Asteroids that are considered near-Earth objects (NEO) can be classified based on their orbits.
Terms:
(1) astonomical unit (AU) = mean distance between the earth and the sun = 93 million miles or 149,597,870,700 meters
(2) semi-major axis (a) = half of the major axis for an object moving in an elliptical path
(3) perihelion is the point in orbit of an object where it is closest to the sun. The perihelion distance is indicated as q.
(4) aphelion is the point in the orbit of an object where it is furthest from the sun. The aphelion distance is indicated as Q.
NEO have a perihelion distance (q) < 1.3 AU (approach to less than 0.3 AU from earth)
1.017 AU is the earth's aphelion distance.
|
Asteroid
|
a in AU
|
q in AU
|
Q in AU
|
closest to earth
|
cross earth's orbit
|
|
Aten
|
< 1.0
|
< 1.3
|
> 0.983
|
aphelion
|
may
|
|
Apohele or Atira
|
< 0.983
|
|
< 0.983
|
aphelion
|
never
|
|
Apollo
|
>= 1.0
|
< 1.017
|
|
perihelion
|
may
|
|
Amor
|
> 1.017 and < 1.523
|
> 1.017 and < 1.3
|
|
perihelion
|
never
|
where:
• Amor asteroids are outside of earth's orbit.
