Editing Near-Earth object (section)

== Number and classification ==
[[File:Known NEAs.svg|thumb|Cumulative discoveries of near-Earth asteroids known by size, 1980–{{#time:Y|-1 year}}|400px]]<!-- The Wikimedia file, which is from the "neo-jpl-stats" source, should be updated (overwritten) annually, using the end-of-the-year version saved from the source at the start of the next year, simultaneously with the other three files & the stats in this article from the "neo-jpl-stats" source -->

When an NEO is detected, like all other small Solar System bodies, its positions and brightness are submitted to the (IAU's) [[Minor Planet Center]] (MPC) for cataloging. The MPC maintains separate lists of confirmed NEOs and potential NEOs.<ref name="MPC-NEO-confirm">{{cite web |title=The NEO Confirmation Page |date=January 1, 2025 |publisher=IAU/MPC |url=http://www.minorplanetcenter.net/iau/NEO/toconfirm_tabular.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250101175114/http://www.minorplanetcenter.net/iau/NEO/toconfirm_tabular.html |archive-date=January 1, 2025}}</ref><ref>{{cite journal |last1=Marsden |first1=B. G. |last2=Williams |first2=G. V. |title=The NEO Confirmation Page |journal=[[Planetary and Space Science]] |volume=46 |issue=2 |pages=299 |bibcode=1998P&SS...46..299M |year=1998 |doi=10.1016/S0032-0633(96)00153-5}}</ref> The MPC maintains a separate list for the potentially hazardous asteroids (PHAs).<ref name="MPC-PHA-list">{{cite web |title=List Of Potentially Hazardous Minor Planets (by designation) |publisher=IAU/MPC |url=https://www.minorplanetcenter.net/iau/lists/t_phas.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250102134144/https://www.minorplanetcenter.net/iau/lists/t_phas.html |archive-date=January 2, 2025}}</ref> NEOs are also catalogued by two separate units of the [[Jet Propulsion Laboratory]] (JPL) of [[NASA]]: the [[Center for Near-Earth Object Studies]] (CNEOS)<ref name="neo-jpl-intro" /> and the Solar System Dynamics Group.<ref name="JPL-SSD-NEA"/> CNEOS's catalog of near-Earth objects includes the approach distances of asteroids and comets.<ref name="NEO-close">{{cite web |url=https://cneos.jpl.nasa.gov/ca/ |title=NEO Earth Close Approaches |date=January 2, 2025 |publisher=NASA/JPL CNEOS |access-date=January 1, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250101175106/https://cneos.jpl.nasa.gov/ca/ |archive-date=January 1, 2025}}</ref> NEOs are also catalogued by a unit of [[European Space Agency|ESA]], the [[Near-Earth Object Coordination Centre]] (NEOCC).<ref name="NEOCC-about">{{cite web |title=About NEOCC |publisher=ESA NEOCC |url=https://neo.ssa.esa.int/about-neocc |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250101040814/https://neo.ssa.esa.int/about-neocc |archive-date=January 1, 2025}}</ref>

Near-Earth objects are classified as [[meteoroid]]s, [[asteroid]]s, or [[comet]]s depending on size, composition, and orbit. Those which are asteroids can additionally be members of an [[asteroid family]], and comets create meteoroid streams that can generate [[meteor shower]]s.

{{As of|2024|12|30|df=US}} and according to statistics maintained by CNEOS, 37,378 NEOs have been discovered. Only 123 (0.33%) of them are comets, whilst 37,255 (99.67%) are asteroids. 2,465 of those NEOs are classified as potentially hazardous asteroids (PHAs).<ref name="neo-jpl-stats" />

{{As of|2025|2|2|df=US}}, 1,886 NEAs appear on the [[Sentry (monitoring system)|Sentry impact risk page]] at the [[NASA]] website.<ref name="Current_Impact_Risks"/> All but 106 of these NEAs are less than 50 meters in diameter, only one recently discovered object has an impact risk meriting a Torino Scale rating higher than zero, while none have a Palermo scale rating higher than zero.<ref name="torino"/>

=== Observational biases ===
The main problem with estimating the number of NEOs is that the probability of detecting one is influenced by a number of  aspects of the NEO, starting naturally with its size but also including the characteristics of its orbit and the reflectivity of its surface.<ref name="science.sciencemag.org">{{Cite journal |last1=Bottke |first1=W. F. Jr. |title=Understanding the Distribution of Near-Earth Asteroids |journal=Science |volume=288 |issue=5474 |pages=2190–2194 |year=2000 |pmid=10864864 |doi=10.1126/science.288.5474.2190 |bibcode=2000Sci...288.2190B}}</ref>  What is easily detected will be more counted, and these [[observational bias]]es need to be compensated when trying to calculate the number of bodies in a population from the list of its detected members.<ref name="science.sciencemag.org"/>
[[File:Artist%E2%80%99s_impression_of_an_asteroid_that_orbits_closer_to_the_Sun_than_Earth%E2%80%99s_orbit.jpg|thumb|right|Artist's impression of an asteroid that orbits closer to the Sun than Earth's orbit, showing its dark side]]

Bigger asteroids reflect more light, and the two biggest near-Earth objects, [[433 Eros]] and [[1036 Ganymed]], were naturally also among the first to be detected.<ref name="Browne">{{Cite news |first=Malcolm W. |last=Browne |title=Mathematicians Say Asteroid May Hit Earth in a Million Years |date=April 25, 1996 |work=The New York Times |url=https://www.nytimes.com/1996/04/25/us/mathematicians-say-asteroid-may-hit-earth-in-a-million-years.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241204111320/https://www.nytimes.com/1996/04/25/us/mathematicians-say-asteroid-may-hit-earth-in-a-million-years.html |archive-date=December 4, 2024}}</ref> 1036 Ganymed is about {{convert|35|km|mi|abbr=on}} in diameter and 433 Eros is about {{convert|17|km|mi|abbr=on}} in diameter.<ref name="Browne"/> Meanwhile, the apparent brightness of objects that are closer is higher, introducing a bias that favours the discovery of NEOs of a given size that get closer to Earth.<ref name="LuuJewitt1989"/>

Earth-based astronomy requires dark skies and hence nighttime observations, and even space-based telescopes avoid looking into directions close to the Sun, thus most NEO surveys are blind towards objects passing Earth on the side of the Sun.<ref name="LuuJewitt1989"/><ref name="NEOS-orbit"/> This bias is further enhanced by the effect of [[planetary phase|phase]]: the narrower the angle of the asteroid and the Sun from the observer, the lesser part of the observed side of the asteroid will be illuminated.<ref name="LuuJewitt1989"/> Another bias results from the different surface brightness or albedo of the objects, which can make a large but low-albedo object as bright as a small but high-albedo object.<ref name="LuuJewitt1989"/><ref name="NEOS-why-infrared"/> In addition, the reflexivity of asteroid surfaces is not uniform but increases towards the direction opposite of illumination, resulting in the phenomenon of phase darkening, which makes asteroids even brighter when the Earth is close to the axis of sunlight.<ref name="LuuJewitt1989"/> An asteroid's observed albedo usually has a strong peak or [[opposition surge]] very close to the direction opposite of the Sun.<ref name="LuuJewitt1989"/> Different surfaces display different levels of phase darkening, and research showed that, on top of albedo bias, this favours the discovery of silicon-rich [[S-type asteroid]]s over carbon-rich [[C-type asteroid|C types]], for example.<ref name="LuuJewitt1989">{{cite journal |first1=Jane |last1=Luu |first2=David |last2=Jewitt |title=On the Relative Numbers of C Types and S Types among Near-Earth Asteroids |journal=[[The Astronomical Journal]] |volume=98 |issue=5 |pages=1905–1911 |date=November 1989 |doi=10.1086/115267 |bibcode=1989AJ.....98.1905L}}</ref> As a result of these observational biases, in Earth-based surveys, NEOs tended to be discovered when they were in opposition, that is, opposite from the Sun when viewed from the Earth.<ref name="Grav2023"/>

The most practical way around many of these biases is to use [[thermal infrared]] telescopes in space that observe their thermal emissions instead of the visible light they reflect, with a sensitivity that is almost independent of the illumination.<ref name="Grav2023"/><ref name="NEOS-why-infrared">{{cite web |title=Why Infrared? |date=13 May 2024 |url=https://neos.epss.ucla.edu/index.php/2024/05/13/why-infared/ |publisher=UCLA |access-date=January 2, 2025}}</ref> In addition, space-based telescopes in an orbit around the Sun in the shadow of the Earth can make observations as close as 45 degrees to the direction of the Sun.<ref name="NEOS-orbit">{{cite web |title=Mission Orbit and Timeline |date=13 May 2024 |url=https://neos.epss.ucla.edu/index.php/2024/05/13/mission-orbit-and-timeline/ |publisher=UCLA |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250102153424/https://neos.epss.ucla.edu/index.php/2024/05/13/mission-orbit-and-timeline/ |archive-date=January 2, 2025}}</ref>

Further observational biases favour objects that have more frequent encounters with the Earth, which makes the detection of [[Aten asteroid|Atens]] more likely than that of [[Apollo asteroid|Apollos]]; and objects that move slower when encountering the Earth, which makes the detection of NEAs with low eccentricities more likely.<ref>{{cite journal |first1=William F. Jr. |last1=Bottke |first2=Michael C. |last2=Nolan |first3=H. Jay |last3=Melosh |first4=Ann M. |last4=Vickery |first5=Richard |last5=Greenberg |title=Origin of the Spacewatch Small Earth-Approaching Asteroids |journal=Icarus |date=August 1996 |volume=122 |issue=2 |pages=406–427 |url=https://www.boulder.swri.edu/~bottke/Reprints/Bottke_1996_Icarus_122_406_Origin_Spacewatch_NEOs.pdf |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241208013553/https://www.boulder.swri.edu/~bottke/Reprints/Bottke_1996_Icarus_122_406_Origin_Spacewatch_NEOs.pdf |archive-date=December 8, 2024 |doi=10.1006/icar.1996.0133 |bibcode=1996Icar..122..406B}}</ref>

Such observational biases must be identified and quantified to determine NEO populations, as studies of asteroid populations then take those known observational selection biases into account to make a more accurate assessment.<ref>{{cite journal |first1=B. |last1=Zellner |first2=E. |last2=Bowell |title=2. Asteroid Compositional Types and their Distributions |journal=International Astronomical Union Colloquium |year=1977 |volume=39 |pages=185–197 |doi=10.1017/S0252921100070093 |s2cid=128650102 |doi-access=free}}</ref> In the year 2000 and taking into account all known observational biases, it was estimated that there are approximately 900 near-Earth asteroids of at least kilometer size, or technically and more accurately, with an [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] brighter than 17.75.<ref name="science.sciencemag.org"/>

=== Near-Earth asteroids{{anchor|Near-Earth_asteroids}}<!--'Near-Earth asteroid' and 'Near-Earth asteroids' redirect here--> ===
[[File:ESO-Asteroid Toutatis-phot-28c-04-normal.jpg|thumb|One-minute path of asteroid [[4179 Toutatis]] in the sky during its September 2004 close approach ([[Paranal Observatory]])]]

These are asteroids in a near-Earth orbit without the tail or coma of a comet. {{As of|2024|12}}, 37,255 '''near-Earth asteroids'''<!--boldface per WP:R#PLA--> (NEAs) are known, 2,465 of which are both sufficiently large and may come sufficiently close to Earth to be classified as potentially hazardous.<ref name="neo-jpl-stats" />

NEAs survive in their orbits for just a few million years.<ref name="MorbidelliAstIII">{{cite book |first1=Alessandro |last1=Morbidelli |first2=William F. Jr. |last2=Bottke |first3=Christiane |last3=Froeschlé |first4=Patrick |last4=Michel |chapter=Origin and Evolution of Near-Earth Objects |editor-first1=W. F. |editor-last1=Bottke Jr. |editor-first2=A. |editor-last2=Cellino |editor-first3=P. |editor-last3=Paolicchi |editor-first4=R. P. |editor-last4=Binzel |display-editors=1 |title=Asteroids III |pages=409–422 |date=January 2002 |doi=10.2307/j.ctv1v7zdn4.33 |bibcode=2002aste.book..409M |url=http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |access-date=December 31, 2024 |url-status=live |archive-url=https://web.archive.org/web/20170809014123/http://www.boulder.swri.edu/%7Ebottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |archive-date=August 9, 2017}}</ref> They are eventually eliminated by planetary [[Perturbation (astronomy)|perturbations]], causing ejection from the Solar System or a [[Impact event|collision]] with the Sun, a planet, or other celestial body.<ref name="MorbidelliAstIII"/> With orbital lifetimes short compared to the age of the Solar System, new asteroids must be constantly moved into near-Earth orbits to explain the observed asteroids. The accepted origin of these asteroids is that [[Asteroid belt|main-belt asteroids]] are moved into the inner Solar System through [[orbital resonance]]s with [[Jupiter]].<ref name="MorbidelliAstIII"/> The interaction with Jupiter through the resonance [[Perturbation (astronomy)|perturb]]s the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as [[Kirkwood gap]]s, where these resonances occur as the asteroids in these resonances have been moved onto other orbits. New asteroids migrate into these resonances, due to the [[Yarkovsky effect]] that provides a continuing supply of near-Earth asteroids.<ref>{{cite journal |title=The Yarkovsky-driven origin of near-Earth asteroids |first1=A. |last1=Morbidelli |first2=D. |last2=Vokrouhlický |journal=Icarus |volume=163 |issue=1 |pages=120–134 |date=May 2003 |doi=10.1016/S0019-1035(03)00047-2 |bibcode=2003Icar..163..120M |citeseerx=10.1.1.603.7624}}</ref> Compared to the entire mass of the asteroid belt, the mass loss necessary to sustain the NEA population is relatively small; totalling less than 6% over the past 3.5 billion years.<ref name="MorbidelliAstIII"/> The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of [[asteroid spectral types]].<ref>{{cite journal |title=On the Origins of Earth-Approaching Asteroids |first1=D.F. |last1=Lupishko |first2=T.A. |last2=Lupishko |name-list-style=amp |journal=[[Solar System Research]] |volume=35 |issue=3 |pages=227–233 |date=May 2001 |doi=10.1023/A:1010431023010 |bibcode = 2001SoSyR..35..227L|s2cid=117912062}}</ref>

A small number of NEAs are [[extinct comets]] that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with [[Jupiter]].<ref name = "MorbidelliAstIII" /><ref>{{cite journal |title=What the physical properties of near-Earth asteroids tell us about sources of their origin? |first1=D.F. |last1=Lupishko |first22=M. |last2=di Martino |first3=T.A. |last3=Lupishko |name-list-style=amp |journal=Kinematika I Fizika Nebesnykh Tel Supplimen |volume=3 |issue=3 |pages=213–216 |date=September 2000 |bibcode=2000KFNTS...3..213L}}</ref>

Many asteroids have [[natural satellite]]s ([[minor-planet moon]]s). {{As of|2024|12|df=US}}, 104 NEAs were known to have at least one moon, including five known to have two moons.<ref>{{cite web |title=Asteroids with Satellites |publisher=Johnston's Archive |date=December 28, 2024 |url=http://www.johnstonsarchive.net/astro/asteroidmoons.html |access-date=January 3, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250103104505/https://www.johnstonsarchive.net/astro/asteroidmoons.html |archive-date=January 3, 2025}}</ref> The asteroid [[3122 Florence]], one of the largest PHAs<ref name="MPC-PHA-list"/> with a diameter of {{convert|4.5|km|mi|abbr=on}}, has two moons measuring {{convert|100–300|m|ft|abbr=on}} across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth.<ref name="Florence-moons">{{cite news |first1=Lance |last1=Benner |first2=Shantanu |last2=Naidu |first3=Marina |last3=Brozovic |first4=Paul |last4=Chodas |title=Radar Reveals Two Moons Orbiting Asteroid Florence |date=September 1, 2017 |work=News |publisher=NASA/JPL CNEOS |url=https://cneos.jpl.nasa.gov/news/news199.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20170903060914/https://cneos.jpl.nasa.gov/news/news199.html |archive-date=September 3, 2017}}</ref>

In May 2022, an algorithm known as Tracklet-less Heliocentric Orbit Recovery or THOR and developed by University of Washington researchers to discover asteroids in the solar system was announced as a success.<ref>{{Cite news |title=UW-developed, cloud-based astrodynamics platform to discover and track asteroids |date=May 31, 2022 |publisher=University of Washington |work=UW News |url=https://www.washington.edu/news/2022/05/31/asteroid-discovery/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241203203327/https://www.washington.edu/news/2022/05/31/asteroid-discovery/ |archive-date=December 3, 2024}}</ref> The International Astronomical Union's Minor Planet Center confirmed a series of first candidate asteroids identified by the algorithm.<ref>{{Cite press release |title=Asteroid Institute Uses Revolutionary Cloud-Based Astrodynamics Platform to Discover and Track Asteroids |date=May 31, 2022 |publisher=B612 Foundation |work=PR Newswire |url=https://www.prnewswire.com/news-releases/asteroid-institute-uses-revolutionary-cloud-based-astrodynamics-platform-to-discover-and-track-asteroids-301557487.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241219010657/https://www.prnewswire.com/news-releases/asteroid-institute-uses-revolutionary-cloud-based-astrodynamics-platform-to-discover-and-track-asteroids-301557487.html |archive-date=December 19, 2024}}</ref>

==== Size distribution ====
[[File:NEA by size.svg|400px|thumb|Known near-Earth asteroids by size]]<!-- The Wikimedia file, which is from the "neo-jpl-stats" source, should be updated (overwritten) annually, using the end-of-the-year version saved from the source at the start of the next year, simultaneously with the other three files & the stats in this article from the "neo-jpl-stats" source -->

While the size of a very small fraction of these asteroids is known to better than 1%, from [[radar]] observations, from images of the asteroid surface, or from [[Occultation#Occultations by asteroids|stellar occultations]], the diameter of the vast majority of near-Earth asteroids has only been estimated on the basis of their brightness and a representative asteroid surface reflectivity or [[albedo]], which is commonly assumed to be 14%.<ref name="neo-jpl-intro">{{cite web |title=Discovery Statistics. Introduction |publisher=NASA/JPL CNEOS |url=https://cneos.jpl.nasa.gov/stats/ |date=2012 |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240126214444/https://cneos.jpl.nasa.gov/stats/ |archive-date=January 26, 2024}}</ref> Such indirect size estimates are uncertain by over a factor of 2 for individual asteroids, since asteroid albedos can range at least as low as 5% and as high as 30%. This makes the volume of those asteroids uncertain by a factor of 8, and their mass by at least as much, since their assumed density also has its own uncertainty. Using this crude method, an [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] of 17.75 roughly corresponds to a diameter of {{convert|1|km|mi|abbr=on|lk=off}}<ref name="neo-jpl-intro"/> and an absolute magnitude of 22.0 to a diameter of {{convert|140|m|ft|abbr=on|lk=off}}.<ref name="CNEOS-NEO-groups"/>  Diameters of intermediate precision, better than from an assumed albedo but not nearly as precise as good direct measurements, can be obtained from the combination of reflected light and thermal infrared emission, using a thermal model of the asteroid to estimate both its diameter and its albedo. The reliability of this method, as applied by the [[Wide-field Infrared Survey Explorer]] and NEOWISE missions, has been the subject of a dispute between experts, with the 2018 publication of two independent analyses, one criticising and another giving results consistent with the WISE method.<ref name="NYT-20180614">{{cite news |last=Chang |first=Kenneth |title=Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks |date=June 14, 2018 |work=The New York Times |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html |access-date=January 2, 2025 |url-access=subscription  |url-status=live |archive-url=https://web.archive.org/web/20241001084248/https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html |archive-date=October 1, 2024}}{{cbignore}}</ref> A 2023 study re-evaluated the relationship of brightness, albedo and diameter. For many objects with a diameter larger than 1&nbsp;km, brightness estimates were reduced slightly. Meanwhile, based on new albedo estimates of smaller objects, the study found that {{nowrap|H {{=}} 23}} best corresponds to a diameter of 140&nbsp;m.<ref name="Grav2023"/>

In 2000, NASA reduced from 1,000–2,000 to 500–1,000 its estimate of the number of existing near-Earth asteroids over one kilometer in diameter, or more exactly brighter than an absolute magnitude of 17.75.<ref>{{cite news |first=Jane |last=Platt |title=Asteroid Population Count Slashed |date=January 12, 2000 |work=Press Releases |publisher=NASA/JPL |url=https://www.jpl.nasa.gov/news/asteroid-population-count-slashed |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240127123244/https://www.jpl.nasa.gov/news/asteroid-population-count-slashed |archive-date=January 27, 2024}}</ref><ref name="Rabinowitzetal00">{{cite journal |title=A reduced estimate of the number of kilometer-sized near-Earth asteroids |journal=Nature |first1=David |last1=Rabinowitz |first2=Eleanor |last2=Helin |first3=Kenneth |last3=Lawrence |first4=Steven |last4=Pravdo |name-list-style=amp |date=January 13, 2000 |volume=403 |pages=165–166 |doi=10.1038/35003128 |pmid=10646594 |issue= 6766 |bibcode=2000Natur.403..165R |s2cid=4303533}}</ref> Shortly thereafter,  the [[LINEAR]] survey provided an alternative estimate  of {{val|1227|+170|-90|fmt=commas}}.<ref name="LINEAR-asteroid-census">{{cite journal |title=A Near-Earth Asteroid Population Estimate from the LINEAR Survey |journal=Science |first=J. S. |last=Stuart |date=November 23, 2001 |volume=294 |issue=5547 |pages=1691–1693 |doi=10.1126/science.1065318 |pmid=11721048 |bibcode=2001Sci...294.1691S |s2cid=37849062}}</ref> In 2011, on the basis of NEOWISE observations, the estimated number of one-kilometer NEAs was narrowed to {{val|981|19}} (of which 93% had been discovered at the time), while the number of NEAs larger than 140 meters across was estimated at {{val|13200|1900|fmt=commas}}.<ref name="pia14734"/><ref name="WISE-asteroid-census">{{cite journal |title=NEOWISE Observations of Near-Earth Objects: Preliminary Results |display-authors=3|journal=[[The Astrophysical Journal]] |first1=A. |last1=Mainzer |first2=T. |last2=Grav |first3=J. |last3=Bauer |first4=J. |last4=Masiero |first5=R. S. |last5=McMillan |first6=R. M. |last6=Cutri |first7=R. |last7=Walker |first8=E. |last8=Wright |first9=P. |last9=Eisenhardt |first10=D. J. |last10=Tholen |first11=T. |last11=Spahr |first12=R. |last12=Jedicke |first13=L. |last13=Denneau |first14=E. |last14=DeBaun |first15=D. |last15=Elsbury |first16=T. |last16=Gautier |first17=S. |last17=Gomillion |first18=E. |last18=Hand |first19=W. |last19=Mo |first20=J. |last20=Watkins |first21=A. |last21=Wilkins |first22=G. L. |last22=Bryngelson |first23=A. |last23=Del Pino Molina |first24=S. |last24=Desai |first25=M. |last25=Go'mez Camus |first26=S. L. |last26=Hidalgo |first27=I. |last27=Konstantopoulos |first28=J. A. |last28=Larsen |first29=C. |last29=Maleszewski |first30=M. A. |last30=Malkan |first31=J.-C. |last31=Mauduit |first32=B. L. |last32=Mullan |first33=E. W. |last33=Olszewski |first34=J. |last34=Pforr |first35=A. |last35=Saro |first36=J. V. |last36=Scotti |first37=L. H. |last37=Wasserman |date=December 20, 2011 |volume=743 |issue=2 |page=156 |arxiv = 1109.6400 |bibcode = 2011ApJ...743..156M |doi = 10.1088/0004-637X/743/2/156 |s2cid=239991}}</ref> The NEOWISE estimate differed from other estimates primarily in assuming a slightly lower average asteroid albedo, which produces larger estimated diameters for the same asteroid brightness. This resulted in 911 then known asteroids at least 1&nbsp;km across, as opposed to the 830 then listed by CNEOS from the same inputs but assuming a slightly higher albedo.<ref>{{cite news |first=Kelly |last=Beatty |title=WISE's Survey of Near-Earth Asteroids |date=September 30, 2011 |work=Sky & Telescope |url=http://www.skyandtelescope.com/astronomy-news/wises-survey-of-near-earth-asteroids/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20231024233026/https://skyandtelescope.org/astronomy-news/wises-survey-of-near-earth-asteroids/ |archive-date=October 24, 2023}}</ref> In 2017, two studies using an improved statistical method reduced the estimated number of NEAs brighter than absolute magnitude 17.75 (approximately over one kilometer in diameter) slightly to {{val|921|20}}.<ref name="NEA1km-est-2017">{{cite news |first=Matt |last=Williams |title=Good News Everyone! There are Fewer Deadly Undiscovered Asteroids than we Thought |date=October 20, 2017 |work=[[Universe Today]] |url=https://www.universetoday.com/137583/good-news-everyone-less-deadly-undiscovered-asteroids-thought/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20171104064609/https://www.universetoday.com/137583/good-news-everyone-less-deadly-undiscovered-asteroids-thought/ |archive-date=November 4, 2017}}</ref><ref name="Tricario"/> The estimated number of near-Earth asteroids brighter than absolute magnitude of 22.0 (approximately over 140&nbsp;m across) rose to {{val|27100|2200|fmt=commas}}, double the WISE estimate, of which about a fourth were known at the time.<ref name="Tricario">{{cite journal |first=Pasquale |last=Tricarico |title=The near-Earth asteroid population from two decades of observations |journal=Icarus |volume=284 |pages=416–423 |date=March 1, 2017 |url=http://orbit.psi.edu/~tricaric/pdf/Tricarico_NEA_population_Icarus_2017.pdf |access-date=March 10, 2018 |url-status=dead |archive-url=https://web.archive.org/web/20180310073945/http://orbit.psi.edu/~tricaric/pdf/Tricarico_NEA_population_Icarus_2017.pdf |archive-date=March 10, 2018 |doi=10.1016/j.icarus.2016.12.008 |arxiv=1604.06328 |bibcode=2017Icar..284..416T |s2cid=85440139}}</ref> The number of asteroids brighter than {{nowrap|H {{=}} 25}}, which corresponds to about {{convert|40|m|ft|abbr=on}} in diameter, is estimated at {{val|840000|23000|fmt=commas}}—of which about 1.3 percent had been discovered by February 2016; the number of asteroids brighter than {{nowrap|H {{=}} 30}} (larger than {{convert|3.5|m|ft|abbr=on}}) is estimated at {{val|400|100}} million—of which about 0.003 percent had been discovered by February 2016.<ref name="Tricario"/>

A September 2021 study revised the estimated number of NEAs with a diameter larger than 1&nbsp;km (using both WISE data and the absolute brightness lower than 17.75 as proxy) slightly upwards to {{val|981|19}}, of which 911 were discovered at the time, but reduced the estimated number of asteroids brighter than absolute magnitude of 22.0 (as proxy for a diameter of 140&nbsp;m) to under 20,000, of which about half were discovered at the time.<ref name="HarrisChodas2021"/><!-- The 140 m numbers can only be read from Figure 15 in the article, the completion ratio is explicitly "~50%" in the text--> The 2023 study that re-evaluated the relationship of average absolute brightness, albedo and diameter confirmed the ratios of the number of discovered and estimated total asteroids of different sizes in the 2021 study<!-- See pages 11 & 12 of the 2023 study. Based solely on brightness-diameter modelling, their completeness ratios are 88% for H <= 17.75 and 38% for H <= 23, but state that NEOWISE data outside of their model adds 6% 1 km objects and the competeness is 44% for 140 m objects.-->, but by changing the proxy for a diameter of 140&nbsp;m to {{nowrap|H {{=}} 23}}, it estimated that only about 44% of the estimated 35,000 total larger than that have been discovered by the end of 2022.<ref name="Grav2023"/> {{As of|2024|1|df=US}}, NEO catalogues still use {{nowrap|H {{=}} 22}} as proxy for a diameter of 140&nbsp;m.<ref name="CNEOS-NEO-groups"/>

{{As of|2024|12|30|df=US}}, and using diameters mostly estimated crudely from a measured absolute magnitude and an assumed albedo, 867 NEAs listed by CNEOS, including 152 PHAs, measure at least 1&nbsp;km in diameter, and 11,167 known NEAs, including 2,465 PHAs, are larger than 140&nbsp;m in diameter.<ref name="neo-jpl-stats"/>

The smallest known near-Earth asteroid is {{mp|2015 FF|415}} with an absolute magnitude of 34.34,<ref name="JPL-SSD-NEA"/> corresponding to an estimated diameter of about {{convert|0.5|m|ft|abbr=on}}.<ref name="h" /> The largest such object is [[1036 Ganymed]],<ref name="JPL-SSD-NEA"/> with an absolute magnitude of 9.18 and directly measured irregular dimensions which are equivalent to a diameter of about {{convert|38|km|mi|abbr=on}}.<ref>{{cite web |title=1036 Ganymed (A924 UB) |date=January 2, 2024 |publisher=NASA/JPL |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=1036 |access-date=January 3, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250103105326/https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1036 |archive-date=January 3, 2025}}</ref>

==== Orbital classification ====
[[File:Neo orbit types.jpg|thumb|upright=1.4|NEA orbital groups (NASA/JPL)]]
Near-Earth asteroids are divided into groups based on their [[semi-major axis]] (a), [[Apsis|perihelion]] distance (q), and [[Apsis|aphelion]] distance (Q):<ref name="CNEOS-NEO-groups"/><ref name="NEOCC-DA"/>
* The ''[[Atira asteroid|Atiras]]'' or ''Apoheles'' have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983&nbsp;AU). That is, {{nowrap|Q < 0.983 AU}}, which implies that the asteroid's semi-major axis is also less than 0.983 AU.<ref name="atiras">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=August 1, 2019 |title=Understanding the evolution of Atira-class asteroid 2019 AQ<sub>3</sub>, a major step towards the future discovery of the Vatira population |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume= 487 |issue= 2 |pages= 2742–2752 |arxiv=1905.08695 |bibcode=2019MNRAS.487.2742D |doi=10.1093/mnras/stz1437 |doi-access=free |s2cid=160009327}}</ref> This group includes asteroids on orbits that never get close to Earth, including the sub-group of [[Atira asteroid#ꞌAylóꞌchaxnim asteroids|ꞌAylóꞌchaxnims]], which orbit the Sun entirely within the orbit of [[Venus (planet)|Venus]]<ref>{{cite journal |last1=Bolin |first1=Bryce T. |display-authors=et al |date=November 2022 |title=The discovery and characterization of (594913) 'Ayló'chaxnim, a kilometre sized asteroid inside the orbit of Venus |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=517 |issue=1 |pages=L49–L54 |doi=10.1093/mnrasl/slac089 |doi-access=free |arxiv=2208.07253}}</ref> and which include the hypothetical sub-group of [[Vulcanoid]]s, which have orbits entirely within the orbit of [[Mercury (planet)|Mercury]].<ref>{{cite news |first=David |last=Dickinson |title=Astronomers Discover Asteroid that Flies Close to the Sun |date=August 25, 2021 |work=Sky & Telescope |url=https://skyandtelescope.org/astronomy-blogs/astronomy-space-david-dickinson/astronomers-discover-asteroid-that-flies-close-to-the-sun/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241130073024/https://skyandtelescope.org/astronomy-blogs/astronomy-space-david-dickinson/astronomers-discover-asteroid-that-flies-close-to-the-sun/ |archive-date=November 30, 2024}}</ref>
* The ''[[Aten asteroid|Atens]]'' have a semi-major axis of less than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a < 1.0 AU}} and {{nowrap|Q > 0.983 AU}}. (0.983 AU is Earth's perihelion distance.)
* The ''[[Apollo asteroid|Apollos]]'' have a semi-major axis of more than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a > 1.0 AU}} and {{nowrap|q < 1.017 AU}}. (1.017&nbsp;AU is Earth's aphelion distance.)
* The ''[[Amor asteroid|Amors]]'' have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017&nbsp;AU). Amor asteroids are also near-Earth objects so {{nowrap|q < 1.3 AU}}. In summary, {{nowrap|1.017 AU < q < 1.3 AU}}. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017&nbsp;AU.) Some Amor asteroid orbits cross the orbit of Mars.

Some authors define Atens differently: they define it as being all the asteroids with a semi-major axis of less than 1&nbsp;AU.<ref>{{cite web |title=Unusual Minor Planets |publisher=IAU/MPC |url=https://minorplanetcenter.net/iau/Unusual.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241130124156/https://minorplanetcenter.net/iau/Unusual.html |archive-date=November 30, 2024}}</ref><ref name="galache"/> That is, they consider the Atiras to be part of the Atens.<ref name="galache">{{cite web |first=J. L. |last=Galache |title=Asteroid Classification I – Dynamics |publisher=IAU/MPC |date=March 5, 2011 |url=http://minorplanetcenter.net/blog/asteroid-classification-i-dynamics/ |access-date=March 9, 2018 |url-status=dead |archive-url=https://web.archive.org/web/20160303235814/http://minorplanetcenter.net/blog/asteroid-classification-i-dynamics/ |archive-date=March 3, 2016}}</ref> Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.

Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.<ref name="MorbidelliAstIII"/><ref>{{Cite journal |last1=Ribeiro |first1=A. O. |last2=Roig |first2=F.|last3=De Prá |first3=M. N. |last4=Carvano |first4=J. M. |last5=DeSouza |first5=S. R. |title=Dynamical study of the Atira group of asteroids |date=March 17, 2016 |journal=Monthly Notices of the Royal Astronomical Society |volume=458 |issue=4 |pages=4471–4476 |doi=10.1093/mnras/stw642 |issn=0035-8711 |doi-access=free}}</ref>

{{As of|2024|12|30|df=US}}, 34 Atiras, 2,952 Atens, 21,132 Apollos and 13,137 Amors have been discovered and cataloged.<ref name="neo-jpl-stats" />

==== Co-orbital asteroids ====
[[File:Lagrange Horseshoe Orbit.jpg|thumb|The five Lagrangian points relative to the Sun and Earth and possible orbits along gravitational contours]]

Most NEAs have orbits that are significantly more [[Orbital eccentricity|eccentric]] than that of the Earth and the other major planets and their orbital planes can [[Orbital inclination|tilt]] several degrees relative to that of the Earth. NEAs which have orbits that do resemble the Earth's in eccentricity, inclination and semi-major axis are grouped as [[Arjuna asteroid]]s.<ref name="2023FY3_2024"/> Within this group are NEAs that have the same orbital period as the Earth, or a [[co-orbital configuration]], which corresponds to an [[orbital resonance]] at a ratio of 1:1. All co-orbital asteroids have special orbits that are relatively stable and, paradoxically, can prevent them from getting close to Earth:
* ''[[Trojan (astronomy)|Trojans]]'': Near the orbit of a planet, there are five gravitational equilibrium points, the [[Lagrangian point]]s, in which an asteroid would orbit the Sun in fixed formation with the planet. Two of these, 60 degrees ahead and behind the planet along its orbit (designated L4 and L5 respectively) are stable; that is, an asteroid near these points would stay there for thousands or even millions of years in spite of light perturbations by other planets and by non-gravitational forces. Trojans circle around L4 or L5 on paths resembling a [[tadpole]].<ref name="Fuentes-horseshoe"/> {{As of|2023|10}}, Earth has two confirmed Trojans:<ref name="CastroCisneros2023"/> {{mpl|(706765) 2010 TK|7}} and {{mpl|(614689) 2020 XL|5}}, both circling Earth's L4 point.<ref name="WISE">{{cite press release |title=NASA's WISE mission finds first Trojan asteroid sharing Earth's orbit |date=July 27, 2011 |work=PR Newswire |publisher=[[NASA]] |url=https://www.prnewswire.com/news-releases/nasas-wise-mission-finds-first-trojan-asteroid-sharing-earths-orbit-126277963.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240127131203/https://www.prnewswire.com/news-releases/nasas-wise-mission-finds-first-trojan-asteroid-sharing-earths-orbit-126277963.html |archive-date=January 27, 2024}}</ref><ref name="Trojan2">{{cite news |first=Chelsea |last=Gohd |title=Earth has an extra companion, a Trojan asteroid that will hang around for 4,000 years |date=February 1, 2022 |work=Space.com |url=https://www.space.com/earth-extra-moon-trojan-asteroid-2020-xl5-discovery |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241208100907/https://www.space.com/earth-extra-moon-trojan-asteroid-2020-xl5-discovery |archive-date=December 8, 2024}}</ref>
* ''[[Horseshoe orbit|Horseshoe librators]]'': The region of stability around L4 and L5 also includes orbits for co-orbital asteroids that run around both L4 and L5. Relative to the Earth and Sun, the orbit can resemble the circumference of a horseshoe, or may consist of annual loops that wander back and forth ([[Libration|librate]]) in a horseshoe-shaped area. In both cases, the Sun is at the horseshoe's center of gravity, Earth is in the gap of the horseshoe, and L4 and L5 are inside the ends of the horseshoe. Among Earth's known co-orbitals, those with the most stable orbits as well as those with the least stable orbits are horseshoe librators.<ref name="Fuentes-horseshoe">{{cite journal |last1=de&nbsp;la&nbsp;Fuente Marcos |first1=C. |last2=de&nbsp;la&nbsp;Fuente Marcos |first2=R. |title=A trio of horseshoes: Past, present, and future dynamical evolution of Earth co-orbital asteroids {{mp|2015 XX|169}}, {{mp|2015 YA}} and {{mp|2015 YQ|1}} |journal=[[Astrophysics and Space Science]] |volume=361 |issue=4 |pages=121–133 |date=April 2016 |doi=10.1007/s10509-016-2711-6 |arxiv=1603.02415 |bibcode=2016Ap&SS.361..121D |s2cid=119222384}}</ref> {{As of|2023|10}}, at least 13 horseshoe librators of Earth have been discovered.<ref name="CastroCisneros2023"/> The most-studied and, at about {{convert|5|km|mi|abbr=on}}, largest is [[3753 Cruithne]], which travels along bean-shaped annual loops and completes its horseshoe libration cycle every 770–780 years.<ref>{{cite journal |title=An asteroidal companion to the Earth |type=letter |last1=Wiegert |first1=Paul A. |last2=Innanen |first2=Kimmo A. |last3=Mikkola |first3=Seppo |journal=Nature |date=June 12, 1997 |volume=387 |issue=6634 |pages=685–686 |doi=10.1038/42662 |bibcode=1997Natur.387..685W |s2cid=4305272 |url=http://www.astro.uwo.ca/~pwiegert/papers/1997Nature.387.685.pdf |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241203022402/https://physics.uwo.ca/~pwiegert/papers/1997Nature.387.685.pdf |archive-date=December 3, 2024}}</ref><ref>{{cite web |first=Brad |last=Snowder |title=Cruithne |publisher=Western Washington University Planetarium |url=https://astro101.wwu.edu/a101_cruithne.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240101150817/https://astro101.wwu.edu/a101_cruithne.html |archive-date=January 1, 2024}}</ref> {{mpl|419624|2010 SO|16}} is an asteroid on a relatively stable circumference-of-a-horseshoe orbit, with a horseshoe [[libration]] period of about 350 years.<ref>{{cite journal |last1=Christou |first1=A.A. |last2=Asher |first2=D.J. |title=A long-lived horseshoe companion to the Earth |date=July 11, 2011 |journal=Monthly Notices of the Royal Astronomical Society |volume=414 |issue=4 |pages=2965–2969 |doi=10.1111/j.1365-2966.2011.18595.x |doi-access=free |arxiv=1104.0036 |bibcode=2011MNRAS.414.2965C |s2cid=13832179}}</ref>
* ''[[Quasi-satellite]]s'': Quasi-satellites are co-orbital asteroids on a normal elliptic orbit with a higher eccentricity than Earth's, which they travel in a way synchronised with Earth's motion. Since the asteroid orbits the Sun slower than Earth when further away and faster than Earth when closer to the Sun, when observed in a rotating frame of reference fixed to the Sun and the Earth, the quasi-satellite appears to orbit Earth in a [[retrograde motion|retrograde]] direction in one year, even though it is not bound gravitationally. {{As of|2023|10}}, six asteroids were known to be a quasi-satellite of Earth.<ref name="CastroCisneros2023"/> [[469219 Kamoʻoalewa]] is Earth's closest quasi-satellite, in an orbit that has been stable for almost a century.<ref name="Fuentes-2016HO3">{{cite journal |last1=de&nbsp;la&nbsp;Fuente Marcos |first1=C. |last2=de&nbsp;la&nbsp;Fuente Marcos |first2=R. |title=Asteroid {{mp|469219|2016 HO|3}}, the smallest and closest Earth quasi-satellite |journal=Monthly Notices of the Royal Astronomical Society |volume=462 |issue=4 |pages=3441–3456 |date=November 11, 2016 |doi=10.1093/mnras/stw1972 |doi-access=free |arxiv=1608.01518 |bibcode=2016MNRAS.462.3441D |s2cid=118580771}}</ref> This asteroid is thought to be a piece of the Moon ejected during an impact.<ref name="CastroCisneros2023">{{cite journal |first1=Jose Daniel |last1=Castro-Cisneros |first2=Renu |last2=Malhotra |first3=Aaron J. |last3=Rosengren |title=Lunar ejecta origin of near-Earth asteroid Kamo'oalewa is compatible with rare orbital pathways |date=October 23, 2023 |journal=[[Communications Earth & Environment]] |volume=4 |issue=1 |at=section 372 |doi=10.1038/s43247-023-01031-w |pmid=39524985 |pmc=11549049 |arxiv=2304.14136 |bibcode=2023ComEE...4..372C}}</ref><ref>{{cite news |first=Robert |last=Lea |title=Earth's weird 'quasi-moon' Kamo'oalewa is a fragment blasted out of big moon crater |date=April 23, 2024 |work=Space.com |url=https://www.space.com/quasi-moon-kamooalewa-giant-lunar-impact |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241224181137/https://www.space.com/quasi-moon-kamooalewa-giant-lunar-impact |archive-date=December 24, 2024}}</ref> Orbit calculations show that almost all quasi-satellites and many horseshoe librators repeatedly transfer between horseshoe and quasi-satellite orbits.<ref name="Fuentes-2016HO3"/><ref name="DiRuzza2023">{{cite journal |first1=Sara |last1=Di Ruzza |first2=Alexandre |last2=Pousse |first3=Elisa Maria |last3=Alessi |title=On the co-orbital asteroids in the solar system: medium-term timescale analysis of the quasi-coplanar objects |date=January 15, 2023 |journal=Icarus |volume=390 |at=section 115330 |doi=10.1016/j.icarus.2022.115330 |arxiv=2209.05219 |bibcode=2023Icar..39015330D}}</ref> One of these objects, {{mpl|2003 YN|107}}, was observed during its transition from a quasi-satellite orbit to a horseshoe orbit in 2006; it is expected to transfer back to a quasi-satellite orbit sometime around year 2066.<ref>{{cite news |first=Tony |last=Phillips |title=Corkscrew asteroid |date=June 9, 2006 |work=Science@NASA |publisher=[[NASA]] |url=http://science.nasa.gov/headlines/y2006/09jun_moonlets.htm |access-date=November 13, 2017 |url-status=dead |archive-url=https://web.archive.org/web/20060929155325/http://science.nasa.gov/headlines/y2006/09jun_moonlets.htm |archive-date=September 29, 2006}}</ref> A quasi-satellite discovered in 2023 but then found in old photographs back to 2012, {{mpl|2023 FW|13}}, was found to have an orbit that is stable for about 4,000 years, from 100 BC to AD 3700.<ref>{{cite news |first=David L. |last=Chandler |title=Astronomers have discovered an asteroid that orbits the Sun with Earth, earning it the moniker "quasi-moon." |date=April 7, 2023 |work=Sky & Telescope |url=https://skyandtelescope.org/astronomy-news/does-earth-have-new-quasi-moon/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240721235051/https://skyandtelescope.org/astronomy-news/does-earth-have-new-quasi-moon/ |archive-date=July 21, 2024}}</ref>
* Asteroids on ''compound orbits'': orbital calculations show that some co-orbital asteroids transit between horseshoe and quasi-satellite orbits during every horseshoe resp. quasi-satellite cycle. Theoretically, similar continuous transitions between Trojan and horseshoe orbits are possible, too. {{As of|2023|1}}, at least 20 Earth co-orbital NEAs are thought to be in the horseshoe-like phase of compound orbits.<ref name="DiRuzza2023"/>
[[File:Animation of 2020 CD3's orbit around Earth.gif|thumb|Animation of {{mpl|2020 CD|3}}'s orbit around Earth<br />{{legend2|Magenta|{{mp|2020 CD|3}}}}{{·}}{{legend2|DarkKhaki|Moon}}{{·}}{{legend2|RoyalBlue|Earth}}]]
* ''[[Temporary satellite]]s'': NEAs can also transfer between solar orbits and distant Earth orbits, becoming gravitationally bound temporary satellites. According to simulations, temporary satellites are typically caught when they pass Earth's L1 or L2 Lagrangian points at the time Earth is either at the point in its orbit closest or farthest from the Sun, complete a couple of orbits around Earth, and then return to a heliocentric orbit due to perturbations from the Moon.<ref name="ST111230">{{cite news |first=Camille M. |last=Carlisle |title=Pseudo-moons orbit Earth |date=December 30, 2011 |work=Sky & Telescope |url=https://skyandtelescope.org/astronomy-news/pseudo-moons-orbit-earth/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240530185354/https://skyandtelescope.org/astronomy-news/pseudo-moons-orbit-earth/ |archive-date=May 30, 2024}}</ref> Strictly speaking, temporary satellites aren't co-orbital asteroids, and they can have orbits of the broader Arjuna type before and after capture by Earth, but simulations show that they can be captured from, or transfer to, horseshoe orbits.<ref name="2023FY3_2024"/> The simulations also indicate that Earth typically has at least one temporary satellite {{convert|1|m|ft|abbr=on}} across at any given time, but they are too faint to be detected by current surveys.<ref name="ST111230"/> {{As of|2024|12}}, five temporary satellites have been observed:<ref name="2023FY3_2024"/> {{mpl|1991 VG|}},<ref name="Fuente-Marcos-2018">{{Cite journal |first1=Carlos |last1=de la Fuente Marcos |first2=Raúl |last2=de la Fuente Marcos |date= January 2018 |title=Dynamical evolution of near-Earth asteroid 1991 VG |journal=Monthly Notices of the Royal Astronomical Society |volume=473 |issue=3 |pages=2939–2948 |bibcode=2018MNRAS.473.2939D |doi=10.1093/mnras/stx2545  |doi-access=free |arxiv=1709.09533}}</ref> {{mpl|2006 RH|120}}, {{mpl|2020 CD|3}},<ref>{{cite news |first=Roger W. |last=Sinnott |title=Earth's "other moon" |date=April 17, 2007 |work=Sky & Telescope |url=https://skyandtelescope.org/astronomy-news/earths-other-moon/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241130072820/https://skyandtelescope.org/astronomy-news/earths-other-moon/ |archive-date=November 30, 2024}}</ref><ref name="Naidu2020">{{cite news |first1=Shantanu |last1=Naidu |last2=Farnocchia |first2=Davide |title=Tiny Object Discovered in Distant Orbit Around the Earth |date=February 27, 2020 |publisher=NASA/JPL CNEOS |url=https://cneos.jpl.nasa.gov/news/news205.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241219113203/https://cneos.jpl.nasa.gov/news/news205.html |archive-date=December 19, 2024}}</ref> {{mpl|2022 NX|1}}<ref name="2023FY3_2024"/> and {{mpl|2024 PT|5}}.<ref>{{cite news |first=Robert |last=Lea |title=Earth's mini-moon has finally departed. Will it ever return as a 'second moon?' |date=November 26, 2024 |work=Space.com |url=https://www.space.com/goodnight-second-moon-asteroid-2024PT5 |access-date=January 1, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250101183548/https://www.space.com/goodnight-second-moon-asteroid-2024PT5 |archive-date=January 1, 2025}}</ref> Calculations for the {{convert|5|m|ft|abbr=on}} asteroid {{mpl|2023 FY|3}} showed repeated transitions into temporary satellite orbits both in the past and the future 10,000 years.<ref name="2023FY3_2024">{{cite journal |first1=R. |last1=de&nbsp;la&nbsp;Fuente Marcos |first2=C. |last2=de&nbsp;la&nbsp;Fuente Marcos |display-authors=etal |title=When the horseshoe fits: Characterizing {{mp|2023 FY|3}} with the 10.4 m Gran Telescopio Canarias and the Two-meter Twin Telescope |date=January 2024 |journal=Astronomy & Astrophysics |volume=681 |at=section A4 |doi=10.1051/0004-6361/202347663 |arxiv=2310.08724 |bibcode=2024A&A...681A...4D}}</ref>

Near-Earth asteroids also include the co-orbitals of Venus. {{As of|2023|1}}, all known co-orbitals of Venus have orbits with high eccentricity, also crossing Earth's orbit.<ref name="DiRuzza2023"/><ref>{{cite journal |first1=Petr |last1=Pokorý |first2=Marc |last2=Kuchner |title=Threat from Within: Excitation of Venus's Co-orbital Asteroids to Earth-crossing Orbits |date=October 2021 |journal=The Planetary Science Journal |volume=2 |issue=5 |at=part 193 |doi=10.3847/PSJ/ac1e9b |doi-access=free |bibcode=2021PSJ.....2..193P}}</ref>

=== Meteoroids ===

In 1961, the IAU defined [[meteoroid]]s as a class of solid interplanetary objects distinct from asteroids by their considerably smaller size.<ref name="Rubin2010"/> This definition was useful at the time because, with the exception of the [[Tunguska event]], all historically observed meteors were produced by objects significantly smaller than the smallest asteroids then observable by telescopes.<ref name="Rubin2010"/> As the distinction began to blur with the discovery of ever smaller asteroids and a greater variety of observed NEO impacts, revised definitions with size limits have been proposed from the 1990s.<ref name="Rubin2010">{{cite journal |last1=Rubin |first1=Alan E. |last2=Grossman |first2=Jeffrey N. |title=Meteorite and meteoroid: New comprehensive definitions |journal=Meteoritics & Planetary Science |volume=45 |issue=1 |pages=114–122 |date=January 2010 |doi=10.1111/j.1945-5100.2009.01009.x |bibcode=2010M&PS...45..114R |s2cid=129972426}}</ref> In April 2017, the IAU adopted a revised definition that generally limits meteoroids to a size between 30&nbsp;μm and 1&nbsp;m in diameter, but permits the use of the term for any object of any size that caused a meteor, thus leaving the distinction between asteroid and meteoroid blurred.<ref name="met-def-2017">{{cite news |first=Vincent |last=Perlerin |title=Definitions of terms in meteor astronomy (IAU) |date=September 26, 2017 |work=News |publisher=[[International Meteor Organization]] |url=https://www.imo.net/definitions-of-terms-in-meteor-astronomy-iau/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20180123072313/https://www.imo.net/definitions-of-terms-in-meteor-astronomy-iau/ |archive-date=January 23, 2018}}</ref>

=== Near-Earth comets<!--'Near-Earth comet' and 'Near-Earth comets' redirect here--> ===

[[File:Halley's Comet - May 29 1910.jpg|thumb|[[Halley's Comet]] during its 0.10&nbsp;AU<ref>{{cite web |first=Donald K. |last=Yeomans |title=Great Comets in History |date=April 2007 |publisher=NASA/JPL |url=https://ssd.jpl.nasa.gov/sb/great_comets.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240126111145/https://ssd.jpl.nasa.gov/sb/great_comets.html |archive-date=January 26, 2024}}</ref> approach of Earth in May 1910]]

'''Near-Earth comets'''<!--boldface per WP:R#PLA--> (NECs) are objects in a near-Earth orbit with a tail or coma made up of dust, gas or ionized particles emitted by a solid nucleus. Comet nuclei are typically less dense than asteroids but they pass Earth at higher relative speeds, thus the impact energy of a comet nucleus is slightly larger than that of a similar-sized asteroid.<ref name="NEOSDT2003"/> NECs may pose an additional hazard due to fragmentation: the meteoroid streams which produce meteor showers may include large inactive fragments, effectively NEAs.<ref name="Jenniksens2005">{{Cite conference |last=Jenniksens |first=Peter |title=Meteor Showers from Broken Comets |date=September 2005 |conference=Workshop on Dust in Planetary Systems (ESA SP-643) |volume=643 |pages=3–6 |bibcode=2007ESASP.643....3J}}</ref> Although no impact of a comet in Earth's history has been conclusively confirmed, the [[Tunguska event]] may have been caused by a fragment of [[Comet Encke]].<ref>{{cite journal |last=Kresak| first=L'.l |title=The Tunguska object – A fragment of Comet Encke |bibcode=1978BAICz..29..129K |volume=29 |date=1978 |pages=129 |journal=Bulletin of the Astronomical Institutes of Czechoslovakia}}</ref>

Comets are commonly divided between short-period and long-period comets. Short-period comets, with an orbital period of less than 200 years, originate in the [[Kuiper belt]], beyond the orbit of [[Neptune]]; while long-period comets originate in the [[Oort Cloud]], in the outer reaches of the Solar System.<ref name="TaskForceReport"/> The orbital period distinction is of importance in the evaluation of the risk from near-Earth comets because short-period NECs are likely to have been observed during multiple apparitions and thus their orbits can be determined with some precision, while long-period NECs can be assumed to have been seen for the first and last time when they appeared since the start of precise observations, thus their approaches cannot be predicted well in advance.<ref name="TaskForceReport">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Report of the Task Force on potentially hazardous Near Earth Objects |location=London |publisher=British National Space Centre |date=September 2000 |url=https://spaceguardcentre.com/wp-content/uploads/2014/04/full_report.pdf |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241208153948/https://spaceguardcentre.com/wp-content/uploads/2014/04/full_report.pdf |archive-date=December 8, 2024}}</ref> Since the threat from long-period NECs is estimated to be at most 1% of the threat from NEAs, and long-period comets are very faint and thus difficult to detect at large distances from the Sun, Spaceguard efforts have consistently focused on asteroids and short-period comets.<ref name="Vulcano1995"/><ref name="NEOSDT2003">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters |date=August 22, 2003 |publisher=NASA |url=https://cneos.jpl.nasa.gov/doc/neoreport030825.pdf |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241212090955/https://cneos.jpl.nasa.gov/doc/neoreport030825.pdf |archive-date=December 12, 2024}}</ref> Both NASA's CNEOS<ref name="CNEOS-NEO-groups"/> and ESA's NEOCC<ref name="NEOCC-DA"/> restrict their definition of NECs to short-period comets. {{As of|2024|12|30|df=US}}, 123 such objects have been discovered.<ref name="neo-jpl-stats" />

[[Comet Swift-Tuttle|Comet 109P/Swift–Tuttle]], which is also the source of the [[Perseids|Perseid meteor shower]] every year in August, has a roughly 130-year orbit that passes close to the Earth. During the comet's September 1992 recovery, when only the two previous returns in 1862 and 1737 had been identified, calculations showed that the comet would pass close to Earth during its next return in 2126, with an impact within the range of uncertainty. By 1993, even earlier returns (back to at least 188 AD) had been identified, and the longer observation arc eliminated the impact risk. The comet will pass Earth in 2126 at a distance of 23 million kilometers. In 3044, the comet is expected to pass Earth at less than 1.6 million kilometers.<ref>{{cite web |first=Sally | last=Stephens |title=What about the comet that's supposed to hit the Earth in 130 years? |date=1993 |work=Cosmic Collisions |publisher=[[Astronomical Society of the Pacific]] |url=https://astrosociety.org/file_download/inline/245c66de-59dd-49e9-8773-16ef08de09ff |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20231003194758/https://www.astrosociety.org/file_download/inline/245c66de-59dd-49e9-8773-16ef08de09ff |archive-date=October 3, 2023}}</ref>

=== Artificial near-Earth objects ===
[[Image:J002e3 animated.gif|thumb|[[J002E3]] discovery images taken on September 3, 2002. J002E3 is in the circle]]
Defunct [[Uncrewed spacecraft|space probes]] and [[Multistage rocket|final stages of rockets]] can end up in near-Earth orbits around the Sun. Examples of such artificial near-Earth objects include a [[Elon Musk's Tesla Roadster|Tesla Roadster]] used as [[Boilerplate (spaceflight)|dummy payload]] in a 2018 rocket test<ref>{{cite web |title=Tesla Roadster (spacecraft) (solution #10) |date=March 27, 2018 |work=[[JPL Horizons On-Line Ephemeris System]] |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=-143205&CENTER='500@10'&MAKE_EPHEM=YES&TABLE_TYPE=ELEMENTS&START_TIME=2018-05-01&STOP_TIME='2018-05-01+00:00:01'&OUT_UNITS=AU-D&REF_PLANE=ECLIPTIC&REF_SYSTEM=J2000&TP_TYPE=ABSOLUTE&ELEM_LABELS=YES&CSV_FORMAT=NO&OBJ_DATA=YES |access-date=December 31, 2024 |url-status=live |archive-date=June 30, 2019 |archive-url=https://web.archive.org/web/20190630211314/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=-143205&CENTER=%27500@10%27&MAKE_EPHEM=YES&TABLE_TYPE=ELEMENTS&START_TIME=2018-05-01&STOP_TIME=%272018-05-01+00:00:01%27&OUT_UNITS=AU-D&REF_PLANE=ECLIPTIC&REF_SYSTEM=J2000&TP_TYPE=ABSOLUTE&ELEM_LABELS=YES&CSV_FORMAT=NO&OBJ_DATA=YES}}</ref> and the [[Kepler space telescope]].<ref>{{cite web |first1=David |last1=Koch |first2=Alan |last2=Gould |title=Kepler Mission: Launch Vehicle and Orbit |date=February 12, 2007 |publisher=NASA |url=http://kepler.nasa.gov/sci/design/orbit.html |access-date=March 14, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20070622161529/http://kepler.nasa.gov/sci/design/orbit.html |archive-date=June 22, 2007}}</ref> Some of these objects have been re-discovered by NEO surveys when they returned to Earth's vicinity and classified as asteroids before their artificial origin was recognised.

An object classified as asteroid [[1991 VG]] was discovered during its transition from a temporary satellite orbit around Earth to a solar orbit in November 1991, and could only be observed until April 1992. Some scientists suspected it to be a returning piece of man-made space debris. After new observations in 2017 provided better data on its orbit and surface characteristics, a new study found the artificial origin unlikely.<ref name="Fuente-Marcos-2018"/>

In September 2002, astronomers found an object designated [[J002E3]]. The object was on a temporary satellite orbit around Earth, leaving for a solar orbit in June 2003. Calculations showed that it was also on a solar orbit before 2002, but was close to Earth in 1971. J002E3 was identified as the third stage of the [[Saturn V]] rocket that carried [[Apollo 12]] to the Moon.<ref>{{cite news |first1=Steve |last1=Chesley |first2=Paul |last2=Chodas |title=J002E3: An Update |date=October 9, 2002 |work=News |publisher=NASA/JPL |url=http://neo.jpl.nasa.gov/news/news136.html |access-date=November 14, 2017 |url-status=dead |archive-url=https://web.archive.org/web/20030503111617/http://neo.jpl.nasa.gov/news/news136.html |archive-date=May 3, 2003}}</ref><ref name="rocket-or-rock"/> In 2006, two more apparent temporary satellites were discovered which were suspected of being artificial.<ref name="rocket-or-rock"/> One of them was eventually confirmed as an asteroid and classified as the temporary satellite {{mp|2006 RH|120}}.<ref name="rocket-or-rock"/> The other, [[6Q0B44E]], was confirmed as an artificial object, but its identity is unknown.<ref name="rocket-or-rock"/> Another temporary satellite was discovered in 2013, and was designated {{mp|2013 QW|1}} as a suspected asteroid. It was later found to be an artificial object of unknown origin. {{mp|2013 QW|1}} is no longer listed as an asteroid by the Minor Planet Center.<ref name="rocket-or-rock">{{cite news |last=Azriel |first=Merryl |title=Rocket or Rock? NEO Confusion Abounds |date=September 25, 2013 |work=[[International Association for the Advancement of Space Safety#Space Safety Magazine|Space Safety Magazine]] |url=http://www.spacesafetymagazine.com/space-debris/satellite-tracking/rocket-or-rock/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20171115143536/http://www.spacesafetymagazine.com/space-debris/satellite-tracking/rocket-or-rock/ |archive-date=November 15, 2017}}</ref><ref name="MPC_QW1">{{Cite web |title=MPC Database Search. Unknown object: 2013 QW1 |publisher=IAU/MPC |url=https://minorplanetcenter.net/db_search/show_object?object_id=2013+QW1 |access-date=January 3, 2025 |url-status=live |archive-url=https://web.archive.org/web/20250103123618/https://minorplanetcenter.net/db_search |archive-date=January 3, 2025}}</ref> In September 2020, an object detected on an orbit very similar to that of the Earth was temporarily designated {{mpl|2020 SO|}}. However, orbital calculations and spectral observations confirmed that the object was the [[Centaur (rocket stage)|Centaur rocket booster]] of the 1966 [[Surveyor 2]] uncrewed lunar lander.<ref>{{cite news |title=Earth May Have Captured a 1960s-Era Rocket Booster |date=November 12, 2020 |work=News |publisher=NASA/JPL |url=https://www.jpl.nasa.gov/news/earth-may-have-captured-a-1960s-era-rocket-booster |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241220135930/https://www.jpl.nasa.gov/news/earth-may-have-captured-a-1960s-era-rocket-booster/ |archive-date=December 20, 2024}}</ref><ref>{{cite news |title=New Data Confirm 2020 SO to Be the Upper Centaur Rocket Booster From the 1960's |date=December 2, 2020 |work=News |publisher=NASA/JPL |url=https://www.jpl.nasa.gov/news/new-data-confirm-2020-so-to-be-the-upper-centaur-rocket-booster-from-the-1960s |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241220224200/https://www.jpl.nasa.gov/news/new-data-confirm-2020-so-to-be-the-upper-centaur-rocket-booster-from-the-1960s/ |archive-date=December 20, 2024}}</ref>

In some cases, active space probes on solar orbits have been observed by NEO surveys and erroneously catalogued as asteroids before identification. During its 2007 flyby of Earth on its route to a comet, ESA's space probe ''[[Rosetta (spacecraft)|Rosetta]]'' was detected unidentified and classified as asteroid {{mp|2007 VN|84}}, with an alert issued due to its close approach.<ref>{{cite news |first=Justin |last= Mullins |title=Astronomers defend asteroid warning mix-up |date=November 13, 2007 |work=New Scientist |url=https://www.newscientist.com/article/dn12914-astronomers-defend-asteroid-warning-mix-up/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20170307062102/https://www.newscientist.com/article/dn12914-astronomers-defend-asteroid-warning-mix-up/ |archive-date=March 7, 2017}}</ref> The designation {{mp|2015 HP|116}} was similarly removed from asteroid catalogues when the observed object was identified with ''[[Gaia (spacecraft)|Gaia]]'', ESA's [[space observatory]] for [[astrometry]].<ref>{{cite web |title=MPEC 2015-H125: Deletion Of 2015 HP116 |date=April 27, 2015 |work=Minor Planet Electronic Circular |publisher=IAU/MPC |url=http://www.minorplanetcenter.net/mpec/K15/K15HC5.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241127001828/https://minorplanetcenter.net/mpec/K15/K15HC5.html |archive-date=November 27, 2024}}</ref>