Aten Asteroids

A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt. The brown band covering area between the Sun and just outside of Earth's orbit are the location of Aten asteroids.

In the previous post, I discussed a little bit about near-Earth asteroids and the three different types that exist. In the next few posts, I will go into more detail about the three families of near-Earth asteroids. Today's topic is about Aten asteroids, the near-Earth asteroids that have a semi-major axis smaller than one AU (the semi-major axis of Earth).

Aten asteroids are named after the first one discovered, 2062 Aten. 2062 Aten was discovered by Eleanor F. Helin at Palomar Observatory in 1976. It is named after the Egyptian Sun god, Aten, whose name is seen in many the names of many pharoahs, most famously Tutankhaten (who renamed himself Tutankhamun to reflect his worship of Amun over Aten).

Despite having semi-major axes smaller than Earth's orbit, Aten asteroids can and do cross Earth's orbit. Remember that to be a near-Earth asteroid, the asteroid must come within 0.3 AU of Earth's orbit which means that Aten asteroids must have an aphelion of 0.7 AU or larger.  Aten asteroids that have an aphelion of less than 1 AU (but still greater than 0.7 AU) belong to a second group of asteroids called Apohele asteroids*.

*Apohele asteroids are asteroids that never cross the Earth’s orbit from within the Earth’s orbit, i.e. their aphelions are always less than 1 AU from the Sun. Another name for Apohele asteroids are inner-Earth objects or Atira asteroids. It should be noted that some Apohele asteroids can be Aten asteroids, but not all. There are only 13 known Apohele asteroids discovered, so far.

Of the known near-Earth asteroids, only 815 (6%) have been identified as Aten asteroids. There are some Aten asteroids that should be noted:

• (325102) 2008 EY5 has the smallest semi-major axis of 0.626 AU (between the orbits of Mercury and Venus)
• (137924) 2000 BD19 has the closest perihelion of 0.092 AU, which takes it within Mercury's orbit. It also has an aphelion of 1.66 AU, which is outside the orbit of Mars, giving it the largest eccentricity of any Aten asteroids (and possibly any solar system body) of e=0.895. It is believed that this may be an extinct comet that was captured by the Sun and was prevented from leaving the inner solar system.
  
• 2002 AA29 (which will have its own post) has a period around the Sun of one year and a semi-major axis of one AU, but does not share the same orbit as the Earth. Its unique position by the Earth actually causes it to have a horseshoe shaped orbit around the Earth.
• 3753 Cruithne has a semi-major axis of one AU and a period around the Sun of about a year as well. However, it has a highly eccentric orbit with its aphelion near 1.51 AU (close to Mars) and a perihelion of 0.484 AU (just outside the aphelion of Mercury). This eccentricity leads it to have a kidney-shaped orbit with respect to the Earth. This will also be discussed in a separate post.