What is "potentially hazardous object"

n. (context astronomy English) 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 (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 and keeps it away from Earth. Potentially hazardous asteroids with a threat of impacting Earth in the next 100 years are listed on the Sentry Risk Table.

1,651 potentially hazardous asteroids (PHAs) are known and only 205 have an observation arc shorter than 30 days. Of the 1,651 known PHAs, 153 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 (1,402) and fewer belong to the group of Aten asteroids (149).

After several astronomical surveys motivated by their search, the number of known PHAs has increased tenfold since the end of the 1990s (see bar charts below). These surveys have led to a total number of 12,933 discovered near-Earth Objects. Most of them are asteroids, with just some 85 near-Earth comets (NECs). The Minor Planet Center's website Unusual Minor Planets also publishes detailed statistics for these objects.

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.