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Saturday, October 19, 2013

99942 Apophis

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99942 Apophis
Discovery[1]
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 PHA[1]
Orbital characteristics[1]
Epoch JD 2454383.5 (October 10, 2007)
(Uncertainty=0)[1]
Aphelion 1.098505744 AU
Perihelion 0.746058621 AU
Semi-major axis 0.9222821826 AU
Eccentricity 0.1910733664
Orbital period 323.5148011 d (0.8857352528 a)
Average orbital speed 30.728 km/s
Mean anomaly 150.996453°
Inclination 3.3312800°
Longitude of ascending node 204.45743°
Argument of perihelion 126.3949°
Physical characteristics
Dimensions 325±15 m[2]
Mass 4×1010 kg (assumed)[3]
Mean density ~3.2 g/cm3[4]
Escape velocity ~0.52 km/h[5]
Rotation period 30.4 h[1][6]
Albedo 0.23[2]
Temperature 270 K
Spectral type Sq [6]
Absolute magnitude (H) 19.7 [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 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 small region no more than about 800 m (half a 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 would 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]
The diameter of Apophis is approximately 325 metres (1,066 ft).[2] As of May 6, 2013 (April 15, 2013 observation arc), the probability of an impact on April 13, 2036 has been eliminated.[3] Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036.[9] Of objects not recently observed, there are about ten asteroids with a more notable Palermo Technical Impact Hazard Scale than Apophis.[10] On average, an asteroid the size of Apophis (325 meters) can be expected to impact Earth about every 80,000 years.[11]

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 somewhat more rough 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,[12] 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.

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 who, on the show, gave rise to the myth of the Egyptian god.[17]

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[12] (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations).[18] 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).[19] Combined with its size, this caused Apophis to be assessed at level 4 on the Torino Scale[8] 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. The potential of an impact in 2029 was eliminated by December 27, 2004.[14] The 2036 passage was lowered to level 0 on the Torino Scale in August 2006.[20] 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.[9] The 2029 pass will be much closer than first predicted. The pass in late March 2036 will be no closer than about 23 million kilometres (14 million mi) — and more likely miss us by something closer to 56 million kilometres (35 million mi).[21]

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

2013 refinement

The close approach in 2029 will substantially alter the object's orbit, making predictions beyond 2029 uncertain without more high quality astrometry. "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.[24] 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][25][26] Just after the close approach on 9 January 2013,[25] the asteroid should peak at about apparent magnitude 15.7.[27] Goldstone observed Apophis during closest approach from January 3 through January 17.[28] Arecibo observed Apophis once it entered Arecibo's declination window after February 13, 2013.[28]
A NASA assessment as of 21 February 2013 that does not use the 2013 radar measurements gives an impact probability of 2.3 out of a million for 2068.[29] 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 is 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.
  • 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; 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.
  • 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, which eliminated the 2029 impact threat.[14] 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.
  • 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 February 2013 the 2053 risk is only 1 in 20 billion.)[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 in January 2005[15] refine the orbit further and show that the April 2029 close approach will occur at only 5.7 Earth radii,[16] approximately one-half the distance previously estimated.
  • By February 6, 2005, Apophis (2004 MN4) had a 1 in 13,000 chance of impacting in April 2036.[30]
  • Radar observation on August 7, 2005,[15] 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).[31]
  • 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 is estimated to occur every 800 years or so.[32]
  • Radar observation at Arecibo Observatory on May 6, 2006,[15] slightly lowered the Palermo scale rating, but the pass in 2036 remained at Torino Scale 1[33] despite the impact probability dropping by a factor of four.
  • Additional observations through 2006 resulted in Apophis being lowered to Torino Scale 0 by August 5, 2006.[20] (The impact probability was 1 in 45,000.)[20]
  • 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.[34]
  • 2009-Apr-29: An animation is released[35] 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
  • 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.[32] Additional factors that can greatly influence the predicted motion in ways that depend on unknown details, are the spin of the asteroid,[36] its precise mass, the way it reflects and absorbs sunlight, radiates heat, and the gravitational pull of other asteroids passing nearby.[32] 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.[32]
  • A statistical impact risk analysis of the data up to January 2013 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.[37]
  • On 9 January 2013, 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.[2] The Herschel Space Observatory observations increased the diameter estimate by 20% from 270 to 325 meters, which translates into a 75% increase in the estimates of the asteroid's volume or mass.[2] Goldstone single-pixel observations of Apophis have ruled out the potential 2036 Earth impact.[9][21][38] Apophis will then come no closer than about 14 million miles — and more likely miss us by something closer to 35 million miles.[21] The radar astrometry is more precise than was expected.[21]

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.[39] 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).
Now obsolete 2008 path of risk where 99942 Apophis had a chance of impacting Earth in 2036.[dated info]
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] 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.[11]
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.[40] 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.[41] 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.[42] An impact in the Atlantic or Pacific oceans would produce a devastating tsunami.[43]

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.[44] 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 under consideration by the European Space Agency as the target of its Don Quijote mission to study the effects of impacting an asteroid.[45]

Proposed deflection strategies

Further information: Asteroid-impact avoidance
Studies by NASA, ESA,[46] and various research groups in addition to the Planetary Society contest teams,[47] 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.[48]
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.[49]

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