A potentially hazardous object (PHO) is an asteroid or comet 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 astronomical units (AU) and its diameter is at least 150 m (nearly 500 ft). 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.
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. 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 potentially hazardous asteroids (PHAs) and 65 potentially hazardous comets (PHCs).[3] The total Solar System inventory continues to grow, with 1244 PHA known by August 2011.[2] 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, infrared and radar observations, 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.
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