An asteroid is considered a PHA 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 493 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.
Near the start of October 2008, NASA had listed 982 PHAs. 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.