A potentially hazardous object (PHO) is a near-Earth asteroid 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 can be known not to be a threat to Earth for the next 100 years or more, if its orbit is reasonably well determined. Potentially hazardous asteroids with some threat of impacting Earth in the next 100 years are listed on the Sentry Risk Table.
As of March 2017 there are 1,786 known potentially hazardous asteroids (PHAs) and only 205 have an observation arc shorter than 30 days. Of the known PHAs, 157 are believed to be larger than one kilometer in diameter. A calculated diameter is only a rough estimate, as it is inferred from the object's varying brightness—observed and measured at various times—and the assumed, yet unknown reflectivity of its surface (albedo). Most of the discovered PHAs are Apollo asteroids (1,516) and fewer belong to the group of Aten asteroids (157).
After several astronomical surveys, the number of known PHAs has increased tenfold since the end of the 1990s (see below). These surveys have led to a total number of 15,802 discovered near-Earth objects. Most of them are asteroids, with just some 106 near-Earth comets (NECs). The Minor Planet Center's website Unusual Minor Planets also publishes detailed statistics for these objects.
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 magnitude H < 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 determined 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. In May 2016, the asteroid size estimates arising from the Wide-field Infrared Survey Explorer and NEOWISE missions have been questioned, but the criticism has yet to undergo peer review.
Several astronomical survey 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.
The lowest numbered PHA is 1566 Icarus. Short-period comets currently with an Earth-MOID less than 0.05AU include: 109P/Swift-Tuttle, 55P/Tempel–Tuttle, 15P/Finlay, 289P/Blanpain, 255P/Levy, 206P/Barnard–Boattini, 21P/Giacobini–Zinner, and 73P/Schwassmann–Wachmann.
The largest known Potentially hazardous asteroid is (53319) 1999 JM8 with a diameter of ~7 km, but it is not currently at risk of any impacts. Below is listed the largest PHAs (based on absolute magnitude H) discovered in a given year. Historical data of the cumulative number of discovered PHA since 1999 are displayed in the bar charts—one for the total number and the other for objects larger than one kilometer.