99942 Apophis

• 1. Introduction

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  The asteroid 99942 Apophis is about 450 metres long and 170 metres wide, and for a few days at the end of 2004 it was the most worrying object in the sky. Early calculations gave it a 2.7 percent chance of striking Earth on Friday, the 13th of April 2029. Astronomers had known about it for only a few months, so very little was certain about its path. Then the numbers began to shift, and they kept shifting for the next two decades.

  What made that first estimate so alarming, and what finally took the danger away? What will Apophis do when it passes Earth anyway? And who is already planning to meet it there?

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• 2. Name of a Serpent

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  Roy A. Tucker, David J. Tholen and Fabrizio Bernardi discovered the asteroid at Kitt Peak National Observatory on the 19th of June 2004. The find was unusual. It lay only 56 degrees from the Sun in the sky, and it was 1.1 astronomical units away. At first it carried just a provisional designation, and early news reports used that code. On the 24th of June 2005 it received the permanent number 99942, which let the discoverers propose a name. They chose Apophis on the 19th of July 2005.

  Apophis is the Greek name of Apep, the enemy of the ancient Egyptian sun god Ra. Apep is the Uncreator, an evil serpent living in the eternal darkness of the Duat. Each night he tries to swallow Ra on his journey, and Set, the god of storms and the desert, holds him back. The choice also follows a naming habit. Apollo-type asteroids usually take Greek deity names, and Aten-type asteroids take Egyptian ones. The 2029 encounter will turn Apophis from an Aten into an Apollo, so a Greek name for an Egyptian deity fits.

  Tholen and Tucker are reportedly fans of the television series Stargate SG-1. Its recurring villain, an alien called Apophis, threatens civilization on Earth through the first few seasons. In the show he lived on Earth in ancient times and posed as a god, which gave rise to the Egyptian myth. Tholen denied that the asteroid was named after the character and called the connection a coincidence.

  The name has two pronunciations, one for the serpent and one for the rock. The mythological figure is said with the stress on the first syllable. The asteroid is generally stressed on the second syllable, and the television series used a short o sound.

  In 2008 Denis Moskowitz, a software engineer who devised most of the dwarf planet symbols in Unicode, proposed a symbol for Apophis. It draws on ancient Egyptian depictions of Apep. He added a star, much like those on many 19th-century asteroid symbols, a practice astronomers dropped once the number of asteroids grew too large.

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• 3. The December Scare

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  On the 21st of December 2004, Apophis passed 0.0964 astronomical units from Earth, and the improved orbit that followed produced a shock. Two automatic systems, NASA's Sentry and NEODyS, run by the University of Pisa and the University of Valladolid, both predicted a very close approach on the 13th of April 2029.

  On the 23rd of December, a NASA report mentioned odds of around 1 in 300, and the media repeated that figure widely. The actual estimate was 1 in 233, which earned a rating of 2 on the Torino scale, the first time any asteroid had scored above 1. Later that day, with 64 observations in hand, the odds were revised to 1 in 62, or 1.6 percent. By the 25th and 26th, new observations had settled the figure near 1 in 45, while the diameter estimate fell from 440 metres to 380 metres.

  On the 27th of December, 176 observations covering 190 days pushed the odds to their peak, with the uncertainty region shrunk to 83,000 kilometres. Combined with its size, that peak rated Level 4 on the Torino scale and 1.10 on the Palermo scale, an impact hazard more than 12 times the background level. These are the highest values any object has received on either scale.

  That same afternoon, an older picture rescued the situation. A precovery image from the 15th of March 2004 stretched the observation arc to 287 days, and it eliminated the 2029 impact. The cumulative impact probability dropped to around 0.004 percent, lower than that of another asteroid, which briefly became the greatest-risk object again.

  Minor ratings of 1 flickered on and off for a few more days, first for passes in 2044 and 2053, then for 2053 alone. By the 30th of December no pass was rated above 0, and the most dangerous one stood at 1 in 7,143,000. In January 2005, precise radar observations at Arecibo Observatory showed that the 2029 approach would be only 5.7 Earth radii away, about half the distance first estimated.

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• 4. Keyholes and Corrections

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  A region of space 800 kilometres across, called a gravitational keyhole, became the next worry. If Apophis passed through it in 2029, Earth's gravity would bend its path so that it returned exactly seven years later, on Easter Sunday, the 13th of April 2036. That scenario kept Apophis at Level 1 on the Torino scale until August 2006. By then, passing through the keyhole was judged very unlikely, and the rating dropped to Level 0, with the impact odds assessed at 1 in 45,000. By 2008 the keyhole had been measured at less than 1 kilometre wide.

  The odds had fallen in steps. In February 2005 the 2036 chance was put at 1 in 13,000. In August 2005 radar observations eliminated 2035 and left 2036 at 1 in 5,560. That July, Rusty Schweickart, a former Apollo astronaut and chairman of the B612 Foundation, formally asked NASA to check whether the post-2029 orbit might be in resonance with Earth. He also asked whether a transponder should be placed on the asteroid to track the Yarkovsky effect, a faint push linked to how an asteroid absorbs and radiates the Sun's heat.

  In April 2008 news outlets carried a story that Nico Marquardt, a 13-year-old German student, had found odds of 1 in 450 for a 2036 impact. NASA denied that it had made an error. It explained that the approach angle to Earth's equator meant the asteroid would not cross the belt of geosynchronous satellites, so no collision risk to them existed.

  By October 2009, refined precovery images from the University of Hawaii's Institute for Astronomy, the 90-inch Bok Telescope and the Arecibo Observatory cut the 2036 odds to about 1 in 250,000. Critics of the older published figures noted that mass and spin had not yet been measured. The standard model also treated Earth as a single point of mass, which could throw off the 2036 approach by up to 2.9 Earth radii.

  In January 2013 the Goldstone radar effectively ruled out a 2036 impact, with a probability below one in a million. Simulations that year then raised a longer chain of events. The Yarkovsky effect might steer Apophis through a keyhole in 2029, bring it close in 2051, and set up a hit in 2068. The chance of the effect having exactly the right value was estimated at two in a million. By early 2015 the Sentry table gave the 12th of April 2068 odds of 6.7 in a million, and a cumulative chance of 9 in a million before 2106.

  Then the asteroid slipped away. From April 2014 to December 2019, its orbit kept it within 60 degrees of the Sun as seen from Earth, which blocked telescopic observation.

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• 5. The Final Clearance

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  In March 2020, astronomers David Tholen and Davide Farnocchia measured the Yarkovsky acceleration of Apophis for the first time. They used observations from the Subaru Telescope in January and March 2020, along with older observations remeasured against the Gaia DR2 star catalog. The effect shrinks the asteroid's semi-major axis by about 170 metres per year. That measurement sharpened every prediction past the 2029 flyby.

  On the 6th of March 2021, Apophis passed 0.11265 astronomical units from Earth and brightened to about magnitude 15. Goldstone radar observed it from the 3rd to the 8th of March. The space telescopes NEOWISE and NEOSSat also watched it. The data showed that the 2029 impact parameter, how far behind Earth the asteroid would pass without Earth's pull, is about 47,363 kilometres. That is 296 kilometres less than the earlier nominal value. The shift means Apophis avoids the keyhole, 212.14 kilometres below nominal, that would have led to a 2068 collision.

  On the 21st of February 2021, Apophis was removed from the Sentry Risk Table, since an impact within 100 years was finally ruled out. In March 2021, the Jet Propulsion Laboratory announced the same conclusion. Its solution of the 15th of March cut the 3-sigma uncertainty in the 2029 approach distance from plus or minus 700 kilometres to about 3 kilometres.

  The International Asteroid Warning Network also used Apophis as a rehearsal. It collected light curves, spectra and astrometry to practice coordinating the response to a real impact threat. In an estimate that ignored all earlier observations, the 2029 impact probability climbed to 16 percent before falling to zero.

  In March and April 2021, Apophis passed in front of several bright stars of magnitude 8 to 11. Five separate occultations were observed, the first time an asteroid as small as Apophis had been tracked this way. The previous record belonged to 3200 Phaethon in 2019, which is more than ten times the size of Apophis. Weather blocked a European attempt on the 11th of March. On the 22nd, a single amateur astronomer, Roger Venable, caught the event, and that detection allowed new predictions. He observed another occultation from New Mexico on the 4th of April, and several observers saw two more from Japan and New Mexico on the 10th and 11th.

  On the 9th of March 2021, Apophis became the asteroid with the most precisely measured Yarkovsky effect, at a signal-to-noise ratio of 186.4. That beat 101955 Bennu, at 181.6. The 2021 appearance was the last chance to observe Apophis until shortly before its 2029 flyby.

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• 6. A Tumbling Shape

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  In 2005, spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii, by Binzel, Rivkin, Bus and Tokunaga, put the diameter at 350 metres. By 2013, NASA's impact risk page listed 330 metres. That January, the Herschel Space Observatory made thermal infrared observations that raised an estimate from 270 to 325 metres. The asteroid was less reflective than expected, and the change meant roughly a 75 percent increase in volume and mass. Apophis's surface probably matches that of LL chondrites, a common class of stony meteorite.

  Radar images from Goldstone and Arecibo in 2012 and 2013 gave Brozović and colleagues a clearer view. They found an elongated object with two lobes, possibly a contact binary, and a relatively bright surface with an albedo of 0.35. The axis of its angular momentum points 59 degrees south of the ecliptic, so Apophis is a retrograde rotator.

  Apophis is also a tumbler, meaning it does not spin around a fixed axis. Instead the rotation axis wanders within the asteroid on a cycle of about 263 hours. The angle between the long axis and the angular momentum vector swings between about 78 and 102 degrees. Meanwhile the principal axis of highest moment of inertia circles the angular momentum axis much faster, with an average period of 27.38 hours. The result is that the asteroid appears to flip, completing a revolution about every 30.56 hours.

  Over one 263-hour cycle, the principal axis goes around about 9.6 times, while the long axis goes around about 8.6 times.

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• 7. The 2029 Flyby

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  Between the 14th of April 1998 and the 13th of April 2029, Apophis comes near Earth four times, on average once every 7.75 years. Its orbit has a low inclination of 3.3 degrees and runs from just outside the orbit of Venus, at 0.746 astronomical units, to just outside Earth's, at 1.099. Because the orbit is eccentric, the close passes are unevenly spaced and tend to fall between December and April. Near the point where it crosses Earth's orbital plane, Apophis lies close to Earth's position around the 13th of April of any year. The orbit also passes south of Earth's mid-December position, which produced close approaches on the 16th of December 1889 and the 18th of December 1939.

  At 21:46 UT on the 13th of April 2029, Apophis will pass about 31,600 kilometres above Earth's surface. The 3-sigma uncertainty, using the June 2024 orbit solution, is only about 3.3 kilometres. That is about five times Earth's radius and one tenth of the distance to the Moon. It is closer than the ring of geostationary satellites. It will be the closest asteroid of its size in recorded history. An asteroid this big or bigger coming this near happens about once in 800 years, and an actual hit is expected about once in 80,000 years.

  The flyby will be visible from Europe, Africa and western Asia. Apophis will brighten to magnitude 3.1, visible to the naked eye from rural and darker suburban areas. Over about a day, it will travel an arc of 205 degrees, moving from Centaurus to Perseus and then to Pisces. At its fastest it will cross the sky at 42 degrees per hour. Its apparent diameter will reach about 2 arcseconds, similar to Neptune seen from Earth. Approaching Earth it will move at 6.0 kilometres per second relative to Earth, speed up to 7.4 at closest approach, and slow again to 6 on departure. Afterward it will pass the Moon at 89,600 kilometres from the lunar surface.

  Earth's tides of gravity are likely to alter the asteroid's rotation axis, though it will not come within the Roche limit where it would be torn apart. A partial resurfacing is possible, which might change its spectral class from a weathered Sq type to an unweathered Q type.

  The orbit will change dramatically. Its period grows from 323.6 days to 423.1 days. Its perihelion lifts from 0.746 to 0.895 astronomical units, and its aphelion from 1.10 to 1.31. On the 27th of March 2036 it will pass at about 0.31 astronomical units, roughly a third of the distance to the Sun. Venus will be closer to Earth than that, at 0.2883 astronomical units on the 30th of May 2036. Around the 19th and the 20th of April 2051, Apophis will pass about 0.04 astronomical units away. On the 11th of September 2102, another close encounter follows, after which the uncertainty in its position grows quickly. Apophis may remain a threat for thousands of years, until a close pass of Venus or Mars removes it from the potentially hazardous category.

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• 8. Impact and Response

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  The Sentry Risk Table estimated in 2021 that an impact by Apophis would release energy equal to 1,200 megatons of TNT. The Krakatoa eruption released about 200 megatons, and the world's nuclear arsenal totals roughly 1,460. The Chicxulub impact that marked a mass extinction released an estimated 100,000,000 megatons. An Apophis impact would devastate thousands of square kilometres but would be unlikely to cause an impact winter. A 370-metre stony asteroid with a density of 3,000 kilograms per cubic metre, striking sedimentary rock, would dig a crater 5.1 kilometres wide.

  In 2008 the B612 Foundation mapped a "path of risk" for a hypothetical 2036 impact, as part of work on deflection strategies. The corridor, only a few kilometres wide, ran across southern Russia and the north Pacific, close to the coasts of California and Mexico. It then passed between Nicaragua and Costa Rica and crossed northern Colombia and Venezuela, ending in the Atlantic just short of Africa. The NEOSim tool estimated more than 10 million casualties for an impact in Colombia and Venezuela. A deep-water strike in either ocean would raise a short-range tsunami with a destructive radius of about 1,000 kilometres for most of North America, Brazil and Africa, 3,000 for Japan, and 4,500 for parts of Hawaii.

  Fiction has borrowed the asteroid freely. In id Software's video game Rage, Apophis hits Earth on the 23rd of August 2029 and nearly wipes out humanity. In Terra Invicta, players can send a scientific expedition to it in 2029. The gothic metal band Type O Negative mentioned it in "The Profit of Doom" on the 2007 album Dead Again.

  In 2007 the Planetary Society, a California-based space advocacy group, ran a $50,000 competition to design a probe that would shadow Apophis for almost a year. It drew 37 entries from 20 countries on 6 continents. The winner was Foresight, from SpaceWorks Engineering, a simple orbiter with two instruments and a radio beacon, costing about US$140 million. The student winner, Pharos, would have carried four science instruments and four BUOI probes to impact the surface.

  The European Space Agency has considered Apophis as a target for its Don Quijote concept, which would study the effects of hitting an asteroid. It also proposes the Ramses mission, with launch in 2026-2028 and rendezvous in 2029, plus a CubeSat mission called Satis. China once planned an Apophis encounter for 2022 under the mission now called Tianwen-2, which launched on the 28th of May 2025 with a different set of targets.

  Proposals for moving Apophis include a gravitational tractor, kinetic impact and nuclear methods. On the 30th of December 2009, Anatoly Perminov, director of the Russian Federal Space Agency, said Roscosmos would study a deflection mission. On the 16th of August 2011, researchers at China's Tsinghua University, led by Shengping Gong, proposed using a solar-sail impactor to push Apophis away from the keyhole. In February 2016, Sabit Saitgarayev of the Makeyev Rocket Design Bureau named Apophis as a possible test object for Russian ICBMs. In October 2022, a method of mapping an asteroid's interior to choose the best impact point was proposed.

  The closest meeting is already scheduled. After returning a sample of Bennu to Earth on the 24th of September 2023, the OSIRIS-REx spacecraft was approved in April 2022 for an extended mission and renamed OSIRIS-APEX. It will rendezvous with Apophis in April 2029, a few days after the flyby, and study it for 18 months. Near the end, it will approach the surface and fire its thrusters to expose the subsurface, as it did at Bennu.

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