View source for Asteroid impact prediction

{{short description|Prediction of the dates and times of asteroids impacting Earth}}
{{Use dmy dates|date=March 2020}}
[[File:2008TC3-groundpath-rev.png|thumb|250px|{{mpl|2008 TC|3}} was the first successfully predicted asteroid impact. This picture shows the estimated path and altitude of the [[meteor]] in red, with the possible location for the METEOSAT IR [[Fireball (meteor)|fireball]] (bolide) as orange crosshairs and the [[Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization#International Monitoring System (IMS)|infrasound detection]] of the explosion in green]]
'''Asteroid impact prediction''' is the prediction of the dates and times of [[asteroid]]s [[Impact event|impacting Earth]], along with the locations and severities of the impacts.

The process of impact prediction follows three major steps:
# Discovery of an asteroid and initial assessment of its [[orbit]] which is generally based on a short [[observation arc]] of less than 2 weeks.
# Follow-up observations to improve the [[orbit determination]]
# Calculating if, when and where the orbit may [[Intersection (Euclidean geometry)|intersect]] with [[Earth]] at some point in the future.<ref name=":1" />
The usual purpose of predicting an impact is to direct an appropriate response.<ref name="NEO Preparedness Plan"/>

Most asteroids are discovered by a camera on a telescope with a wide [[field of view]]. [[Image differencing]] software compares a recent image with earlier ones of the same part of the sky, detecting objects that have moved, brightened, or appeared. Those systems usually obtain a few observations per night, which can be linked up into a very preliminary [[orbit determination]]. This predicts approximate positions over the next few nights, and follow-ups can then be carried out by any telescope powerful enough to see the newly detected object. Orbit intersection calculations are then carried out by two independent systems, one ([[Sentry (monitoring system)|Sentry]]) run by [[NASA]] and the other ([[NEODyS]]) by [[ESA]].

Current systems only detect an arriving object when several factors are just right, mainly the direction of approach relative to the Sun, the weather, and phase of the Moon. The overall success rate is around 1% and is lower for the smaller objects.<ref group="note">Smaller asteroids which are only bright enough to observe briefly. Larger asteroids are visible for long enough to overcome the more temporary conditions that prevent observation such as bad weather or a bright moon in the sky, however close proximity to the Sun in the sky can prevent all sizes of asteroid from being discovered. This is particularly true for [[Aten asteroid]]s, which spend the majority of their time closer to the Sun than Earth and are therefore difficult to detect without a space based system orbiting inside of Earth's orbit.</ref>  A few [[List of asteroid close approaches to Earth#Predicted encounters|near misses]] by medium-size asteroids have been predicted years in advance, with a tiny chance of actually striking Earth, and a handful of small actual impactors have successfully been detected hours in advance. All of the latter struck wilderness or ocean, and hurt no one.  The majority of impacts are by small, undiscovered objects.  They rarely hit a populated area, but can cause [[Chelyabinsk meteor|widespread damage]] when they do.  Performance is improving in detecting smaller objects as existing systems are upgraded and new ones come on line, but all current systems have a blind spot around the Sun that can only be overcome by a dedicated space based system or by discovering objects on a previous approach to Earth many years before a potential impact.

== History ==
In 1992 a report to [[NASA]] recommended a coordinated survey (christened [[Spaceguard]]) to discover, verify and provide follow-up observations for [[Earth-crossing asteroid]]s.<ref name="morrison_1992">Morrison, D., 25 January 1992, [https://archive.org/details/nasa_techdoc_19920025001 ''The Spaceguard Survey: Report of the NASA International Near-Earth-Object Detection Workshop''] {{webarchive |url=https://web.archive.org/web/20161013041451/https://archive.org/details/nasa_techdoc_19920025001 |date=13 October 2016 }}, [[NASA]], Washington, D.C.</ref> This survey was scaled to discover 90% of all objects larger than one kilometer within 25 years. Three years later, a further NASA report recommended search surveys that would discover 60–70% of the short-period, near-Earth objects larger than one kilometer within ten years and obtain 90% completeness within five more years.<ref>Shoemaker, E.M., 1995, ''Report of the Near-Earth Objects Survey Working Group'', NASA Office of Space Science, Solar System Exploration Office</ref>

In 1998, NASA formally embraced the goal of finding and cataloging, by 2008, 90% of all near-Earth objects (NEOs) with diameters of 1&nbsp;km or larger that could represent a collision risk to Earth. The 1&nbsp;km diameter metric was chosen after considerable study indicated that an impact of an object smaller than 1&nbsp;km could cause significant local or regional damage but is unlikely to cause a worldwide catastrophe.<ref name="morrison_1992"/> The impact of an object much larger than 1&nbsp;km diameter could well result in worldwide damage up to, and potentially including, [[Human extinction|extinction of the human race]]. The NASA commitment has resulted in the funding of a number of NEO search efforts, which made considerable progress toward the 90% goal by the target date of 2008 and also produced the first ever successful prediction of an asteroid impact (the 4-meter {{mpl|2008 TC|3}} was detected 19 hours before impact). However, the 2009 discovery of several NEOs approximately 2 to 3 kilometers in diameter (e.g. {{mp|2009 CR|2}}, {{mpl|2009 HC|82}}, {{mp|2009 KJ}}, {{mp|2009 MS}} and {{mp|2009 OG}}) demonstrated there were still large objects to be detected.

[[File:Цинковый завод Челябинска.jpg|thumb|250px|right|One of the 7,000 buildings damaged by the 2013 [[Chelyabinsk meteor]]]]

Three years later, in 2012, the 40 meter diameter asteroid [[367943 Duende]] was discovered and successfully predicted to be on close but non-colliding approach to Earth again just 11 months later. This was a landmark prediction as the object was only {{Val|20|x|40|u=m}}, and it was closely monitored as a result. On the day of its closest approach and by coincidence, a smaller asteroid was also approaching Earth, unpredicted and undetected, from a direction close to the Sun. Unlike [[367943 Duende]] it was on a collision course and it impacted Earth 16 hours before [[367943 Duende]] passed, becoming the [[Chelyabinsk meteor]]. It injured 1,500 people and damaged over 7,000 buildings, raising the profile of the dangers of even small asteroid impacts if they occur over populated areas. The asteroid is estimated to have been 17&nbsp;m across.

In April 2018, the [[B612 Foundation]] stated "It's 100 per cent certain we'll be hit [by a devastating asteroid], but we're not 100 per cent sure when."<ref name="INQ-20180428">{{cite news |last=Homer |first=Aaron |title=Earth Will Be Hit By An Asteroid With 100 Percent Certainty, Says Space-Watching Group B612{{Snd}} The group of scientists and former astronauts is devoted to defending the planet from a space apocalypse. |url=https://www.inquisitr.com/4881237/earth-will-be-hit-by-an-asteroid-with-100-percent-certainty-says-space-watching-group-b612/ |date=28 April 2018 |work=[[Inquisitr]] |access-date=24 November 2018 |archive-date=27 March 2019 |archive-url=https://web.archive.org/web/20190327150135/https://www.inquisitr.com/4881237/earth-will-be-hit-by-an-asteroid-with-100-percent-certainty-says-space-watching-group-b612/ |url-status=dead }}</ref> Also in 2018, [[physicist]] [[Stephen Hawking]], in his final book ''[[Brief Answers to the Big Questions (book)|Brief Answers to the Big Questions]]'', considered an asteroid collision to be the biggest threat to the planet.<ref name="WP-20181015">{{cite news |last=Stanley-Becker |first=Isaac |title=Stephen Hawking feared race of 'superhumans' able to manipulate their own DNA |url=https://www.washingtonpost.com/news/morning-mix/wp/2018/10/15/stephen-hawking-feared-race-of-superhumans-able-to-manipulate-their-own-dna/ |date=15 October 2018 |newspaper=[[The Washington Post]] |access-date=24 November 2018 |archive-date=28 June 2019 |archive-url=https://web.archive.org/web/20190628213202/https://www.washingtonpost.com/news/morning-mix/wp/2018/10/15/stephen-hawking-feared-race-of-superhumans-able-to-manipulate-their-own-dna/ |url-status=live }}</ref><ref name="QZ-20181014">{{cite web |last=Haldevang |first=Max de |title=Stephen Hawking left us bold predictions on AI, superhumans, and aliens |url=https://qz.com/1423685/stephen-hawking-says-superhumans-will-take-over-ai-is-a-threat-and-humans-will-conquer-space/ |date=14 October 2018 |work=[[Quartz (publication)|Quartz]] |access-date=24 November 2018 |archive-date=4 March 2020 |archive-url=https://web.archive.org/web/20200304115756/https://qz.com/1423685/stephen-hawking-says-superhumans-will-take-over-ai-is-a-threat-and-humans-will-conquer-space/ |url-status=live }}</ref> In June 2018, the US [[National Science and Technology Council]] warned that America is unprepared for an asteroid impact event, and has developed and released the National Near-Earth Object Preparedness Strategy Action Plan to better prepare.<ref name="WH-20180621">{{cite web |author=Staff |title=National Near-Earth Object Preparedness Strategy Action Plan |url=https://trumpwhitehouse.archives.gov/wp-content/uploads/2018/06/National-Near-Earth-Object-Preparedness-Strategy-and-Action-Plan-23-pages-1MB.pdf |date=21 June 2018 |via=[[NARA|National Archives]] |work=[[whitehouse.gov]] |access-date=24 November 2018 |archive-date=27 March 2021 |archive-url=https://web.archive.org/web/20210327095519/https://trumpwhitehouse.archives.gov/wp-content/uploads/2018/06/National-Near-Earth-Object-Preparedness-Strategy-and-Action-Plan-23-pages-1MB.pdf |url-status=live }}</ref><ref name="GIZ-20180621">{{cite news |last=Mandelbaum |first=Ryan F. |title=America Isn't Ready to Handle a Catastrophic Asteroid Impact, New Report Warns |url=https://gizmodo.com/america-isnt-ready-to-handle-a-catastrophic-asteroid-im-1827014709 |date=21 June 2018 |work=[[Gizmodo]] |access-date=24 November 2018 |archive-date=6 November 2019 |archive-url=https://web.archive.org/web/20191106085535/https://gizmodo.com/america-isnt-ready-to-handle-a-catastrophic-asteroid-im-1827014709 |url-status=live }}</ref><ref name="ICARUS-220180522">{{cite journal |last=Myhrvold |first=Nathan |author-link=Nathan Myhrvold |title=An empirical examination of WISE/NEOWISE asteroid analysis and results |date=22 May 2018 |journal=[[Icarus (journal)|Icarus]] |volume=314 |pages=64–97 |doi=10.1016/j.icarus.2018.05.004 |bibcode=2018Icar..314...64M |doi-access=free }}</ref><ref name="NYT-20180614">{{cite news |last=Chang |first=Kenneth |title=Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks{{Snd}} Two years ago, NASA dismissed and mocked an amateur's criticisms of its asteroids database. Now Nathan Myhrvold is back, and his papers have passed peer review. |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html |date=14 June 2018 |work=[[The New York Times]] |access-date=24 November 2018 |archive-date=23 April 2020 |archive-url=https://web.archive.org/web/20200423070930/https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html |url-status=live }}</ref><ref name="NYT-20180614c">{{cite news |last=Chang |first=Kenneth |title=Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks{{Snd}} Relevant Comments |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html#permid=27500228:27506217 |date=14 June 2018 |work=[[The New York Times]] |access-date=24 November 2018 |archive-date=23 April 2020 |archive-url=https://web.archive.org/web/20200423070930/https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html#permid=27500228:27506217 |url-status=live }}</ref>

== Discovery of near-Earth asteroids ==
The first step in predicting impacts is detecting asteroids and determining their orbits. Finding faint [[near-Earth object]]s against the much more numerous background [[star]]s is very much a [[wikt:Special:Search/needle in a haystack|needle in a haystack]] search. It is achieved by [[Astronomical survey|sky surveys]] that are designed to discover near Earth asteroids. Unlike the majority of telescopes that have a narrow [[field of view]] and high magnification, survey telescopes have a wide field of view to scan the entire sky in a reasonable amount of time with enough sensitivity to pick up the faint near-Earth objects they are searching for.

[[Near-Earth object|NEO]] focused surveys revisit the same area of sky several times in succession. Movement can then be detected using [[image differencing]] techniques. Anything that moves from image to image against the background of stars is compared to a catalogue of all known objects, and if it is not already known is reported as a new discovery along with its [[astrometry|precise position]] and the observation time. This then allows other observers to confirm and add to the data about the newly discovered object.<ref name=":1" /><ref name="JPL-2017-SDT-Update" />

=== Cataloging vs warning surveys ===
Asteroid surveys can be broadly classified as either ''cataloging surveys'', which use larger telescopes to mostly identify larger asteroids well before they come notably close to Earth, or ''warning surveys'', which use smaller telescopes to mostly look for smaller asteroids within several million kilometers of Earth. Cataloging systems focus on finding larger asteroids years in advance and they scan the sky slowly (of the order of once per month), but deeply. Warning systems focus on scanning the sky relatively quickly (of the order of once per night). They typically cannot detect objects that are as faint as cataloging systems but they will not miss an asteroid that dramatically brightens for just a few days when it passes very close to Earth. Some systems compromise and scan the sky approximately once per week.{{citation needed|date=October 2020}}

====Cataloging systems====
For larger [[asteroid]]s (> 100&nbsp;[[metre|m]] to 1&nbsp;[[kilometre|km]] across), prediction is based on cataloging the asteroid, years to centuries before it could impact. This technique is possible as their size makes them bright enough to be seen from a long distance. Their orbits therefore can be measured and any future impacts predicted long before they are on an impact approach to Earth. This long period of warning is important as an impact from a 1&nbsp;km object would cause worldwide damage and a minimum of around a decade of lead time would be needed to deflect it away from Earth. As of 2018, the inventory is nearly complete for the kilometer-size objects (around 900) which would cause global damage, and approximately one third complete for 140 meter objects (around 8500) which would cause major regional damage.<ref group="note">Incompleteness refers to the fraction of undiscovered asteroids, not the amount of time remaining to achieve completeness. The asteroids remaining to be discovered are the ones which are hardest to find.</ref><ref group="note">The exact percentage of objects discovered is uncertain but is estimated using statistical techniques. 2018 estimates for objects at least 1&nbsp;km in size put the figure somewhere between 89% and 99%, with an expected value of 94%. This matches the figure from a 2017 NASA report which was estimated independently using a different technique</ref><ref name="JPL-2017-SDT-Update"/><ref name="Orbit distributions">{{cite journal |last1=Granvik |first1=Mikael |last2=Morbidelli |first2=Alessandro |last3=Jedicke |first3=Robert |last4=Bolin |first4=Bryce |last5=Bottke |first5=William F. |last6=Beshore |first6=Edward |last7=Vokrouhlický |first7=David |last8=Nesvorný |first8=David |last9=Michel |first9=Patrick |title=Debiased orbit and absolute-magnitude distributions for near-Earth objects |journal=Icarus |date=25 April 2018 |volume=312 |pages=181–207 |publisher=Elsevier / Science Direct|doi=10.1016/j.icarus.2018.04.018 |arxiv=1804.10265 |bibcode=2018Icar..312..181G |s2cid=118893782 }}</ref><ref name="Asteroid analytics">{{cite web |last1=David |first1=Rich |title=The "Threat" of Asteroid Impacts{{Snd}} Breaking Down the Comprehensive Chart by the US Government |url=https://asteroidanalytics.com/neo-threat-summary-chart-2018 |website=Asteroid Analytics |access-date=14 December 2018 |date=22 June 2018 |archive-date=15 December 2018 |archive-url=https://web.archive.org/web/20181215065912/https://asteroidanalytics.com/neo-threat-summary-chart-2018/ |url-status=live }}</ref> The effectiveness of the cataloging is somewhat limited by the fact that some proportion of the objects have been [[lost minor planet|lost]] since their discovery, due to insufficient observations to accurately determine their orbits.

====Warning systems====
Smaller [[near-Earth object]]s number into millions and therefore impact Earth much more often, though obviously with much less damage. The vast majority remain undiscovered.<ref name="Asteroid analytics"/> They seldom pass close enough to Earth that they become bright enough to observe, and so most can only be observed when within a few million kilometers of Earth. They therefore cannot usually be catalogued well in advance and can only be warned about, a few weeks to days in advance.

Current mechanisms for detecting asteroids on approach rely on ground based visible-light [[telescopes]] with wide fields of view. Those currently can monitor the sky at most every night, and therefore miss most of the smaller asteroids which are bright enough to detect for less than a day. Such very small asteroids much more commonly [[Impact event|impact]] Earth than larger ones, but they make little damage. Missing them therefore has limited consequences. Much more importantly, ground-based telescopes are blind to most of the asteroids which impact the [[Day#Daytime|day]] side of the planet and will miss even large ones. These and other problems mean very few impacts are successfully predicted (see [[#Effectiveness of the current system|§Effectiveness of the current system]] and [[#Improving impact prediction|§Improving impact prediction]]).<ref name="JPL-2017-SDT-Update">{{cite news
 |title        = Update to Determine the Feasibility of Enhancing the Search and Characterization of NEOs
 |work         = Near-Earth Object Science Definition Team Report 2017
 |publisher    = NASA
 |url          = https://www.nasa.gov/sites/default/files/atoms/files/2017_neo_sdt_final_e-version.pdf
 |access-date  = 7 July 2018
 |archive-date = 20 November 2017
 |archive-url  = https://web.archive.org/web/20171120074246/https://www.nasa.gov/sites/default/files/atoms/files/2017_neo_sdt_final_e-version.pdf
 |url-status   = live
}}</ref>

Asteroids detected by warning systems are much too close to their time of potential impact to deflect them away from Earth, but there is still enough time to mitigate the consequences of the impact by evacuating and otherwise preparing the affected area. Warning systems can also detect asteroids which have been successfully catalogued as existing, but whose orbit was insufficiently well determined to allow a prediction of where they are now. {{See|Lost minor planet}}

=== Surveys ===

The main NEO focussed surveys are listed below, along with future telescopes that are already funded.

{{wide image|Global NEO survey sites.jpg|600px|align-cap=center|Locations of the major near Earth asteroid surveys. The addition of Kiso, SST and the Southern ATLAS locations mean the surveys are no longer solely clustered in the north west of the globe}}

Originally all the surveys were clustered together in a relatively small part of the Northern Hemisphere. This meant that around 15% of the sky at extreme Southern declination was never monitored,<ref name="ATLAS-SA-Physics-World">{{cite web|last1=Makoni|first1=Munyaradzi|date=4 September 2018|title=NASA's next-generation asteroid telescope set for South Africa|url=https://physicsworld.com/a/nasas-next-generation-asteroid-telescope-set-for-south-africa/|access-date=10 December 2018|website=Physics World|publisher=IOP Publishing|archive-date=10 December 2018|archive-url=https://web.archive.org/web/20181210202646/https://physicsworld.com/a/nasas-next-generation-asteroid-telescope-set-for-south-africa/|url-status=live}}</ref> and that the rest of the Southern sky was observed over a shorter season than the Northern sky. Moreover, as the hours of darkness are fewer in summertime, the lack of a balance of surveys between North and South meant that the sky was scanned less often in the Northern summer. The ATLAS telescopes now operating at the [[South African Astronomical Observatory]] and El Sauce observatory in Chile now cover this gap in the south east of the globe.<ref name="nature-ATLAS-southern">{{cite journal|last1=Watson|first1=Traci|date=2018-08-14|title=Project that spots city-killing asteroids expands to Southern Hemisphere|url=https://www.nature.com/articles/d41586-018-05969-2|access-date=17 October 2018|journal=Nature|publisher=Springer Nature Limited|doi=10.1038/d41586-018-05969-2|s2cid=135330315|archive-date=17 April 2023|archive-url=https://web.archive.org/web/20230417020439/https://www.nature.com/articles/d41586-018-05969-2|url-status=live}}</ref><ref name="ATLAS-SA-Physics-World" /> Once it is completed, the [[Large Synoptic Survey Telescope]] will improve the existing cover of the southern sky. The 3.5&nbsp;m [[Space Surveillance Telescope]], which was originally also in the [[southwest United States]], was dismantled and moved to [[Western Australia]] in 2017. When completed, this should also improve the global coverage. Construction has been delayed due to the new site being in a [[cyclone]] region, but was completed in September 2022.<ref name="SSTA-Cyclonic-Delay">{{Cite book |last1=Terán |first1=José |last2=Hill |first2=Derek |last3=Ortega Gutiérrez |first3=Alan |last4=Lindh |first4=Cory
 |editor-first1=Jason |editor-first2=Heather K. |editor-first3=Roberto |editor-last1=Spyromilio |editor-last2=Marshall |editor-last3=Gilmozzi |title=Ground-based and Airborne Telescopes VII |chapter=Design and construction of the SST Australia Observatory in a cyclonic region |volume=10700 |pages=1070007 |publisher=The international society for optics and photonics |doi=10.1117/12.2314722 |date=2018-07-06 |isbn=9781510619531 |s2cid=115871925 }}</ref><ref>{{cite web |last1=Courtney |first1=Albon |title=Space Force surveillance telescope now operational in Australia |url=https://www.defensenews.com/space/2022/09/30/space-force-surveillance-telescope-now-operational-in-australia/ |website=Defense News |date=30 September 2022 |publisher=Sightline Media Group |access-date=10 October 2022 |archive-date=4 September 2024 |archive-url=https://web.archive.org/web/20240904144151/https://www.defensenews.com/space/2022/09/30/space-force-surveillance-telescope-now-operational-in-australia/ |url-status=live }}</ref>

{| class="wikitable sortable"
|-
! Survey
! data-sort-type="number" | Telescope diameter ([[Metre|m]])
! data-sort-type="number" | Number of telescopes
! data-sort-type="number" | Time to scan entire visible sky (when clear)<ref group="note">Because of daylight telescopes cannot see the portion of the sky around the [[Sun]] and because the [[Earth]] is in the way can only see so far North and South of the latitude that they are positioned at. The time given is the time it takes for a survey to complete coverage of the part of the sky that it ''can'' see from where it is located, assuming good weather.</ref>
! data-sort-type="number" | Limiting [[Apparent magnitude|magnitude]]<ref group="note">The limiting magnitude indicates how bright an object needs to be before the telescope can detect it, larger numbers are better (fainter objects can be detected).</ref>
! Hemisphere
! data-sort-type="number" | Activity
! data-sort-type="number" | Peak yearly observations<ref name="MPC-Residuals">{{cite web |title=Residuals |url=https://minorplanetcenter.net/iau/special/residuals.txt |website=Minor Planet Center |publisher=International Astronomical Union |access-date=22 October 2018 |archive-date=18 November 2020 |archive-url=https://web.archive.org/web/20201118190502/https://minorplanetcenter.net/iau/special/residuals.txt |url-status=live }}</ref><ref group="note">This total includes all asteroid observations, not just [[Near-Earth asteroids]]</ref>
! Survey category
|-
| rowspan=2 | [[Asteroid Terrestrial-impact Last Alert System|ATLAS]]
| 0.5 || 2 || 2 nights || 19 || Northern || 2016–present || 1,908,828 || Warning survey
|-
| 0.5 || 2 || 1 night || 19 || Southern || 2022–present<ref name=":2">{{Cite news|title=Just Look Up: Expanded UH asteroid tracking system can monitor entire sky|work=University of Hawaiʻi System News|url=https://www.hawaii.edu/news/2022/01/27/expanded-uh-asteroid-tracking-monitor-entire-sky/|access-date=2022-01-28|language=en-US|archive-date=7 December 2022|archive-url=https://web.archive.org/web/20221207105248/https://www.hawaii.edu/news/2022/01/27/expanded-uh-asteroid-tracking-monitor-entire-sky/|url-status=live}}</ref>|| NA || Warning survey
|-
| rowspan=3 | [[Catalina Sky Survey]]
| 1.5 || 1 || 30 nights || 21.5 || Northern || 1998–present || see Mount Lemmon Survey || Cataloging survey
|-
| 0.7 || 1 || 7 nights || 19.5 || Northern || 1998–present || 1,934,824 || Cataloging survey
|- bgcolor="#c0c0c0"
| 0.5 || 1 || ? || ? || Southern || 2004–2013 || 264,634 || Warning survey
|-
| [[Kiso Observatory]]
| 1.05 || 1 || 0.2 nights (2 hours) || 18 || Northern || 2019–present || ? || Warning survey
|- bgcolor="#ffcc00"
| [[Large Synoptic Survey Telescope|Large Synoptic<br />Survey Telescope]]
| 8.4 || 1 || 3–4 nights || 27 || Southern || '''2022''' || NA || Both
|- bgcolor="#c0c0c0"
| [[Lincoln Near-Earth Asteroid Research]]
| 1.0 || 2 || ? || ? || Northern || 1998–2012 || 3,346,181 || Cataloging survey
|- bgcolor="#c0c0c0"
| [[Lowell Observatory Near-Earth-Object Search]]
| 0.6 || 1 || 41 nights || 19.5 || Northern || 1998–2008 || 836,844 || Cataloging survey
|-
| [[Mount Lemmon Survey]]
| 1.52 || 1 || ? || ~21 || Northern || 2005–present || 2,920,211 || Cataloging survey
|- bgcolor="#c0c0c0"
| [[Near-Earth Asteroid Tracking]]
| ? || 2 || ? || ? || Northern || 1995–2007 || 1,214,008 || Cataloging survey
|- bgcolor="#ffcc00"
| [[NEOSM]]
| 0.5 || 1 || ? || ? || [[Lagrangian point#L1|SEL1]] || '''2026'''<ref name="NASA-20210611"/><ref name="neosm-planet-soc"/> || NA || Cataloging survey
|- bgcolor="#ffcc00"
| [[NEOSTEL|NEO Survey Telescope]]
| 1 || 1 || 1 night || 21 || Northern || '''2022'''<ref name=":0">{{Cite web|title=An Agency for asteroids|url=http://spaceref.com/news/viewpr.html?pid=55886|access-date=2020-07-18|website=spaceref.com|date=23 June 2020}}{{Dead link|date=December 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> || NA || Warning survey
|-
| [[NEOWISE]]
| 0.4 || 1 || ~6 months || ~22 || Earth Orbit || 2009–present || 2,279,598 || Cataloging survey
|-
| [[Pan-STARRS]]
| 1.8 || 2 || 30 nights || 23 || Northern || 2010–present || 5,254,605 || Cataloging survey
|-
|rowspan=2 | [[Space Surveillance Telescope]]
| rowspan=2 | 3.5 || rowspan=2 | 1 || rowspan=2 | 6 nights || rowspan=2 | 20.5 || bgcolor="#c0c0c0" | Northern || bgcolor="#c0c0c0" | 2014–2017 || bgcolor="#c0c0c0" | 6,973,249 || bgcolor="#c0c0c0" | Warning survey
|- 
| Southern ||2022–present<ref>{{Cite web |last=Albon |first=Courtney |date=2022-09-30 |title=Space Force surveillance telescope now operational in Australia |url=https://www.defensenews.com/space/2022/09/30/space-force-surveillance-telescope-now-operational-in-australia/ |access-date=2022-10-08 |website=Defense News |language=en |archive-date=4 September 2024 |archive-url=https://web.archive.org/web/20240904144151/https://www.defensenews.com/space/2022/09/30/space-force-surveillance-telescope-now-operational-in-australia/ |url-status=live }}</ref>|| NA || Warning survey
|- bgcolor="#c0c0c0"
|rowspan=2 | [[Spacewatch]]
| 1.8 || 1 || ? || ? ||rowspan=2 | Northern ||rowspan=2 | 1980–1998<ref group="note">Spacewatch is still operational, however in 1998 [[Catalina Sky Survey]] (which is also run by the [[University of Arizona]]) took over survey duties. Since then Spacewatch has focused on follow-up observations.</ref><ref name="spacewatch"/> ||rowspan=2 | 1,532,613 ||rowspan=2 | Cataloging survey
|- bgcolor="#c0c0c0"
| 0.9 || 1 || ? || 22
|-
| [[Zwicky Transient Facility]]
| 1.2 || 1 || 3 nights || 20.5 || Northern || 2018–present || 483,822 || Warning survey
|}

==== ATLAS ====
{{main|Asteroid Terrestrial-impact Last Alert System}}
ATLAS, the "Asteroid Terrestrial-impact Last Alert System" uses four 0.5-metre telescopes. Two are located on the [[Hawaiian Islands]], at [[Haleakala Observatory|Haleakala]] and [[Mauna Loa Observatory|Mauna Loa]], one at the [[South African Astronomical Observatory]], and one in Chile.<ref name="nature-ATLAS-southern" /><ref name="ATLAS-how-it-works">{{cite web|title=Atlas: How it works|url=http://fallingstar.com/how_atlas_works.php|access-date=20 December 2019|website=Asteroid Terrestrial-impact Last Alert System|publisher=University of Hawaii|archive-date=1 March 2023|archive-url=https://web.archive.org/web/20230301150131/http://fallingstar.com/how_atlas_works.php|url-status=live}}</ref><ref name=":2" /> With a field of view of 30 [[square degrees]] each, the telescopes survey the observable sky down to [[apparent magnitude]] 19 with 4 exposures every night.<ref name="CESG-Recorder">{{cite web |last1=Heinze |first1=Aren (Ari) |title=The Last Alert: A New Battle Front in Asteroid Defense |url=https://csegrecorder.com/articles/view/the-last-alert-a-new-battle-front-in-asteroid-defense |website=CSEG Recorder |publisher=Canadian Society of Exploration Geophysicists |access-date=17 October 2018 |archive-date=17 October 2018 |archive-url=https://web.archive.org/web/20181017163144/https://csegrecorder.com/articles/view/the-last-alert-a-new-battle-front-in-asteroid-defense |url-status=live }}</ref><ref name=ArXib2018>{{Cite journal |author= Tonry|display-authors=et al |title= ATLAS: A High-Cadence All-Sky Survey System|journal= Publications of the Astronomical Society of the Pacific|volume= 130|issue= 988|pages= 064505| date = 28 March 2018 |arxiv= 1802.00879|bibcode= 2018PASP..130f4505T|doi= 10.1088/1538-3873/aabadf|s2cid=59135328 }} Accessed 2018-04-14.</ref> The survey has been operational with the two Hawaii telescopes since 2017, and in 2018 obtained NASA funding for two additional telescopes sited in the Southern hemisphere. They were expected to take 18 months to build.<ref name="ATLAS-specs">{{cite web|url=http://fallingstar.com/specifications.php|title=ATLAS specifications|access-date=9 December 2018|archive-date=8 December 2018|archive-url=https://web.archive.org/web/20181208063649/http://fallingstar.com/specifications.php|url-status=live}}</ref> Their southern locations provide coverage of the 15% of the sky that cannot be observed from Hawaii, and combined with the Northern hemisphere telescopes give non-stop coverage of the equatorial night sky (the South African location is not only in the opposite hemisphere to Hawaii, but also at an opposing longitude).<ref name="ATLAS-how-it-works"/> The full ATLAS concept consists of eight of its 50-centimeter diameter f/2 [[Wright camera|Wright]]-[[Schmidt corrector plate|Schmidt telescopes]], spread over the globe for 24h/24h coverage of the full-night-sky.

====Catalina Sky Survey (including Mount Lemmon Survey)====
{{main|Catalina Sky Survey}}
In 1998, the Catalina Sky Survey (CSS) took over from Spacewatch in surveying the sky for the [[University of Arizona]]. It uses two telescopes, a 1.5&nbsp;m [[Cassegrain reflector]] telescope on the peak of [[Mount Lemmon]] (also known as a survey in its own right, the [[Mount Lemmon Survey]]), and a 0.7&nbsp;m [[Schmidt camera|Schmidt]] telescope near [[Mount Bigelow (Arizona)|Mount Bigelow]] (both in the Tucson, Arizona area in the south west of the [[United States]]). Both sites use identical cameras which provide a [[field of view]] of 5 [[square degrees]] on the 1.5&nbsp;m telescope and 19 [[square degrees]] on the Catalina Schmidt. The Cassegrain reflector telescope takes three to four weeks to survey the entire sky, detecting objects fainter than [[apparent magnitude]] 21.5. The 0.7&nbsp;m telescope takes a week to complete a survey of the sky, detecting objects fainter than [[apparent magnitude]] 19.<ref name="CSS-Facilities">{{cite web |last1=UA Lunar & Planetary Laboratory |title=Catalina Sky Survey Telescopes |url=https://catalina.lpl.arizona.edu/about/facilities/telescopes |website=Catalina Sky Survey |publisher=The University of Arizona |access-date=17 October 2018 |archive-date=19 July 2020 |archive-url=https://web.archive.org/web/20200719112914/https://catalina.lpl.arizona.edu/about/facilities/telescopes |url-status=live }}</ref> This combination of telescopes, one slow and one medium, has so far detected more [[near Earth Object]]s than any other single survey. This shows the need for a combination of different types of telescopes.

CSS used to include a telescope in the Southern Hemisphere, the [[Siding Spring Survey]]. However operations ended in 2013 after funding was discontinued.<ref name=guard-closure>{{cite news |last1=Safi |first1=Michael |title=Earth at risk after cuts close comet-spotting program, scientists warn |url=https://www.theguardian.com/science/2014/oct/20/funding-cuts-leave-earth-with-blind-spot-for-potentially-catastrophic-comets |access-date=25 November 2015 |work=[[The Guardian]] |date=20 October 2014 |archive-date=25 November 2015 |archive-url=https://web.archive.org/web/20151125210751/http://www.theguardian.com/science/2014/oct/20/funding-cuts-leave-earth-with-blind-spot-for-potentially-catastrophic-comets |url-status=live }}</ref>

==== Kiso Observatory (Tomo-e Gozen) ====
{{main|Kiso Observatory}}
The Kiso Observatory uses a 1.05m [[Schmidt camera|Schmidt]] telescope on [[Mt. Ontake]] near [[Tokyo]] in [[Japan]].<ref name="kiso-observatory-overview">{{cite web |title=Kiso Observatory Overview |url=http://www.ioa.s.u-tokyo.ac.jp/kisohp/OVERVIEW/overview_e.html |website=Kiso Observatory |publisher=The Institute of Astronomy, the University of Tokyo |access-date=2 July 2021 |archive-date=8 August 2018 |archive-url=https://web.archive.org/web/20180808220332/http://www.ioa.s.u-tokyo.ac.jp/kisohp/OVERVIEW/overview_e.html |url-status=live }}</ref> In late 2019 the Kiso Observatory added a new instrument to the telescope, "Tomo-e Gozen", designed to detect fast moving and rapidly changing objects. It has a wide field of view (20 [[square degrees]]) and scans the sky in just 2 hours, far faster than any other survey as of 2021.<ref>{{cite web |last1=the Tomo-e Gozen project |title=UTokyo starts a full operation of Tomo-e Gozen |url=http://www.mtk.ioa.s.u-tokyo.ac.jp/kisohp/NEWS/pr20190930/pr20190930e.html |website=Kiso Observatory |publisher=The Institute of Astronomy, the University of Tokyo |access-date=2 July 2021 |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709181900/http://www.mtk.ioa.s.u-tokyo.ac.jp/kisohp/NEWS/pr20190930/pr20190930e.html |url-status=live }}</ref><ref name="canon-tomoe-gozen">{{cite web |title=Kiso Observatory, University of Tokyo, commences full operation of 'Tomo-e Gozen', a new observation system equipped with 84 Canon ultra-high-sensitivity CMOS sensors |url=https://global.canon/en/news/2019/20190930.html |website=Canon Global |publisher=Canon Inc |access-date=2 July 2021 |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709181752/https://global.canon/en/news/2019/20190930.html |url-status=live }}</ref> This puts it squarely in the warning survey category. In order to scan the sky so quickly, the camera captures 2 frames per second, which means the sensitivity is lower than other metre class telescopes (which have much longer exposure times), giving a limiting magnitude of just 18.<ref name="iau-kiso-apmag-18">{{cite web |last1=Staff |first1=M.P.C. |title=Minor Planet Electronic Circular MPEC 2020-Q81: 2020 QW |url=https://www.minorplanetcenter.net/mpec/K20/K20Q81.html |website=Minor Planet Center |publisher=International Astronomical Union |access-date=2 July 2021 |archive-date=4 September 2024 |archive-url=https://web.archive.org/web/20240904144313/https://www.minorplanetcenter.net/mpec/K20/K20Q81.html |url-status=live }}</ref><ref name="kiso-18thmag-slide">{{cite web |last1=the Tomo-e Gozen project |title=Field-of-view |url=http://www.mtk.ioa.s.u-tokyo.ac.jp/kisohp/NEWS/pr20190930/fig19en.PNG |website=Kiso Observatory |publisher=The Institute of Astronomy, the University of Tokyo |access-date=2 July 2021 |archive-date=4 September 2024 |archive-url=https://web.archive.org/web/20240904144149/http://www.mtk.ioa.s.u-tokyo.ac.jp/kisohp/NEWS/pr20190930/fig19en.PNG |url-status=live }}</ref> However, despite not being able to see dimmer objects which are detectable by other surveys, the ability to scan the entire sky several times per night allows it to spot fast moving asteroids that other surveys miss. It has discovered a significant number of near-Earth asteroids as a result (for example see [[List of asteroid close approaches to Earth in 2021]]).

==== Large Synoptic Survey Telescope ====
{{main|Large Synoptic Survey Telescope}}
The Large Synoptic Survey Telescope (LSST) is a wide-field survey reflecting telescope with an 8.4&nbsp;meter primary mirror, currently under construction on [[Cerro Pachón]] in [[Chile]]. It will survey the entire available sky around every three nights. Science operations are due to begin in 2022.<ref name="LSST-summary-factsheet">{{cite web |last1=LSST Project Office |title=LSST Project Summary |url=https://docushare.lsst.org/docushare/dsweb/Get/Document-13936 |website=Large Synoptic Survey Telescope |access-date=17 October 2018 |archive-date=18 October 2018 |archive-url=https://web.archive.org/web/20181018003101/https://docushare.lsst.org/docushare/dsweb/Get/Document-13936 |url-status=live }}</ref> Scanning the sky relatively fast but also being able to detect objects down to [[apparent magnitude]] 27, it should be good at detecting nearby fast moving objects as well as excellent for larger slower objects that are currently further away.

==== Near-Earth Object Surveillance Mission ====
{{main|NEOSM}}
A planned space-based 0.5m [[infrared telescope]] designed to survey the [[Solar System]] for [[potentially hazardous object|potentially hazardous asteroids]].<ref name='NEO Home'>[https://neocam.ipac.caltech.edu/ Finding Asteroids Before They Find Us] {{Webarchive|url=https://web.archive.org/web/20201129080112/https://neocam.ipac.caltech.edu/ |date=29 November 2020 }}. NEOCam Home site at NASA's Jet Propulsion Laboratory – Caltech.</ref> The telescope will use a passive cooling system, and so unlike its predecessor [[NEOWISE]], it will not suffer from a performance degradation due to running out of coolant. It does still have a limited mission duration however as it needs to use propellant for [[orbital station keeping]] in order to maintain its position at [[Lagrange point#Sun–Earth|SEL1]]. From here, the mission will search for asteroids hidden from Earth based satellites by the Sun's glare. It is planned for launch in 2026.<ref name="NASA-20210611">{{cite web|last=Talbert|first=Tricia|url=https://www.nasa.gov/feature/nasa-approves-asteroid-hunting-space-telescope-to-continue-development|title=NASA Approves Asteroid Hunting Space Telescope to Continue Development|publisher=NASA|date=11 June 2021|access-date=11 June 2021|archive-date=30 September 2022|archive-url=https://web.archive.org/web/20220930131330/https://www.nasa.gov/feature/nasa-approves-asteroid-hunting-space-telescope-to-continue-development/|url-status=live}}</ref><ref name="neosm-planet-soc">{{cite web |title=NEO Surveyor, Protecting Earth from Dangerous Asteroids |url=https://www.planetary.org/space-missions/neosm |website=Space Missions |publisher=The Planetary Society |access-date=8 February 2022 |archive-date=8 February 2022 |archive-url=https://web.archive.org/web/20220208083110/https://www.planetary.org/space-missions/neosm |url-status=live }}</ref>

==== NEO Survey Telescope ====
{{main|NEOSTEL}}
The Near Earth Object Survey TELescope ([[NEOSTEL]]) is an [[ESA]] funded project, starting with an initial prototype currently under construction. The telescope is of a new "fly-eye" design that combines a single reflector with multiple sets of optics and CCDs, giving a very wide field of view (around 45 [[square degrees]]). When complete it will have the widest field of view of any telescope and will be able to survey the majority of the visible sky in a single night. If the initial prototype is successful, three more telescopes are planned for installation around the globe. Because of the novel design, the size of the primary mirror is not directly comparable to more conventional telescopes, but is equivalent to a conventional 1–metre telescope.<ref name="ESA-NEOSTEL">{{cite web |title=Flyeye Telescope |url=https://www.esa.int/spaceinimages/Images/2017/02/Flyeye_telescope |website=ESA |publisher=European Space Agency |access-date=10 December 2018 |archive-date=10 December 2018 |archive-url=https://web.archive.org/web/20181210150418/https://www.esa.int/spaceinimages/Images/2017/02/Flyeye_telescope |url-status=live }}</ref><ref>{{cite web |title=ESA's bug-eyed telescope to spot risky asteroids |url=http://www.esa.int/Our_Activities/Operations/Space_Situational_Awareness/ESA_s_bug-eyed_telescope_to_spot_risky_asteroids |website=ESA Space Situational Awareness |publisher=European Space Agency |access-date=10 December 2018 |archive-date=29 October 2017 |archive-url=https://web.archive.org/web/20171029065234/http://www.esa.int/Our_Activities/Operations/Space_Situational_Awareness/ESA_s_bug-eyed_telescope_to_spot_risky_asteroids |url-status=live }}</ref>

The telescope itself should be complete by end of 2019, and installation on Mount Mufara, Sicily should be complete in 2020 but was pushed back to 2022.<ref name=":0" /><ref name="ESA-NEOSTEL"/><ref name="Newsweek-Flyeye">{{cite web |last1=Hugo |first1=Kristin |title=European Space Agency's 'Flyeye' Telescope Could Spot Asteroids Before They Destroy Life on Earth |url=https://www.newsweek.com/telescope-mimics-fly-eyes-scan-sky-asteroids-974852 |website=Newsweek Tech & Science |date=13 June 2018 |publisher=Newsweek |access-date=10 December 2018 |archive-date=11 December 2018 |archive-url=https://web.archive.org/web/20181211010131/https://www.newsweek.com/telescope-mimics-fly-eyes-scan-sky-asteroids-974852 |url-status=live }}</ref>

==== NEOWISE ====
[[File:418872main wise20100122-full.jpg|thumb|250px|Viewed from space by [[Wide-field Infrared Survey Explorer|WISE]] using a [[thermal camera]], asteroid [[2010 AB78]] appears redder than the background stars as it emits most of its light at longer infrared wavelengths. In visible light it is very faint and difficult to see.]]
{{main|Wide-field Infrared Survey Explorer}}
The Wide-field Infrared Survey Explorer is a 0.4&nbsp;m [[infrared|infrared-wavelength]] [[Space observatory|space telescope]] launched in December 2009,<ref name="WISE-MSC">{{cite news |first=Justin |last=Ray |title=Mission Status Center: Delta/WISE |url=http://www.spaceflightnow.com/delta/d347/status.html |work=Spaceflight Now |date=14 December 2008 |access-date=26 December 2009 |archive-date=31 December 2009 |archive-url=https://web.archive.org/web/20091231004927/http://spaceflightnow.com/delta/d347/status.html |url-status=live }}</ref><ref>{{cite web |author=Rebecca Whatmore |author2=Brian Dunbar |title=WISE |publisher=NASA |url=http://www.nasa.gov/mission_pages/WISE/main/index.html |date=14 December 2009 |access-date=26 December 2009 |archive-date=9 November 2009 |archive-url=https://web.archive.org/web/20091109045853/http://www.nasa.gov/mission_pages/WISE/main/index.html |url-status=dead }}</ref><ref name=launch>{{cite web |first=Whitney |last=Clavin |title=NASA's WISE Eye on the Universe Begins All-Sky Survey Mission |publisher=NASA Jet Propulsion Laboratory |url=http://www.nasa.gov/mission_pages/WISE/news/wise20091214.html |date=14 December 2009 |access-date=26 December 2009 |archive-date=18 December 2009 |archive-url=https://web.archive.org/web/20091218051231/http://www.nasa.gov/mission_pages/WISE/news/wise20091214.html |url-status=dead }}</ref> and placed in hibernation in February 2011.<ref name=wright>{{cite web |url=http://www.astro.ucla.edu/~wright/WISE/ |title=Wide-field Infrared Survey Explorer |publisher=Astro.ucla.edu |access-date=24 August 2013 |archive-date=16 April 2003 |archive-url=https://web.archive.org/web/20030416193853/http://www.astro.ucla.edu/~wright/WISE/ |url-status=live }}</ref> It was re-activated in 2013 specifically to search for near-Earth objects under the [[NEOWISE]] mission.<ref name="cbc20130822">{{cite news |url=https://www.cbc.ca/news/science/nasa-space-telescope-rebooted-as-asteroid-hunter-1.1394598 |title=NASA space telescope rebooted as asteroid hunter |work=[[CBC News]] |agency=Reuters |date=22 August 2013 |access-date=22 August 2013 |archive-date=4 September 2024 |archive-url=https://web.archive.org/web/20240904144153/https://www.cbc.ca/news/science/nasa-space-telescope-rebooted-as-asteroid-hunter-1.1394598 |url-status=live }}</ref> By this stage, the spacecraft's [[cryogen]]ic coolant had been depleted and so only two of the spacecraft's four sensors could be used. Whilst this has still led to new discoveries of asteroids not previously seen from ground-based telescopes, the productivity has dropped significantly. In its peak year when all four sensors were operational, [[Wide-field Infrared Survey Explorer|WISE]] made 2.28&nbsp;million asteroid observations. In recent years, with no cryogen, NEOWISE typically makes approximately 0.15&nbsp;million asteroid observations annually.<ref name="MPC-Residuals"/> The next generation of infrared space telescopes has been designed so that they do not need cryogenic cooling.<ref name="NEOCAM-Instrument">{{cite web |title=NEOCam Instrument |url=https://neocam.ipac.caltech.edu/page/Instrument |website=Jet Propulsion Laboratory |publisher=NASA |access-date=23 October 2018 |archive-date=23 October 2018 |archive-url=https://web.archive.org/web/20181023195732/https://neocam.ipac.caltech.edu/page/Instrument |url-status=dead }}</ref>

==== Pan-STARRS ====
{{main|Pan-STARRS}}
[[Pan-STARRS]], the "Panoramic Survey Telescope And Rapid Response System", currently (2018) consists of two 1.8&nbsp;m [[Ritchey–Chrétien telescope]]s located at [[Haleakala]] in [[Hawaii]]. It has discovered a large number of new asteroids, [[comet]]s, [[variable star]]s, [[supernova]]e and other celestial objects.<ref name="MPC-Discoverers">{{cite web
 |title        = Minor Planet Discoverers (by number)
 |publisher    = [[IAU Minor Planet Center]]
 |url          = http://www.minorplanetcenter.net/iau/lists/MPDiscsNum.html
 |date         = 12 March 2017
 |access-date  = 28 March 2017
 |archive-date = 24 December 2018
 |archive-url  = https://web.archive.org/web/20181224182955/https://minorplanetcenter.net//iau/lists/MPDiscsNum.html%20
 |url-status   = live
}}</ref> Its primary mission is now to detect near-Earth objects that threaten [[impact event]]s, and it is expected to create a database of all objects visible from Hawaii (three-quarters of the entire sky) down to [[apparent magnitude]] 24. The Pan-STARRS NEO survey searches all the sky north of [[declination]] −47.5.<ref>{{cite tweet |user=astrokiwi |number=483572110179459073 |title=Twitter |author=Michele Bannister |date=30 June 2014 |access-date=1 May 2016}}</ref> It takes three to four weeks to survey the entire sky.<ref name="Pan-STARRS">{{cite web |title=Pan-STARRS |url=https://www.ifa.hawaii.edu/research/Pan-STARRS.shtml |website=UoH Institute for Astronomy |publisher=University of Hawaii |access-date=17 October 2018 |archive-date=18 October 2018 |archive-url=https://web.archive.org/web/20181018070831/https://www.ifa.hawaii.edu/research/Pan-STARRS.shtml |url-status=live }}</ref><ref name=Astronomy>{{cite news | author = University of Hawaii at Manoa's Institute for Astronomy | title = ATLAS: The Asteroid Terrestrial-impact Last Alert System | date = 18 February 2013 | url = http://www.astronomy.com/News-Observing/News/2013/02/ATLAS%20-%20The%20Asteroid%20Terrestrial-Impact%20Last%20Alert%20System.aspx | publisher = Astronomy Magazine | access-date = 2018-10-17 | archive-date = 2 August 2017 | archive-url = https://web.archive.org/web/20170802165254/http://www.astronomy.com/News-Observing/News/2013/02/ATLAS%20-%20The%20Asteroid%20Terrestrial-Impact%20Last%20Alert%20System.aspx | url-status = dead }}</ref>

==== Space Surveillance Telescope ====
{{main|Space Surveillance Telescope}}
The [[Space Surveillance Telescope]] (SST) is a 3.5&nbsp;m telescope that detects, tracks, and can discern small, obscure objects, in deep space with a wide [[field of view]] system. The SST mount uses an advanced servo-control technology, that makes it one of the quickest and most agile telescopes of its size.<ref name=SST-GS>{{Cite web| last =Pike| first =John| title =Space Surveillance Telescope| publisher =GlobalSecurity.org| date =2010| url =http://www.globalsecurity.org/space/systems/sst.htm| format =Basic overview| access-date =2010-05-20| archive-date =21 October 2018| archive-url =https://web.archive.org/web/20181021151410/https://www.globalsecurity.org/space/systems/sst.htm| url-status =live}}</ref><ref name=DARPA-SST>{{Cite web |author=Travis Blake | title =Space Surveillance Telescope (SST)| website=[[DARPA]]| date =2010| url =http://www.darpa.mil/tto/programs/sst/index.html | access-date =2010-05-20| archive-url =https://web.archive.org/web/20100112071232/http://www.darpa.mil/tto/programs/sst/index.html| archive-date =2010-01-12}}</ref> It has a field of view of 6 [[square degrees]] and can scan the visible sky in 6 clear nights down to [[apparent magnitude]] 20.5. Its primary mission is tracking orbital debris. This task is similar to that of spotting near-Earth asteroids and so it is capable of both.<ref name="DTIC-SST-Results">{{cite web |last1=Ruprecht |first1=Jessica D |last2=Ushomirsky |first2=Greg |last3=Woods |first3=Deborah F |last4=Viggh |first4=Herbert E M |last5=Varey |first5=Jacob |last6=Cornell |first6=Mark E |last7=Stokes |first7=Grant |title=Asteroid Detection Results Using the Space Surveillance Telescope |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/1001992.pdf |archive-url=https://web.archive.org/web/20200511012906/http://www.dtic.mil/dtic/tr/fulltext/u2/1001992.pdf |url-status=live |archive-date=11 May 2020 |website=Defense Technical Information Center |publisher=DTIC |access-date=20 October 2018}}</ref>

The SST was initially deployed for testing and evaluation at the [[White Sands Missile Range]] in [[New Mexico]]. On 6 December 2013, it was announced that the telescope system would be moved to the [[Naval Communication Station Harold E. Holt]] in [[Exmouth, Western Australia]]. The SST was moved to [[Australia]] in 2017, captured first light in 2020 and after a two and a half year testing programme became operational in September 2022.<ref>{{cite web |last1=Erwin |first1=Sandra |title=Space surveillance telescope developed by the U.S. begins operations in Australia |url=https://spacenews.com/space-surveillance-telescope-developed-by-the-u-s-begins-operations-in-australia/ |website=SpaceNews |date=30 September 2022 |publisher=Pocket Ventures LLC |access-date=10 October 2022 |archive-date=4 September 2024 |archive-url=https://web.archive.org/web/20240904144254/https://spacenews.com/space-surveillance-telescope-developed-by-the-u-s-begins-operations-in-australia/ |url-status=live }}</ref><ref>{{cite web |title=Space surveillance telescope is declared operational |url=https://news.defence.gov.au/media/media-releases/space-surveillance-telescope-declared-operational |website=Defence News |publisher=Australian Government |access-date=10 October 2022 |archive-date=10 October 2022 |archive-url=https://web.archive.org/web/20221010030853/https://news.defence.gov.au/media/media-releases/space-surveillance-telescope-declared-operational |url-status=live }}</ref>

==== Spacewatch ====
{{main|Spacewatch}}
Spacewatch was an early sky survey focussed on finding near Earth asteroids, founded in 1980. It was the first to use [[Charge-coupled device|CCD]] [[image sensor]]s to search for them, and the first to develop [[software]] to detect moving objects automatically in [[real-time computing|real-time]]. This led to a huge increase in productivity. Before 1990 a few hundred observations were made each year. After automation, annual productivity jumped by a factor of 100 leading to tens of thousands of observations per year. This paved the way for the surveys we have today.<ref name="spacewatch"/>

Although the survey is still in operation, in 1998 it was superseded by Catalina Sky Survey. Since then it has focused on following up on discoveries by other surveys, rather than making new discoveries itself. In particular it aims to prevent high priority [[Potentially hazardous object|PHO]]s from being lost after their discovery. The survey telescopes are 1.8&nbsp;m and 0.9&nbsp;m. The two follow-up telescopes are 2.3&nbsp;m and 4&nbsp;m.<ref name="spacewatch"/>

==== Zwicky Transient Facility ====
{{main|Zwicky Transient Facility}}
The [[Zwicky Transient Facility]] (ZTF) was commissioned in 2018, superseding the [[Palomar Transient Factory|Intermediate Palomar Transient Factory]] (2009–2017). It is designed to detect [[Transient astronomical event|transient objects]] that rapidly change in brightness, for example [[supernovae]], [[gamma ray burst]]s, collisions between two [[neutron star]]s, as well as moving objects such as [[comet]]s and [[asteroid]]s. The ZTF is a 1.2&nbsp;m telescope that has a field of view of 47 [[square degrees]], designed to image the entire northern sky in three nights and scan the plane of the [[Milky Way]] twice each night to a limiting [[Apparent magnitude|magnitude]] of 20.5.<ref name="unblinking-eye-in-sky">{{cite journal | last1=Bellm | first1=Eric | last2=Kulkarni | first2=Shrinivas | title=The unblinking eye on the sky | journal=Nature Astronomy | volume=1 | issue=3 | date=2017-03-02 | issn=2397-3366 | doi=10.1038/s41550-017-0071 | page=0071|arxiv=1705.10052|bibcode=2017NatAs...1E..71B| s2cid=119365778 }}</ref><ref>{{Cite journal|last1=Smith|first1=Roger M.|last2=Dekany|first2=Richard G.|last3=Bebek|first3=Christopher|last4=Bellm|first4=Eric|last5=Bui|first5=Khanh|last6=Cromer|first6=John|last7=Gardner|first7=Paul|last8=Hoff|first8=Matthew|last9=Kaye|first9=Stephen|date=2014-07-14|title=The Zwicky transient facility observing system|url=https://authors.library.caltech.edu/58413/1/Smith_2014p914779.pdf|journal=Ground-based and Airborne Instrumentation for Astronomy V|volume=9147|pages=914779|doi=10.1117/12.2070014|bibcode=2014SPIE.9147E..79S|s2cid=9106668|editor1-last=Ramsay|editor1-first=Suzanne K|editor2-last=McLean|editor2-first=Ian S|editor3-last=Takami|editor3-first=Hideki|access-date=20 October 2018|archive-date=14 August 2017|archive-url=https://web.archive.org/web/20170814041537/http://authors.library.caltech.edu/58413/1/Smith_2014p914779.pdf|url-status=live}}</ref> The amount of data produced by ZTF is expected to be 10 times larger than its predecessor.<ref>{{Cite journal| last1=Cao| first1=Yi| last2=Nugent|first2=Peter E.| last3=Kasliwal|first3=Mansi M.| title=Intermediate Palomar Transient Factory: Realtime Image Subtraction Pipeline| journal=Publications of the Astronomical Society of the Pacific| volume=128| issue=969|pages=114502| doi=10.1088/1538-3873/128/969/114502| arxiv=1608.01006| bibcode=2016PASP..128k4502C| year=2016| s2cid=39571681}}</ref>

== Follow-up observations ==
[[File:NEA-sizes-vs-uncertainties-compact.PNG|thumb|upright=1.25| The orbits of kilometre class [[Near-Earth asteroid|NEAs]] are generally well known as there are usually many follow-up observations. Large numbers of smaller NEAs however have highly uncertain orbits due to insufficient follow-up after discovery. Many have been [[Lost minor planet|lost]].
<ref name="MPC-NEA">{{cite web
  |title=Orbits for Near Earth Asteroids (NEAs)
  |url=https://www.minorplanetcenter.net/iau/MPCORB/NEA.txt
  |website=IAU Minor Planet Center
  |publisher=International Astronomical Union
  |access-date=25 June 2020
  |archive-date=30 October 2010
  |archive-url=https://web.archive.org/web/20101030080833/https://www.minorplanetcenter.net/iau/MPCORB/NEA.txt
  |url-status=live
  }}
* {{cite web |title=Export Format for Minor-Planet Orbits |website=IAU Minor Planet Center |url=https://www.minorplanetcenter.net/iau/info/MPOrbitFormat.html |access-date=26 June 2020 |archive-date=30 October 2010 |archive-url=https://web.archive.org/web/20101030092325/https://www.minorplanetcenter.net/iau/info/MPOrbitFormat.html |url-status=live }}</ref>]]

Once a new [[asteroid]] has been discovered and reported, other observers can confirm the finding and help define the orbit of the newly discovered object. The [[International Astronomical Union]] [[Minor Planet Center]] (MPC) acts as the global clearing house for information on asteroid orbits. It publishes lists of new discoveries that need verifying and still have uncertain orbits, and it collects the resulting follow-up observations from around the world. Unlike the initial discovery, which typically requires unusual and expensive wide-field telescopes, ordinary telescopes can be used to confirm the object as its position is now approximately known. There are far more of these around the globe, and even a well equipped [[Amateur astronomy|amateur astronomer]] can contribute valuable follow-up observations of moderately bright asteroids. For example, the [[Great Shefford Observatory]] in the back garden of amateur [[Peter Birtwhistle]] typically submits thousands of observations to the [[Minor Planet Center]] every year.<ref>{{Cite web| url=http://www.birtwhistle.org.uk/Location.htm| title=Great Shefford Location and situation| last=Birtwhistle| first=Peter| website=Great Shefford Observatory| access-date=24 October 2018| archive-date=4 September 2024| archive-url=https://web.archive.org/web/20240904144154/https://www.birtwhistle.org.uk/Location.htm| url-status=live}}</ref><ref name="MPC-Residuals" /> Nonetheless, some surveys (for example CSS and Spacewatch) have their own dedicated follow-up telescopes.<ref name="spacewatch">{{cite web |title=Spacewatch |url=https://spacewatch.lpl.arizona.edu/ |website=UA Lunar & Planetary Lab |publisher=University of Arizona |access-date=7 December 2018 |archive-date=8 June 2023 |archive-url=https://web.archive.org/web/20230608123107/https://spacewatch.lpl.arizona.edu/ |url-status=live }}</ref>

Follow-up observations are important because once a sky survey has reported a discovery it may not return to observe the object again for days or weeks. By this time it may be too faint for it to detect, and in danger of becoming a [[lost asteroid]]. The more observations and the longer the [[observation arc]], the greater the accuracy of the [[Orbit determination|orbit model]]. This is important for two reasons:

# for imminent impacts it helps to make a better prediction of where the impact will occur and whether there is any danger of hitting a populated area.
# for asteroids that will miss Earth this time round, the more accurate the orbit model is, the further into the future its position can be predicted. This allows recovery of the asteroid on its subsequent approaches, and impacts to be predicted years in advance.<ref name="JPL-2017-SDT-Update" />

=== Estimating size and impact severity ===
Assessing the size of the asteroid is important for predicting the severity of the impact, and therefore the actions that need to be taken (if any). With just observations of reflected visible light by a conventional telescope, the object could be anything from 50% to 200% of the estimated diameter, and therefore anything from one-eighth to eight times the estimated volume and mass.<ref name="NEOCAM-Infrared">{{cite web |title=NEOCam Infrared |url=https://neocam.ipac.caltech.edu/page/whyinfrared |website=Jet Propulsion Laboratory |publisher=NASA |access-date=30 October 2018 |archive-date=29 November 2020 |archive-url=https://web.archive.org/web/20201129080826/https://neocam.ipac.caltech.edu/page/whyinfrared |url-status=dead }}</ref> Because of this, one key follow-up observation is to measure the asteroid in the [[thermal infrared]] spectrum (long-wavelength infrared), using an [[infrared telescope]]. The amount of thermal radiation given off by an asteroid together with the amount of reflected visible light allows a much more accurate assessment of its size than just how bright it appears in the visible spectrum. Jointly using thermal infrared and visible measurements, a thermal model of the asteroid can estimate its size to within about 10% of the true size.

One example of such a follow-up observation was for [[3671 Dionysus]] by [[UKIRT]], the world's largest [[infrared telescope]] at the time (1997).<ref>{{cite web |title=Discovery of a Satellite Around a Near-Earth Asteroid |url=https://www.eso.org/public/unitedkingdom/news/eso9718/ |website=European Southern Observatory |access-date=30 October 2018 |date=22 July 1997 |archive-date=30 October 2018 |archive-url=https://web.archive.org/web/20181030170654/https://www.eso.org/public/unitedkingdom/news/eso9718/ |url-status=live }}</ref> A second example was the 2013 ESA [[Herschel Space Observatory]] follow-up observations of [[99942 Apophis]], which showed it was 20% larger and 75% more massive than previously estimated.<ref name=esanews1>{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Herschel_intercepts_asteroid_Apophis|title=Herschel intercepts asteroid Apophis|author=ESA|publisher=[[European Space Agency]] (ESA)|date=9 January 2013|access-date=2013-01-09|archive-date=3 August 2013|archive-url=https://web.archive.org/web/20130803012352/http://www.esa.int/Our_Activities/Space_Science/Herschel_intercepts_asteroid_Apophis|url-status=live}}</ref> However such follow-ups are rare. The size estimates of most near-Earth asteroids are based on visible light only.<ref name="NASA-NEO-Close-Approach-Data">{{cite web |title=NEO Earth Close Approach data |url=https://cneos.jpl.nasa.gov/ca/ |website=NASA JPL |publisher=NASA |access-date=7 July 2018 |archive-date=27 January 2023 |archive-url=https://archive.today/20230127163841/https://cneos.jpl.nasa.gov/ca/ |url-status=live }}</ref>

If the object was discovered by an infrared survey telescope initially, then an accurate size estimate will become available with visible light follow-up, and infrared follow-up will not be needed. However, none of the ground-based survey telescopes listed above operate at thermal infrared wavelengths. The [[NEOWISE]] satellite had two thermal infrared sensors but they stopped working when the [[cryogen]] ran out. There are therefore currently no active thermal infrared sky surveys which are focused on discovering near-Earth objects. There are plans for a new space based thermal infrared survey telescope, [[Near-Earth Object Surveillance Mission]], due to launch in 2025.

== Impact calculation ==

=== Minimum orbit intersection distance ===
{{main|Minimum orbit intersection distance}}
The minimum orbit intersection distance (MOID) between an asteroid and the Earth is the distance between the closest points of their [[osculating orbit|orbits]]. This first check is a coarse measure that does not allow an impact prediction to be made, but is based solely on the [[orbit]] parameters and gives an initial measure of how close to Earth the asteroid could come. If the MOID is large then the two objects never come near each other. In this case, unless the orbit of the asteroid is [[Perturbation (astronomy)|perturbed]] so that the MOID is reduced at some point in the future, it will never impact Earth and can be ignored. However, if the MOID is small then it is necessary to carry out more detailed calculations to determine if an impact will happen in the future. Asteroids with a MOID of less than 0.05&nbsp;[[Astronomical Unit|AU]] and an [[absolute magnitude]] brighter than 22 are categorized as a [[potentially hazardous asteroid]].<ref name="NEO-Groups">{{cite web |title=NEO Basics{{Snd}} NEO Groups |url=https://cneos.jpl.nasa.gov/about/neo_groups.html |website=Center for Near Earth Object Studies |publisher=NASA JPL |access-date=25 October 2018 |archive-date=11 November 2021 |archive-url=https://web.archive.org/web/20211111141623/https://cneos.jpl.nasa.gov/about/neo_groups.html |url-status=live }}</ref>

=== Projecting into the future ===
[[File:2018 LA-orbit.png|thumb|250px|[[Orbit]] and positions of [[2018 LA]] and [[Earth]], 30 days before impact. The diagram shows how orbit data can be used to predict impacts well in advance. This particular asteroid's orbit was only known a few hours before impact. The diagram was made later.|alt=]]
Once the initial [[orbit]] is known, the potential positions can be forecast years into the future and compared to the future position of Earth. If the distance between the asteroid and the centre of the Earth is less than [[Earth radius]] then a potential impact is predicted. To take account of the uncertainties in the orbit of the asteroid, many future projections are made (simulations) with slightly different parameters within the range of the uncertainty. This allows a percentage chance of impact to be estimated. For example, if 1,000 simulations are carried out and 73 result in an impact, then the prediction would be a 7.3% chance of impact.<ref name="Sentry-Intro">{{cite web |title=Sentry: Earth Impact Monitoring Introduction |url=https://cneos.jpl.nasa.gov/sentry/intro.html |website=Center for Near Earth Object Studies |publisher=NASA JPL |access-date=25 October 2018 |archive-date=6 January 2017 |archive-url=https://web.archive.org/web/20170106082258/http://neo.jpl.nasa.gov/risk/doc/sentry.html |url-status=live }}</ref>

=== NEODyS ===
{{main|NEODyS}}
NEODyS (Near Earth Objects Dynamic Site) is a [[European Space Agency]] service that provides information on near Earth objects. It is based on a continually and (almost) automatically maintained database of near Earth asteroid orbits. The site provides a number of services to the NEO community. The main service is an impact monitoring system (CLOMON2) of all near-Earth asteroids covering a period until the year 2100.<ref name="NEODyS-2">{{cite web |title=Near Earth Objects{{Snd}} Dynamic Site |url=https://newton.spacedys.com/neodys2/ |website=NEODyS-2 |publisher=European Space Agency |access-date=25 October 2018 |archive-date=6 December 2019 |archive-url=https://web.archive.org/web/20191206143936/https://newton.spacedys.com/neodys2/ |url-status=live }}</ref>

The NEODyS website includes a Risk Page where all NEOs with probabilities of hitting the Earth greater than 10<sup>−11</sup> from now until 2100 are shown in a risk list. In the table of the risk list the NEOs are divided into:
* "special", as was the case of [[(99942) Apophis]]
* "observable", objects which are presently observable and which critically need a follow-up in order to improve their orbit
* "possible recovery", objects which are not visible at present, but which are possible to recover in the near future
* "lost", objects which have an [[absolute magnitude]] (H) brighter than 25 but which are virtually lost, their orbit being too uncertain; and
* "small", objects with an absolute magnitude fainter than 25; even when those are "lost", they are considered too small to result in heavy damage on the ground (though the [[Chelyabinsk meteor]] would have been fainter than this).

Each object has its own impactor table (IT) which shows many parameters useful to determine the risk assessment.<ref name="NEODyS-Risk-List">{{cite web |title=NEODyS-2 Risk List |url=https://newton.spacedys.com/neodys2/index.php?pc=4.1 |website=NEODyS-2 |publisher=European Space Agency |access-date=25 October 2018 |archive-date=19 June 2021 |archive-url=https://web.archive.org/web/20210619132148/https://newton.spacedys.com/neodys2/index.php?pc=4.1 |url-status=live }}</ref>

=== Sentry prediction system ===
{{main|Sentry (monitoring system)}}
[[NASA]]'s [[Sentry (monitoring system)|Sentry System]] continually scans the MPC catalog of known asteroids, analyzing their orbits for any possible future impacts.<ref name=":1">{{YouTube|id=53Js-_vo3mo|title=How Does NASA Spot a Near-Earth Asteroid?}}</ref> Like [[ESA]]'s [[NEODyS]], it gives a list of possible future impacts, along with the probability of each. It uses a slightly different [[algorithm]] to [[NEODyS]], and so provides a useful cross-check and corroboration.

Currently, no impacts are predicted (the single highest probability impact currently listed is ~7&nbsp;m asteroid {{mpl|2010 RF|12}}, which is due to pass Earth in September 2095 with only a 10% predicted chance of impacting; its size is also small enough that any damage from an impact would be minimal).<ref>{{cite web|title=Sentry: Earth Impact Monitoring|url=https://cneos.jpl.nasa.gov/sentry/|website=Jet Propulsion Laboratory|publisher=NASA|access-date=25 August 2018|archive-date=11 September 2016|archive-url=https://web.archive.org/web/20160911172609/http://neo.jpl.nasa.gov/risk/|url-status=live}}</ref><ref name="Vox article">{{cite web|title=How NASA hunts the asteroids that could smash into Earth|url=https://www.vox.com/a/asteroid-day|website=Vox.com|publisher=Vox Media Inc|access-date=4 September 2018|date=2017-06-30|archive-date=16 October 2018|archive-url=https://web.archive.org/web/20181016093253/https://www.vox.com/a/asteroid-day|url-status=live}}</ref>

=== Impact probability calculation pattern ===
[[Image:AsteroidImpactProb.png|thumb|250px|Why predicted asteroid impact probability often goes up, then down]]
The ellipses in the diagram on the right show the predicted position of an example asteroid at closest Earth approach. At first, with only a few asteroid observations, the error ellipse is very large and includes the Earth. The impact prediction probability is small because the Earth cover a small fraction of the large error ellipse. (Often times the error ellipse extends for tens if not hundreds of millions of km.) Further observations shrink the error ellipse. If it still includes the Earth, this raises the predicted impact probability, since the fixed-size Earth now covers a larger fraction of the smaller error region.  Finally, yet more observations (often radar observations, or discovery of a previous sighting of the same asteroid on much older archival images) shrink the ellipse, usually revealing that the Earth is outside the smaller error region and the impact probability is then near zero.<ref>{{cite web|url=http://www.spaceguarduk.com/scares.htm |title=Why we have Asteroid "Scares" |publisher=Spaceguard UK |url-status=bot: unknown |archive-url=https://web.archive.org/web/20071222114100/http://www.spaceguarduk.com/scares.htm |archive-date=22 December 2007 }} (Original Site is no longer available, see Archived Site at )</ref> In rare cases, the Earth remains in the ever shrinking error ellipse and the impact probability then approaches one.

For asteroids that are actually on track to hit Earth, the predicted probability of impact never stops increasing as more observations are made. This initially very similar pattern makes it difficult to quickly differentiate between asteroids which will be millions of kilometres from Earth and those which will actually hit it. This in turn makes it difficult to decide when to raise an alarm as gaining more certainty takes time, which reduces the time available to react to a predicted impact. However raising the alarm too soon has the danger of causing a [[false alarm]] and creating a [[Boy Who Cried Wolf]] effect if the asteroid in fact misses Earth. NASA will raise an alert if an asteroid has a better than 1% chance of impacting.

In December 2004 when [[99942 Apophis|Apophis]] was estimated to have a 2.7% chance of impacting Earth on 13 April 2029, the uncertainty region for this asteroid had shrunk to 82,818&nbsp;km.<ref name=VI2029>[https://web.archive.org/web/20050314032111/https://astro.uni-bonn.de/~dfischer/mirror/285neo041227.html Virtual Impactor for 2029-04-13] (Stretch LOV = 12.9) * Earth radius of 6,420&nbsp;km = 82,818&nbsp;km</ref>

== Response to predicted impact ==
Once an impact has been predicted the potential severity needs to be assessed, and a response plan formed.<ref name="NEO Preparedness Plan"/> Depending on the time to impact and the predicted severity this may be as simple as giving a warning to citizens. For example, although unpredicted, the 2013
impact at Chelyabinsk was spotted through the window by teacher Yulia Karbysheva. She thought it prudent to take precautionary measures by ordering her students to stay away from the room's windows and to perform a [[duck and cover]] maneuver. The teacher, who remained standing, was seriously lacerated when the blast arrived and window glass severed a [[tendon]] in one of her arms and left [[thigh]], but none of her students, whom she ordered to hide under their desks, suffered lacerations.<ref name="NYT-20130217">{{citation  | last=Kramer | first=Andrew E. | title=After Assault From the Heavens, Russians Search for Clues and Count Blessings | url=https://www.nytimes.com/2013/02/17/world/europe/russians-seek-clues-and-count-blessings-after-meteor-blast.html | date=17 February 2013 | work=[[New York Times]] | archive-url=https://web.archive.org/web/20130217150820/http://www.nytimes.com/2013/02/17/world/europe/russians-seek-clues-and-count-blessings-after-meteor-blast.html | archive-date=17 February 2013 | url-status=live }}</ref><ref name="chelyabinsk">{{Cite news|url=https://interfax.com.ua/news/general/140940.html|title=Челябинская учительница спасла при падении метеорита более 40 детей|work=Интерфакс-Украина|access-date=2018-09-28|language=ru|archive-date=28 September 2018|archive-url=https://web.archive.org/web/20180928121347/https://interfax.com.ua/news/general/140940.html|url-status=live}}</ref> If the impact had been predicted and a warning had been given to the entire population, similar simple precautionary actions could have vastly reduced the number of injuries. Children who were in other classes were injured.<ref name="spiegel883565">{{cite news | first=Benjamin | last=Bidder | language=de |title=Meteoriten-Hagel in Russland: 'Ein Knall, Splittern von Glas' |trans-title=Meteorite hail in Russia: 'A blast, splinters of glass' | work=Der Spiegel | date=15 February 2013 | url=http://www.spiegel.de/wissenschaft/weltall/meteoriten-hagel-in-russland-a-883565.html | archive-url=https://web.archive.org/web/20130218003330/http://www.spiegel.de/wissenschaft/weltall/meteoriten-hagel-in-russland-a-883565.html | archive-date=18 February 2013 | url-status=live }}</ref>

If a more severe impact is predicted, the response may require evacuation of the area, or with sufficient lead time available, an avoidance mission to repel the asteroid. According to expert testimony in the [[United States Congress]] in 2013, [[NASA]] would require at least five years of preparation before a mission to intercept an asteroid could be launched which was demonstrated by kinetically deflecting a [[minor planet moon]], non-hazardous [[near-Earth Object#Near-Earth asteroids|NEO]] Asteroid called [[Dimorphos]] with the help of the [[Double Asteroid Redirection Test|DART]] spacecraft.<ref name="US-Congress-20130410">{{cite web |author=U.S.Congress |title=Threats From Space: a Review of U.S. Government Efforts to Track and mitigate Asteroids and Meteors (Part I and Part II){{Snd}} Hearing Before the Committee on Science, Space, and Technology House of Representatives One Hundred Thirteenth Congress First Session |url=http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |date=19 March 2013 |page=147 |work=[[United States Congress]] |access-date=24 November 2018 |archive-date=10 March 2017 |archive-url=https://web.archive.org/web/20170310212940/https://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |url-status=live }}</ref> Following a ten-month journey to the Didymos system, the impactor collided with Dimorphos on 26 September 2022 at a speed of around {{Convert|15000|mph|km/h|abbr=off}}.<ref name=Rincon-2021-11-24>{{cite news |last=Rincon |first=Paul |date=2021-11-24 |title=NASA DART asteroid spacecraft: Mission to smash into Dimorphos space rock launches |lang=en-GB |website=[[BBC News]] |url=https://www.bbc.com/news/science-environment-59327293 |access-date=2021-11-25 |archive-date=1 January 2022 |archive-url=https://web.archive.org/web/20220101073839/https://www.bbc.com/news/science-environment-59327293 |url-status=live }}</ref><ref>{{cite press release |last=Potter |first=Sean |date=2021-11-23 |title=NASA, SpaceX Launch DART: First test mission to defend planet Earth |website=NASA |publisher=[[National Aeronautics and Space Administration]] |url=http://www.nasa.gov/press-release/nasa-spacex-launch-dart-first-test-mission-to-defend-planet-earth |access-date=2021-11-25 |archive-date=24 November 2021 |archive-url=https://web.archive.org/web/20211124233245/http://www.nasa.gov/press-release/nasa-spacex-launch-dart-first-test-mission-to-defend-planet-earth/ |url-status=live }}</ref> The collision successfully decreased Dimorphos's [[orbital period]] around Didymos by {{val|32|2}} minutes.<ref name="DartPressUpdateNASA">{{cite web |title=NASA Confirms DART Mission Impact Changed Asteroid's Motion in Space |url=https://www.nasa.gov/press-release/nasa-confirms-dart-mission-impact-changed-asteroid-s-motion-in-space |last=Dunbar |first=Brian |website=NASA |date=11 October 2022 |access-date=2022-10-11 |language=en |archive-date=7 March 2023 |archive-url=https://web.archive.org/web/20230307153625/https://www.nasa.gov/press-release/nasa-confirms-dart-mission-impact-changed-asteroid-s-motion-in-space/ |url-status=live }}</ref><ref name=Temming-2020-06-29>{{cite news |last=Temming |first=Maria |date=29 June 2020 |title=An asteroid's moon got a name so NASA can bump it off its course |website=Science News |url=https://www.sciencenews.org/article/asteroid-moon-name-nasa-course-deflection-mission |access-date=1 July 2020 |archive-date=12 September 2021 |archive-url=https://web.archive.org/web/20210912104908/https://www.sciencenews.org/article/asteroid-moon-name-nasa-course-deflection-mission |url-status=live }}</ref><ref name=Crane-2021-11-23>{{cite magazine |last=Crane |first=Leah |date=2021-11-23 |title=NASA's DART mission will try to deflect an asteroid by flying into it |magazine=[[New Scientist]] |lang=en-UK |url=https://www.newscientist.com/article/2298530-nasas-dart-mission-will-try-to-deflect-an-asteroid-by-flying-into-it/ |url-status=live |access-date=2021-11-25 |archive-url=https://web.archive.org/web/20211123132010/https://www.newscientist.com/article/2298530-nasas-dart-mission-will-try-to-deflect-an-asteroid-by-flying-into-it/ |archive-date=2021-11-23 }}</ref><ref name="DartUpdateNASA">{{cite web |last1=Nelson |first1=Bill |last2=Saccoccia |first2=Giorgio |title=Update on DART Mission to Asteroid Dimorphos (NASA News Conference) |url=https://www.youtube.com/watch?v=Zhzn0U2m5wQ |website=YouTube |date=11 October 2022 |access-date=2022-10-11 |language=en |archive-date=30 November 2022 |archive-url=https://web.archive.org/web/20221130135608/https://www.youtube.com/watch?v=Zhzn0U2m5wQ |url-status=live }}</ref>

{{See|Asteroid impact avoidance}}

== Effectiveness of the current system ==
The effectiveness of the current system can be assessed a number of ways. The diagram below illustrates the number of successfully predicted impacts each year compared to the number of unpredicted [[asteroid]] [[Impact events|impacts]] recorded by [[Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization#International Monitoring System (IMS)|infrasound sensors designed to detect detonation of nuclear devices]].<ref name="NASA JPL Impact Reports">{{cite web |title=Fireball and bolide reports |url=http://neo.jpl.nasa.gov/fireballs/ |archive-url=https://web.archive.org/web/20130306210754/http://neo.jpl.nasa.gov/fireballs/ |archive-date=6 March 2013 |website=[[Jet Propulsion Laboratory]] |access-date=1 February 2019}}</ref> It shows that the success rate is increasing over time, but that the vast majority are still missed.
<!-- 2018-11-09 Changed style of bar graph as previous format was too wide for many devices, here is the original graph (if restoring, note data is out of date):
{{Bar graph
| title       = Impact prediction annual success rates<ref name="NASA JPL Impact Reports"/>
| float       = center
| bar_width   = 20
| width_units = em
| data_max    = 50
| label_type  = #
| data_type   = Successfully predicted impacts
| label1      = 2006
| data1       = 0
| label2      = 2007
| data2       = 0
| label3      = 2008
| data3       = 1
| label4      = 2009
| data4       = 0
| label5      = 2010
| data5       = 0
| label6      = 2011
| data6       = 0
| label7      = 2012
| data7       = 0
| label8      = 2013
| data8       = 0
| label9      = 2014
| data9       = 1
| label10     = 2015
| data10      = 0
| label11     = 2016
| data11      = 0
| label12     = 2017
| data12      = 0
| label13     = 2018
| data13      = 1
| col2_data_type= Unpredicted impacts
| col2_data_max = 50
| col2_data1    = 37
| col2_data2    = 23
| col2_data3    = 34
| col2_data4    = 32
| col2_data5    = 34
| col2_data6    = 30
| col2_data7    = 35
| col2_data8    = 24
| col2_data9    = 33
| col2_data10   = 43
| col2_data11   = 32
| col2_data12   = 29
| col2_data13   = 30
| col3_data_type= Success rate
| col3_data_max = 1
| col3_data1    = 0.000
| col3_data2    = 0.000
| col3_data3    = 0.029
| col3_data4    = 0.000
| col3_data5    = 0.000
| col3_data6    = 0.000
| col3_data7    = 0.000
| col3_data8    = 0.000
| col3_data9    = 0.029
| col3_data10   = 0.000
| col3_data11   = 0.000
| col3_data12   = 0.000
| col3_data13   = 0.032
}}
2018-11-09 Changed style of bargraph as previous format was too wide for many devices. Above is the original graph (if restoring, note data is out of date) -->

{{ #invoke:Chart | bar chart
| height = 300
| width = 600
| stack = 1
<!--         2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 -->
| group 1 =    1 :  0 :  0 :  0 :  0 :  0 :  1 :  0 :  0 :  0 :  1 :  1 :  0 :  0 :  2 :  1
| group 2 =   34 : 32 : 34 : 30 : 35 : 24 : 33 : 43 : 32 : 29 : 38 : 41 : 41 : 32 : 39 :  34
| group names = Successfully predicted impacts
              : Unpredicted impacts
| colors = #00FF00 : red
| x legends = [[List of asteroid close approaches to Earth in 2008|2008]]
            : [[List of asteroid close approaches to Earth in 2009|2009]]
            : [[List of asteroid close approaches to Earth in 2010|2010]]
            : [[List of asteroid close approaches to Earth in 2011|2011]]
            : [[List of asteroid close approaches to Earth in 2012|2012]]
            : [[List of asteroid close approaches to Earth in 2013|2013]]
            : [[List of asteroid close approaches to Earth in 2014|2014]]
            : [[List of asteroid close approaches to Earth in 2015|2015]]
            : [[List of asteroid close approaches to Earth in 2016|2016]]
            : [[List of asteroid close approaches to Earth in 2017|2017]]
            : [[List of asteroid close approaches to Earth in 2018|2018]]
            : [[List of asteroid close approaches to Earth in 2019|2019]]
            : [[List of asteroid close approaches to Earth in 2020|2020]]
            : [[List of asteroid close approaches to Earth in 2021|2021]]
            : [[List of asteroid close approaches to Earth in 2022|2022]]
            : [[List of asteroid close approaches to Earth in 2022|2023]]
}}

One problem with assessing effectiveness this way is that the sensitivity of infrasound sensors extends to small asteroids, which generally do very little damage. The missed asteroids do tend to be small, and missing small asteroids is relatively unimportant. By contrast, missing a large day-side impacting asteroid is highly problematic, with the  unpredicted mid-size [[Chelyabinsk meteor]] providing a mild real-life example. In order to assess the effectiveness for detecting the (rare) larger asteroids which do matter, a different approach is needed.

That effectiveness for larger asteroid can be assessed by looking at warning times for asteroids which did not impact Earth but came close. The below diagram for asteroids which came closer than the [[Moon]] shows how far in advance of closest approach they were first detected. Unlike actual asteroid impacts, where infrasound sensors provide  [[ground truth]], it is impossible to know for sure how many close approaches were undetected. The below chart therefore and by construction excludes any asteroids which went completely undetected. Of the asteroids that were detected, the diagram shows that about half were not detected until after they had passed Earth. If they had been on course to impact Earth, they would not have been spotted before they hit, primarily because they approached from a direction close to the Sun. This includes larger asteroids such as [[2018 AH]], which approached from a direction close to the Sun and wasn't detected until 2 days after it had passed. It is estimated to be around 100 times more massive than the [[Chelyabinsk meteor]].

{{See|List of asteroid close approaches to Earth}}

{{ #invoke:Chart | bar chart
| height = 300
| width = 600
| stack = 1
<!--         200x 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 -->
| group 1 =    0 :  0 :  2 :  1 :  0 :  0 :  0 :  0 :  1 :  2 :  0
| group 2 =    2 :  0 :  0 :  0 :  1 :  0 :  0 :  0 :  0 :  0 :  0
| group 3 =    0 :  0 :  0 :  2 :  0 :  1 :  1 :  0 :  1 :  0 :  4
| group 4 =   41 : 12 :  9 : 12 :  9 :  8 :  6 : 17 : 23 : 26 : 20
| group 5 =    9 :  0 :  1 :  1 :  2 :  5 :  3 :  5 :  6 : 11 : 11
| group 6 =   41 : 11 : 19 :  6 : 12 : 18 : 17 : 38 : 26 : 54 : 47
| group names = Discovered > 1 year in advance
              : Discovered > 7 weeks in advance
              : Discovered > 1 week in advance
              : Discovered up to 1 week in advance
              : < 24 hours warning
              : No warning
| colors = blue : turquoise : green : gray : yellow : red
| x legends = 2000–2009
            : [[List of asteroid close approaches to Earth in 2010|2010]]
            : [[List of asteroid close approaches to Earth in 2011|2011]]
            : [[List of asteroid close approaches to Earth in 2012|2012]]
            : [[List of asteroid close approaches to Earth in 2013|2013]]
            : [[List of asteroid close approaches to Earth in 2014|2014]]
            : [[List of asteroid close approaches to Earth in 2015|2015]]
            : [[List of asteroid close approaches to Earth in 2016|2016]]
            : [[List of asteroid close approaches to Earth in 2017|2017]]
            : [[List of asteroid close approaches to Earth in 2018|2018]]
            : [[List of asteroid close approaches to Earth in 2019|2019]]
}}

It is worth noting that the number of detections is increasing as more survey sites come on line (for example ATLAS in 2016 and ZTF in 2018), but that approximately half of the detections are invariably made after the asteroid passes the Earth. The below charts visualise the warning times of the close approaches listed in the above bargraph, by the size of the asteroid instead of by the year they occurred in. The sizes of the charts show the relative sizes of the asteroids to scale. This is based the [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] of each asteroid, an approximate measure of size based on brightness. For comparison, the approximate size of a person is also shown.

'''Abs Magnitude 30 and greater'''
[[File:Silhouette of man standing and facing forward.svg|Silhouette of man standing and facing forward|4px|left]] (size of a person for comparison)
{| class="wikitable"
| {{#invoke:Chart
  |pie chart
  |radius = 11
  |slices =
    (  4 : After closest approach  : red )
    (  3 : < 24 hours before       : yellow )
    (  0 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
| {{#invoke:Chart
  |pie chart
  |radius = 11
  |slices =
    ( 33 : After closest approach  : red )
    ( 19 : < 24 hours before       : yellow )
    ( 12 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
|-
! 2000–2009 !! 2010–2019
|}
'''Abs Magnitude 29–30'''
{| class="wikitable"
| {{#invoke:Chart
  |pie chart
  |radius = 17
  |slices =
    (  8 : After closest approach  : red )
    (  2 : < 24 hours before       : yellow )
    (  3 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  2 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
| {{#invoke:Chart
  |pie chart
  |radius = 17
  |slices =
    ( 57 : After closest approach  : red )
    ( 15 : < 24 hours before       : yellow )
    ( 34 : up to 7 days before     : lightgrey )
    (  2 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
|-
! 2000–2009 !! 2010–2019
|}
'''Absolute Magnitude 28–29'''
{| class="wikitable"
| {{#invoke:Chart
  |pie chart
  |radius = 27
  |slices =
    (  7 : After closest approach  : red )
    (  2 : < 24 hours before       : yellow )
    (  7 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
| {{#invoke:Chart
  |pie chart
  |radius = 27
  |slices =
    ( 73 : After closest approach  : red )
    (  9 : < 24 hours before       : yellow )
    ( 47 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  1 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
|-
! 2000–2009 !! 2010–2019
|}
'''Absolute Magnitude 27–28'''
{| class="wikitable"
| {{#invoke:Chart
  |pie chart
  |radius = 43
  |slices =
    ( 13 : After closest approach  : red )
    (  2 : < 24 hours before       : yellow )
    ( 12 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
| {{#invoke:Chart
  |pie chart
  |radius = 43
  |slices =
    ( 55 : After closest approach  : red )
    (  5 : < 24 hours before       : yellow )
    ( 33 : up to 7 days before     : lightgrey )
    (  2 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  1 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
|-
! 2000–2009 !! 2010–2019
|}
'''Absolute Magnitude 26–27'''

(probable size of the [[Chelyabinsk meteor]])
{| class="wikitable"
| {{#invoke:Chart
  |pie chart
  |radius = 68
  |slices =
    (  5 : After closest approach  : red )
    (  0 : < 24 hours before       : yellow )
    ( 15 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
| {{#invoke:Chart
  |pie chart
  |radius = 68
  |slices =
    ( 30 : After closest approach  : red )
    (  2 : < 24 hours before       : yellow )
    ( 13 : up to 7 days before     : lightgrey )
    (  4 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  1 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
|-
! 2000–2009 !! 2010–2019
|}
'''Absolute Magnitude 25–26'''
{| class="wikitable"
| {{#invoke:Chart
  |pie chart
  |radius = 107
  |slices =
    (  3 : After closest approach  : red )
    (  0 : < 24 hours before       : yellow )
    (  3 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
| {{#invoke:Chart
  |pie chart
  |radius = 107
  |slices =
    (  8 : After closest approach  : red )
    (  1 : < 24 hours before       : yellow )
    (  7 : up to 7 days before     : lightgrey )
    (  1 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  1 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
|-
! 2000–2009 !! 2010–2019
|}
'''Absolute Magnitude less than 25 (largest)'''
{| class="wikitable"
| {{#invoke:Chart
  |pie chart
  |radius = 170
  |slices =
    (  1 : After closest approach  : red )
    (  0 : < 24 hours before       : yellow )
    (  1 : up to 7 days before     : lightgrey )
    (  0 : > one week before       : green )
    (  0 : > 7 weeks before        : turquoise )
    (  0 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
| {{#invoke:Chart
  |pie chart
  |radius = 170
  |slices =
    (  7 : After closest approach  : red )
    (  0 : < 24 hours before       : yellow )
    (  5 : up to 7 days before     : lightgrey )
    (  1 : > one week before       : green )
    (  1 : > 7 weeks before        : turquoise )
    (  2 : > one year before       : blue )
  |percent = true
  |hide group legends = true
  }}
|-
! 2000–2009 !! 2010–2019
|}

As can be seen, the ability to predict larger asteroids has significantly improved since the early years of the 21st century, with some now being catalogued (predicted more than 1 year in advance), or having usable early warning times (greater than a week).

One final statistic which casts some light on the effectiveness of the current system is the average warning time for an asteroid impact. Based on the few successfully predicted asteroid impacts, the average time between initial detection and impact is currently around 9 hours. There is however some delay between the initial observation of the asteroid, data submission, and the follow-up observations and calculations which lead to an impact prediction being made.

{{See|#List of successfully predicted asteroid impacts}}

{{ #invoke:Chart | bar chart
| height = 300
| width = 600
| stack = 1
| colors = gray
<!--         <30m <3.5h <1d <1w   1w+ -->
| group 1 =    1 :  3 :  4 :  0 :  0
| group names = Successfully predicted impacts
| x legends = under 30 minutes notice
            : 0.5–3.5 hours notice (30–210 minutes)
            : 3.5–24 hours notice (210–1440 minutes)
            : 1–7 days notice
            : 1 week notice or more (over 10000 minutes)
}}

== Improving impact prediction ==
In addition to the already-funded telescopes mentioned above, two separate approaches have been suggested by NASA to improve impact prediction. Both approaches focus on the first step in impact prediction (discovering near-Earth asteroids) as this is the largest weakness in the current system. The first approach uses more powerful ground-based telescopes similar to the [[Large Synoptic Survey Telescope|LSST]].<ref name="LSST schedule">{{cite web
 |url=https://www.lsst.org/about/timeline
 |title=LSST Project Schedule
 |date=8 June 2015
 |access-date=24 August 2018
 |archive-date=21 August 2018
 |archive-url=https://web.archive.org/web/20180821160508/https://www.lsst.org/about/timeline
 |url-status=live
 }}</ref>  Being ground-based, such telescopes will still only observe part of [[Celestial sphere|the sky around Earth]]. In particular, all ground-based telescopes have a large blind spot for any asteroids coming from the direction of the [[Sun]].<ref name="JPL-2017-SDT-Update"/> In addition, they are affected by weather conditions, [[airglow]] and the [[Lunar phase|phase of the Moon]].

{{wide image|SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|600px|align-cap=center|Ground based telescopes can only detect objects approaching on the night-side of the planet, away from the [[Sun]]. Roughly half of impacts occur on the day-side of the planet.}}

To get around all of these issues, the second approach suggested is the use of [[space telescopes|space-based telescopes]] which can observe a much larger region of [[Celestial sphere|the sky around Earth]]. Although they still cannot point directly towards the Sun, they do not have the problem of [[daylight|blue sky]] to overcome and so can detect asteroids much closer in the sky to the Sun than ground-based telescopes.<ref name="Vox article"/> Unaffected by weather or [[airglow]] they can also operate 24 hours per day all year round. Finally, telescopes in [[Outer space|space]] have the advantage of being able to use [[infrared]] sensors without the interference of the [[Atmosphere of Earth|Earth's atmosphere]]. These sensors are better for detecting asteroids than optical sensors, and although there are some ground based [[infrared telescope]]s such as [[UKIRT]],<ref>[http://www.ukidss.org UKIDSS Home Page] {{Webarchive|url=https://web.archive.org/web/20131104011541/http://www.ukidss.org/ |date=4 November 2013 }}. Retrieved 30 April 2007.</ref> they are not designed for detecting asteroids. Space-based telescopes are more expensive, however, and tend to have a shorter lifespan. Therefore, Earth-based and space-based technologies complement each other to an extent.<ref name="JPL-2017-SDT-Update"/> Although the majority of the IR spectrum is blocked by Earth's atmosphere, the very useful [[thermal infrared|thermal]] (long-wavelength infrared) [[frequency band]] is not blocked (see gap at 10 μm in the diagram below). This allows for the possibility of ground based [[thermal image|thermal imaging]] surveys designed for detecting near earth asteroids, though none are currently planned.

{{wide image|Atmospheric electromagnetic opacity.svg|600px|align-cap=center|A diagram of the [[electromagnetic spectrum]] and the types of [[telescope]] used to view different parts of it}}

=== Opposition effect ===
There is a further issue that even telescopes in Earth orbit do not overcome (unless they operate in the [[thermal infrared]] spectrum). This is the issue of illumination. Asteroids go through [[Lunar phase|phases]] similar to the [[lunar phase]]s. Even though a telescope in orbit may have an unobstructed view of an object that is close in the sky to the Sun, it will still be looking at the dark side of the object. This is because the Sun is shining primarily on the side facing away from the Earth, as is the case with the [[Moon]] when it is in a [[new moon]] phase. Because of this [[opposition effect]], objects are far less bright in these phases than when fully illuminated, which makes them difficult to detect (see chart and diagram below).

{|
|{{Bar graph
| title       = Known asteroids passing <1 [[Lunar distance (astronomy)|LD]] from Earth
| float       = left
| data_max    = 100
| bar_width   = 15
| width_units = em
| label_type  = year
| data_type   = discovered on [[Opposition (astronomy)|opposition]]
| label1 = 2001
| data1 = 1
| label2 = 2002
| data2 = 2
| label3= 2003
| data3 = 5
| label4= 2004
| data4 = 10
| label5 = 2005
| data5 = 6
| label6 = 2006
| data6 = 12
| label7 = 2007
| data7 = 15
| label8 = [[List of asteroid close approaches to Earth in 2008|2008]]
| data8 = 20
| label9 = [[List of asteroid close approaches to Earth in 2009|2009]]
| data9 = 19
| label10 = [[List of asteroid close approaches to Earth in 2010|2010]]
| data10 = 22
| label11 = [[List of asteroid close approaches to Earth in 2011|2011]]
| data11 = 27
| label12 = [[List of asteroid close approaches to Earth in 2012|2012]]
| data12 = 22
| label13 = [[List of asteroid close approaches to Earth in 2013|2013]]
| data13 = 23
| label14 = [[List of asteroid close approaches to Earth in 2014|2014]]
| data14 = 32
| label15 = [[List of asteroid close approaches to Earth in 2015|2015]]
| data15 = 27
| label16 = [[List of asteroid close approaches to Earth in 2016|2016]]
| data16 = 60
| label17 = [[List of asteroid close approaches to Earth in 2017|2017]]
| data17 = 56
| label18 = [[List of asteroid close approaches to Earth in 2018|2018]]
| data18 = 91
| col2_data_type= discovered later
| col2_data_max = 100
| col2_data1 = 1
| col2_data2 = 0
| col2_data3 = 0
| col2_data4 = 0
| col2_data5 = 0
| col2_data6 = 0
| col2_data7 = 0
| col2_data8 = 0
| col2_data9 = 0
| col2_data10 = 0
| col2_data11 = 0
| col2_data12 = 0
| col2_data13 = 0
| col2_data14 = 0
| col2_data15 = 0
| col2_data16 = 0
| col2_data17 = 0
| col2_data18 = 0
| col3_data_type = discovered prior
| col3_data_max = 100
| col3_data1 = 0
| col3_data2 = 0
| col3_data3 = 0
| col3_data4 = 0
| col3_data5 = 0
| col3_data6 = 0
| col3_data7 = 1
| col3_data8 = 0
| col3_data9 = 0
| col3_data10 = 0
| col3_data11 = 3
| col3_data12 = 0
| col3_data13 = 1
| col3_data14 = 0
| col3_data15 = 0
| col3_data16 = 0
| col3_data17 = 1
| col3_data18 = 2
}}
|}
{{clear}}
This problem can be solved by the use of thermal infrared surveys (either ground based or space based). Ordinary telescopes depend on observing light reflected from the Sun, which is why the opposition effect occurs. Telescopes which [[Thermal imager|detect thermal infrared light]] depend only on the temperature of the object. Its thermal glow can be detected from any angle, and is particularly useful for differentiating asteroids from the background stars, which have a different thermal signature.<ref name="NEOCAM-Infrared"/>

This problem can also be solved without using thermal infrared, by positioning a [[space telescope]] away from Earth, closer to the Sun. The telescope can then look back towards Earth from the same direction as the Sun, and any asteroids closer to Earth than the telescope will then be in [[wikt:opposition|opposition]], and much better illuminated. There is a point between the Earth and Sun where the gravities of the two bodies are perfectly in balance, called the Sun-Earth L1 [[Lagrange point]] (SEL1). It is approximately {{convert|1|e6mi|e6km|order=flip|abbr=off}} from Earth, about four times as far away as the Moon, and is ideally suited for placing such a space telescope.<ref name="JPL-2017-SDT-Update"/> One problem with this position is Earth glare. Looking outward from SEL1, Earth itself is at full brightness, which prevents a telescope situated there from seeing that area of sky. Fortunately, this is the same area of sky that ground-based telescopes are best at spotting asteroids in, so the two complement each other.

Another possible position for a space telescope would be even closer to the Sun, for example in a [[Venus]]-like orbit. This would give a wider view of Earth orbit, but at a greater distance. Unlike a telescope at the SEL1 [[Lagrange point]], it would not stay in sync with Earth but would orbit the Sun at a similar rate to Venus. Because of this, it would not often be in a position to provide any warning of asteroids shortly before impact, but it would be in a good position to catalog objects before they are on final approach, especially those which primarily orbit closer to the Sun.<ref name="JPL-2017-SDT-Update"/> One issue with being as close to the Sun as Venus is that the craft may be too warm to use [[infrared]] wavelengths. A second issue would be communications. As the telescope will be a long way from Earth for most of the year (and even behind the Sun at some points) communication would often be slow and at times impossible, without expensive improvements to the [[Deep Space Network]].<ref name="JPL-2017-SDT-Update"/>

=== Solutions to problems: summary table ===
This table summarises which of the various problems encountered by current telescopes are solved by the various different solutions.

{| class="wikitable sortable" style="text-align:center"
! width=100 | Proposed solution
! Global<br />coverage
! [[Weather|Clouds]]
! [[Daylight|Blue<br />sky]]
! [[Lunar phase|Full<br />moon]]<br /><ref group="note">Around the time of the full Moon, the Moon is so bright that it lights up the atmosphere making faint objects impossible to see for several days per month</ref>
! [[Opposition effect|Opposition<br />Effect]]<br /><ref group="note">This refers to the opposition effect as seen from Earth, the fact that objects outside of the narrow cone centered on Earth are much fainter and harder to spot without using thermal infrared (see diagram above)</ref>
! [[Infrared telescope|Thermal<br />Infrared]]<br /><ref group="note">Use of thermal infrared allows objects to be seen at all angles as detection doesn't depend on reflected sunlight. It also allows an accurate size estimate of the object which is important for predicting the severity of an impact.</ref>
! [[Airglow]]
|-
! Geographically separated ground based survey telescopes
| {{Marked}} || || || || || ||
|-
! More powerful ground based survey telescopes
| || || || || {{Marked}} || ||
|-
! Infrared ground based NEO survey telescopes<ref group="note">Although many IR wavelengths are blocked by the atmosphere, there is a window from 8 μm to 14 μm that allows detection of IR at useful wavelengths such as 12 μm. A 12 μm sensor was used by WISE to detect asteroids during its space based mission. Although some ground based IR surveys exist which can detect 12 μm (such as [[UKIRT Infrared Deep Sky Survey]]), none are designed to detect moving objects such as asteroids.</ref>
| || || || || {{Marked}} || {{Marked}} ||
|-
! Telescope in Earth orbit
| {{Marked}} || {{Marked}} || {{Marked}} || {{Marked}}<br /><ref group="note" name="orbit moonglow">Telescopes in Earth orbit are affected to an extent by the glow of the moon, but not in the same way as ground based telescopes where the light from the moon is scattered across the sky by the atmosphere</ref>|| || || {{Marked}}
|-
! Infrared Telescope in Earth orbit
| {{Marked}} || {{Marked}} || {{Marked}} || {{Marked}}<br /><ref group="note" name="orbit moonglow"/> || {{Marked}} || {{Marked}} || {{Marked}}
|-
! Telescope at SEL1
| {{Marked}} || {{Marked}} || {{Marked}} || {{Marked}}<br /><ref group="note" name="SEL1 earthglow">Telescopes at SEL1 are primarily affected by the glare of the Earth rather than the Moon, but not in the same way as ground based telescopes where the light from the moon is scattered across the sky by the atmosphere</ref> || {{Marked}} || || {{Marked}}
|-
! Infrared Telescope at SEL1
| {{Marked}} || {{Marked}} || {{Marked}} || {{Marked}}<br /><ref group="note" name="SEL1 earthglow"/> || {{Marked}} || {{Marked}} || {{Marked}}
|-
! Telescope in Venus-like orbit
| || {{Marked}} || {{Marked}} || {{Marked}} || {{Marked}} || <ref group="note">Telescopes in a Venus-like orbit have no problems with atmosphere but being closer to the Sun, may be too warm to effectively use thermal infrared sensors. This problem could be overcome by using [[Cryogenics|cryogenic]] coolant but this increases cost and gives the telescope a limited lifespan due to the coolant running out</ref> || {{Marked}}
|}

==== Near-Earth Object Surveillance Mission ====
{{main|NEOSM}}
In 2017, NASA proposed a number of alternative solutions to detect 90% of near-Earth objects of size 140&nbsp;m or larger over the next few decades. As the detection sensitivity drops off with size but does not cut off, this will also improve the detection rates for the smaller objects which impact Earth much more often. Several of the proposals use a combination of an improved ground-based telescope and a space-based telescope positioned at the SEL1 [[Lagrange point]].<ref name="JPL-2017-SDT-Update"/><ref name="NEO Preparedness Plan">{{cite news
 |title        = Federal Government Releases National Near-Earth Object Preparedness Plan
 |work         = Centre for NEO Studies
 |publisher    = Interagency Working Group for Detecting and Mitigating the Impact of Earth-Bound Near-Earth Objects
 |url          = https://www.jpl.nasa.gov/news/news.php?feature=7166
 |access-date  = 24 August 2018
 |archive-date = 11 December 2019
 |archive-url  = https://web.archive.org/web/20191211045655/https://www.jpl.nasa.gov/news/news.php?feature=7166
 |url-status   = live
}}</ref><ref name="Vox article"/> A number of large ground based telescopes are already in the late stages of construction (see above). A space based mission situated at SEL1, [[NEOSM]] has now also been funded. It is planned for launch in 2026.<ref name="NASA-20210611"/><ref name="neosm-planet-soc"/>

== {{anchor|List}} List of successfully predicted asteroid impacts ==
<!--
Pie chart commented out for now after review comments pointing out not necessary whilst there are only 5 successful predictions.
{| class="infobox" style="width: 220px;"
|-
|
{| style="width: 211px;" cellpadding=0 cellspacing=2
|-
|Warning times for successfully predicted impacts
|-
| colspan=2 align=center | {{#invoke:Chart
  |pie chart
  |radius = 100
  |slices =
    ( 10 : < 24 hours   : red )
    (  0 : up to a week : yellow )
    (  0 : > a week     : green )
    (  0 : > a year     : blue )
  |units suffix =
  |percent = true
  }}
|}
|}
-->
Below is the list of all near-Earth objects which have or may have impacted the Earth and which were predicted beforehand.<ref name="NEODyS Past Impactors">{{cite web |title=Past Impactors |url=https://newton.spacedys.com/neodys/index.php?pc=4.5 |website=NEODyS (Near Earth Objects Dynamic Site) |publisher=ESA (European Space Agency) and University of Pisa |access-date=27 June 2020 |archive-date=2 July 2020 |archive-url=https://web.archive.org/web/20200702164015/https://newton.spacedys.com/neodys/index.php?pc=4.5 |url-status=live }}</ref> This list would also include any objects identified as having greater than 50% chance of impacting in the future, but no such future [[Impact event|impacts]] are predicted at this time.<ref name="Sentry">{{cite news
  |title      = Impact Risk Data
  |work       = Sentry: Earth Impact Monitoring
  |publisher  = Jet Propulsion Lab
  |url        = https://cneos.jpl.nasa.gov/sentry/vi.html
  |access-date = 7 July 2018}}</ref> As [[asteroid]] detection ability increases it is expected that prediction will become more successful in the future.
{{See|#Effectiveness of the current system}}
<!--
Commented out for now after review comments pointing out not necessary whilst there are only 3 successful predictions. Note to future editors who may reinstate this section, it was also noted that the colors used are indistinct and hard to differentiate on some screens, suggest making them more distinct
{{color box|#ffdddd}} Rows highlighted red indicate objects which were discovered less than 24 hours before impact

{{color box|#ffffdd}} Rows highlighted yellow indicate objects discovered up to a week before impact

{{color box|#ddffdd}} Rows highlighted green indicate objects discovered more than one week before impact

{{color box|#ddeeff}} Rows highlighted blue indicate objects discovered more than one year before impact (objects successfully cataloged on a previous orbit, rather than being detected during final approach)-->

{| class="wikitable sortable" style="font-size: 0.9em;"
! width=100 | Date of<br />impact
! Date<br />discovered
! width=100 | [[Near-Earth object|Object]]
! data-sort-type="number"| [[Observation arc]]<br />(minutes)
! data-sort-type="number"| Warning<br />period<br />(days)<ref group="note">The listed warning period is the time between the first observation and the impact. The time between the first impact prediction and the impact is necessarily shorter, and some of the impacts were actually predicted after they occurred.</ref>
! [[List of minor planets|Cataloged]]<br /><ref group="note">There are two main strategies for predicting [[asteroid]] impacts with [[Earth]], the Cataloging Strategy and the Warning Strategy. The Cataloging Strategy aims to detect all near Earth objects which could at some point in the future impact Earth. Accurate orbit predictions are made which can then anticipate any future impact years in advance. The larger and therefore most dangerous objects are amenable to this strategy as they can be observed from a sufficient distance. The more numerous but less dangerous smaller objects cannot so easily be detected this way as they are fainter and cannot be seen until they are relatively close by. The Warning Strategy aims to detect impactors months or days before they reach Earth ([https://www.nasa.gov/sites/default/files/atoms/files/2017_neo_sdt_final_e-version.pdf NASA 2017 Update on Enhancing the Search and Characterization of Near Earth Objects] {{Webarchive|url=https://web.archive.org/web/20171120074246/https://www.nasa.gov/sites/default/files/atoms/files/2017_neo_sdt_final_e-version.pdf |date=20 November 2017 }})</ref>
! data-sort-type="number"|Size ([[Metre|m]])<br />
! ([[Absolute magnitude#Solar System bodies (H)|H]])<br />(abs. mag)
! data-sort-type="number"| Velocity<br />[[With respect to|wrt]] Earth<br />(km/s)
! data-sort-type="number"| Velocity<br />wrt Sun<br />(km/s)
! Impact<br />Location
! data-sort-type="number"| Explosion<br />Altitude<br />(km)
! data-sort-type="number"| Impact<br />Energy<br />([[Kiloton|kt]])
! [[Meteorite fall|Meteorites recovered]]
|- <!-- bgcolor="#ffdddd" -->
| 2008-10-07 02:46  || 2008-10-06 || {{mpl|2008 TC|3}} ||align=center | 1,145 || align=center | 0.837 || align=center | No || align=center | 4.1 || align=center | 30.4 || align=center | 12.8 || align=center | 31.5 ||align=center | Northern [[Sudan]] || align=center | 37 || align=center | 0.98<ref name="Jenniskens2009">
{{Cite journal
 |last=Jenniskens |first=P.
 |date=2009
 |volume=458 |issue=7237 |pages=485–488
 |title=The impact and recovery of asteroid 2008 TC<sub>3</sub>
 |journal=[[Nature (journal)|Nature]]
 |bibcode=2009Natur.458..485J
 |doi=10.1038/nature07920
 |pmid=19325630
 |s2cid=7976525
 |display-authors=etal}}</ref> || align=center | {{Marked}}
|- <!-- bgcolor="#ffdddd" -->
| 2014-01-02 03:04 || 2014-01-01 || [[2014 AA]] ||align=center | 69 || align=center | 0.867 || align=center | No || align=center | 1.6–3.9 || align=center | 30.9 || align=center | 12.2 || align=center | 35.0 ||align=center | Central [[Atlantic Ocean|Atlantic]] || align=center | unknown || align=center | unknown<ref group="note">[[2014 AA]] exploded over the [[Atlantic Ocean|mid-Atlantic]], far from the nearest infrasound detectors. Although some detections were made, reliable figures are not known</ref><ref name="Farnocchia2016">{{cite journal|title=The trajectory and atmospheric impact of asteroid 2014 AA|first1=Davide|last1=Farnocchia|first2=Steven R.|last2=Chesley|first3=Peter G.|last3=Brown|first4=Paul W.|last4=Chodas|date=1 August 2016|journal=[[Icarus (journal)|Icarus]]|volume=274|pages=327–333|bibcode=2016Icar..274..327F|doi=10.1016/j.icarus.2016.02.056 }}</ref> || align=center | —
|- <!-- bgcolor="#ffdddd" -->
| 2018-06-02 16:45 || 2018-06-02 || [[2018 LA]]<ref group="note">[[2018 LA]] was estimated to have an 82% chance of impacting Earth somewhere between the central Pacific ocean and Africa ([https://projectpluto.com/temp/z.png Impact path] {{Webarchive|url=https://web.archive.org/web/20180613084708/https://www.projectpluto.com/temp/z.png |date=13 June 2018 }}). Several reports from South Africa and Botswana confirmed that it did indeed impact in South-central Africa and additional observations that came in after the impact post-predicted a consistent impact location.</ref> ||align=center | 227 || align=center | 0.354 || align=center | No || align=center | 2.7 || align=center | 30.6 || align=center | 16.6 || align=center | 37.6 ||align=center | [[Botswana]]-[[South Africa]] border || align=center | 28.7 || align=center | 1<ref name="JPL-new-discovery">{{cite news
 |title        = Tiny Asteroid Discovered Saturday Disintegrates Hours Later Over Southern Africa
 |work         = NASA/JPL
 |publisher    = Jet Propulsion Laboratory
 |url          = https://www.jpl.nasa.gov/news/news.php?feature=7148
 |access-date  = 4 June 2018
 |archive-date = 4 June 2018
 |archive-url  = https://web.archive.org/web/20180604133333/https://www.jpl.nasa.gov/news/news.php?feature=7148
 |url-status   = live
}}</ref> || align=center | {{Marked}}
|- <!-- bgcolor="#ffdddd" -->
| 2019-06-22 21:26 || 2019-06-22 || [[2019 MO]] ||align=center | 138 || align=center | 0.564 || align=center | No || align=center | 5.3–7.2 || align=center | 29.3 || align=center | 16.0 || align=center | 42.5 ||align=center | [[Caribbean Sea]],<br />South of [[Puerto Rico]] || align=center | 25 || align=center | 6<ref name="remanzacco">{{cite web
 |title        = Small Asteroid (NEOCP A10eoM1) impacted Earth on June 22
 |last1        = Guido
 |first1       = Ernesto
 |website      = Comets & Asteroids news (remanzacco)
 |date         = 25 June 2019
 |url          = https://remanzacco.blogspot.com/2019/06/small-asteroid-neocp-a10eom1-impacted.html
 |access-date  = 25 June 2019
 |archive-date = 2 July 2019
 |archive-url  = https://web.archive.org/web/20190702202128/https://remanzacco.blogspot.com/2019/06/small-asteroid-neocp-a10eom1-impacted.html
 |url-status   = live
}}</ref><ref name="UH">{{cite web
 |title        = Breakthrough: UH team successfully locates incoming asteroid
 |last1        = Gal
 |first1       = Roy
 |website      = University of Hawaii
 |url          = http://www.ifa.hawaii.edu/info/press-releases/ATLAS_2019MO/
 |access-date  = 26 June 2019
 |archive-date = 1 July 2019
 |archive-url  = https://web.archive.org/web/20190701050848/http://www.ifa.hawaii.edu/info/press-releases/ATLAS_2019MO/
 |url-status   = live
}}</ref> || align=center | — 
|- <!-- bgcolor="#ffdddd" -->
| 2022-03-11 21:22 || 2022-03-11 || {{mpl|2022 EB|5}} ||align=center| 112 || align=center | 0.082 || align=center | No || align=center | 4.2–5.7 || align=center | 31.3 || align=center | 18.5 || align=center | 41.4 ||align=center | [[Arctic Ocean]],<br />South of [[Jan Mayen]] || align=center | 33.3 || align=center | 4 || align=center | —
|- <!-- bgcolor="#ffdddd" -->
| 2022-11-19 08:26 || 2022-11-19 || {{mpl|2022 WJ|1}}<ref name=ESA-C8FF042>{{Cite web |url=https://twitter.com/esaoperations/status/1593912958509367296 |title=ESA C8FF042 (2:23 AM 19 Nov 2022) |access-date=19 November 2022 |archive-date=19 November 2022 |archive-url=https://web.archive.org/web/20221119122049/https://twitter.com/esaoperations/status/1593912958509367296 |url-status=live }}</ref> ||align=center| 185 || align=center | 0.149 || align=center | No || align=center | 0.5–1.2<ref name=ESA-C8FF042/> || align=center | 33.5 || align=center | 14.3 || align=center | 37.9 ||align=center | [[Brantford]], [[Ontario]], [[Canada]] || align=center | unknown || align=center | unknown || align=center | —
|- <!-- bgcolor="#ffdddd" -->
| 2023-02-13 02:59 || 2023-02-12 || {{mpl|2023 CX|1}} ||align=center| 394 || align=center | 0.279 || align=center | No || align=center | 0.7 || align=center | 32.8 || align=center | 14.2 || align=center | 37.8 ||align=center | [[English Channel]] || align=center | 28 || align=center | unknown || align=center | {{Marked}}
|- <!-- bgcolor="#ffdddd" -->
| 2024-01-21 00:32 || 2024-01-20 || {{mpl|2024 BX|1}} ||align=center| 156 || align=center | 0.114 || align=center | No || align=center | 0.4 || align=center | 32.8 || align=center | 15.3 || align=center | 35.7 ||align=center | [[Berlin, Germany]] || align=center | unknown || align=center | unknown || align=center | {{Marked}}
|- <!-- bgcolor="#ffdddd" -->
| 2024-09-04 16:39 || 2024-09-04 || {{mpl|2024 RW|1}}<ref>{{Cite web |url=https://minorplanetcenter.net/iau/NEO/ToConfirm_PrevDes.html |title=Previous NEO Confirmation Page Objects |access-date=4 September 2024 |archive-date=28 August 2024 |archive-url=https://web.archive.org/web/20240828160321/https://minorplanetcenter.net/iau/NEO/ToConfirm_PrevDes.html |url-status=live }}</ref> <ref>{{Cite web |url=https://minorplanetcenter.net/mpec/K24/K24R68.html |title=MPEC 2024-R68 : 2024 RW1 |access-date=4 September 2024 |archive-date=4 September 2024 |archive-url=https://web.archive.org/web/20240904144200/https://minorplanetcenter.net/mpec/K24/K24R68.html |url-status=live }}</ref> ||align=center| 618 || align=center | 0.455 || align=center | No || align=center | 1.5–2.0 || align=center | 32.1 || align=center | 20.8 || align=center | 38.9 ||align=center | Northern [[Luzon]], [[Philippines]] || align=center | 25.0 || align=center | 0.2 || align=center |  —
|- <!-- bgcolor="#ffdddd" -->
| 2024-10-22 10:54 || 2024-10-22 || [[2024 UQ]]|| align="center" | 14 || align=center | 0.074 || align=center | No || align=center | 1.2-1.5 || align=center | 32.7 || align=center | 23.6 || align=center | 38.1 ||align=center | Eastern [[Pacific Ocean]],<br />between [[Hawaii]] and [[California]] || align=center | 38.2 || align=center | 0.15 || align=center |  —
|}

In addition to these objects, the meteoroid CNEOS20200918 was found in 2022 in archival ATLAS data, imaged 10 minutes before its 2020/09/18 impact. Although it technically could have been discovered before impact, it was only noticed in retrospect.<ref>{{cite journal |last1=Clark |first1=David L. |last2=Wiegert |first2=Paul A. |last3=Brown |first3=Peter G. |last4=Vida |first4=Denis |last5=Heinze |first5=Aren |last6=Denneau |first6=Larry |title=Exoatmospheric detection of a meter-sized Earth impactor |journal=The Planetary Science Journal |date=9 September 2022 |volume=4 |issue=6 |page=103 |doi=10.3847/PSJ/acc9b1 |arxiv=2209.04583 |bibcode=2023PSJ.....4..103C |s2cid=252200041 |doi-access=free }}</ref>

There are also a number of objects which have been observed in orbit which may have impacted shortly after being observed, but may not have. It is difficult to know the true number of these possible impactors as unconfirmed tracklets have a wide range of possible orbits, and only a portion of these are consistent with earth impact. One example is [[A106fgF]], an object observed on January 22, 2018 with an observation arc of only 39 minutes. <ref group="note">Using the observation arc, it was only possible to estimate a 9% chance of impact between the South Atlantic, southern Africa, the Indian ocean, Indonesia, or the Pacific ocean. Whether the asteroid did impact Earth or not remains uncertain due to its small size and since much of the potential impact area is at sea or sparsely populated.</ref>

== See also ==
* [[Earth-grazing fireball]]
* [[List of asteroid close approaches to Earth]]
* [[List of bolides]]{{Snd}}asteroids and meteoroids that impacted Earth

== Notes ==
{{Reflist|group="note"}}

== References ==
{{reflist}}

==External links==
* [http://www.passc.net/EarthImpactDatabase/ Earth Impact Database]
* [http://www.lpl.arizona.edu/impacteffects/ Earth Impact Effects Program]
* [https://cneos.jpl.nasa.gov/ca/ NASA JPL Predicted Close Approaches (including impacts)]

{{Politics of outer space}}
{{Navboxes|list=
{{Modern impact events}}
}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System|Science}}

[[Category:Astronomical events]]
[[Category:Impact events|Impact events]]
[[Category:Lists of asteroids]]
[[Category:Near-Earth asteroids]]
[[Category:Planetary defense]]
[[Category:Predicted impact events|*]]