I just read in the EarthSky.org article Hours after discovery, asteroid swept by that the near earth asteroid 2016 QA2 recently passed the earth at about 85,000 km. The article says that it is a member of the Aten group of asteroids. That article links to this very cool GIF http://www.virtualtelescope.net/2016qa2_28aug2016.gif, however it's 40MB so it takes a while to load.

Wikipedia says:

Aten asteroids are defined by having a semi-major axis of less than one astronomical unit (AU), the distance from the Earth to the Sun. They also have an aphelion (furthest distance from the Sun) greater than 0.983 AU${}^2$.


That link shows the following table and drawings. The text says:

NEAs are divided into groups (Aten, Apollo, Amor) according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a).

Are $q$ and $Q$ really perihelion and aphelion respectively? The table says Aten asteroids have $Q>0.983$ but in the drawing for Aten asteroids it says $Aphelion<1.0167$. I noticed these values could be reciprocals, so there may be something more going on here. But at least on the surface there seems to be some contradiction.

Could someone confirm these definitions with an external source - and help me understand the basis for these limits that deviate from 1 AU by only +/- 1.7%?

enter image description here

The image below is from this explanation: Near Earth Asteroid / NEO Classifications Based on Locations. It might be helpful for discussion.

enter image description here


1 Answer 1


An Earth-crossing asteroid has either an aphelion larger than Earth's perihelion ($Q \gt q_\oplus$, 0.983 AU), or a perihelion smaller than Earth's aphelion ($q < Q_\oplus$, 1.017 AU). For those which have both, the semimajor axis $a$ determines whether it is an Aten or an Apollo - subject to change by a close encounter with Earth.

This Minor Planet Center blog post is consistent with the JPL table. I think the diagram used the wrong inequality for Aten aphelia.

  • 2
    $\begingroup$ Bingo! So the 1.7% I mentioned turns out to be the eccentricity of the Earth's orbit - I was concentrating on the asteroid's orbitt. It also explains now why the two limit values are reciprocals - when expressed in AU. Thank you also for the link. $\endgroup$
    – uhoh
    Commented Aug 29, 2016 at 17:45

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