Editing Near-Earth object (section)

==== 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>