99942 Apophis

99942 Apophis (provisional designation 2004 MN₄) is a near-Earth asteroid and a potentially hazardous object with a diameter of 370 metres (1,210 feet)^[3] that caused a brief period of concern in December 2004 when initial observations indicated a probability 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 in 2029. Until 2006, a small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole of no more than about 800 kilometres (500 mi) in diameter,^[11][12] which would have 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 and Apophis's rating on the Torino scale was lowered to zero. By 2008, the keyhole had been determined to be less than 1 km wide.^[11] During the short time when it had been of greatest concern, Apophis set the record for highest rating ever on the Torino scale, reaching level 4 on December 27, 2004.^[13]

Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036.^[14] By May 6, 2013 (April 15, 2013, observation arc), the possibility of an impact on April 13, 2036 had been eliminated altogether.^[3] In 2036, Apophis will approach the Earth at a third the distance of the Sun in both March and December,^[1] but this is about the distance of the planet Venus when it overtakes Earth every 1.6 years. On April 12, 2068, the nominal trajectory has Apophis 1.87 AU (280 million km) from Earth.^[15] Entering March 2021, six asteroids each had a more notable cumulative Palermo Technical Impact Hazard Scale rating than Apophis, and none of those has a Torino level above 0.^[16][a] On average, an asteroid the size of Apophis (370 metres) is expected to impact Earth once in about 80,000 years.^[17] Observations in 2020 by the Subaru telescope confirmed David Vokrouhlický's 2015 Yarkovsky effect predictions.^[18] The Goldstone radar observed Apophis March 3–11, 2021, helping to refine the orbit again,^[19] and on March 25, 2021, the Jet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years.^[20][21] The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometers to now just a couple of kilometers,^[22] greatly enhancing predictions of future approaches.

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 0.0964 AU (14.42 Gm; 8.96 million mi) from Earth.^[1] Precovery observations from March 15, 2004, were identified on December 27, and an improved orbit solution was computed.^[24][25] Radar astrometry in January 2005 further refined its orbit solution.^[26][27] The discovery was notable in that it was at a very low solar elongation (56°) and at very long range (1.1 AU).^{[citation needed]}

When first discovered, the object received the provisional designation 2004 MN₄, and early news and scientific articles naturally referred to it by that name. Once 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 they chose the name "Apophis" on July 19, 2005.^[28] Apophis is the Greek name of Apep, an enemy of the Ancient Egyptian sun-god Ra. He is 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.^[29]

Tholen and Tucker, two of the co-discoverers of the asteroid, are reportedly fans of the television 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.^[28]

The mythological creature Apophis is pronounced with the accent on the first syllable (/ˈæpəfɪs/).^{[citation needed]} In contrast, the asteroid's name is generally accented on the second syllable (/əˈpɒfɪs/) as the name was pronounced in the TV series.^[30]

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] Apophis's surface composition probably matches that of LL chondrites.^[31]

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 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,^[32] easily visible to the naked eye, with a maximum angular speed of 42° per hour. The maximum apparent angular diameter will be approximately 2 arcseconds. This is roughly equivalent to the angular diameter of Neptune from earth. Therefore, the asteroid will be barely resolved by ground-based telescopes not equipped with adaptive optics but very well resolved by those that are.^[33] Because the approach will be so close, tidal forces are likely to 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][31]

Orbit

Apophis has a low inclination orbit (3.3°) that varies from just outside the orbit of Venus (0.746 AU) to just outside the orbit of Earth (1.099 AU).^[1] After the 2029 Earth approach, the orbit will vary from just inside of Earth's to just inside of Mars's.

2029 close approach

The closest known approach of Apophis occurs at April 13, 2029 21:46 UT, when Apophis will pass Earth closer than geosynchronous communication satellites, but will come no closer than 31,600 kilometres (19,600 mi) above Earth's surface.^[37][38] Using the June 2021 orbit solution which includes the Yarkovsky effect, the 3-sigma uncertainty region in the 2029 approach distance is about ±3.4 km.^[22][1] The distance, a hair's breadth in astronomical terms, is five times the radius of the Earth, ten times closer than the Moon,^[38] and closer than the ring of geostationary satellites currently orbiting the Earth.^[39][40] It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude 3.1^[32] (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations).^[41] The close approach will be visible from Europe, Africa, and western Asia. During the approach, 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.^[42] Perihelion will lift from 0.746 AU to 0.895 AU and aphelion will lift from 1.10 AU to 1.31 AU.^[42]