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Near Earth Object Program
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NEO Basics Search Programs Discovery Statistics Accessible NEAs News Frequently Asked Questions
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Orbit Diagrams Orbit Elements Close Approaches Impact Risk Images Related LInks
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NEO GROUPS
In terms of orbital elements, NEOs are asteroids and comets with perihelion distance q less than 1.3 AU. Near-Earth Comets (NECs) are further restricted to include only short-period comets (i.e orbital period P less than 200 years). The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). NEAs are divided into groups (Aten, Apollo, Amor) according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a).

Group Description Definition
NECs Near-Earth Comets q<1.3 AU, P<200 years
NEAs Near-Earth Asteroids q<1.3 AU
Atiras NEAs whose orbits are contained entirely with the orbit of the Earth (named after asteroid 163693 Atira). a<1.0 AU, Q<0.983 AU
Atens Earth-crossing NEAs with semi-major axes smaller than Earth's (named after asteroid 2062 Aten). a<1.0 AU, Q>0.983 AU
Apollos Earth-crossing NEAs with semi-major axes larger than Earth's (named after asteroid 1862 Apollo). a>1.0 AU, q<1.017 AU
Amors Earth-approaching NEAs with orbits exterior to Earth's but interior to Mars' (named after asteroid 1221 Amor). a>1.0 AU, 1.017<q<1.3 AU
PHAs Potentially Hazardous Asteriods: NEAs whose Minimum Orbit Intersection Distance (MOID) with the Earth is 0.05 AU or less and whose absolute magnitude (H) is 22.0 or brighter. MOID<=0.05 AU, H<=22.0

Near Earth Asteroid Orbit Types

WHAT IS A PHA?
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 1491 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.


Near-Earth Objects | NEO Groups | NEOs and Life on Earth | Target Earth | NEOs As Future Resources

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