Near-Earth_object - Wikiparency

# Near-Earth object  **Microsim — three.js (Wikitube framework):** *Near-Earth objects in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=neo&embed=1" data-title="Near-Earth objects in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=neo`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: set the speed to 1 month/s and follow the three labelled bodies around the inner system: Apophis mostly inside Earth's orbit, Bennu crossing it, and Eros staying just outside it; switch the scale to true to see how closely the whole swarm of near-Earth paths crowds around Earth's orbit at real distances; then set show to small bodies to hide the planets and leave the near-Earth population beside the main belt it comes from.* A **near-Earth object** (**NEO**) is any [[Small_Solar_System_body|small Solar System body]], an [[Asteroid|asteroid]] or a [[Comet|comet]], whose [[Orbit|orbit]] around the [[Sun]] brings it within 1.3 [[Astronomical_unit|AU]] of the Sun at perihelion.[^iau-neo][^cneos-groups] The label describes the orbit, not the object's present position, so a NEO counts as one even while it is far from [[Earth]]. Objects whose orbits pass within 0.05 AU of Earth's and which are large enough to be dangerous, roughly 140 m across or more, are singled out as potentially hazardous.[^cneos-groups] By the end of 2024 about 37,400 NEOs were known, all but 123 of them asteroids.[^cneos-stats] NEOs matter because some of them hit Earth. Impacts have shaped the planet's geological and biological history, bodies as small as about 20 m can do serious local damage, and larger ones can dig craters or raise tsunamis.[^monastersky1997][^rumpf2017] Since the 1990s, national surveys coordinated under the name Spaceguard have catalogued the population systematically; the goal of finding 90% of NEOs larger than 1 km was met in 2011, and the search has since moved to smaller bodies.[^nasa2004][^jpl-wise2011] Their low gravity and Earth-like orbits also make NEOs cheap to reach: spacecraft have visited several, returned samples from three and, in 2022, deliberately changed the orbit of one.[^vergano2007][^nasa-dart2022b] The explorer at the top of this page shows the near-Earth population threading the inner Solar System. Its sampled points are ILLUSTRATIVE, but three members follow real JPL orbital elements on two-body paths: 99942 Apophis, 101955 Bennu and 433 Eros, which happen to belong to three different orbital classes.[^jpl-sbdb] ## Definitions The International Astronomical Union defines NEOs as small Solar System bodies whose orbits come at least partly within 1.3 AU of the Sun.[^iau-neo] That excludes planets such as [[Venus]], moons of bodies other than the Sun, such as the [[Moon]], and spacecraft. Because a small body is either an asteroid or a comet, every NEO is a near-Earth asteroid (NEA) or a near-Earth comet (NEC). The catalogues kept by NASA's Center for Near-Earth Object Studies (CNEOS) and ESA's Near-Earth Object Coordination Centre also require an orbital period shorter than 200 years, a limit that in practice removes long-period comets; the IAU itself does not impose it.[^cneos-groups][^esa-neocc-def][^iau-neo] Some authors add a further condition that the orbit reach beyond 0.983 AU, Earth's perihelion distance, so that the object can actually meet Earth.[^morbidelli2002][^waszczak2017] Membership is set by geometry, and the geometry changes. Close passes past Earth disturb NEO orbits, and a few NEOs are briefly captured into loose orbits around Earth before escaping again; the definitions are applied flexibly to such cases.[^carlisle2011] A second category ranks danger. A potentially hazardous asteroid (PHA) must satisfy two tests: its minimum orbit intersection distance with Earth (MOID) is 0.05 AU or less, and its absolute magnitude H is 22.0 or brighter.[^cneos-groups] The first test is about 7.5 million km, some 19 times the Moon's distance (derived); the second corresponds to a diameter of about 140 m if the surface reflects 14% of the light.[^cneos-groups][^cneos-size] An object that fails either test, because it cannot come within 0.05 AU or because it is fainter than H = 22.0, is not a PHA. The broader term potentially hazardous object also covers comets.[^mpc-pha][^perna2013] ## History of human awareness of NEOs [[Comet|Comets]] were the first NEOs anyone saw, though for most of history they were not understood as bodies in space. Tycho Brahe's attempt to measure the parallax of the comet of 1577 put it far beyond the [[Moon]], and Edmond Halley's orbit calculations of 1705 showed that some comets return; the 1758–1759 reappearance of [[Halley's_Comet|Halley's Comet]] was the first return predicted in advance.[^halley1705][^stoyan2015] Meteors took longer. Denison Olmsted's analysis of the great Leonid [[Meteor_shower|meteor storm]] of 1833 established that they come from space, and in 1867 astronomers matched the Leonids' orbit to that of the newly found comet 55P/Tempel–Tuttle, a comet that would now be classed as an NEO.[^dick1998] The first near-Earth asteroid, 433 Eros, was found in 1898, and its close passes were soon used to pin down the distance from [[Earth]] to the [[Sun]].[^scholl2002][^apl-eros] ### Encounters with Earth A close approach happens when an NEO and Earth reach the nearest parts of their two orbits at the same time. Historical records go back far: Lexell's [[Comet]] passed 0.0151 AU from [[Earth]] on 1 July 1770, still the closest comet approach observed.[^mpc-comets] Among asteroids, 69230 Hermes passed at about twice the [[Moon]]'s distance in 1937, and 1566 Icarus, 1.4 km across, came within 0.0425 AU in June 1968 and became the first minor planet observed by radar.[^ucla-hermes][^jpl-icarus][^pettengill1969][^goldstein1968] Improved surveys now record sub-lunar passes every year, more than a hundred a year since 2020.[^mpc-closest][^cneos-ca] On 15 February 2013, 367943 Duende (2012 DA14), about 30 m across, passed about 27,700 km above the surface, inside the ring of geosynchronous satellites; it was the first such pass predicted from an earlier apparition.[^palmer2013][^chodas2012] The closest non-impacting pass on record, by 2025 UC11 on 30 October 2025, came within about 6,600 km of Earth's centre, some 240 km above the surface.[^mpc-2025uc11] Collisions are the extreme case. Bodies up to a few tens of metres usually explode high in the [[Atmosphere_of_Earth|atmosphere]], while larger ones reach the ground or the sea.[^chapman1994] One impact model gives a stony impactor of 4 m about once a year, of 7 m (about 15 kilotonnes, like the Hiroshima bomb) every five years, of 60 m (10 megatonnes, like the Tunguska event of 1908) every 1,300 years, and of 1 km every 440,000 years.[^marcus2010] The Chelyabinsk fireball of 15 February 2013, from a 20 m asteroid, released 400–500 kilotonnes, about thirty Hiroshimas (derived).[^david2013] On 7 October 2008 the 4 m asteroid 2008 TC3 became the first impactor spotted before it struck; it exploded 37 km above the Nubian Desert and meteorites were recovered.[^shaddad2010] ### Risk How people judge the danger has shifted with science and culture, from comets read as omens to the crater-forming impacts recognised from the 1980s onward.[^fernandez2012] The case that an [[Asteroid|asteroid]] impact ended the Cretaceous, and the collision of the fragments of Comet Shoemaker–Levy 9 with [[Jupiter]] in July 1994, brought the threat to wide attention.[^chapman1998] Two scales translate orbital predictions into risk. The Torino scale, set up in 1999, rates impacts in the next century from 0 to 10 by probability and energy; the Palermo scale, from 2002, compares an impact's probability with the background rate of similar impacts, as a logarithm, so values above zero deserve attention.[^binzel2000][^cneos-torino][^cneos-palermo] NASA's Sentry system keeps a running table of possible impacts over the next 100 years; nearly every entry drops off as more observations shrink the uncertainty.[^cneos-sentry] Early in that process, the computed probability often rises before it falls, which is what happened with 99942 Apophis: in December 2004 it reached Torino 4 with a chance of up to 2.7% of striking in 2029, and three days of further data ruled that out.[^yeomans2004a][^yeomans2004b] A NASA analysis in 2021 found no impact risk from Apophis for at least a century.[^jpl-apophis2021] The asteroid 2024 YR4 repeated the pattern in early 2025, its 2032 impact probability rising above 2% before falling to effectively zero.[^bassi2025][^lea2025] ### Projects to minimize the threat The first survey devoted to planet-crossing asteroids ran at Palomar, and a 1981 conference in Snowmass, Colorado, first joined impact hazard, dedicated telescopes and deflection in one discussion.[^chapman1998] NASA planned the Spaceguard Survey from 1992 at the request of the US Congress, and in 1998 was directed to find 90% of near-Earth [[Asteroid|asteroids]] larger than 1 km within a decade.[^chapman1998b][^nasa2004] Surveys such as LINEAR, Spacewatch, NEAT, LONEOS and the Catalina Sky Survey raised the known fraction from about 20% in 1998 to 93% in 2011, when NEOWISE results put the total at 981.[^nasa2004][^jpl-wise2011][^mainzer2011] A 2005 act extended the goal to 90% of NEOs of 140 m or more, and NASA's Planetary Defense Coordination Office, created in 2016, took it over.[^pl109-155][^nasa-pdco] By 2023 the completeness for that size was estimated at 38%; the Vera C. Rubin Observatory and the NEO Surveyor telescope, planned for launch in 2027, are expected to raise it to well over half.[^grav2023][^rubin-goals] The ATLAS system aims instead to catch small impactors days before they arrive, too late to deflect but in time to warn.[^atlas2013] All serious mitigation schemes deflect rather than destroy, because the fragments of a shattered body would still hit.[^bnsc2000] ## Number and classification Observations of new NEOs go to the IAU's Minor Planet Center, which keeps lists of confirmed and candidate objects and of potentially hazardous asteroids; CNEOS at JPL and ESA's NEOCC maintain parallel catalogues and close-approach tables.[^marsden1998][^mpc-pha][^cneos-ca][^esa-neocc] At 30 December 2024 CNEOS counted 37,378 NEOs: 37,255 [[Asteroid|asteroids]], of which 2,465 were PHAs, and 123 [[Comet|comets]].[^cneos-stats] ### Observational biases The catalogue over-represents what is easy to see. Large and bright bodies were found first; bodies that come close to Earth appear brighter than equally large ones that do not; and ground-based surveys work at night, so objects approaching from the Sun's side are missed.[^bottke2000][^luu1989] Reflectivity adds further bias: a dark body looks smaller than it is, and phase effects favour the discovery of stony S-types over carbon-rich C-types.[^luu1989] Infrared telescopes in space measure an asteroid's own [[Thermal_radiation|thermal emission]], which depends little on reflectivity, and can look closer to the Sun than ground telescopes, which is the design choice behind NEO Surveyor.[^ucla-ir][^ucla-orbit] Correcting for these effects, Bottke and colleagues estimated in 2000 that about 900 NEAs are brighter than H = 17.75, roughly 1 km across.[^bottke2000] ### Near-Earth asteroids NEAs fall into four classes by orbit, defined against [[Earth]]'s semi-major axis of 1 [[Astronomical_unit|AU]] and its perihelion and aphelion distances of 0.983 and 1.017 AU.[^cneos-groups] Atiras stay entirely inside Earth's orbit; Atens have semi-major axes under 1 AU but aphelia beyond 0.983 AU; Apollos have semi-major axes over 1 AU and perihelia inside 1.017 AU; and Amors approach from outside, with perihelia between 1.017 and 1.3 AU. At the end of 2024 the counts were about 21,100 Apollos, 13,100 Amors, 3,000 Atens and 34 Atiras.[^cneos-stats] The explorer's three real bodies span three of these classes: Apophis is an Aten (a = 0.922 AU), Bennu an Apollo (a = 1.13 AU, perihelion 0.897 AU) and Eros an Amor (perihelion 1.13 AU).[^jpl-sbdb] These orbits are short-lived, lasting a few million years before a planetary encounter, a plunge into the [[Sun]] or ejection from the [[PORTAL_Solar_System|Solar System]] ends them, so the population must be resupplied.[^morbidelli2002] The supply comes from the [[Asteroid_belt|asteroid belt]]: the Yarkovsky push of absorbed and re-emitted sunlight slowly drifts main-belt bodies into resonances with [[Jupiter]], the same resonances that clear the [[Kirkwood_gap|Kirkwood gaps]], and the resonances then pump their eccentricities until they reach the inner system.[^morbidelli2003][^morbidelli2002] A few NEAs are extinct comets. Size estimates are rough for most objects, since they rest on brightness and an assumed reflectivity; asteroid albedos range from about 5% to 30%, so a diameter can be off by a factor of two.[^cneos-stats-intro] At least 104 NEAs have moons, among them 3122 Florence, 4.5 km across, whose two moons were found by radar in 2017.[^johnston-moons][^benner2017] ### Meteoroids [[Meteoroid|Meteoroids]] are the smallest NEOs. The IAU distinguished them from asteroids by size in 1961, and in 2017 adopted a definition that generally limits them to between 30 μm and 1 m, while allowing the word for any object that produces a meteor.[^rubin2010][^imo2017] ### Near-Earth comets Near-Earth comets are NEOs with a coma or tail. Their nuclei are less dense than asteroids but arrive faster, so their impact energy for a given size is somewhat larger, and their debris streams produce [[Meteor_shower|meteor showers]] that may carry large, inert fragments.[^nasa2003][^jenniskens2005] Short-period comets come from the [[Kuiper_belt|Kuiper belt]] and long-period ones from the [[Oort_cloud|Oort cloud]]; because long-period comets are seen once and are faint far from the Sun, surveys have concentrated on asteroids and short-period comets.[^bnsc2000][^nasa2003] Comet 109P/Swift–Tuttle, parent of the Perseids, was briefly a concern in 1992, until older sightings fixed its orbit; it will pass 23 million km from Earth in 2126.[^stephens1993] ### Artificial near-Earth objects Spent rocket stages and retired probes can also end up on NEO-like orbits around the [[Sun]]. J002E3, found in 2002 on a temporary orbit around Earth, turned out to be the third stage of the Saturn V that launched Apollo 12, and 2020 SO was identified as the Centaur upper stage from Surveyor 2 in 1966.[^chesley2002][^jpl-2020so] Active spacecraft have been mistaken too: ESA's Rosetta was briefly catalogued as an asteroid during its 2007 Earth flyby.[^mullins2007] ## Exploratory missions Some NEOs are easier to reach than the [[Moon]], measured by the total change of [[Orbit|orbital]] velocity a spacecraft needs, because they move slowly relative to Earth and have almost no gravity to fight when landing or leaving.[^vergano2007][^xu2007] That makes them useful for science and, some have argued, as future sources of raw materials.[^vergano2007] ### Missions to NEAs NASA's NEAR Shoemaker entered orbit around the [[Asteroid|asteroid]] 433 Eros in February 2000 and landed on it in February 2001.[^nasa-near2001] JAXA's Hayabusa explored the 535 m 25143 Itokawa in 2005 and returned the first asteroid samples; Hayabusa2 studied 162173 Ryugu from June 2018 to November 2019 and delivered its sample capsule in December 2020.[^jaxa-hayabusa][^clark2018][^rincon2020] China's Chang'e 2 flew past 4179 Toutatis in December 2012.[^lakdawalla2012] NASA's OSIRIS-REx reached 101955 Bennu in December 2018, touched its surface in October 2020 and returned samples in 2023; redirected after the sample return, it is due to orbit Apophis from April 2029.[^tillman2023][^loeffler2024] Planetary defence has been tested directly. On 26 September 2022 NASA's DART spacecraft struck Dimorphos, the moon of the asteroid 65803 Didymos; the impact shortened Dimorphos's orbit around Didymos by 33 minutes, and the momentum transferred was about 3.6 times that of the spacecraft, most of it from ejected debris.[^nasa-dart2022a][^nasa-dart2022b] ESA's Hera, launched in October 2024, is to reach Didymos in December 2026 to survey the result.[^esa-hera] Commercial mining ventures have mostly stopped at early studies.[^dorminey2021] ### Missions to NECs The first near-Earth [[Comet|comet]] visited was 21P/Giacobini–Zinner, whose tail the International Cometary Explorer crossed in 1985; six spacecraft met [[Halley's_Comet|Halley's Comet]] in 1986, and Giotto went on to 26P/Grigg–Skjellerup in 1992.[^bnsc2000] Deep Impact flew by 103P/Hartley in November 2010, and ESA's Rosetta orbited 67P/Churyumov–Gerasimenko from August 2014, landed Philae on it that November and ended its mission on the comet's surface in 2016.[^beatty2010][^aron2016] ## See also - [[Asteroid]] · [[Comet]] · [[Meteoroid]] - [[Asteroid_belt]] · [[Kirkwood_gap]] - [[Jupiter_trojan]] - [[Interstellar_object]] - List of Earth-crossing asteroids · Asteroid impact avoidance ## References [^iau-neo]: International Astronomical Union. 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"Does commercial asteroid mining still have a future?". *Forbes*. https://www.forbes.com/sites/brucedorminey/2021/08/31/does-commercial-asteroid-mining-still-have-a-future/ [^beatty2010]: Beatty, K. (4 November 2010). "Mr. Hartley's amazing comet". *Sky & Telescope*. https://skyandtelescope.org/astronomy-news/mr-hartleys-amazing-comet/ [^aron2016]: Aron, J. (30 September 2016). "Rosetta lands on 67P in grand finale to two year comet mission". *New Scientist*. https://www.newscientist.com/article/2107585-rosetta-lands-on-67p-in-grand-finale-to-two-year-comet-mission/ ## External links - NASA/JPL Center for Near-Earth Object Studies. https://cneos.jpl.nasa.gov/ - ESA Near-Earth Object Coordination Centre. https://neo.ssa.esa.int/ - IAU Minor Planet Center: NEO Confirmation Page. https://www.minorplanetcenter.net/iau/NEO/toconfirm_tabular.html - NASA Planetary Defense Coordination Office. https://science.nasa.gov/planetary-defense/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Near-Earth_object) : [Wikitube](https://en.wikitube.io/wiki/Near-Earth_object) · pinned revision [1375277926](https://en.wikipedia.org/w/index.php?oldid=1375277926) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. 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