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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 site	Kitt Peak[1]
Discovery date	19 June 2004
Designations
MPC designation	(99942) Apophis
Named after	Apep
Alternative designations	2004 MN4
Minor planet category	Aten NEO, PHA[1]
Orbital characteristics[1]
Epoch 24 September 2008 (JD 2454733.5)
Uncertainty parameter 0
Observation arc	3946 days (10.80 yr)
Aphelion	1.09851 AU (164.335 Gm)
Perihelion	0.74607 AU (111.610 Gm)
Semi-major axis	0.92244 AU (137.995 Gm)
Eccentricity	0.19120
Orbital period	0.89 yr (323.6 d)
Average orbital speed	30.728 km/s
Mean anomaly	180.42937°
Mean motion	1.11250°/day
Inclination	3.33137°
Longitude of ascending node	204.454603°
Argument of perihelion	126.40188°
Earth MOID	0.000315683 AU (47,225.5 km; 29,344.6 mi)
Jupiter MOID	4.12582 AU (617.214 Gm)
TJupiter	6.466
Physical characteristics
Dimensions	0.370 km (0.230 mi) 0.45 × 0.17 km[2]
Mean radius	0.185 km (0.115 mi) 7002170000000000000♠0.17±0.02 km[2]
Mass	6.1×1010 kg (assumed)[3]
Mean density	~3.2 g/cm3[4] 2.6 g/cm3 (assumed)[3]
Rotation period	30.4 h (1.27 d)[1][5] tumbling:[6] precession period: 27.38±0.07 h[6] rotation period: 263±6 h[6] period of harmonic with strongest lightcurve amplitude: 30.56±0.01 h[6]
Geometric albedo	0.23[7] 6999350000000000000♠0.35±0.10[2]
Temperature	270 K
Spectral type	Sq [5]
Absolute magnitude (H)	7001197000000000000♠19.7±0.4[1][5] 7001190900000000000♠19.09±0.19[2]

99942 Apophis (/əˈpɒfɪs/, previously known by its provisional designation 2004 MN₄) is a 370 meter diameter 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^[8][9] 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.^[8] 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.^[10]

As of 2014, 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.^[11] By May 6, 2013 (April 15, 2013 observation arc), the probability of an impact on April 13, 2036 had been eliminated.^[3] 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.^[3] As of March 2018^[update], there were seven asteroids with a more notable cumulative Palermo Technical Impact Hazard Scale than Apophis.^[12] On average, one asteroid the size of Apophis (370 metres) can be expected to impact Earth about every 80,000 years.^[13]

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 solution was computed.^[14][15]Radar astrometry in January 2005 further refined its orbit solution.^[16][17]. 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 the Earth and Sun, compared to the positions of other NEOs at the time of their discovery^[18]

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.^[19]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.^[19]

Physical characteristics

Comparison between the best-fit convex and nonconvex shape models, and some of the available radar images of (99942) Apophis.

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.^[20]

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 6999350000000000000♠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,^[21] 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, so that it will be barely resolved by ground-based telescopes not equipped with adaptive optics. Due to the small distance, 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][20]

Orbit

Close approaches

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 3.1^[21] (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations).^[22] 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.^[23][24] This probability, combined with its size, caused Apophis to be assessed at level 4 on the Torino Scale^[10] 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.^[15] The danger of a 2036 passage was lowered to level 0 on the Torino Scale in August 2006.^[25] 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]

2029/2036/2068 approaches

On April 13, 2029, Apophis will pass Earth within the orbits of geosynchronous communication satellites, but will come no closer than 31,200 kilometres (19,400 mi) above Earth's surface.^[11] The 2029 pass will be much closer than had first been predicted in 2004 when the observation arc was very short.^[10] 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 about 56 million kilometres (35×10^⁶ mi).^[27][28]

• Sample gallery


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  Close approach of Apophis on April 13, 2029 (as known in February 2005)^[17]

  
  

  
  


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  The white bar indicates uncertainty in the range of positions (as known in February 2005)^[17]

  
  

  
  

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.^[29] On January 31, 2011, astronomers took the first new images of Apophis in more than 3 years.^[30]

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: "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."^[31] 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.^[7][32][33] Just after the closest approach on 9 January 2013,^[32] the asteroid peaked at an apparent magnitude  of about 15.7.^[34]Goldstone observed Apophis during that approach from January 3 through January 17.^[35] The Arecibo Observatory observed Apophis once it entered Arecibo's declination window after February 13, 2013.^[35]

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.^[36] 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]

2015 observations

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

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.^[52] 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 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 kilometres, but would be unlikely to have long-lasting global effects, such as the initiation of an impact winter.^{[citation needed]} 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.^[13]

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.^[53] 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.^[54] 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.^[55] 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.^[56]

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.^[57] 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.^[58]

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.^[59]

Proposed deflection strategies

Studies by NASA, ESA,^[60] and various research groups in addition to the Planetary Society contest teams,^[61] 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.^[62]

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.^[63]

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.^[64]

Popular culture

• 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.^[65]
  


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  Type O Negative mentions this asteroid in the song Profit of Doom, from its album Dead Again.


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  Enter Shikari mention the asteroid in the song Zzzonked, from their album Common Dreads.

References

External links

Wikimedia Commons has media related to 99942 Apophis.

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  Thermal Infrared Observations of Asteroid (99942) Apophis with Herschel (Müller : 23 Apr 2014 : arXiv:1404.5847)


• Apophis Asteroid


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


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  Diagrams and orbits of Apophis (Sormano Astronomical Observatory)

Risk assessment

• Apophis Orbital Prediction Page at NASA JPL


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  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

NASA

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  Possibility of an Earth Impact in 2029 Ruled Out for Asteroid 2004 MN₄ (JPL)


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  Radar Observations Refine the Future Motion of Asteroid 2004 MN₄ (JPL)


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  Animation explaining how impact risk is determined from Impact Probability
  


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  99942 Apophis at the JPL Small-Body Database
  
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    Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters