Potentially hazardous object

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Plot of orbits of known Potentially Hazardous Asteroids (size over 460 feet (140 m) and passing within 4.7 million miles (7.6×10^6 km) of Earth's orbit) as of early 2013 (alternative image).

A potentially hazardous object (PHO) is a near-Earth asteroid or comet with an orbit such that it has the potential to make close approaches to the Earth and is of a size large enough to cause significant regional damage in the event of impact. A potentially hazardous object with a reasonably well determined orbit can be known not to be a threat to Earth for the next 100 years or more.


An object is considered a PHO[1] 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 diameter is at least 100 to 150 meters (330 to 490 ft) (corresponding to absolute magnitude, H < 22). 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.[2] As of 2012, an estimated 20 to 30 percent of these objects have been found.[2] Asteroids larger than 35 meters across can pose a threat to a town or city.[3]

The diameter of most small asteroids is not well known and can only be estimated based on their brightness and distance. 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,[4][5] although 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 0.13 for this purpose.[4]

By January 2009, NASA had listed 1006 potentially hazardous asteroids (PHAs) and 85 near-Earth comets (NECs).[6] As of February 2015 there are 1544 known PHAs[4][7] and only 208 have an observation arc less than 30 days. There are 1313 known apollo-class PHAs[8] and only 145 known Aten-class PHAs.[9] Projects such as Lincoln Near-Earth Asteroid Research and Catalina Sky Survey 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 monitoring.

During an asteroid's close approaches to planets or moons 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.

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

Largest PHA[edit]

The largest PHA (based on absolute magnitude H) discovered in a given year:[10]

Name/Year (H)
(4179) Toutatis (1989 AC) 15.3
(4953) 1990 MU 14.1
(7341) 1991 VK 16.7
(5604) 1992 FE 16.4
(39572) 1993 DQ1 16.4
(136618) 1994 CN2 16.6
(243566) 1995 SA 17.3
(8566) 1996 EN 16.5
(35396) 1997 XF11 16.9
(16960) 1998 QS52 14.3
(137427) 1999 TF211 15.0
(23187) 2000 PN9 16.1
(111253) 2001 XU10 14.9
(89830) 2002 CE 14.7
(242216) 2003 RN10 15.7
(242450) 2004 QY2 14.7
(308242) 2005 GO21 16.4
(374851) 2006 VV2 16.8
(214869) 2007 PA8 16.2
(294739) 2008 CM 17.15
(369264) 2009 MS 16.0
(381906) 2010 CL19 17.55
(415029) 2011 UL21 15.7
2012 LK9 17.8
2013 UP8 16.3
2014 LJ21 16.1


See also[edit]


  1. ^ Task Force on potentially hazardous Near Earth Objects (September 2000). "Report of the Task Force on potentially hazardous Near Earth Objects" (PDF). Retrieved 2010-06-28. 
  2. ^ a b "NASA Survey Counts Potentially Hazardous Asteroids". NASA/JPL. May 16, 2012. Retrieved 2012-05-17. 
  3. ^ Will Ferguson (January 22, 2013). "Asteroid Hunter Gives an Update on the Threat of Near-Earth Objects". Scientific American. Retrieved 2013-01-23. 
  4. ^ a b c "Potentially Hazard Asteroids". Retrieved 2011-08-06. 
  5. ^ "Conversion of Absolute Magnitude to Diameter". Minor Planet Center. Retrieved 2009-05-13. 
  6. ^ "NEO Discovery Statistics". Retrieved October 8, 2007. 
  7. ^ "Unusual Minor Planets". Minor Planet Center. Retrieved 2012-05-17. 
  8. ^ "JPL Small-Body Database Search Engine: PHAs and orbital class (APO)". JPL Solar System Dynamics. Retrieved 2014-02-23. 
  9. ^ "JPL Small-Body Database Search Engine: PHAs and orbital class (ATE)". JPL Solar System Dynamics. Retrieved 2014-02-23. 
  10. ^ "JPL Small-Body Database Search Engine: PHAs and H < 18 (mag)". JPL Solar System Dynamics. Retrieved 2012-06-13. 
  11. ^ U.S.Congress (Spring 2013). "Threats From Space: a Review of U.S. Government Efforts to Track and mitigate Asteroids and Meteors (Part I and Part II) - Hearing Before the Committee on Science, Space, and Technology House of Representatives One Hundred Thirteenth Congress First Session". United States Congress (Hearings held 19 March 2013 and 10 April 2013). p. 147. Retrieved 3 May 2014. 

External links[edit]