Editing Near-Earth object (section)

=== 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"/>