Potentially hazardous object

File:Toutatis.jpg

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.)

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. They are conventionally defined as having a minimum orbit intersection distance with Earth of less than and an absolute magnitude of 22 or brighter, the latter of which roughly corresponds to a size larger than 140 meters. More than 99% of the known potentially hazardous objects are no impact threat over the next 100 years. As of February 2025, 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. 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 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).Template:Efn Most of the discovered PHAs are Apollo asteroids (1,965) and fewer belong to the group of Aten asteroids (185).

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

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.

Overview

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Plot of orbits of known potentially hazardous asteroids, with sizes over and that pass within of Earth's orbit. 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 – approximately 19.5 lunar distances – and its absolute magnitude is brighter than 22, approximately corresponding to a diameter above . 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.

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

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Detected NEAs by various projects. The broader class of NEAs includes all PHAs as a subset.

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In 2012 NASA estimated 20 to 30 percent of these objects have been found. 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. 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.

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. Although the early original criticism had not undergone peer review, a more recent peer-reviewed study was subsequently published.

Largest PHAs

With a mean diameter of approximately 7 kilometers, Apollo asteroid Template:Mpl 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.). Template:Sort under

Brightest Potentially Hazardous Asteroids

Designation	Discovery			(H) (mag)	D (km)	Orbital description							Remarks	References
	Year	Place	Discoverer			Class	a (AU)	e	i (°)	q (AU)	Q (AU)	MOID (AU)
Template:Mpl	1990	413	R. H. McNaught	14.1	3	APO	1.621	0.658	24.4	0.555	2.687	0.02640	—	Template:MPC · JPL · Template:LoMP
3122 Florence	1981	413	S. J. Bus	14.1	5	AMO	1.769	0.423	22.2	1.020	2.518	0.04430	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	1998	704	LINEAR	14.3	4	APO	2.203	0.858	17.5	0.313	4.093	0.01443	—	Template:MPC · JPL · Template:LoMP
4183 Cuno	1959	074	C. Hoffmeister	14.4	4	APO	1.982	0.634	6.7	0.725	3.240	0.02825	—	Template:MPC · JPL · Template:LoMP
3200 Phaethon	1983	500	IRAS	14.6	5.8	APO	1.271	0.890	22.3	0.140	2.402	0.01945	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	2004	E12	Siding Spring Survey	14.7	3	APO	1.084	0.477	37.0	0.567	1.601	0.04686	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	2002	704	LINEAR	14.9	3.1	AMO	2.077	0.507	43.7	1.023	3.131	0.02767	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	1999	704	LINEAR	15.1	2.9	APO	2.448	0.610	39.2	0.955	3.942	0.01787	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	2001	704	LINEAR	15.2	3	APO	1.754	0.439	42.0	0.983	2.524	0.02934	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	1999	704	LINEAR	15.2	7	APO	2.726	0.641	13.8	0.978	4.474	0.02346	Likely largest PHO	Template:MPC · JPL · Template:LoMP
1981 Midas	1973	675	C. T. Kowal	15.2	2	APO	1.776	0.650	39.8	0.621	2.931	0.00449	—	Template:MPC · JPL · Template:LoMP
2201 Oljato	1947	690	H. L. Giclas	15.25	2.1	APO	2.175	0.713	2.5	0.624	3.726	0.00305	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	2002	644	NEAT	15.3	2.2	APO	2.134	0.576	8.9	0.904	3.363	0.03010	—	Template:MPC · JPL · Template:LoMP
4179 Toutatis	1989	010	C. Pollas	15.30	2.5	APO	2.536	0.629	0.4	0.940	4.132	0.00615	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	2004	704	LINEAR	15.4	3	APO	2.264	0.593	23.1	0.920	3.607	0.01682	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	1998	704	LINEAR	15.6	3.5	APO	1.924	0.639	10.8	0.694	3.154	0.00234	—	Template:MPC · JPL · Template:LoMP
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	—	Template:MPC · JPL · Template:LoMP
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	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	2011	703	CSS	15.7	2.5	APO	2.122	0.653	34.9	0.736	3.509	0.01925	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	2003	699	LONEOS	15.7	2.5	AMO	2.231	0.541	39.6	1.024	3.438	0.00956	—	Template:MPC · JPL · Template:LoMP
12923 Zephyr	1999	699	LONEOS	15.8	2	APO	1.962	0.492	5.3	0.996	2.927	0.02115	—	Template:MPC · JPL · Template:LoMP
Template:Mpl	1998	566	NEAT	15.8	2	APO	2.380	0.573	5.9	1.017	3.743	0.01573	—	Template:MPC · JPL · Template:LoMP

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. PHAs brighter than absolute magnitude 17.75 are likely larger than 1 km in size.

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.

Cumulative number of all discovered PHA by end of year (first of December). As of September 2020, there are 2115 PHAs.

Gallery

• Radar image of the 350-meter PHO Template:Mpl
  Radar image of the 350-meter PHO Template:Mpl

See also

• Asteroid impact avoidance
• Earth-grazing fireball
• Global catastrophic risk
• List of asteroid close approaches to Earth
• Template:Annotated link; short term warnings
• Template:Annotated link
• Template:Annotated link

External links

• Sentry: Earth Impact Monitoring (current) (NASA NEO Program)
• Very Close Approaches (<0.01 AU) of PHAs to Earth 1900-2200
• TECA Table of Asteroids Next Closest Approaches to the Earth, Sormano Astronomical Observatory
• MBPL – Minor Body Priority List (PHA Asteroids), Sormano Astronomical Observatory
• Responding to the Threat of Potentially-Hazardous Near Earth Objects (PDF)
• Risk of comet hitting Earth is greater than previously thought, say researchers, The Guardian, 22 December 2015
• Conversion of Absolute Magnitude to Diameter, Minor Planet Center

Minor Planet Center

• List of the Potentially Hazardous Asteroids (PHAs)
• Template:Replace on YouTube (min. 7:14)
• Template:Replace on YouTube (min. 5:38)