Potentially hazardous asteroid
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A Potentially Hazardous Asteroid (PHA) is a Near-Earth asteroid with a size and an orbit such that it has a potential to make threateningly close approaches to the Earth.
An asteroid is considered a PHA[1] if its Minimum Orbit Intersection Distance (MOID) with respect to Earth is less than 0.05 AU and its diameter at least 150 m (nearly 500 ft). This is big enough to cause unprecedented regional devastation for a land impact or the threat of a major tsunami for an ocean impact, if it were to hit the Earth. Such impact events occur on average once per 10,000 years or less. The Torino Scale is used to categorize the impact hazard associated with PHAs.
By the start of November 2006, NASA had listed 825 PHAs.[2] The total Solar System inventory is believed to be somewhere between 1,000 and 1,100. Searches for yet undiscovered PHAs are ongoing, with the most prolific the year prior to June 2005 being the LINEAR- and Catalina-surveys. Once found, each PHA is being studied by various means, including optical, infrared and radar observations, to further determine its characteristics, such as size, composition, rotation state, and more accurately determine its orbit. Both professional and amateur astronomers participate in such monitoring.
During asteroid close approaches to planets or moons, it will be subject to gravitational perturbation, modifying the orbit, and sometimes, a previously non-threatening asteroid may become a PHA or vice versa. This is a result of the dynamic character of the Solar system.
[edit] References
- ^ Task Force on potentially hazardous Near Earth Objects (September 2000). "Report of the Task Force on potentially hazardous Near Earth Objects" (PDF). Retrieved on 2006-08-31.
- ^ NEO Discovery Statistics. Retrieved on August 5, 2005.
Vulcanoids · Near-Earth asteroids · Main belt · Jupiter Trojans · Centaurs · Damocloids · Comets · Trans-Neptunians (Kuiper belt • Scattered disc • Oort cloud)
For other objects and regions, see Asteroid groups and families, Binary asteroids, Asteroid moons and the Solar System.
For a complete listing, see List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
