Potentially hazardous object

The asteroid Toutatis is listed as a potentially hazardous near-Earth asteroid, yet poses no immediate threat to Earth. (Radar image taken by GDSCC in 1996.)

百度 Kaufman笑道。

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 event of impact.^[1] They are conventionally defined as having a minimum orbit intersection distance with Earth of less than 0.05 astronomical units (19.5 lunar distances) and an absolute magnitude of 22 or brighter, the latter of which roughly corresponds to a size larger than 140 meters.^[2] More than 99% of the known potentially hazardous objects are no impact threat over the next 100 years.^[3] As of February 2025^[update], just 21 of the known potentially hazardous objects listed on the Sentry Risk Table could not be excluded as potential threats over the next hundred years.^[4] 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 comets. As of November 2022^[update] 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 below).^[5]^[6]^[a] Most of the discovered PHAs are Apollo asteroids (1,965) and fewer belong to the group of Aten asteroids (185).^[7]^[8]

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 September 2022^[update], only 17 potentially hazardous asteroids are listed on the Sentry Risk Table.^[4] 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 surveys, the number of known PHAs has increased tenfold since the end of the 1990s (see bar charts below).^[5] The Minor Planet Center's website List of the Potentially Hazardous Asteroids also publishes detailed information for these objects.^[9]

In May 2021, NASA astronomers reported that 5 to 10 years of preparation may be needed to avoid a potential impactor, as most recently based on a simulated exercise conducted by the 2021 Planetary Defense Conference.^[10]^[11]^[12]

Overview

An object is considered a PHO if its minimum orbit intersection distance (MOID) with respect to Earth is less than 0.05 AU (7,500,000 km; 4,600,000 mi) – approximately 19.5 lunar distances – and its absolute magnitude is brighter than 22, approximately corresponding to a diameter above 140 meters (460 ft).^[1]^[2] 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 years. NEOWISE data estimates that there are 4,700 ± 1,500 potentially hazardous asteroids with a diameter greater than 100 meters.^[13]

Levels of hazard

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

Potentially hazardous comet

A potentially hazardous comet (PHC) is a short-period comet which currently has an Earth-MOID less than 0.05 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 AU. It now has an MOID of 0.075 AU.

Numbers

In 2012 NASA estimated 20 to 30 percent of these objects have been found.^[13] During an asteroid's close approaches to another planet it will be subject to gravitational 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, and infrared to determine its characteristics, such as size, composition, rotation state, and to more accurately determine its orbit. Both professional and amateur astronomers participate in such observation and tracking.

Size

Asteroids larger than approximately 35 meters across can pose a threat to a town or city.^[14] 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 (H). Any asteroid with an absolute magnitude of 22.0 or brighter is assumed to be of the required size.^[2]

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 NEOWISE missions have been questioned.^[15]^[16]^[17] Although the early original criticism had not undergone peer review,^[18] a more recent peer-reviewed study was subsequently published.^[19]^[20]

Largest PHAs

With a mean diameter of approximately 7 kilometers, Apollo asteroid (53319) 1999 JM8 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.).

Brightest Potentially Hazardous Asteroids^[9]
Designation	Discovery			(H) (mag)	D (km)	Orbital description							Remarks	References
Designation	Year	Place	Discoverer	(H) (mag)	D (km)	Class	a (AU)	e	i (°)	q (AU)	Q (AU)	MOID (AU)	Remarks	References
(4953) 1990 MU	1990	413	R. H. McNaught	14.1	3	APO	1.621	0.658	24.4	0.555	2.687	0.02640	—	MPC · JPL · catalog
3122 Florence	1981	413	S. J. Bus	14.1	5	AMO	1.769	0.423	22.2	1.020	2.518	0.04430	—	MPC · JPL · catalog
(16960) 1998 QS52	1998	704	LINEAR	14.3	4	APO	2.203	0.858	17.5	0.313	4.093	0.01443	—	MPC · JPL · catalog
4183 Cuno	1959	074	C. Hoffmeister	14.4	4	APO	1.982	0.634	6.7	0.725	3.240	0.02825	—	MPC · JPL · catalog
3200 Phaethon	1983	500	IRAS	14.6	5.8	APO	1.271	0.890	22.3	0.140	2.402	0.01945	—	MPC · JPL · catalog
(242450) 2004 QY2	2004	E12	Siding Spring Survey	14.7	3	APO	1.084	0.477	37.0	0.567	1.601	0.04686	—	MPC · JPL · catalog
(89830) 2002 CE	2002	704	LINEAR	14.9	3.1	AMO	2.077	0.507	43.7	1.023	3.131	0.02767	—	MPC · JPL · catalog
(137427) 1999 TF211	1999	704	LINEAR	15.1	2.9	APO	2.448	0.610	39.2	0.955	3.942	0.01787	—	MPC · JPL · catalog
(111253) 2001 XU10	2001	704	LINEAR	15.2	3	APO	1.754	0.439	42.0	0.983	2.524	0.02934	—	MPC · JPL · catalog
(53319) 1999 JM8	1999	704	LINEAR	15.2	7	APO	2.726	0.641	13.8	0.978	4.474	0.02346	Likely largest PHO	MPC · JPL · catalog
1981 Midas	1973	675	C. T. Kowal	15.2	2	APO	1.776	0.650	39.8	0.621	2.931	0.00449	—	MPC · JPL · catalog
2201 Oljato	1947	690	H. L. Giclas	15.25	2.1	APO	2.175	0.713	2.5	0.624	3.726	0.00305	—	MPC · JPL · catalog
(90075) 2002 VU94	2002	644	NEAT	15.3	2.2	APO	2.134	0.576	8.9	0.904	3.363	0.03010	—	MPC · JPL · catalog
4179 Toutatis	1989	010	C. Pollas	15.30	2.5	APO	2.536	0.629	0.4	0.940	4.132	0.00615	—	MPC · JPL · catalog
(159857) 2004 LJ1	2004	704	LINEAR	15.4	3	APO	2.264	0.593	23.1	0.920	3.607	0.01682	—	MPC · JPL · catalog
(85713) 1998 SS49	1998	704	LINEAR	15.6	3.5	APO	1.924	0.639	10.8	0.694	3.154	0.00234	—	MPC · JPL · catalog
4486 Mithra	1987	071	E. W. Elst V. G. Shkodrov	15.6	2	APO	2.200	0.663	3.0	0.742	3.658	0.04626	—	MPC · JPL · catalog
1620 Geographos	1951	675	A. G. Wilson R. Minkowski	15.60	2.5	APO	1.245	0.335	13.3	0.828	1.663	0.03007	—	MPC · JPL · catalog
(415029) 2011 UL21	2011	703	CSS	15.7	2.5	APO	2.122	0.653	34.9	0.736	3.509	0.01925	—	MPC · JPL · catalog
(242216) 2003 RN10	2003	699	LONEOS	15.7	2.5	AMO	2.231	0.541	39.6	1.024	3.438	0.00956	—	MPC · JPL · catalog
12923 Zephyr	1999	699	LONEOS	15.8	2	APO	1.962	0.492	5.3	0.996	2.927	0.02115	—	MPC · JPL · catalog
(52768) 1998 OR2	1998	566	NEAT	15.8	2	APO	2.380	0.573	5.9	1.017	3.743	0.01573	—	MPC · JPL · catalog

Statistics

Below is a list of the largest PHAs (based on 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.^[5] PHAs brighter than absolute magnitude 17.75 are likely larger than 1 km in size.

Brightest PHA discoveries of each calendar year since 1989^[9]
Number	Name	Year	(H)	Refs
4179	Toutatis	1989	15.3	MPC · JPL · catalog
4953	1990 MU	1990	14.9	MPC · JPL · catalog
7341	1991 VH	1991	17.0	MPC · JPL · catalog
10115	1992 SK	1992	17.2	MPC · JPL · catalog
39572	1993 DQ₁	1993	16.6	MPC · JPL · catalog
7482	1994 PC₁	1994	16.7	MPC · JPL · catalog
243566	1995 SA	1995	17.4	MPC · JPL · catalog
8566	1996 EN	1996	16.3	MPC · JPL · catalog
35396	1997 XF11	1997	17.0	MPC · JPL · catalog
16960	1998 QS₅₂	1998	14.4	MPC · JPL · catalog
137427	1999 TF₂₁₁	1999	15.3	MPC · JPL · catalog
138095	2000 DK₇₉	2000	16.0	MPC · JPL · catalog
111253	2001 XU₁₀	2001	15.3	MPC · JPL · catalog
89830	2002 CE	2002	15.0	MPC · JPL · catalog
242216	2003 RN₁₀	2003	15.7	MPC · JPL · catalog
242450	2004 QY₂	2004	14.6	MPC · JPL · catalog
308242	2005 GO₂₁	2005	16.3	MPC · JPL · catalog
374851	2006 VV₂	2006	16.7	MPC · JPL · catalog
214869	2007 PA₈	2007	16.5	MPC · JPL · catalog
294739	2008 CM	2008	17.1	MPC · JPL · catalog
523630	2009 OG	2009	16.2	MPC · JPL · catalog
458122	2010 EW₄₅	2010	17.6	MPC · JPL · catalog
415029	2011 UL₂₁	2011	15.9	MPC · JPL · catalog
746849	2012 HJ1	2012	17.9	MPC · JPL · catalog
507716	2013 UP₈	2013	16.4	MPC · JPL · catalog
533671	2014 LJ21	2014	16.1	MPC · JPL · catalog
–	2015 HY116	2015	17.6	MPC · JPL · —
620095	2016 CB194	2016	17.6	MPC · JPL · catalog
–	2017 CH1	2017	17.9	MPC · JPL · —
756316	2018 XV5	2018	17.7	MPC · JPL · catalog
–	2019 RU3	2019	18.1	MPC · JPL · —
–	2020 SL1	2020	17.7	MPC · JPL · —
–	2021 HK12	2021	17.7	MPC · JPL · —
–	2022 AP7	2022	17.3	MPC · JPL · —
–	2023 QF5	2023	18.3	MPC · JPL · —
–	2024 YU4	2024	19.0	MPC · JPL · —

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1999

2001

2003

2005

2007

2009

2011

2013

2015

2017

2019

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.^[5]

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1,000

1,500

2,000

2,500

3,000