Potentially Hazardous Object

Asteroid 4179 Toutatis

A Potentially Hazardous Object (PHO) is an asteroid (PHA) or comet (PHC) with an orbit such that it has the potential to make close approaches to the Earth and a size large enough to cause significant regional damage in the event of impact.

An object is considered a PHO^[1], if its Minimum Orbit Intersection Distance (MOID) with respect to Earth is less than 0.05 AU and its diameter is at least 150 m (nearly 500 ft). This is big enough to cause unprecedented regional devastation in the case of a land impact, or a major tsunami in the case of an ocean impact. Such impact events occur on average once per 10,000 years or less.

The diameter of most asteroids is not known with any accuracy. For this reason NASA and JPL use the more practical measure of absolute magnitude. Any asteroid with an absolute magnitude of 22.0 or brighter is assumed to be of the required size, 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.^[2]

Near the start of October 2008, NASA had listed 982 PHAs and 65 PHCs^[3]. The total Solar System inventory is believed to be somewhere between 1,000 and 1,100. Searches for yet-undiscovered PHOs are ongoing, with the most prolific the year prior to June 2005 being the LINEAR and Catalina surveys. Once found, each PHO is studied by various means, including optical, infrared and radar observations, to further 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.

References

See also

• Near earth object

External links

• Very Close Approaches (<0.01 A.U.) of PHAs to Earth 1800-2200

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