Potentially hazardous object

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 large enough to cause significant regional damage in the event of impact.[1] They are defined as having a minimum orbital intersection distance with Earth of less than 0.05 astronomical units (19.5 lunar distances) and an absolute magnitude of 22 or brighter.[2] 98% of the known potentially hazardous objects are not an impact threat over the next 100 years.[3] Only about 38 potentially hazardous objects are listed on the Sentry Risk Table as entries are removed when they become known not to be a threat over the next hundred years or more.[4]

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.)

Most of these objects are potentially hazardous asteroids (PHAs), and a few are comets. As of September 2020 there are 2,116 known PHAs (about 9% of the total near-Earth population), of which 157 are estimated to be larger than one kilometer in diameter (see list of largest PHAs below).[5][6][lower-alpha 1] Most of the discovered PHAs are Apollo asteroids (1,730) and fewer belong to the group of Aten asteroids (171).[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 June 2020, only about 38 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 become sufficiently divergent, 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]

Overview
Plot of orbits of known potentially hazardous asteroids, with sizes over 140 metres (460 ft) and that pass within 7.6 million kilometres (4.7×10^6 mi) 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 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.[10]

Levels of hazard
Main articles: Torino Scale and 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 comets

Short-period comets currently with an Earth-MOID less than 0.05 AU include: 109P/Swift-Tuttle, 55P/Tempel–Tuttle, 15P/Finlay, 289P/Blanpain, 255P/Levy, 206P/Barnard–Boattini, 21P/Giacobini–Zinner, and 73P/Schwassmann–Wachmann.

Numbers
Detected NEAs by various projects. The broader class of NEAs includes all PHAs as a subset.[5]
  LINEAR
  NEAT
  Spacewatch
  LONEOS
  CSS		   Pan-STARRS
  NEOWISE
  ATLAS
  Others

In 2012 NASA estimated 20 to 30 percent of these objects have been found.[10] During an asteroid's close approaches to planets or moons other than the Earth, 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.[11] 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.[12][13][14] Although the early original criticism had not undergone peer review,[15] a more recent peer-reviewed study was subsequently published.[16][17]

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 objects brightness and its (assumed) albedo. They are only an approximation.). The lowest numbered PHA is 1566 Icarus.[9]

Brightest Potentially Hazardous Asteroids [9]
DesignationDiscovery(H) (mag)D (km)Orbital descriptionRemarksRefs
YearPlaceDiscovererClassa
(AU)		ei
(°)		q
(AU)		Q
(AU)		MOID
(AU)
(4953) 1990 MU 1990 413 R. H. McNaught 14.1 3 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km AMO 2.231 0.541 39.6 1.024 3.438 0.00956 — MPC · JPL · catalog
12923 Zephyr 1999 699 LONEOS 15.8 2 km 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 km 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.[18] 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]

Brightest PHA discoveries of each calendar year since 1989 [18]
Number Name Year (H) Refs
4179 Toutatis 1989 15.3 MPC · JPL · catalog
4953 1990 MU 1990 14.1 MPC · JPL · catalog
7341 1991 VK 1991 16.7 MPC · JPL · catalog
5604 1992 FE 1992 16.4 MPC · JPL · catalog
39572 1993 DQ1 1993 16.4 MPC · JPL · catalog
136618 1994 CN2 1994 16.6 MPC · JPL · catalog
243566 1995 SA 1995 17.3 MPC · JPL · catalog
8566 1996 EN 1996 16.5 MPC · JPL · catalog
35396 1997 XF11 1997 16.9 MPC · JPL · catalog
16960 1998 QS52 1998 14.3 MPC · JPL · catalog
137427 1999 TF211 1999 15.0 MPC · JPL · catalog
23187 2000 PN9 2000 16.1 MPC · JPL · catalog
111253 2001 XU10 2001 14.9 MPC · JPL · catalog
89830 2002 CE 2002 14.7 MPC · JPL · catalog
242216 2003 RN10 2003 15.7 MPC · JPL · catalog
242450 2004 QY2 2004 14.7 MPC · JPL · catalog
308242 2005 GO21 2005 16.4 MPC · JPL · catalog
374851 2006 VV2 2006 16.8 MPC · JPL · catalog
214869 2007 PA8 2007 16.2 MPC · JPL · catalog
294739 2008 CM 2008 17.1 MPC · JPL · catalog
369264 2009 MS 2009 16.0 MPC · JPL · catalog
381906 2010 CL19 2010 17.8 MPC · JPL · catalog
415029 2011 UL21 2011 15.7 MPC · JPL · catalog
482467 2012 LK9 2012 17.8 MPC · JPL · catalog
507716 2013 UP8 2013 16.5 MPC · JPL · catalog
533671 2014 LJ21 2014 16.0 MPC · JPL · catalog
– 2015 HY116 2015 17.5 MPC · JPL · —
– 2016 CB194 2016 17.6 MPC · JPL · —
– 2017 CH1 2017 17.9 MPC · JPL · —
– 2018 XV5 2018 17.5 MPC · JPL · —
– 2019 CE4 2019 18.0 MPC · JPL · —
– 2020 SL1 2020 17.4 MPC · JPL · —
50
100
150
200
prev.
1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
2019
PHA-KM: potentially hazardous asteroids larger than 1 kilometer since 1999 – Cumulative number of discovered PHA by end of year (first of December). As of September 2020, there are 157 known PHAs larger than one kilometer.[5]
500
1,000
1,500
2,000
2,500
3,000
prev.
1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
2019
PHA: total number of potentially hazardous asteroids since 1999 – Cumulative number of all discovered PHA by end of year (first of December). As of September 2020, there are 2115 PHAs.[5]

See also

Notes
  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 Asteroid Size Estimator is a tool for a generic absolute magnitude-to-diameter conversion for an assumed geometric albedo.

References
  1. Task Force on potentially hazardous Near Earth Objects (September 2000). "Report of the Task Force on potentially hazardous Near Earth Objects" (PDF). Archived from the original (PDF) on 10 December 2016. Retrieved 22 January 2018. Cite journal requires |journal= (help)
  2. "NEO Basics – Potentially Hazardous Asteroids (PHAs)". CNEOS NASA/JPL. Retrieved 16 January 2020.
  3. (38 Sentry risk-listed PHAs / 2093 PHAs) = 2%
  4. "Sentry Risk Table". Retrieved 2019-08-15. (Click "Use Unconstrained Settings" AND select "H<=22" for list of PHAs)
  5. "Discovery Statistics – Cumulative Totals". CNEOS NASA/JPL. 7 January 2020. Retrieved 16 January 2020.
  6. "Unusual Minor Planets – Overview". Minor Planet Center. Retrieved 16 January 2020.
  7. "JPL Small-Body Database Search Engine: PHAs and orbital class (APO)". JPL Solar System Dynamics. Retrieved 16 January 2020.
  8. "JPL Small-Body Database Search Engine: PHAs and orbital class (ATE)". JPL Solar System Dynamics. Retrieved 16 January 2020.
  9. "List of the Potentially Hazardous Asteroids (PHAs)". Minor Planet Center. Retrieved 16 January 2018.
  10. "NASA news – NASA Survey Counts Potentially Hazardous Asteroids". NASA/JPL. 16 May 2012. Retrieved 22 January 2018.
  11. Will Ferguson (22 January 2013). "Asteroid Hunter Gives an Update on the Threat of Near-Earth Objects". Scientific American. Retrieved 2013-01-23.
  12. Chang, Kenneth (23 May 2016). "How Big Are Those Killer Asteroids? A Critic Says NASA Doesn't Know". New York Times. Retrieved 24 May 2016.
  13. Myhrvold, Nathan (23 May 2016). "Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data". arXiv:1605.06490v2 [astro-ph.EP].
  14. Billings, Lee (27 May 2016). "For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling". Scientific American. Retrieved 28 May 2016.
  15. NASA Administrator (25 May 2016). "NASA Response to Recent Paper on NEOWISE Asteroid Size Results". NASA. Retrieved 29 May 2016.
  16. Myhrvold, Nathan (2018). "An empirical examination of WISE/NEOWISE asteroid analysis and results". Icarus. 314: 64–97. Bibcode:2018Icar..314...64M. doi:10.1016/j.icarus.2018.05.004.
  17. Chang, Kenneth (14 June 2018). "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". The New York Times. Retrieved 14 June 2018.
  18. "JPL Small-Body Database Search Engine: PHAs and H < 18 (mag)". JPL Solar System Dynamics. Retrieved 2012-06-13.

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