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).
||q<1.3 AU, P<200 years
||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
||Earth-crossing NEAs with semi-major axes smaller than Earth's
(named after asteroid 2062 Aten).
||a<1.0 AU, Q>0.983 AU
||Earth-crossing NEAs with semi-major axes larger than Earth's
(named after asteroid 1862 Apollo).
||a>1.0 AU, q<1.017 AU
||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
||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
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 1556 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.