Predicting Apophis' Earth

99942 Apophis

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99942 Apophis

Discovery [2]
Discovered by	Roy A. Tucker David J. Tholen Fabrizio Bernardi
Discovery site	Kitt Peak[1]
Discovery date	June 19, 2004
Designations
Named after	Apep
Alternative names	2004 MN4
Minor planet category	Aten[2]
Orbital characteristics[2]
Epoch January 4, 2010 (JD 2455200.5) (Uncertainty=0)[2]
Aphelion	1.0987 AU
Perihelion	0.74604 AU
Semi-major axis	0.92241 AU
Eccentricity	0.19121
Orbital period	323.58 d (0.89 a)
Average orbital speed	30.728 km/s
Mean anomaly	339.94°
Inclination	3.3315°
Longitude of ascending node	204.43°
Argument of perihelion	126.42°
Physical characteristics
Dimensions	~270 m[2]
Mass	2.7×1010 kg (assumed)[3]
Mean density	~3.2 g/cm3[4]
Escape velocity	~0.52 km/h[5]
Rotation period	30.4 h[2][6]
Albedo	0.33 [2][6]
Temperature	270 K
Spectral type	Sq [6]
Absolute magnitude (H)	19.7 [2][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 small probability (up to 2.7%) that it would strike the Earth in 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 precise region in space no more than about a half-mile wide,^[7] that would set up a future impact on April 13, 2036. This possibility kept the asteroid at Level 1 on the Torino impact hazard scale until August 2006, when the probability that Apophis will pass through the keyhole was determined to be very small. Apophis broke the record for the highest level on the Torino Scale, being, for only a short time, a level 4, before it was lowered.^[8] Its diameter is approximately 270 meters (885 ft).^[2] As of October 7, 2009, the probability of an April 13, 2036 impact is considered to be 1 in 250,000.^[3][9] Of objects not recently observed, there are 7 asteroids with a more notable Palermo Technical Impact Hazard Scale than Apophis.^[10]

Physical characteristics

Based upon the observed brightness, Apophis' 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 currently lists the diameter at 0.270km (270m) or roughly the size of two football fields.
During the 2029 approach, Apophis’ brightness will peak at magnitude 3.4,^[11] 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 telescopes not equipped with adaptive optics.

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] Precovery observations from March 15, 2004, were identified on December 27, and an improved orbit was computed.^[12] Radar astrometry further refined the orbit.
When first discovered, the object received the provisional designation 2004 MN₄ (sometimes written 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. 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 (earth's middle) 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. 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 who, on the show, gave rise to the myth of the Egyptian god.^[13]

Close approaches

Close approach of Apophis on April 13, 2029

The white bar indicates uncertainty in the range of positions

99942 Apophis

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^[11] (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations^[14]). This close approach will be visible from Europe, Africa, and western Asia. As a result of its close passage, it will move from the Aten to the Apollo class.
After Sentry and NEODyS announced the possible impact, additional observations decreased the uncertainty in Apophis' trajectory. As they did, the probability of an impact event temporarily climbed, peaking at 2.7% (1 in 37). Combined with its size, this 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 the danger of an asteroid hitting Earth. These are the highest values for which any object has been rated on either scale. It has since been downgraded to level 0 on the Torino Scale for both the 2029 and 2036 passes.^[3] With a cumulative Palermo Scale rating of -2.9,^[3] the risk of impact from Apophis is less than the background hazard level.^[3]
On Friday, April 13, 2029, Apophis will pass Earth within the orbits of geosynchronous communication satellites.^[15] It will return for another close Earth approach in 2036.
The 2029 pass will actually be much closer than the first predictions, but the uncertainty is such that an impact is ruled out. Similarly, the pass on April 13, 2036, carries little risk of an impact.

2013 refinement

The close approach in 2029 will substantially alter the object's orbit, making predictions uncertain without more data. "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." said Jon Giorgini of JPL.^[16] Apophis will pass within 0.0966 AU (14,450,000 km; 8,980,000 mi) of the Earth in 2013, allowing astronomers to refine the trajectory for future close passes.^[17][18] Just after the close approach on 9 January 2013,^[17] the asteroid should peak at about apparent magnitude 15.7.^[19] On January 31, 2011, astronomers took the first new images of Apophis in more than 3 years.^[20]
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 asked for an investigation of the necessity of placing a transponder on the asteroid for more accurate tracking of how its orbit is affected by the Yarkovsky effect.^[21]

History of impact estimates

This section needs additional citations for verification. (July 2008)

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

• The original NASA report on December 23, 2004, mentioned impact chances of "around 1 in 300" in 2029, which was widely reported in the media.^[8] The actual NASA estimates at the time were 1 in 233; they resulted in the 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.
• On December 25, 2004, 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.
• On December 26, 2004 (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.
• On December 27, 2004 (based on a total of 176 observations), the impact probability was raised to 1 in 37 (2.7%); diameter was increased to 390 m, and mass to 7.9×10¹⁰ kg.
• On December 27, 2004, in the afternoon, a precovery increased the span of observations to 287 days and allowed more accurate calculations to re-rate the asteroid's 2029 approach as level zero on the Torino scale (no threat). The cumulative impact probability was estimated to be around 0.004%, a lower risk than 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, and thus remains that way.
• On December 28, 2004 at 12:23 GMT and (based on a total of 139 observations), produced a value of one on the Torino scale for 2044-04-13.29 and 2053-04-13.51.
• By 01:10 GMT on December 29, 2004 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 2010, the 2053 is now a 2056-04-13 risk of 1 in 10 million.)^[3]
• By 19:18 GMT on December 29, 2004 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.
• By 13:46 GMT on December 30, 2004 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.
• By 22:34 GMT on December 30, 2004, 157 observations spanning 289.33 days (2004-Mar-15.1104 to 2004-Dec-29.44434). One pass at 1 (Torino scale) 3 other passes.
• By 03:57 GMT on January 2, 2005, 182 observations spanning 290.97 days (2004-Mar-15.1104 to 2004-Dec-31.07992) One pass at 1 (Torino scale) 19 other passes.
• By 14:49 GMT on January 3, 2005, observations spanning 292.72 days (2004-Mar-15.1104 to 2005-Jan-01.82787) One pass at 1 (Torino scale) 15 other passes.
• Extremely precise radar observations at Arecibo Observatory on January 27, 28, and 30 refine the orbit further and show that the April, 2029 close approach will occur at only 5.6 Earth radii, approximately one-half the distance previously estimated.
• A radar observation on August 7, 2005, refines the orbit further and eliminates the possibility of an impact in 2035. Only the pass in 2036 remains at Torino Scale 1.
• In October 2005 it is predicted that the asteroid will pass just below the altitude of geosynchronous satellites, which are at 35,786 kilometres (22,236 mi).^{[citation needed]} Such a close approach by an asteroid of this size is expected to occur every 800 years or so.^[22]
• A new radar observation at Arecibo Observatory on May 6, 2006, 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.^[23]
• Additional observations through 2006 resulted in Apophis being lowered to Torino Scale 0 on August 6, 2006. Around this time, the impact probability was lowered to 1 in 45,000.
• As of October 7, 2009, 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.^[3][9]
• 2009-Apr-29: An animation is released^[24] 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 the 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 the Earth, producing a negligible impact probability

• Criticism of all published impact probabilites 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 the Earth is a point mass; this can introduce up to 2.9 Earth radii of prediction error for the 2036 approach, and the Earth's oblateness must be considered to predict an impact reliably.^[22] Additional factors that can greatly influence the predicted motion in ways that depend on unknown details, are the spin of the asteroid,^[25] its precise mass, the way it reflects and absorbs sunlight, radiates heat, and the gravitational pull of other asteroids passing nearby.^[22] Small uncertainties in the masses and positions of the planets and Sun can cause up to a 23 Earth radii of prediction error for Apophis by 2036.^[22]

Possible impact effects

NASA initially estimated the energy that Apophis would have released if it struck Earth as the equivalent of 1480 megatons of TNT. A later, more refined NASA estimate was 880 megatons, then revised to 510 megatons.^[3] The impacts which created the Barringer Crater or the Tunguska event are estimated to be in the 3–10 megaton range.^[26] The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons and the biggest hydrogen bomb ever exploded, the Tsar Bomba, was around 50 megatons. In comparison, the Chicxulub impact, believed by many to be a significant factor in the extinction of the dinosaurs, has been estimated to have released about as much energy as 100,000,000 megatons (100 teratons).

Path of risk where 99942 Apophis may impact Earth in 2036.

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.^{[citation needed]}
The B612 Foundation made estimates of Apophis' path if a 2036 Earth impact were to occur, as part of an effort to develop viable deflection strategies.^[27] The result is 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.^[28][29] Using the computer simulation tool NEOSim, it was estimated that the hypothetical impact of Apophis in countries such as Colombia and Venezuela, which are in the path of risk, could have more than 10 million casualties.^[30] An impact in the Atlantic or Pacific oceans would produce a devastating tsunami.^[31]

Potential space missions

Planetary Society competition

This section needs additional citations for verification. (April 2008)

In 2008, 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.^[32] 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.