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 (Atira, Aten, Apollo and Amor) according to their
perihelion distance (
q
),
aphelion distance (
Q
) and
their semi-major axes (
a
).
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 140 m (~500 ft) in diameter
(i.e.,
H
= 22.0 with assumed
albedo
of 14%) are not considered PHAs.