99942 Apophis

99942 Apophis is a near-Earth asteroid and a potentially hazardous object, 450 metres by 170 metres in size. During a brief period of concern in December 2004, initial observations indicated a probability of 0.027 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 38,000 kilometres above the surface. It will also have a close encounter with the Moon, passing about 96,000 km from the lunar surface.
A small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole estimated to be 800 kilometres in diameter, which would have set up a future impact exactly seven years later on Easter Sunday, April 13, 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 very small and Apophis's rating on the Torino scale was consequently lowered to Level 0. By 2008, the keyhole had been determined to be less than 1 km wide. 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.
It is estimated that an asteroid as big or bigger coming so close to Earth happens only once in 800 years on average. Such an asteroid is expected to actually hit Earth once in about 80,000 years.
Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036. In February 2013 the estimated probability of an impact in 2036 was further reduced to. It is now known that in 2036, Apophis will approach the Earth at a third the distance of the Sun in both March and December, about the distance of the planet Venus when it overtakes Earth every 1.6 years. Simulations in 2013 showed that the Yarkovsky effect might cause Apophis to hit a "keyhole" in 2029 so that it will come close to Earth in 2051, and then could hit another keyhole and hit Earth in 2068. The chance of the Yarkovsky effect having exactly the right value for this was however estimated as two in a million. Radar observations in March 2021 helped refine the orbit again, and in March 2021 the Jet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years. The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometres to now just a couple of kilometres, greatly enhancing predictions of future approaches. Entering March 2021, six asteroids each had a more notable cumulative Palermo scale rating than Apophis, and none of those has a Torino level above 0. In the longer term, Apophis will however continue to be a threat for possibly thousands of years, until it is removed from being a potentially hazardous object, for instance by passing close to Venus or Mars.

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. On December 21, 2004, Apophis passed from Earth. Precovery observations from March 15, 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 and at very long range.
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. Receiving a permanent number made it eligible for naming by its discoverers, and they chose the name "Apophis" on July 19, 2005. 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. 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.
File:Apep 1.jpg|thumb|upright=1.2|Atum facing Apep, tomb of Ramesses I, 19th Dynasty
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. In contrast, the asteroid's name is generally accented on the second syllable.
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 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 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. 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. During this period, the angle between the long axis of Apophis and the angular momentum vector swings between around 78° and 102°. 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. 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, whereas the long axis goes around 263/30.56 times.

Orbit

Apophis has a low inclination orbit that varies from just outside the orbit of Venus to just outside the orbit of Earth. 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. 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 Apophis is always receding from Earth around May 1 and is always approaching around December 2. At the ascending node Apophis is very close to where Earth is around April 13 of any year, and this is what gives rise to close encounters such as the one on April 13, 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 September 11, 2102, after which the uncertainty in the location of Apophis will increase rapidly with time.

Date	JPL SBDB nominal geocentric distance	uncertainty region
2004-12-21		n/a
2013-01-09		n/a
2029-04-13		±3.3 km
2036-03-27		±130 thousand km
2051-04-20		±240 thousand km
2066-09-16		±870 thousand km
2116-04-12
2117-10-07

2029 close approach

The closest known approach of Apophis will occur on April 13, 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. 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.
Apophis also encounters the Moon at ~95,000 km from the lunar surface, ~17 hours after 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.