HOME
        TheInfoList






A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and large enough to cause significant regional damage in the event of impact.[1] They are defined as having a minimum orbital intersection distance with Earth of less than 0.05 astronomical units (19.5 lunar distances) and an absolute magnitude of 22 or brighter.[2] 98% of the known potentially hazardous objects are not an impact threat over the next 100 years.[3] Only about 38 potentially hazardous objects are listed on the Sentry Risk Table as entries are removed when they become known not to be a threat over the next hundred years or more.[4]

Most of these objects are potentially hazardous asteroids (PHAs), and a few are comets. As of September 2020 there are 2,116 known PHAs (about 9% of the total near-Earth population), of which 157 are estimated to be larger than one kilometer in diameter (see list of largest PHAs below).[5][6][a] Most of the discovered PHAs are Apollo asteroids (1,730) and fewer belong to the group of Aten asteroids (171).[7][8]

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 June 2020, only about 38 potentially hazardous asteroids are listed on the Sentry Risk Table.[4] Most potentially hazardous asteroids are ruled out as hazardous to at least several hundreds of years when their competing best orbit models become sufficiently divergent, but recent discoveries whose orbital constraints are little-known have divergent or incomplete mechanical models until observation yields further data. After several astronomical surveys, the number of known PHAs has increased tenfold since the end of the 1990s (see bar charts below).[5] The Minor Planet Center's website List of the Potentially Hazardous Asteroids also publishes detailed information for these objects.[9]

Overview

Plot of orbits of known potentially hazardous asteroids, with sizes over 140 metres (460 ft) and that pass within 7.6 million kilometres (4.7×10^6 mi) of Earth's orbit. Epoch as of early 2013.

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 comets. As of September 2020 there are 2,116 known PHAs (about 9% of the total near-Earth population), of which 157 are estimated to be larger than one kilometer in diameter (see list of largest PHAs below).[5][6][a] Most of the discovered PHAs are Apollo asteroids (1,730) and fewer belong to the group of Aten asteroids (171).[7][8]

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 June 2020, only about 38 potentially hazardous asteroids are listed on the Sentry Risk Table.[4] Most potentially hazardous asteroids are ruled out as hazardous to at least several hundreds of years when their competing best orbit models become sufficiently divergent, but recent discoveries whose orbital constraints are little-known have divergent or incomplete mechanical models until observation yields further data. After several astronomical surveys, the number of known PHAs has increased tenfold since the end of the 1990s (see bar charts below).[5] The Minor Planet Center's website List of the Potentially Hazardous Asteroids also publishes detailed information for these objects.[9]

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 absolute magnitude is brighter than 22, approximately corresponding to a diameter above 140 meters (460 ft).[1][2] 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.[10]

Levels of hazard

The two main scales used to categorize the impact hazards of asteroids are the Palermo Technical Impact Hazard Scale and the Torino Scale.

Potentially hazardous comets

Short-period comets currently with an Earth-MOID less than 0.05 AU include: 109P/Swift-Tuttle, 55P/Tempel–Tuttle, 15P/Finlay, 289P/Blanpain, 255P/Levy, 206P/Barnard–Boattini, 21P/Giacobini–Zinner, and 73P/Schwassmann–Wachmann.

Numbers