::Potentially hazardous object


Align::right    Orbit::earth    Title::format    Class::group    Asteroid::width    Number::impact

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 with a reasonably well determined orbit can be known not to be a threat to Earth for the next 100 years or more.

As of 1 August 2015 NASA had listed a cumulative total of 1,605 discovered potentially hazardous asteroids (PHAs). Among these, there are 154 PHAs believed to be larger than one kilometer in diameter. However 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).<ref name=neo-jpl-stats /> Most of the discovered PHAs are Apollo asteroids (1,363) and fewer belong to the group of Aten asteroids (147).<ref name="pha-apollo" /><ref name="pha-aten" /><ref name=NASA_PHA />

Due to 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 12,933 discovered near-Earth Objects, most of them being asteroids and some 85 near-Earth comets (NECs).<ref name=neo-jpl-stats /> The Minor Planet Center's website Unusual Minor Planets also publishes detailed statistics for these objects.<ref name=unusual />

On 19 August 2015, NASA reported that there was "no scientific basis" that the world will end due to a rumored impact of an asteroid near Puerto Rico between 15 and 28 September 2015.<ref name="NASA-20150819-dca">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="NYT-20150820-kc">{{#invoke:citation/CS1|citation |CitationClass=news }}</ref>

Potentially hazardous object sections
Intro   Overview    Largest PHA    See also    References    External links   

PREVIOUS: IntroNEXT: Overview