|Discovered by||Lincoln Near-Earth Asteroid Research|
|Discovery site||Socorro, New Mexico|
|Discovery date||February 11, 2003|
|Alternative names||2003 CP20|
|Minor planet category||Aten, Atira
|Epoch October 22, 2004 (JD 2453300.5)|
|Semi-major axis||0.741 AU
|Orbital period||233.023 d (0.64 a)|
|Average orbital speed||33.68 km/s|
|Longitude of ascending node||103.952°|
|Argument of perihelion||252.914°|
|Mass||1.0×1012 kg (assumed)|
|Mean density||2.0? g/cm³|
|Equatorial surface gravity||0.0003 m/s²|
|Escape velocity||0.0005 km/s|
|Rotation period||2.97 hr|
|Absolute magnitude (H)||16.28|
Discovered February 11, 2003, by the Lincoln Near-Earth Asteroid Research project's station at Socorro, New Mexico, (163693) Atira (previously named 2003 CP20), was the first asteroid known to have an orbit entirely within that of Earth. Together with 2004 JG6, and 2007 EB26, both of which have even smaller orbits, forms a new subclass of Aten asteroids. Following tradition, the new subclass is named after the first known object in that class, and therefore is known as Atiras.
Atira takes slightly over 233 days to orbit the Sun. It is a Venus-crosser asteroid, but does not get as close to the Sun as Mercury. With a diameter of about 2 km, it is the largest of the 12 currently known Atiras and one of the larger Aten asteroids.
Knowing that traditionally the first known object in a new class of asteroids will become the name of the new class of asteroids, due consideration was given to the name for (163693). The other classes of near-Earth asteroids are Amors, Apollos, and Atens, named after a Roman, Greek, and Egyptian god, so a preference was given to a god or goddess beginning with the letter "A". Given (163693) was discovered by the LINEAR program which operates out of the southwestern United States, preference was also given to a Native American god or goddess. Atira is a Pawnee goddess of the Earth and the evening star.
The official citation:
Discovered 2003 Feb. 11 by the Lincoln Laboratory Near-Earth Asteroid Research Team at Socorro.
Atira is the Pawnee goddess of Earth and the evening star. This is the first numbered minor planet to have an orbit completely interior to that of the earth."
- "Discovery Circumstances: Numbered Minor Planets (160001)-(165000)". Minor Planet Center. Retrieved 2010-10-25.
- "JPL Small-Body Database Browser: 163693 Atira (2003 CP20)". Jet Propulsion Laboratory. 2012-01-20 last obs (arc=8.9 years). Retrieved 2013-03-14.
- Wm. Robert Johnston. "Names of Solar System objects and features". Archived from the original on 12 April 2010. Retrieved 2010-04-03.
- Shoemaker E M (1983). "Asteroid and Comet Bombardment of the Earth". Annual Review of Earth and Planetary Sciences 11: 461–494. Bibcode:1983AREPS..11..461S. doi:10.1146/annurev.ea.11.050183.002333.
- "Near-Earth Object Groups". NASA. Archived from the original on 25 March 2010. Retrieved 2010-04-03.
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