99942 Apophis (provisional designation ) is a near-Earth asteroid and a potentially hazardous object, 450 metres (1,480 ft) by 170 metres (560 ft) in size. During a brief period of concern in December 2004, initial observations indicated a probability of 2.7% that the asteroid would hit Earth on Friday, April 13, 2029. Later observations eliminated that possibility of an impact with Earth, and it will pass Earth in 2029 at a distance of about above the surface. During the short time when it had been of greatest concern, Apophis set the record for highest rating ever on the Torino scale by reaching Level 4 on 27 December 2004.
It took until 2021 to rule out the possibility that Apophis might hit Earth at some later date in the next century. Observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036. Radar observations in early 2021 helped refine the orbit, and in March 2021 the Jet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years. By 2021, the uncertainty in the 2029 approach distance had been reduced from hundreds of kilometres to now just a couple of kilometres, greatly enhancing predictions of future approaches.
Apophis is the target of a number of planetary defense missions to study the asteroid during its 2029 close approach. Those include the OSIRIS-REx and Ramses spacecraft.
Discovery and naming
Apophis was discovered on 19 June 2004, by Roy A. Tucker, David J. Tholen, and Fabrizio Bernardi at the Kitt Peak National Observatory. On 21 December 2004, Apophis passed from Earth. Precovery observations from 15 March 2004 were identified on December 27, and an improved orbit solution was computed. Radar astrometry in January 2005 further refined its orbit solution. The discovery was notable in that it was at a very low solar elongation (56°) and at very long range (1.1 AU).
When first discovered, the object received the provisional designation , 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 24 June 2005). Receiving a permanent number made it eligible for naming by its discoverers, and they chose the name "Apophis" on 19 July 2005. Apophis is the Greek name of Apep, an enemy of the Ancient Egyptian sun-god Ra. He was the uncreator, an evil serpent that dwelled in the eternal darkness of the Duat and tried to swallow and kill Ra during his nightly passage by devouring him as his food. Apep was held at bay by Set and Horus, the Ancient Egyptian gods of storms and the desert with skies and the plain respectively with Ra's entourage, and Ra beheaded Apep to death forever once in the form of the great feline in the shade of Heliopolis sun temple's persea tree. Apophis will evolve from an Aten- into an Apollo-type asteroid as a result of the 2029 encounter. Apollo asteroids are usually named after Greek deities, Aten asteroids after Egyptian ones. The name Apophis is a nod to that: It is the Greek name of an Egyptian deity.
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, and may have inspired the naming. Tholen denied reports that the asteroid was named after the TV character, stating that the connection is coincidental. 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.
The mythological creature Apophis is pronounced with the accent on the first syllable (/ˈæpəfɪs/). In contrast, the asteroid's name is generally accented on the second syllable (, or as the name was pronounced in the TV series).
Symbols were given to the first few asteroids in the 19th century, though this practice faded when it became clear that there were a great number of them: such symbols are now extremely rarely used by astronomers. In 2008, Denis Moskowitz, a software engineer who devised most of the dwarf planet symbols in Unicode, proposed a symbol for Apophis. His symbol is based on ancient Egyptian depictions of Apep. The added star is similar to many of the 19th-century asteroid symbols.
Physical characteristics and rotation
Based on the observed brightness, Apophis's diameter was initially estimated at ; a more refined estimate based on spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is . As of 2013, NASA's impact risk page listed the diameter at , and an assumed mass of 4 kg. The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three. Apophis's surface composition probably matches that of LL chondrites.
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 . The axis of its angular momentum points 59° south of the ecliptic, which means that Apophis is a retrograde rotator.
Apophis is a tumbler, which means that it does not rotate around a fixed axis. Rather, the axis of rotation moves in the frame of reference of the asteroid with a period of around 263 hours (called the rotation period). The angle between it and the principal axis of highest moment of inertia varies, as does the angle between that principal axis and the vector of angular momentum (from around 12° to around 55° twice every period). During this period, the angle between the long axis of Apophis and the angular momentum vector swings between around 78° and 102° (90°±12°). But the principal axis of highest moment and the rotation axis both move around the constant axis of angular momentum much faster, with a time-averaged period of 27.38 hours (this is called precession). The result is that Apophis appears to be "flipping", making a revolution on average every 30.56 hours. Every 263 hours, the principal axis with highest moment goes around 263/27.28 times (ca 9.6), whereas the long axis goes around 263/30.56 times (ca 8.6).
Orbit Apophis has a low inclination orbit (3.3°) that varies from just outside the orbit of Venus (0.746 AU, as compared to the aphelion of Venus, 0.728) to just outside the orbit of Earth (1.099 AU). Although its orbit changes slightly each time it comes close to Earth, at present it comes near Earth once in 7.75 years on average (four times between April 14, 1998, and April 13, 2029). Because of its eccentric orbit, these moments are not evenly spaced and tend to occur between December and April, when Apophis is in the outer portions of its orbit. In fact, the eccentricity and semi-major axis are such that (before 2029) Apophis is always receding from Earth around May 1 and is always approaching around December 2. At the ascending node (where Apophis crosses the plane of Earth's orbit from south to north) Apophis is very close to where Earth is around 13 April of any year, and this is what gives rise to close encounters such as the one on 13 April 2029. The orbit also passes south of where the earth is in mid December, producing for example the close approaches of December 16, 1889, and December 18, 1939. After the 2029 Earth approach, the orbit will change dramatically. The period will change from around of a year to a bit under . It will still come very close to Earth's yearly April 13 location. It will no longer pass close to Earth's yearly mid-December location, but will then pass close to Earth's mid-September location. This will cause a close encounter on 11 September 2102, after which the uncertainty in the location of Apophis will increase rapidly with time.
By 2116, the JPL Small-Body Database and NEODyS close approach data start to become divergent. In April 2116, Apophis is expected to pass about from Earth, but could pass as close as or as far as .
+Selected approaches to Earth until 2117 Date JPL SBDBnominal geocentricdistance (AU) uncertaintyregion(3-sigma)2004-12-21 n/a 2013-01-09 n/a 2029-04-13 ±3.3 km2036-03-27 ±130 thousand km2051-04-20 ±240 thousand km2066-09-16 ±870 thousand km2116-04-12 2117-10-07
2029 close approach
The closest known approach of Apophis will occur on 13 April 2029, at 21:46 UT, when Apophis will pass Earth at a distance of about above the surface. Using the June 2024 orbit solution which includes the Yarkovsky effect, the 3-sigma uncertainty region in the 2029 approach distance is about ±3.3 km. The distance, a hair's breadth in astronomical terms, is five times the radius of the Earth, one tenth the distance to the Moon, and closer than the ring of geostationary satellites currently orbiting the Earth. It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude 3.1 (visible to the naked eye from rural as well as darker suburban areas, visible with binoculars from most locations). The close approach will be visible from Europe, Africa, and western Asia. Over the course of about a day, Apophis will move northwest from Centaurus to Perseus and then southwest to Pisces, an arc of 205°. Approaching Earth its speed relative to Earth will be 6.0 km/s. Earth's gravity will accelerate it to 7.4 km/s at the time of closest approach, and then slow it back down to 6 as it departs. 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. Perihelion will lift from 0.746 AU to 0.895 AU and aphelion will lift from 1.10 AU to 1.31 AU.
+Orbital elements for 2029 (pre-flyby) and 2030 (post-flyby) Parameter Epoch Orbittype Orbitalperiod Semi-majoraxis Perihelion Aphelion Inclination Eccentricity UnitsAU(°) Pre-flyby 2029 Aten 0.922 0.746 1.10 3.34° 0.191 Post-flyby 2030 Apollo 1.103 0.895 1.31 2.22° 0.189
Apophis also encounters the Moon at from the lunar surface, after the encounter with Earth.
During the 2029 approach, Apophis's brightness will peak at magnitude 3.1, 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. Because the approach will be so close, tidal forces are likely to alter Apophis's rotation axis, but Apophis will not approach within the Roche limit where it would be broken up by tidal forces. A partial resurfacing of the asteroid is possible, which might change its spectral class from a weathered Sq- to an unweathered Q-type.
2036 approaches
After the possibility of an impact in 2029 was ruled out, a small probability remained that during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole estimated to be less than 1 km wide, which would have set up a future impact exactly seven years later on Easter Sunday, 13 April 2036. This possibility kept it at Level 1 on the 0 to 10 Torino impact hazard scale until August 2006, when the probability that Apophis would pass through the keyhole was determined to be negligible. Apophis's rating on the Torino scale was consequently lowered to Level 0.