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The asteroid Toutatis is listed as a potentially hazardous near-Earth asteroid, yet poses no immediate threat to Earth. (Radar image taken by GDSCC in 1996.)
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)[1][2] 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.[3] 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).[3] Most of the discovered PHAs are Apollo asteroids (1,516)[4] and fewer belong to the group of Aten asteroids (157).[5]
After several astronomical surveys, 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 15,802 discovered near-Earth objects. Most of them are asteroids, with just some 106 near-Earth comets (NECs).[3] The Minor Planet Center's website Unusual Minor Planets also publishes detailed statistics for these objects.[2]
References[]
- ↑ Potentially Hazard Asteroids. Retrieved on 2011 August 6.
- ↑ 2.0 2.1 Unusual Minor Planets. Minor Planet Center. Retrieved on 2012 May 17.
- ↑ 3.0 3.1 3.2 Near-Earth Asteroid Discovery Statistics. Retrieved on 2017 March 1.
- ↑ JPL Small-Body Database Search Engine: PHAs and orbital class (APO). JPL Solar System Dynamics. Retrieved on 2014 February 23.
- ↑ JPL Small-Body Database Search Engine: PHAs and orbital class (ATE). JPL Solar System Dynamics. Retrieved on 2014 February 23.
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