A potentially hazardous object (PHO) is a near-Earthasteroid 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 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 magnitudeH < 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. As of 2012, an estimated 20 to 30 percent of these objects have been found. Asteroids larger than 35 meters across can pose a threat to a town or city.
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, 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.
By January 2009, NASA had listed 1006 potentially hazardous asteroids (PHAs) and 85 near-Earth comets (NECs). As of February 2015[update] there are 1544 known PHAs and only 208 have an observation arc less than 30 days. There are 1313 known apollo-class PHAs and only 145 known Aten-class PHAs. 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.