99942 Apophis

99942 Apophis
Discovery[1]
Discovered by Roy A. Tucker
David J. Tholen
Fabrizio Bernardi
Discovery site Kitt Peak[1]
Discovery date 19 June 2004
Designations
Named after
 Apep
2004 MN4
Aten
NEO, PHA[1]
Orbital characteristics[1]
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)
Eccentricity 0.19108
0.89 yr (323.5 d)
30.728 km/s
215.53998°
1.11278°/day
Inclination 3.33129°
204.45719°
126.39364°
Earth MOID 0.000659446 AU (98,651.7 km)
Jupiter MOID 4.12656 AU (617.325 Gm)
Jupiter Tisserand parameter 6.467
Physical characteristics
Dimensions 325±15 m[2]
Mean radius
 0.1625 ± 0.0075 km
Mass 4×1010 kg (assumed)[3]
Mean density
 ~3.2 g/cm3[4]
Equatorial surface gravity
 0.00027 m/s2 (average)
Equatorial escape velocity
 ~0.52 km/h[5]
30.4 h (1.27 d)[1][6]
tumbling:[7]
precession period:
27.38±0.07 h[7]
rotation period:
263±6 h[7]
period of harmonic with strongest lightcurve amplitude:
30.56±0.01 h[7]
0.23[2]
Temperature 270 K
Sq [6]
19.7 ± 0.4[1][6]

    99942 Apophis (/əˈpɒfɪs/, 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, 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 800 m (~0.5 mi) wide,[8] 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. 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.[9]

    The diameter of Apophis is, as of the most recent 2013 observations, approximately 325 metres (1,066 ft).[2] Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036.[10] By May 6, 2013 (April 15, 2013 observation arc), the probability of an impact on April 13, 2036 had been eliminated.[3] As of October 8, 2014, using observations through February 26, 2014, the odds of an impact on April 12, 2068, as calculated by the JPL Sentry risk table is (1 in 149,000).[3] Of objects not recently observed, there are about ten asteroids with a more notable Palermo Technical Impact Hazard Scale than Apophis.[11] On average, an asteroid the size of Apophis (325 meters) can be expected to impact Earth about every 80,000 years.[12]

    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 0.0963 AU (14,410,000 km; 8,950,000 mi) from Earth.[1] Precovery observations from March 15, 2004, were identified on December 27, and an improved orbit was computed.[13][14] Radar astrometry in January 2005 further refined the orbit.[15][16]

    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, and it received the name "Apophis" on July 19, 2005.[17] 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. 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.[17]

    Physical characteristics

    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]

    During the 2029 approach, Apophis's brightness will peak at magnitude 3.4,[18] 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.

    Orbit

    Close approaches

    Close approach of Apophis on April 13, 2029 (as known in February 2005)[16]
    The white bar indicates uncertainty in the range of positions (as known in February 2005)[16]

    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[18] (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations).[19] 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 temporarily climbed, peaking at 2.7% (1 in 37).[20] This probability, combined with its size, caused Apophis to be assessed at level 4 on the Torino Scale[9] 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 December 27, 2004.[14] The danger of a 2036 passage was lowered to level 0 on the Torino Scale in August 2006.[21] With a cumulative Palermo Scale rating of −3.2,[3] the risk of impact from Apophis is less than one thousandth the background hazard level.[3]

    On April 13, 2029, Apophis will pass Earth within the orbits of geosynchronous communication satellites, but will come no closer than 19,400 miles (31,300 kilometers) above Earth's surface.[10] 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 (14×10^6 mi)—and will most likely miss Earth by something closer to 56 million kilometres (35×10^6 mi).[22]

    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.[23] On January 31, 2011, astronomers took the first new images of Apophis in more than 3 years.[24]

    2013 refinement

    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."[25] 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.[2][26][27] Just after the closest approach on 9 January 2013,[26] the asteroid peaked at an apparent magnitude  of about 15.7.[28] Goldstone observed Apophis during that approach from January 3 through January 17.[29] The Arecibo Observatory observed Apophis once it entered Arecibo's declination window after February 13, 2013.[29]

    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.[30] 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).[3]

    History of impact estimates

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

    Possible impact effects

    The Sentry Risk Table estimates that Apophis would make atmospheric entry with 750 megatons of kinetic energy.[3] The impacts that created Meteor Crater or the Tunguska event are estimated to be in the 3–10 megaton range.[46] The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons and the biggest hydrogen bomb ever exploded, the Tsar Bomba, was around 57 megatons. In comparison, the Chicxulub impact has been estimated to have released about as much energy as 100,000,000 megatons (100 teratons).

    The B612 foundation produced a paper for the 2007 Planetary Defense Conference with a calculated path of risk for 2036 before it was ruled out in 2013.

    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 kilometers, but would be unlikely to have long-lasting global effects, such as the initiation of an impact winter. Assuming Apophis is a 325-meter wide stony asteroid, if it were to impact into sedimentary rock, Apophis would create a 4.3 kilometers (2.7 mi) impact crater.[12]

    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.[47] The result was a narrow corridor a few kilometers 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.[48] 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.[49] 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 1000 km for the most of North America, Brazil and Africa, 3000 km for Japan and 4500 km for some areas in Hawaii.[50]

    Potential space missions

    Planetary Society competition

    In 2007, The Planetary Society, a California-based space advocacy group, organized a $50,000 competition to design an unmanned space probe that would 'shadow' Apophis for almost a year, taking measurements that would "determine whether it will impact Earth, thus helping governments decide whether to mount a deflection mission to alter its orbit". The society received 37 entries from 20 countries on 6 continents.

    The commercial competition was won by a design called 'Foresight' created by SpaceWorks Enterprises, Inc.[51] 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 kilometers (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.

    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.[52]

    Proposed deflection strategies

    Further information: Asteroid-impact avoidance

    Studies by NASA, ESA,[53] and various research groups in addition to the Planetary Society contest teams,[54] 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.[55]

    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.[56]

    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 meters in size, it was also stated that 99942 Apophis would be an object subject to tests by the program.[57]

    Popular culture

    References

    1. 1 2 3 4 5 6 7 8 "JPL Small-Body Database Browser: 99942 Apophis (2004 MN4)" (last observation: 2014-02-26; arc: 9.95 years). Retrieved 12 April 2016.
    2. 1 2 3 4 5 6 ESA (January 9, 2013). "Herschel intercepts asteroid Apophis". European Space Agency (ESA). Retrieved 2013-01-09.
    3. 1 2 3 4 5 6 7 8 9 10 "99942 Apophis (2004 MN4) Earth Impact Risk Summary". NASA. Archived from the original on 2013-05-11. Retrieved 2015-03-03.
    4. ↑ 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.
    5. ↑ 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.
    6. 1 2 3 "99942 Apophis". The Near-Earth Asteroids Data Base at E.A.R.N. Retrieved 2009-10-15.
    7. 1 2 3 4 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.
    8. ↑ David Noland (November 7, 2006). "5 Plans to Head Off the Apophis Killer Asteroid". Popular Mechanics. Retrieved 2011-04-05.
    9. 1 2 3 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.
    10. 1 2 3 "NASA Rules Out Earth Impact in 2036 for Asteroid Apophis". NASA. January 10, 2013. Retrieved 2013-01-10.
    11. ↑ "Sentry Risk Table". NASA/JPL Near-Earth Object Program Office. 14 Oct 2011. Retrieved 2013-01-13.
    12. 1 2 Robert Marcus; H. Jay Melosh & Gareth Collins (2010). "Earth Impact Effects Program". Imperial College London / Purdue University. Retrieved 2013-02-07. (solution using 330 meters, 2600 kg/m3, 12.6 km/s, 45 degrees)
    13. ↑ "MPEC 2004-Y70 : 2004 MN4". IAU Minor Planet Center. 2004-12-27.
    14. 1 2 3 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.
    15. 1 2 3 4 "Scheduled Arecibo Radar Asteroid Observations". National Astronomy and Ionosphere Center.
    16. 1 2 3 4 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.
    17. 1 2 Bill Cooke (August 18, 2005). "Asteroid Apophis set for a makeover". Astronomy Magazine. Retrieved 2009-10-08.
    18. 1 2 "(99942) Apophis Ephemerides for 13 Apr 2029". NEODyS (Near Earth Objects – Dynamic Site). Retrieved 2011-05-05.
    19. ↑ The astronomical magnitude scale. International Comet Quarterly
    20. ↑ Dwayne Brown (Oct 7, 2009). "NASA Refines Asteroid Apophis' Path Toward Earth". NASA's Near Earth Object Program Office. Retrieved 2013-01-18.
    21. 1 2 3 "WayBack Machine archive from 5 Aug 2006". Wayback Machine. 2006-08-05. Archived from the original on August 5, 2006. Retrieved 2013-01-13.
    22. 1 2 3 4 Kelly Beatty (January 9, 2013). "Asteroid Apophis Takes a Pass in 2036". Sky & Telescope. Retrieved 2014-11-10.
    23. ↑ David Morrison (July 22, 2005). "Schweickart Proposes Study of Impact Risk from Apophis (MN4)". NASA. Retrieved 2009-10-08.
    24. ↑ "Hawaii astronomers keep tabs on asteroid Apophis". Astronomy Magazine. 2011-03-10. Retrieved 2011-03-10.
    25. ↑ David Morrison (April 6, 2011). "Asteroid 2004 MN4 will come scarily close to Earth on April 13, 2029, but it will not hit.". Science@NASA.
    26. 1 2 "NEODyS : (99942) Apophis (Close Approaches)". NEODyS (Near Earth Objects—Dynamic Site). Retrieved 2009-02-25.
    27. ↑ Dan Vergano (2010-11-10). "Apophis asteroid encounter in 2013 should help answer impact worries". USA Today ScienceFair. Retrieved 2010-11-10.
    28. ↑ "99942 Apophis Ephemerides for 9 Jan 2013". NEODyS (Near Earth Objects – Dynamic Site). Retrieved 2011-10-19.
    29. 1 2 Dr. Lance A. M. Benner (2013-01-09). "99942 Apophis 2013 Goldstone Radar Observations Planning". NASA/JPL Asteroid Radar Research. Retrieved 2013-01-09.
    30. ↑ "Apophis Risk Assessment Updated".
    31. ↑ "WayBack Machine archive from 2 Feb 2005". Wayback Machine. 2005-02-05. Archived from the original on February 6, 2005. Retrieved 2013-01-13.
    32. ↑ "WayBack Machine archive from 18 Oct 2005". Wayback Machine. 2005-10-18. Archived from the original on October 18, 2005. Retrieved 2013-01-13.
    33. ↑ A geostationary Earth orbit satellite model using Easy Java Simulation, Loo Kang Wee and Giam Hwee Goh 2013 Phys. Educ. 48 72
    34. 1 2 3 4 "Predicting Apophis' Earth Encounters in 2029 and 2036".
    35. ↑ "WayBack Machine archive from 1 Jul 2006". Wayback Machine. 2006-07-01. Archived from the original on July 1, 2006. Retrieved 2013-01-13.
    36. ↑ "Ich habe den Weltuntergang ausgerechnet". Bild. Retrieved 2015-03-27.
    37. ↑ "NASA refutes story of boy who predicted asteroid collision". Radio-Canada. 16 April 2008. Retrieved 8 November 2015.
    38. ↑ http://afp.google.com/article/ALeqM5g6fIS_34_CxE8-vcC5GvbjD4MIOQ
    39. ↑ http://www.bild.de/BILD/news/2008/04/04/ich-hab-den/weltuntergang-ausgerechnet.html
    40. 1 2 Dwayne Brown (2008-04-16). "NASA Statement on Student Asteroid Calculations". NASA. Retrieved 2008-04-28.
    41. ↑ Brown, Dwayne (2009-10-07). "NASA Refines Asteroid Apophis' Path Toward Earth". Archived from the original on 9 October 2009. Retrieved 2009-10-07.
    42. ↑ Apophis Unknown Bias animation (NASA)
    43. ↑ J. Giorgini. "Apophis Trajectory Change 2018–2036: Energy Reflection, Absorption, and Emission". NASA.
    44. ↑ Farnocchia, D.; Chesley, S. R.; Chodas, P. W.; Micheli, M.; Tholen, D. J.; Milani, A.; Elliott, G. T.; Bernardi, F. (2013). "Yarkovsky-driven impact risk analysis for asteroid (99942) Apophis". Icarus 224: 192. arXiv:1301.1607. Bibcode:2013Icar..224..192F. doi:10.1016/j.icarus.2013.02.020.
    45. ↑ Phil Plait (Jan 10, 2013). "Impact Threat from Near-Earth Asteroid Apophis in 2036 Now Ruled Out". Bad Astronomy blog. Retrieved 2013-01-10.
    46. ↑ "Sandia supercomputers offer new explanation of Tunguska disaster". Sandia National Laboratories. December 17, 2007. Archived from the original on 18 January 2008. Retrieved 2008-01-29. The asteroid that caused the extensive damage was much smaller than we had thought,” says Sandia principal investigator Mark Boslough of the impact that occurred June 30, 1908.
    47. ↑ Russell Schweickart; et al. "Threat Characterization: Trajectory dynamics (White Paper 39)" (PDF). Figure 4, pp. 9. B612 Foundation. Archived (PDF) from the original on 28 February 2008. Retrieved 2008-02-22.
    48. ↑ Range of Possible Impact Points on April 13, 2036 in Scenarios for Dealing with Apophis, by Donald B. Gennery, presented at the Planetary Defense Conference. Washington, DC. March 5–8, 2007 (archived from the original on 2012-04-12).
    49. ↑ Nick J. Baileya (2006). "Near Earth Object impact simulation tool for supporting the NEO mitigation decision making process" (PDF). Cambridge University Press. doi:10.1017/S1743921307003614. Retrieved 2009-10-08.
    50. ↑ Michael P. Paine (January 1999). "The Threat is Out There" (PDF). Archived from the original on 23 February 2008. Retrieved 2016-02-29.
    51. ↑ Paul Rincon (2008-02-26). "US team wins asteroid competition". Retrieved 2009-03-25.
    52. ↑ esa. "Don Quijote concept". European Space Agency.
    53. ↑ Izzo, D.; Bourdoux, A.; Walker, R. & Ongaro, F. (2006). "Optimal Trajectories for the Impulsive Deflection of NEOs" (PDF). Acta Astronautica 59: 294. Bibcode:2006AcAau..59..294I. doi:10.1016/j.actaastro.2006.02.002.
    54. ↑ "Scenarios for Dealing with Apophis" (PDF). The Aerospace Corporation. Archived (PDF) from the original on 27 August 2008. Retrieved 2008-07-18.
    55. ↑ ISACHENKOV, VLADIMIR (2009-12-30). "Russia may send spacecraft to knock away asteroid". Yahoo! News. Archived from the original on 2 January 2010. Retrieved 2009-12-31.
    56. ↑ "China Reveals Solar Sail Plan To Prevent Apophis Hitting Earth in 2036". Technology Review Physics arXiv Blog. 2011-08-18. Retrieved 2011-08-18.
    57. ↑ "Russia wants to target near-Earth objects with its ICBMs". TASS in foxnews.com. 2016-02-15. Retrieved 2016-02-15.
    58. ↑ "Rage Interview: We speak with id's Tim Willits about their new IP, Rage.". Gamespot.

    External links

    Wikimedia Commons has media related to 99942 Apophis.

    Risk assessment

    NASA

    Preceded by
    (153814) 2001 WN5    		 Large NEO Earth close approach
    (inside the orbit of the moon)
    13 April 2029     		Succeeded by
    2012 UE34
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