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Potentially Hazardous Asteroids (PHAs) are currently defined
based on parameters that measure the asteroid's potential to make
threatening close approaches to the Earth.
Specifically, all asteroids with an Earth Minimum Orbit Intersection Distance
(MOID) of 0.05 AU or less
and an absolute magnitude (H) of 22.0 or less
are considered PHAs.
In other words, asteroids that can't get any closer to the Earth
(i.e. MOID) than 0.05 AU
(roughly 7,480,000 km or 4,650,000 mi)
or are smaller than about 150 m (500 ft) in diameter
(i.e. H = 22.0 with assumed albedo of 13%)
are not considered PHAs.
There are currently 952 known PHAs.
This ``potential'' to make close Earth approaches does not mean a
PHA will impact the Earth. It only means there is a possibility for
such a threat. By monitoring these PHAs and updating their orbits as new
observations become available, we can better predict the close-approach
statistics and thus their Earth-impact threat.
To learn more about the Earth impact threat, visit the NASA Ames
Asteroid and Comet Impact Hazards site.
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