting in April 2036.|
|2005-08-07||Radar observation 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).|
|2005-10||It is predicted that the asteroid will pass just below the altitude of geosynchronous satellites, which are at approximately 35,900 kilometres (22,300 mi). Such a close approach by an asteroid is estimated to occur every 800 years or so.|
|2006-05-06||Radar observation at Arecibo Observatory slightly lowered the Palermo scale rating, but the pass in 2036 remained at Torino Scale 1 despite the impact probability dropping by a factor of four.|
|2006-08-05||Additional observations through 2006 resulted in Apophis being lowered to Torino Scale 0. (The impact probability was 1 in 45,000.)|
|2008-04||Nico Marquardt published a research paper in which he calculated the probability of Apophis to collide with a geosynchronous satellite during its flyby on April 13, 2029 and the consequences of this event to the likelihood of an Earth-collision 2036. Afterwards, the German newspaper Bild published an article stating a 100-times higher probability of an Earth-collision in the year 2036 than Marquardt calculated. Nearly all international press reported the news with false data caused by the review from Bild even though Marquardt denied. This estimate was allegedly confirmed by ESA and NASA but in an official statement, NASA denied the wrong statement. The release went on to explain that since the angle of Apophis's approach to the Earth's equator means the asteroid will not travel through the belt of current equatorial geosynchronous satellites, there is currently no risk of collision; and the effect on Apophis' orbit of any such impact would be insignificant.|
|2008-04-16||NASA News Release 08-103 reaffirmed that its estimation of a 1 in 45,000 chance of impact in 2036 remained valid.|
|2009-04-29||An animation is released that shows how unmeasured physical parameters of Apophis bias the entire statistical uncertainty region. If Apophis is a retrograde rotator on the small, less-massive end of what is possible, the measurement uncertainty region will get pushed back such that the center of the distribution encounters Earth's orbit. This would result in an impact probability much higher than computed with the Standard Dynamical Model. Conversely, if Apophis is a small, less-massive prograde rotator, the uncertainty region is advanced along the orbit. Only the remote tails of the probability distribution could encounter Earth, producing a negligible impact probability.|
|2009-10-07||Refinements to the precovery images of Apophis by the University of Hawaii's Institute for Astronomy, the 90-inch Bok Telescope, and the Arecibo Observatory have generated a refined path that reduces the odds of an April 13, 2036 impact to about 1 in 250,000.|
|Criticism of older published impact probabilities rests on the fact that important physical parameters such as mass and spin that affect its precise trajectory have not yet been accurately measured and hence there are no associated probability distributions. The Standard Dynamical Model used for making predictions simplifies calculations by assuming Earth is a point mass; this can introduce up to 2.9 Earth radii of prediction error for the 2036 approach, and Earth's oblateness must be considered for the 2029 passage to predict a potential impact reliably. Additional factors that can greatly influence the predicted motion in ways that depend on unknown details, are the spin of the asteroid, its precise mass, the way it reflects and absorbs sunlight, radiates heat, and the gravitational pull of other asteroids passing nearby. Small uncertainties in the masses and positions of the planets and Sun can cause up to 23 Earth radii of prediction error for Apophis by 2036.|
|2013-01||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.|
|2013-01-09||The 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. The Herschel Space Observatory observations increased the diameter estimate by 20% from 270 to 325 metres, which translates into a 75% increase in the estimates of the asteroid's volume or mass. Goldstone single-pixel observations of Apophis have ruled out the potential 2036 Earth impact. Apophis will then come no closer than about 14 million miles—and more likely miss us by something closer to 35 million miles. The radar astrometry is more precise than was expected.|
|2016-03-25||The 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 at any point between 2060 and 2105.|
Possible impact effects
This section needs to be updated.(September 2013)
The Sentry Risk Table estimates that Apophis would make atmospheric entry with 750 megatons of kinetic energy. The impacts that created Meteor Crater or the Tunguska event are estimated to be in the 3–10 megaton range. The biggest hydrogen bomb ever exploded, the Tsar Bomba, was around 57 megatons while the 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons. In comparison, the Chicxulub impact has been estimated to have released about as much energy as 100,000,000 megatons (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 325-metre-wide (1,066 ft) stony asteroid, if it were to impact into sedimentary rock, Apophis would create a 4.3-kilometre (14,000 ft) impact crater.
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. However, the exact location of the impact would be known weeks or even months in advance, allowing any nearby inhabited areas to be completely evacuated and significantly decreasing the potential loss of life and property. A deep-water impact in the Atlantic or Pacific oceans would produce an incoherent short-range tsunami with a potential destructive radius (inundation height of >2 m) of roughly 1,000 kilometres (620 mi) for most of North America, Brazil and Africa, 3,000 kilometres (1,900 mi) for Japan and 4,500 kilometres (2,800 mi) for some areas in Hawaii.
Potential space missions
Planetary Society competition
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In 2007, The Planetary Society, a California-based space advocacy group, organized a $50,000 competition to design an unmanned space probe that would 'shadow' Apophis for almost a year, taking measurements that would "determine whether it will impact Earth, thus helping governments decide whether to mount a deflection mission to alter its orbit". The society received 37 entries from 20 countries on 6 continents.
The commercial competition was won by a design called 'Foresight' created by SpaceWorks Enterprises, Inc. SpaceWorks proposed a simple orbiter with only two instruments and a radio beacon at a cost of ~140 million USD, 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 Università di Pisa, Italy. Juan L. Cano was principal investigator.
Another European team took home $5,000 for third place. Their team lead was EADS Astrium Ltd, United Kingdom, in conjunction with EADS Astrium SAS, France; IASF-Roma, INAF, Rome, Italy; Open University, UK; Rheinisches Institut für Umweltforschung, Germany; Royal Observatory of Belgium; and Telespazio, Italy. The principal investigator was Paolo D'Arrigo.
Two teams tied for second place in the Student Category: Monash University, Clayton Campus, Australia, with Dilani Kahawala as principal investigator; and University of Michigan, with Jeremy Hollander as principal investigator. Each second place team won $2,000. A team from Hong Kong Polytechnic University and Hong Kong University of Science and Technology, under the leadership of Peter Weiss, received an honorable mention and $1,000 for the most innovative student proposal.
Planned Chinese mission
China plans an exploration fly-by mission to Apophis after 2020 when the asteroid comes to within a distance of 30,000 kilometers of Earth. The distance, a hair's breadth in astronomical terms, is within the orbit of the moon, and even closer than some man-made satellites. It will be the closest asteroid of its size in recorded history. This fly by mission to Apophis is part of an asteroid exploration mission planned after China's Mars mission in 2020 currently in development, according to Ji Jianghui, a researcher at the Purple Mountain Observatory of the Chinese Academy of Sciences and a member of the expert committee for scientific goal argumentation of deep space exploration in China. The whole mission will include exploration and close study of three asteroids by sending a probe to fly side by side with Apophis for a period to conduct close observation, and land on the asteroid 1996 FG3 to conduct in situ sampling analysis on the surface. The probe is also expected to conduct a fly-by of a third asteroid to be determined at a later time. The whole mission would last around six years, said Ji.
Don Quijote mission
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 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 Id Software's video game Rage, the back-story involves asteroid Apophis colliding with Earth, nearly wiping out humanity and ushering in a post-apocalyptic age.
- Type O Negative mentions this asteroid in the song Profit of Doom, from its album Dead Again.
- Enter Shikari mention the asteroid in the song Zzzonked, from their album Common Dreads.
- "JPL Small-Body Database Browser: 99942 Apophis (2004 MN4)" (last observation: 2014-02-26; arc: 9.95 years). Retrieved 12 April 2016.
- "99942 Apophis (2004 MN4) Earth Impact Risk Summary". NASA. 2016-03-25. Archived from the original on 2013-05-11. Retrieved 2018-03-20.
- Binzel, Richard P. (2007). "Can NEAs be Grouped by Their Common Physical Characteristics?" (PDF). Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology. aero.org. Archived from the original (PDF) on 2012-04-12.
- assuming radius of 0.135 km and mass of 2.1e10 kg yields an escape velocity of 0.14 m/s or 0.52 km/h.
- "99942 Apophis". The Near-Earth Asteroids Data Base at E.A.R.N. Retrieved 2009-10-15.
- Pravec, P.; Scheirich, P.; Ďurech, J.; Pollock, J.; Kušnirák, P.; Hornoch, K.; Galád, A.; Vokrouhlický, D.; Harris, A.W.; Jehin, E.; Manfroid, J.; Opitom, C.; Gillon, M.; Colas, F.; Oey, J.; Vraštil, J.; Reichart, D.; Ivarsen, K.; Haislip, J.; LaCluyze, A. (2014). "The tumbling spin state of (99942) Apophis" (PDF). Icarus. 233: 48–60. Bibcode:2014Icar..233...48P. doi:10.1016/j.icarus.2014.01.026.
- ESA (January 9, 2013). "Herschel intercepts asteroid Apophis". European Space Agency (ESA). Retrieved 2013-01-09.
- David Noland (November 7, 2006). "5 Plans to Head Off the Apophis Killer Asteroid". Popular Mechanics. Retrieved 2011-04-05.
- deGrasse Tyson, Neil. "Neil deGrasse Tyson - Death By Giant Meteor". youtube.com. Retrieved 9 August 2016.
- Don Yeomans; Steve Chesley & Paul Chodas (December 23, 2004). "Near-Earth Asteroid 2004 MN4 Reaches Highest Score To Date On Hazard Scale". NASA's Near Earth Object Program Office. Archived from the original on 18 August 2007. Retrieved 2007-08-16.
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.
- "NASA Rules Out Earth Impact in 2036 for Asteroid Apophis". NASA. January 10, 2013. Retrieved 2013-01-10.
- "Sentry: Earth Impact Monitoring". NASA/JPL Center for NEO Studies. Retrieved 2018-03-20. (Use Unconstrained Settings)
- Robert Marcus; H. Jay Melosh & Gareth Collins (2010). "Earth Impact Effects Program". Imperial College London / Purdue University. Retrieved 2013-02-07. (solution using 330 metres, 2600 kg/m3, 12.6 km/s, 45 degrees)
- "MPEC 2004-Y70 : 2004 MN4". IAU Minor Planet Center. 2004-12-27.
- Don Yeomans; Paul Chodas & Steve Chesley (December 27, 2004). "Possibility of an Earth Impact in 2029 Ruled Out for Asteroid 2004 MN4". NASA's Near Earth Object Program Office. Retrieved 2013-01-18.
- "Scheduled Arecibo Radar Asteroid Observations". National Astronomy and Ionosphere Center.
- Paul Chodas; Steve Chesley; Jon Giorgini & Don Yeomans (February 3, 2005). "Radar Observations Refine the Future Motion of Asteroid 2004 MN4". NASA's Near Earth Object Program Office. Retrieved 2013-01-18.
- Bill Cooke (August 18, 2005). "Asteroid Apophis set for a makeover". Astronomy Magazine. Retrieved 2009-10-08.
- "(99942) Apophis Ephemerides for 13 Apr 2029". NEODyS (Near Earth Objects – Dynamic Site). Retrieved 2011-05-05.
- The astronomical magnitude scale. International Comet Quarterly
- Dwayne Brown (Oct 7, 2009). "NASA Refines Asteroid Apophis' Path Toward Earth". NASA's Near Earth Object Program Office. Retrieved 2013-01-18.
- Daniel Fischer (Dec 27, 2004). "2004 MN4 Earth Impact Risk Summary (computed on Dec 27, 2004)". The Cosmic Mirror. Archived from the original on 14 March 2005. Retrieved 2011-11-04.
- "WayBack Machine archive from 5 Aug 2006". Wayback Machine. 2006-08-05. Archived from the original on August 5, 2006. Retrieved 2013-01-13.
- Kelly Beatty (January 9, 2013). "Asteroid Apophis Takes a Pass in 2036". Sky & Telescope. Retrieved 2014-11-10.
- "99942 Apophis Ephemerides for March/April 2036". NEODyS (Near Earth Objects – Dynamic Site). Retrieved 2017-12-09.
- David Morrison (July 22, 2005). "Schweickart Proposes Study of Impact Risk from Apophis (MN4)". NASA. Archived from the original on 2009-09-29. Retrieved 2009-10-08.
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|Wikimedia Commons has media related to 99942 Apophis.|
- Thermal Infrared Observations of Asteroid (99942) Apophis with Herschel (Müller : 23 Apr 2014 : arXiv:1404.5847)
- Apophis Asteroid
- Asteroid Apophis orbit from recent observations, EPSC Abstracts Vol. 6, EPSC-DPS2011-1212, 2011, EPSC-DPS Joint Meeting 2011
- Diagrams and orbits of Apophis (Sormano Astronomical Observatory)
- Apophis Orbital Prediction Page at NASA JPL
- 2004 MN4 page and 2004 MN4 impactor table from NEODyS.
- MBPL – Minor Body Priority List (technical List) at Sormano Observatory
- TECA – Table of Asteroids Next Closest Approaches to the Earth at Sormano Observatory
- Possibility of an Earth Impact in 2029 Ruled Out for Asteroid 2004 MN4 (JPL)
- Radar Observations Refine the Future Motion of Asteroid 2004 MN4 (JPL)
- Animation explaining how impact risk is determined from Impact Probability
- 99942 Apophis at the JPL Small-Body Database
(153814) 2001 WN5
| Large NEO Earth close approach
(inside the orbit of the moon)
13 April 2029