View source for Potentially hazardous object

{{Short description|Hazardous near-Earth asteroid or comet}}
[[Image:Toutatis.jpg|thumb|250px|The asteroid [[4179 Toutatis|Toutatis]] is listed as a potentially hazardous [[Near-Earth object|near-Earth]] [[asteroid]], yet poses no immediate threat to Earth. ([[radar astronomy|Radar image]] taken by [[Goldstone Deep Space Communications Complex|GDSCC]] in 1996.)]]

A '''potentially hazardous object''' ('''PHO''') is a [[near-Earth object]] – either an [[asteroid]] or a [[comet]] – with an [[orbit]] that can make close approaches to the [[Earth]] and which is large enough to cause significant regional damage in the [[Impact event|event of impact]].<ref name="Task-Force-Report" /> They are conventionally defined as having a [[minimum orbit intersection distance]] with Earth of less than {{convert|0.05|AU|LD|lk=on|sigfig=3|abbr=off}} and an [[absolute magnitude]] of 22 or brighter, the latter of which roughly corresponds to a size larger than 140 meters.<ref name="CNEOS-Basics" /> More than 99% of the known potentially hazardous objects are no impact threat over the next 100 years.<ref name=sentry/> {{As of|2022|09}}, just 17 of the known potentially hazardous objects listed on the [[Sentry Risk Table]] could not be excluded as potential threats over the next hundred years.<ref name="risk-table"/> Over hundreds if not thousands of years though, the orbits of some "potentially hazardous" asteroids can evolve to live up to their namesake.

Most of these objects are '''potentially hazardous asteroids''' ('''PHAs'''), and a few are [[comet]]s. {{As of|2022|11}} there are 2,304 known PHAs (about 8% of the total near-Earth population), of which 153 are estimated to be larger than one kilometer in diameter ''(see list of [[#Largest PHAs|largest PHAs]] below)''.<ref name="CNEOS-totals" /><ref name="MPC-unusual-overview" />{{efn|name=note-diameter-estimates}} Most of the discovered PHAs are [[Apollo asteroids]] (1,965) and fewer belong to the group of [[Aten asteroid]]s (185).<ref name="SBDB-query-all-PHA-Apollos"/><ref name="SBDB-query-all-PHA-Atens"/>

A potentially hazardous object can be known not to be a threat to Earth for the next 100 years or more, if its orbit is reasonably well determined. Potentially hazardous asteroids with some threat of impacting Earth in the next 100 years are listed on the Sentry Risk Table. {{As of|2022|09}}, only 17 potentially hazardous asteroids are listed on the Sentry Risk Table.<ref name="risk-table"/> Most potentially hazardous asteroids are ruled out as hazardous to at least several hundreds of years when their competing best orbit models are sufficiently constrained, but recent discoveries whose orbital constraints are little-known have divergent or incomplete mechanical models until observation yields further data. After several [[astronomical survey]]s, the number of known PHAs has increased tenfold since the end of the 1990s ''(see [[#Statistics|bar charts]] below)''.<ref name="CNEOS-totals" /> The [[Minor Planet Center]]'s website ''List of the Potentially Hazardous Asteroids'' also publishes detailed information for these objects.<ref name="MPC-PHA-list" />

In May 2021, NASA astronomers reported that 5 to 10 years of preparation may be needed to avoid a [[Asteroid impact avoidance|potential impactor]], as most recently based on a simulated exercise conducted by the 2021 Planetary Defense Conference.<ref name="BUS-20210512">{{cite news |last1=McFall-Johnsen |first1=Morgan |last2=Woodward |first2=Aylin |title=A NASA simulation revealed that 6 months' warning isn't enough to stop an asteroid from hitting Earth. We'd need 5 to 10 years.|url=https://www.businessinsider.com/nasa-asteroid-simulation-reveals-need-years-of-warning-2021-5 |date=12 May 2021 |work=[[Business Insider]] |accessdate=14 May 2021 }}</ref><ref name="SPC-20210501">{{cite news |last=Bartels |first=Meghan |title=How did you spend your week? NASA pretended to crash an asteroid into Earth. |url=https://www.space.com/planetary-defense-asteroid-impact-scenario-exercise-2021 |date=1 May 2021 |work=[[Space.com]] |accessdate=14 May 2021 }}</ref><ref name="NASA-20210430">{{cite news |last1=Chodas |first1=Paul |last2=Khudikyan |first2=Shakeh |last3=Chamberlin |first3=Alan |title=Planetary Defense Conference Exercise – 2021 Planetary Defense Conference (virtually) in Vienna, Austria, April 26–April 30, 2021. |url=https://cneos.jpl.nasa.gov/pd/cs/pdc21/ |date=30 April 2021 |work=[[NASA]] |accessdate=14 May 2021 }}</ref>

== Overview ==
[[Image:Potentially Hazardous Asteroids 2013.png|thumb|left|245px|Plot of orbits of known potentially hazardous asteroids, with sizes over {{convert|460|ft|m|order=flip}} and that pass within {{convert|4.7|e6mi|e6km|order=flip}} of Earth's orbit. [[Epoch (astronomy)|Epoch]] as of early 2013.]]

An object is considered a PHO if its [[minimum orbit intersection distance]] (MOID) with respect to Earth is less than {{convert|0.05|AU|km mi|abbr=on|lk=on}} – approximately 19.5&nbsp;[[Lunar distance (astronomy)|lunar distances]] – and its [[absolute magnitude]] is brighter than 22, approximately corresponding to a diameter above {{convert|140|m|ft|sp=us}}.<ref name="Task-Force-Report" /><ref name="CNEOS-Basics" /> This is big enough to cause regional devastation to human settlements unprecedented in human history in the case of a land impact, or a major [[tsunami]] in the case of an [[ocean]] impact. Such impact events occur on average around once per 10,000&nbsp;years. [[NEOWISE]] data estimates that there are 4,700&nbsp;±&nbsp;1,500 potentially hazardous asteroids with a diameter greater than 100 meters.<ref name="NASA-news" />

=== Levels of hazard ===
{{main|Torino scale|Palermo Technical Impact Hazard Scale}}

The two main scales used to categorize the impact hazards of asteroids are the Palermo Technical Impact Hazard Scale and the Torino scale.

=== Potentially hazardous comet ===
{{Unreferenced section|date=May 2022|find=Potentially hazardous comets}}
A '''potentially hazardous comet''' ('''PHC''') is a short-period comet which currently has an Earth-[[Minimum orbit intersection distance|MOID]] less than 0.05&nbsp;AU. Known PHCs include: [[109P/Swift-Tuttle]], [[55P/Tempel–Tuttle]], [[15P/Finlay]], [[289P/Blanpain]], [[255P/Levy]], [[206P/Barnard–Boattini]], [[21P/Giacobini–Zinner]], and [[73P/Schwassmann–Wachmann]]. [[Halley's Comet]] fit the criteria before AD 837, when it passed the earth at a distance of 0.033&nbsp;AU. It now has an MOID of 0.075&nbsp;AU.

=== Numbers ===
[[File:NEA by survey.png|thumb|Detected [[Near-Earth object|NEA]]s by various [[Astronomical survey|projects]]. The broader class of NEAs includes all PHAs as a subset.<ref name="CNEOS-totals" />
{| style="width: 100%; margin-bottom: 8px 0 6px; background-color:white;"
|-
| valign=top width=50% |
{{legend2|#0000FF|border=1px solid #0000B3|[[Lincoln Near-Earth Asteroid Research|LINEAR]]}}<br />
{{legend2|#FFA500|border=1px solid #B37400|[[Near Earth Asteroid Tracking|NEAT]]}}<br />
{{legend2|#FF0000|border=1px solid #800000|[[Spacewatch]]}}<br />
{{legend2|#FFFF00|border=1px solid #B3B300|[[LONEOS]]}}<br />
{{legend2|#008B00|border=1px solid #003D00|[[Catalina Sky Survey|CSS]]}}
| valign=top |
{{legend2|#9400D3|border=1px solid #390052|[[Pan-STARRS]]}}<br />
{{legend2|#00CCCC|border=1px solid #333|[[NEOWISE]]}}<br />
{{legend2|#FFE4C4|border=1px solid #333|[[Asteroid Terrestrial-impact Last Alert System|ATLAS]]}}<br />
{{legend2|#993300|border=1px solid #333|Others}}
|}]]

In 2012 [[NASA]] estimated 20 to 30&nbsp;percent of these objects have been found.<ref name="NASA-news" /> During an asteroid's close approaches to another planet it will be subject to gravitational [[perturbation (astronomy)|perturbation]], modifying its orbit, and potentially changing a previously non-threatening asteroid into a PHA or vice versa. This is a reflection of the dynamic character of the Solar System.

Several [[astronomical survey]] projects such as [[Lincoln Near-Earth Asteroid Research]], [[Catalina Sky Survey]] and [[Pan-STARRS]] continue to search for more PHOs. Each one found is studied by various means, including optical, [[Radar astronomy|radar]], and [[infrared]] to determine its characteristics, such as size, composition, rotation state, and to more accurately determine its orbit. Both professional and [[amateur astronomer]]s participate in such observation and tracking.

=== Size ===

Asteroids larger than approximately 35&nbsp;meters across can pose a threat to a town or city.<ref name="SA2013" /> However the diameter of most small asteroids is not well determined, as it is usually only estimated based on their brightness and distance, rather than directly measured, e.g. from radar observations. For this reason [[NASA]] and the [[Jet Propulsion Laboratory]] use the more practical measure of [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] (''H''). Any asteroid with an absolute magnitude of 22.0 or brighter is assumed to be of the required size.<ref name="CNEOS-Basics" />

Only a coarse estimation of size can be found from the object's magnitude because an assumption must be made for its [[albedo]] which is also not usually known for certain. The NASA [[near-Earth object]] program uses an assumed [[albedo]] of 0.14 for this purpose. In May 2016, the asteroid size estimates arising from the [[Wide-field Infrared Survey Explorer]] and [[Wide-field Infrared Survey Explorer#NEOWISE|NEOWISE]] missions have been questioned.<ref name="NYT-20160523">{{cite news |last=Chang |first=Kenneth |title=How Big Are Those Killer Asteroids? A Critic Says NASA Doesn't Know. |url=https://www.nytimes.com/2016/05/24/science/asteroids-nathan-myhrvold-nasa.html |date=23 May 2016 |work=The New York Times |access-date=24 May 2016 }}</ref><ref name="ARX-20160523">{{cite arXiv|last=Myhrvold |first=Nathan |title=Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data |date=23 May 2016 |eprint=1605.06490v2|class=astro-ph.EP }}</ref><ref name="SA-20160527">{{cite web |last=Billings |first=Lee |title=For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling |url=http://www.scientificamerican.com/article/for-asteroid-hunting-astronomers-nathan-myhrvold-says-the-sky-is-falling1/ |date=27 May 2016 |work=[[Scientific American]] |access-date=28 May 2016 }}</ref> Although the early original criticism had not undergone peer review,<ref name="NASA-20160525">{{cite news |author=NASA Administrator |title=NASA Response to Recent Paper on NEOWISE Asteroid Size Results |url=https://www.nasa.gov/feature/nasa-response-to-recent-paper-on-neowise-asteroid-size-results |date=25 May 2016 |work=[[NASA]] |access-date=29 May 2016 }}</ref> a more recent peer-reviewed study was subsequently published.<ref name="ICARUS-20180522">{{cite journal |last=Myhrvold |first=Nathan |author-link=Nathan Myhrvold |title=An empirical examination of WISE/NEOWISE asteroid analysis and results |journal=[[Icarus (journal)|Icarus]] |volume=314 |pages=64–97 |doi=10.1016/j.icarus.2018.05.004 |bibcode=2018Icar..314...64M |year=2018 |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 – 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=14 June 2018 }}</ref>

== Largest PHAs ==

With a [[mean diameter]] of approximately 7 kilometers, Apollo asteroid {{mpl|(53319) 1999 JM|8}} is likely the largest known potentially hazardous object, despite its fainter [[absolute magnitude]] of 15.2, compared to other listed objects in the table below ''(note: calculated mean-diameters in table are inferred from the object's brightness and its (assumed) albedo. They are only an approximation.)''. The lowest [[Minor planet designation|numbered]] PHA is [[1566&nbsp;Icarus]].<ref name="MPC-PHA-list" />
{{sort under}}
{| class="wikitable sortable sort-under-center"
|+ Brightest Potentially Hazardous Asteroids&thinsp;<ref name="MPC-PHA-list" />
! width= 140 rowspan=2 | [[Minor planet designation|Designation]] !! colspan=3 | Discovery !! rowspan=2 | [[Absolute magnitude#Solar System bodies (H)|(H)]]<br>(mag) !! rowspan=2 | [[Diameter|D]]<br>(km) !! colspan=7 | Orbital description !! rowspan=2 width=240 class=unsortable| Remarks !! rowspan=2 width=150 class="unsortable" | References
|-
! width=45 | Year !! [[List of observatory codes|Place]] !! width=115 | [[List of minor planet discoverers|Discoverer]] !! Class !! [[Semi-major axis|a]]<br />{{small|([[Astronomical unit|AU]])}} !! [[Orbital eccentricity|e]] !! [[Orbital inclination|''i'']]<br />{{small|([[Degree (angle)|°]])}} !! [[Perihelion|q]]<br />{{small|(AU)}} !! [[Aphelion|Q]]<br />{{small|(AU)}} !! [[Minimum orbital intersection distance|MOID]]<br />{{small|(AU)}}
|-
| data-sort-value="4953" | {{mpl|(4953) 1990 MU}}
| 1990
| [[IAU code#413|413]]
| R. H. McNaught
| 14.1
| 3
| [[Apollo asteroid|APO]]
| 1.621
| 0.658
| 24.4
| 0.555
| 2.687
| 0.02640
| —
|{{MPC|4953}}{{·}}{{JPL|4953}}{{·}}{{LoMP|4953|catalog}}
|-
| data-sort-value="3122" | [[3122 Florence]]
| 1981
| [[IAU code#413|413]]
| S. J. Bus
| 14.1
| 5
| [[Amor asteroid|AMO]]
| 1.769
| 0.423
| 22.2
| 1.020
| 2.518
| 0.04430
| —
|{{MPC|3122}}{{·}}{{JPL|3122}}{{·}}{{LoMP|3122|catalog}}
|-
| data-sort-value="16960" | {{mpl|(16960) 1998 QS|52}}
| 1998
| [[IAU code#704|704]]
| LINEAR
| 14.3
| 4
| [[Apollo asteroid|APO]]
| 2.203
| 0.858
| 17.5
| 0.313
| 4.093
| 0.01443
| —
|{{MPC|16960}}{{·}}{{JPL|16960}}{{·}}{{LoMP|16960|catalog}}
|-
| data-sort-value="4183" | [[4183 Cuno]]
| 1959
| [[IAU code#074|074]]
| C. Hoffmeister
| 14.4
| 4
| [[Apollo asteroid|APO]]
| 1.982
| 0.634
| 6.7
| 0.725
| 3.240
| 0.02825
| —
|{{MPC|4183}}{{·}}{{JPL|4183}}{{·}}{{LoMP|4183|catalog}}
|-
| data-sort-value="3200" | [[3200 Phaethon]]
| 1983
| [[IAU code#500|500]]
| IRAS
| 14.6
| 5.8
| [[Apollo asteroid|APO]]
| 1.271
| 0.890
| 22.3
| 0.140
| 2.402
| 0.01945
| —
|{{MPC|3200}}{{·}}{{JPL|3200}}{{·}}{{LoMP|3200|catalog}}
|-
| data-sort-value="242450" | {{mpl|(242450) 2004 QY|2}}
| 2004
| [[IAU code#E12|E12]]
| Siding Spring Survey
| 14.7
| 3
| [[Apollo asteroid|APO]]
| 1.084
| 0.477
| 37.0
| 0.567
| 1.601
| 0.04686
| —
|{{MPC|242450}}{{·}}{{JPL|242450}}{{·}}{{LoMP|242450|catalog}}
|-
| data-sort-value="89830" | {{mpl|(89830) 2002 CE}}
| 2002
| [[IAU code#704|704]]
| LINEAR
| 14.9
| 3.1
| [[Amor asteroid|AMO]]
| 2.077
| 0.507
| 43.7
| 1.023	
| 3.131
| 0.02767
| —
|{{MPC|89830}}{{·}}{{JPL|89830}}{{·}}{{LoMP|89830|catalog}}
|-
| data-sort-value="137427" | {{mpl|(137427) 1999 TF|211}}
| 1999
| [[IAU code#704|704]]
| LINEAR
| 15.1
| 2.9
| [[Apollo asteroid|APO]]
| 2.448
| 0.610
| 39.2
| 0.955
| 3.942
| 0.01787
| —
|{{MPC|137427}}{{·}}{{JPL|137427}}{{·}}{{LoMP|137427|catalog}}
|-
| data-sort-value="111253" | {{mpl|(111253) 2001 XU|10}}
| 2001
| [[IAU code#704|704]]
| LINEAR
| 15.2
| 3
| [[Apollo asteroid|APO]]
| 1.754
| 0.439
| 42.0
| 0.983
| 2.524
| 0.02934
| —
|{{MPC|111253}}{{·}}{{JPL|111253}}{{·}}{{LoMP|111253|catalog}}
|-
| data-sort-value="53319" | {{mpl|(53319) 1999 JM|8}}
| 1999
| [[IAU code#704|704]]
| LINEAR
| 15.2
| 7
| [[Apollo asteroid|APO]]
| 2.726
| 0.641
| 13.8	
| 0.978	
| 4.474
| 0.02346
| Likely largest PHO
|{{MPC|53319}}{{·}}{{JPL|53319}}{{·}}{{LoMP|53319|catalog}}
|-
| data-sort-value="1981" | [[1981 Midas]]
| 1973
| [[IAU code#675|675]]
| C. T. Kowal
| 15.2
| 2
| [[Apollo asteroid|APO]]
| 1.776
| 0.650
| 39.8
| 0.621
| 2.931
| 0.00449
| —
|{{MPC|1981}}{{·}}{{JPL|1981}}{{·}}{{LoMP|1981|catalog}}
|-
| data-sort-value="2201" | [[2201 Oljato]]
| 1947
| [[IAU code#690|690]]
| H. L. Giclas
| 15.25
| 2.1
| [[Apollo asteroid|APO]]
| 2.175
| 0.713
| 2.5
| 0.624
| 3.726
| 0.00305
| —
|{{MPC|2201}}{{·}}{{JPL|2201}}{{·}}{{LoMP|2201|catalog}}
|-
| data-sort-value="90075" | {{mpl|(90075) 2002 VU|94}}
| 2002
| [[IAU code#644|644]]
| NEAT
| 15.3
| 2.2
| [[Apollo asteroid|APO]]
| 2.134
| 0.576
| 8.9
| 0.904
| 3.363
| 0.03010
| —
|{{MPC|90075}}{{·}}{{JPL|90075}}{{·}}{{LoMP|90075|catalog}}
|-
| data-sort-value="4179" | [[4179 Toutatis]]
| 1989 
| [[IAU code#010|010]]
| C. Pollas
| 15.30
| 2.5
| [[Apollo asteroid|APO]]
| 2.536
| 0.629
| 0.4
| 0.940
| 4.132
| 0.00615
| —
|{{MPC|4179}}{{·}}{{JPL|4179}}{{·}}{{LoMP|4179|catalog}}
|-
| data-sort-value="159857" | {{mpl|(159857) 2004 LJ|1}}
| 2004
| [[IAU code#704|704]]
| LINEAR
| 15.4
| 3
| [[Apollo asteroid|APO]]
| 2.264
| 0.593
| 23.1
| 0.920
| 3.607
| 0.01682
| —
|{{MPC|159857}}{{·}}{{JPL|159857}}{{·}}{{LoMP|159857|catalog}}
|-
| data-sort-value="85713" | {{mpl|(85713) 1998 SS|49}}
| 1998
| [[IAU code#704|704]]
| LINEAR
| 15.6
| 3.5
| [[Apollo asteroid|APO]]
| 1.924
| 0.639
| 10.8
| 0.694
| 3.154
| 0.00234
| —
|{{MPC|85713}}{{·}}{{JPL|85713}}{{·}}{{LoMP|85713|catalog}}
|-
| data-sort-value="4486" | [[4486 Mithra]]
| 1987
| [[IAU code#071|071]]
| E. W. Elst<br />V. G. Shkodrov
| 15.6
| 2
| [[Apollo asteroid|APO]]
| 2.200
| 0.663
| 3.0
| 0.742
| 3.658
| 0.04626
| —
|{{MPC|4486}}{{·}}{{JPL|4486}}{{·}}{{LoMP|4486|catalog}}
|-
| data-sort-value="1620" | [[1620 Geographos]]
| 1951
| [[IAU code#675|675]]
| A. G. Wilson<br />R. Minkowski
| 15.60
| 2.5
| [[Apollo asteroid|APO]]
| 1.245
| 0.335
| 13.3
| 0.828
| 1.663
| 0.03007
| —
|{{MPC|1620}}{{·}}{{JPL|1620}}{{·}}{{LoMP|1620|catalog}}
|-
| data-sort-value="415029" | {{mpl|(415029) 2011 UL|21}}
| 2011
| [[IAU code#703|703]]
| CSS
| 15.7
| 2.5
| [[Apollo asteroid|APO]]
| 2.122
| 0.653
| 34.9
| 0.736
| 3.509
| 0.01925
| —
|{{MPC|415029}}{{·}}{{JPL|415029}}{{·}}{{LoMP|415029|catalog}}
|-
| data-sort-value="242216" | {{mpl|(242216) 2003 RN|10}}
| 2003
| [[IAU code#699|699]]
| LONEOS
| 15.7
| 2.5
| [[Amor asteroid|AMO]]
| 2.231
| 0.541
| 39.6
| 1.024
| 3.438
| 0.00956
| —
|{{MPC|242216}}{{·}}{{JPL|242216}}{{·}}{{LoMP|242216|catalog}}
|-
| data-sort-value="12923" | [[12923 Zephyr]]
| 1999
| [[IAU code#699|699]]
| LONEOS
| 15.8
| 2
| [[Apollo asteroid|APO]]
| 1.962
| 0.492
| 5.3
| 0.996
| 2.927
| 0.02115
| —
|{{MPC|12923}}{{·}}{{JPL|12923}}{{·}}{{LoMP|12923|catalog}}
|-
| data-sort-value="12923" | {{mpl|(52768) 1998 OR|2}}
| 1998
| [[IAU code#566|566]]
| NEAT
| 15.8
| 2
| [[Apollo asteroid|APO]]
| 2.380
| 0.573
| 5.9
| 1.017
| 3.743
| 0.01573
| —
|{{MPC|52768}}{{·}}{{JPL|52768}}{{·}}{{LoMP|52768|catalog}}
|}

=== Statistics ===

Below is a list of the largest PHAs (based on [[Absolute magnitude#Solar System bodies (H)|absolute magnitude H]]) discovered in a given year. Historical data of the cumulative number of discovered PHA since 1999 are displayed in the bar charts—one for the total number and the other for objects larger than one kilometer.<ref name="CNEOS-totals" /> PHAs brighter than absolute magnitude 17.75 are likely larger than 1&nbsp;km in size.
{| class="wikitable floatleft sortable"
|+ Brightest PHA discoveries of each calendar year since 1989&thinsp;<ref name="MPC-PHA-list" />
! data-sort-type="number" | Number
! width=85 | Name
! width=35 | Year
! width=35 | [[Absolute magnitude#Solar System bodies (H)|(H)]]
! Refs
|-
| align=right | 4179
| [[4179 Toutatis|Toutatis]]
| 1989
| 15.3
| {{MPC|4179}}{{·}}{{JPL|4179}}{{·}}{{LoMP|4179|catalog}}
|-
| align=right | 4953
| {{mpl-|4953|1990 MU}}
| 1990
| 14.9
| {{MPC|4953}}{{·}}{{JPL|4953}}{{·}}{{LoMP|4953|catalog}}
|-
| align=right | 7341
| {{mpl|1991 VH}}
| 1991
| 17.0
| {{MPC|35107}}{{·}}{{JPL|35107}}{{·}}{{LoMP|35107|catalog}}
|-
| align=right | 10115
| {{mpl|1992 SK}}
| 1992
| 17.2
| {{MPC|10115}}{{·}}{{JPL|10115}}{{·}}{{LoMP|10115|catalog}}
|-
| align=right | 39572
| {{mpl-|39572|1993 DQ1}}
| 1993
| 16.6
| {{MPC|39572}}{{·}}{{JPL|39572}}{{·}}{{LoMP|39572|catalog}}
|-
| align=right | 7482
| {{mpl-|7482|1994 PC|1}}
| 1994
| 16.7
| {{MPC|7482}}{{·}}{{JPL|7482}}{{·}}{{LoMP|7482|catalog}}
|-
| align=right | 243566
| {{mpl-|243566|1995 SA}}
| 1995
| 17.4
| {{MPC|243566}}{{·}}{{JPL|243566}}{{·}}{{LoMP|243566|catalog}}
|-
| align=right | 8566
| {{mpl-|8566|1996 EN}}
| 1996
| 16.3
| {{MPC|8566}}{{·}}{{JPL|8566}}{{·}}{{LoMP|8566|catalog}}
|-
| align=right | 35396
| {{mpl|1997 XF|11}}
| 1997
| 17.0
| {{MPC|35396}}{{·}}{{JPL|35396}}{{·}}{{LoMP|35396|catalog}}
|-
| align=right | 16960
| {{mpl-|16960|1998 QS|52}}
| 1998
| 14.4
| {{MPC|16960}}{{·}}{{JPL|16960}}{{·}}{{LoMP|16960|catalog}}
|-
| align=right | 137427
| {{mpl-|137427|1999 TF|211}}
| 1999
| 15.3
| {{MPC|137427}}{{·}}{{JPL|137427}}{{·}}{{LoMP|137427|catalog}}
|-
| align=right | 138095
| {{mpl-|138095|2000 DK|79}}
| 2000
| 16.0
| {{MPC|138095}}{{·}}{{JPL|138095}}{{·}}{{LoMP|138095|catalog}}
|-
| align=right | 111253
| {{mpl-|111253|2001 XU|10}}
| 2001
| 15.3
| {{MPC|111253}}{{·}}{{JPL|111253}}{{·}}{{LoMP|111253|catalog}}
|-
| align=right | 89830
| {{mpl-|89830|2002 CE}}
| 2002
| 15.0
| {{MPC|89830}}{{·}}{{JPL|89830}}{{·}}{{LoMP|89830|catalog}}
|-
| align=right | 242216
| {{mpl-|242216|2003 RN|10}}
| 2003
| 15.7
| {{MPC|242216}}{{·}}{{JPL|242216}}{{·}}{{LoMP|242216|catalog}}
|-
| align=right | 242450
| {{mpl-|242450|2004 QY|2}}
| 2004
| 14.6
| {{MPC|242450}}{{·}}{{JPL|242450}}{{·}}{{LoMP|242450|catalog}}
|-
| align=right | 308242
| {{mpl-|308242|2005 GO|21}}
| 2005
| 16.3
| {{MPC|308242}}{{·}}{{JPL|308242}}{{·}}{{LoMP|308242|catalog}}
|-
| align=right | 374851
| {{mpl-|374851|2006 VV|2}}
| 2006
| 16.7
| {{MPC|374851}}{{·}}{{JPL|374851}}{{·}}{{LoMP|374851|catalog}}
|-
| align=right | 214869
| {{mpl-|214869|2007 PA|8}}
| 2007
| 16.5
| {{MPC|214869}}{{·}}{{JPL|214869}}{{·}}{{LoMP|214869|catalog}}
|-
| align=right | 294739
| {{mpl-|294739|2008 CM}}
| 2008
| 17.1
| {{MPC|294739}}{{·}}{{JPL|294739}}{{·}}{{LoMP|294739|catalog}}
|-
| align=right | 523630
| {{mpl-|523630|2009 OG}}
| 2009
| 16.2
| {{MPC|523630}}{{·}}{{JPL|523630}}{{·}}{{LoMP|523630|catalog}}
|-
| align=right | 458122
| {{mpl-|381906|2010 EW|45}}
| 2010
| 17.6
| {{MPC|458122}}{{·}}{{JPL|458122}}{{·}}{{LoMP|458122|catalog}}
|-
| align=right | 415029
| {{mpl-|415029|2011 UL|21}}
| 2011
| 15.9
| {{MPC|415029}}{{·}}{{JPL|415029}}{{·}}{{LoMP|415029|catalog}}
|-
| align=right | –
| {{mpl|2012 HJ|1}}
| 2012
| 17.9
| {{MPC|2012+HJ1}}{{·}}{{JPL|2012+HJ1}}{{·}} —
|-
| align=right | 507716
| {{mpl-|507716|2013 UP|8}}
| 2013
| 16.4
| {{MPC|507716}}{{·}}{{JPL|507716}}{{·}}{{LoMP|507716|catalog}}
|-
| align=right | 533671
| {{mpl|2014 LJ|21}}
| 2014
| 16.1
| {{MPC|533671}}{{·}}{{JPL|533671}}{{·}}{{LoMP|533671|catalog}}
|-
| align=right | –
| {{mpl|2015 HY|116}}
| 2015
| 17.6
| {{MPC|2015+HY116}}{{·}}{{JPL|2015+HY116}}{{·}} —
|-
| align=right | 620095
| {{mpl|2016 CB|194}}
| 2016
| 17.6
| {{MPC|620095}}{{·}}{{JPL|620095}}{{LoMP|620095|catalog}}
|-
| align=right | –
| {{mpl|2017 CH|1}}
| 2017
| 17.9
| {{MPC|2017+CH1}}{{·}}{{JPL|2017+CH1}}{{·}} —
|-
| align=right | –
| {{mpl|2018 XV|5}}
| 2018
| 17.7
| {{MPC|2018+XV5}}{{·}}{{JPL|2018+XV5}}{{·}} —
|-
| align=right | –
| {{mpl|2019 RU|3}}
| 2019
| 18.1
| {{MPC|2019+RU3}}{{·}}{{JPL|2019+RU3}}{{·}} —
|-
| align=right | –
| {{mpl|2020 SL|1}}
| 2020
| 17.7
| {{MPC|2020+SL1}}{{·}}{{JPL|2020+SL1}}{{·}} —
|-
| align=right | –
| {{mpl|2021 HK|12}}
| 2021
| 17.7
| {{MPC|2021+HK12}}{{·}}{{JPL|2021+HK12}}{{·}} —
|-
| align=right | –
| {{mpl|2022 AP|7}}
| 2022
| 17.3
| {{MPC|2022+AP7}}{{·}}{{JPL|2022+AP7}}{{·}} —
|}

<!-- Bar-Chart: PHA-KM; figures as per December 1, of each year; current year displays most recent figures-->
{{Image frame
 |width = 500
 |align=right
 |pos=bottom
 |content=<div style="margin: 15px 5px -30px -50px; font-size: 80%;">{{ #invoke:Chart | bar-chart
   | width = 500
   | height = 300
   | group 1 =50 : 59 : 71 : 83 : 98 : 105 : 121 : 130 : 134 : 138 : 142 : 145 : 146 : 150 : 150 : 152 : 153 : 155 : 156 : 156 : 156 : 157 : 157
   | colors = #FF3366
   | group names = 
   | units suffix = _PHA-KM discovered
   | x legends = prev.<!--before 1999--> : 1999 : : 2001: : 2003 : : 2005: : 2007 : : 2009 : : 2011 : : 2013 : : 2015 : : 2017 : : 2019 : 
   }}</div>
 |caption =Cumulative number of discovered potentially hazardous asteroids larger than 1 kilometer by end of year (first of December). As of September 2020, there are 157 known PHAs larger than one kilometer.<ref name="CNEOS-totals" />
|max-width=}}

<!-- Bar-Chart: total number of PHAs (as per December 1, of each year)-->
{{Image frame
 |width = 500
 |align=right
 |pos=bottom
 |content=<div style="margin: 15px 5px -30px -50px; font-size: 80%;">{{ #invoke:Chart | bar-chart
   | width = 500
   | height = 300
   | group 1=162 : 209 : 286 : 360 : 466 : 536 : 636 : 729 : 819 : 897 : 993 : 1078 : 1158 : 1265 : 1344 : 1436 : 1524 : 1629 : 1736 : 1857 : 1944 : 2036 : 2115
   | colors = #003DF5
   | group names = 
   | units suffix = _PHA discovered
   | x legends = prev.<!--before 1999--> : 1999 : : 2001: : 2003 : : 2005: : 2007 : : 2009 : : 2011 : : 2013 : : 2015 : : 2017 : : 2019 : 
   }}</div>
 |caption = Cumulative number of all discovered PHA by end of year (first of December). As of September 2020, there are 2115 PHAs.<ref name="CNEOS-totals" />
|max-width=}}
{{clear}}

== Gallery ==
<gallery>
File:2005YU55-20111107.jpg|Radar image of the 350-meter PHO {{mpl|(308635) 2005 YU|55}}
<!-- Commented out: File:Toutatis from Chang'e 2.jpg|[[4179 Toutatis|Toutatis]] seen by [[Chang'e 2]] -->
</gallery>

== See also ==
{{div col}}
* [[Asteroid impact avoidance]]
* [[Earth-grazing fireball]]
* [[Global catastrophic risk]]
* [[List of asteroid close approaches to Earth]]
* {{annotated link|Scout (NASA program)}}; short term warnings
* {{annotated link|Sentry (monitoring system)}}
* {{annotated link|Spaceguard}}
{{div col end}}

== Notes ==
{{notelist|refs=

{{efn|name=note-diameter-estimates|1=An object's calculated mean-diameter is only a rough estimate. It is inferred from the object's varying brightness—observed and measured at various times—and the assumed, yet often unknown reflectivity of its surface. NASA's [https://cneos.jpl.nasa.gov/tools/ast_size_est.html Asteroid Size Estimator] is a tool for a generic [[absolute magnitude]]-to-diameter conversion for an assumed [[geometric albedo]].}}

}} <!-- end of notelist -->

== References ==
{{reflist|refs=

<ref name="MPC-PHA-list">{{cite web
  |title      = List of the Potentially Hazardous Asteroids (PHAs)
  |publisher       = Minor Planet Center
  |url        = https://www.minorplanetcenter.net/iau/lists/t_phas.html
  |access-date = 16 January 2018}}</ref>

<ref name="Task-Force-Report">{{cite journal
 |title        = Report of the Task Force on potentially hazardous Near Earth Objects
 |author       = Task Force on potentially hazardous Near Earth Objects
 |date         = September 2000
 |url          = http://www.nss.org/resources/library/planetarydefense/2000-ReportOfTheTaskForceOnPotentiallyHazardousNearEarthObjects-UK.pdf
 |access-date   = 22 January 2018
 |archive-url  = https://web.archive.org/web/20161210142717/http://nss.org/resources/library/planetarydefense/2000-ReportOfTheTaskForceOnPotentiallyHazardousNearEarthObjects-UK.pdf
 |archive-date = 10 December 2016
 |url-status   = dead
}}</ref>

<ref name="CNEOS-Basics">{{cite web
  |title      = NEO Basics – Potentially Hazardous Asteroids (PHAs) 
  |publisher  = CNEOS NASA/JPL
  |url        = https://cneos.jpl.nasa.gov/about/neo_groups.html
  |access-date = 16 January 2020}}</ref>

<ref name="MPC-unusual-overview">{{cite web
  |title      = Unusual Minor Planets – Overview
  |publisher       = Minor Planet Center
  |url        = https://minorplanetcenter.net/iau/lists/Unusual.html
  |access-date = 16 January 2020}}</ref>

<ref name="CNEOS-totals">{{cite web
  |title      = Discovery Statistics – Cumulative Totals
  |publisher  = CNEOS NASA/JPL
  |date       = 7 January 2020
  |url        = https://cneos.jpl.nasa.gov/stats/totals.html
  |access-date = 16 January 2020}}</ref>

<ref name="NASA-news">{{cite web
  |title      = NASA news – NASA Survey Counts Potentially Hazardous Asteroids
  |date       = 16 May 2012
  |publisher  = NASA/JPL
  |url        = https://www.jpl.nasa.gov/news/news.php?release=2012-138
  |access-date = 22 January 2018}}</ref>

<ref name="SA2013">{{cite magazine
  |title      = Asteroid Hunter Gives an Update on the Threat of Near-Earth Objects
  |date       = 22 January 2013
  |magazine  = [[Scientific American]]
  |author     = Will Ferguson
  |url        = https://blogs.scientificamerican.com/observations/asteroid-hunter-gives-an-update-on-the-threat-of-near-earth-objects/
  |access-date=2013-01-23}}</ref>

<ref name="SBDB-query-all-PHA-Apollos">{{cite web
  |title      = JPL Small-Body Database Search Engine: PHAs and orbital class (APO)
  |publisher  = JPL Solar System Dynamics
  |url        = https://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=pha;obj_kind=all;obj_numbered=all;ast_orbit_class=APO;OBJ_field=0;ORB_field=0;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsCjCpAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AcA
  |access-date = 16 January 2020}}</ref>

<ref name="SBDB-query-all-PHA-Atens">{{cite web
  |title      = JPL Small-Body Database Search Engine: PHAs and orbital class (ATE)
  |publisher  = JPL Solar System Dynamics
  |url        = https://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=pha;obj_kind=all;obj_numbered=all;ast_orbit_class=ATE;OBJ_field=0;ORB_field=0;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsCjCpAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AcA
  |access-date = 16 January 2020}}</ref>

<ref name="risk-table">{{cite web
  |title=Sentry Risk Table
  |url=https://cneos.jpl.nasa.gov/sentry/
  |access-date=2022-09-29}} (Click "Use Unconstrained Settings" and select "H<=22" for list of PHAs)</ref>

<ref name=sentry>(17 [[Sentry (monitoring system)#Numbers|Sentry "risk-listed"]] PHAs / [https://cneos.jpl.nasa.gov/stats/totals.html 2289 known PHAs]) ≈ 0.74%</ref>

}} <!-- end of reflist -->

== External links ==
* [https://cneos.jpl.nasa.gov/sentry/ Sentry: Earth Impact Monitoring (current)] ([[NASA]] NEO Program)
* [http://andrew-lowe.ca/ALL-PHAS.HTM#Earth Very Close Approaches (<0.01 AU) of PHAs to Earth 1900-2200]
* [http://www.brera.mi.astro.it/sormano/teca.html TECA Table of Asteroids Next Closest Approaches to the Earth], ''[[Sormano Astronomical Observatory]]''
* [http://www.brera.mi.astro.it/sormano/mbpl.html MBPL – Minor Body Priority List (PHA Asteroids)], ''Sormano Astronomical Observatory''
* [http://swfound.org/media/16114/iac-10.e3.1b.4.pdf Responding to the Threat of Potentially-Hazardous Near Earth Objects] (PDF)
* [https://www.theguardian.com/science/2015/dec/23/risk-of-comet-hitting-earth-as-bad-as-asteroids-or-worse-say-researchers Risk of comet hitting Earth is greater than previously thought, say researchers], ''The Guardian'', 22 December 2015
* [https://www.minorplanetcenter.net/iau/lists/Sizes Conversion of Absolute Magnitude to Diameter], ''Minor Planet Center''

=== Minor Planet Center === 
* [http://www.minorplanetcenter.net/iau/lists/t_phas.html List of the Potentially Hazardous Asteroids (PHAs)]
* {{YouTube|id=Ar_h-zAccsY|title=Asteroid Hazards, Part 2: ''The Challenge of Detection''}} {{small|(min. 7:14)}}
* {{YouTube|id=8M_27st1ZCI|title=Asteroid Hazards, Part 3: ''Finding the Path''}} {{small|(min. 5:38)}}

{{Natural disasters}}
{{Small Solar System bodies}}
{{Solar System}}
{{Modern impact events}}
{{Impact cratering on Earth}}
{{Planetary defense}}
{{Doomsday}}
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[[Category:Near-Earth objects]]
[[Category:Planetary defense]]
[[Category:Space hazards]]
[[Category:Potentially hazardous asteroids|*]]