Number 21: 05/08/2003
A scientific publication by SGF and NEODyS

Discovery of the First Asteroid with an Interior Earth Orbit: 2003 CP20
by Jennifer Evans and Grant Stokes -MIT Lincoln Laboratory

Image of 2003 CP20 (arrowed) taken on 2003 February 14.0 U at Klet Observatory with an exposure of 30s with north to the top and west to the right. (click for a bigger version)

Copyright - Klet Observatory

Historically, the populations of asteroids have been categorized by their orbital characteristics. Near Earth Asteroids (NEAs) have been divided into 3 families - Apollos, Amors and Atens - depending essentially on the fraction of their orbit that is contained inside the orbit of the Earth. Until recently, there have been no asteroids known that have their entire orbit contained inside the Earth's orbit. That changed on the evening of February 10, 2003. In Socorro, NM, the observers at MIT's Lincoln Near Earth Asteroid Research (LINEAR) survey began another night of routine observing. Only three nights were left before the full Moon and the end of a productive observing run. The skies had scattered clouds, but the night's observing was started as usual in hopes of making whatever detections possible despite the clouds. However, an hour and a half later the observing was stopped due to heavy cloud coverage. Not a single useful set of fields had been acquired yet, so the observers waited. Three hours later the skies finally began to clear and the observing was restarted. Ecliptic opposition was (RA, Dec) = (144.5°, 14.1°). The search region to be covered during the remaining dark time was 30.6° to 41.6° declination and from 2° to 147° from opposition, with the area to be shared by the two operational LINEAR search telescopes, designated L1 and L2. Eventually 240 detections were made before daybreak, with 48 deemed as near-Earth object candidates by the LINEAR system, including a very special one detected by L2.
Two objects from the night's observing were placed on the MPC confirmation page. The most interesting was object AF42951, detected 121° from opposition at (RA, Dec) = (265.4°, 32.94°), with an apparent velocity of 0.795 deg/day, an apparent magnitude of 17.3, and an absolute magnitude of 16.3. Two days later, thanks to follow-up observations by multiple observers, the provisional designation 2003 CP20 was assigned to the first confirmed asteroid to have an orbit entirely interior to the Earth's orbit.
While the discovery of asteroid 2003 CP20 is newsworthy because it is the first in a new class of asteroids, it almost certainly won't be the last such discovery. The existence of asteroids with their entire orbit interior to Earth's has been predicted for quite some time and a number of asteroids, called Atens, have been found with the majority of their orbit contained inside the Earth's. LINEAR search pattern includes the entire sky available from our observatory in the desert southwest of the United States each month, including as close to the east and west horizons as feasible every night of observing. This observing philosophy is responsible for LINEAR's prolific discovery of Atens, (LINEAR has discovered 110 of the currently known population of 179 Atens) and has now resulted in the discovery of the first member of the wholly interior population. Since LINEAR began observing 5 years ago, it has discovered 14 of the 15 cataloged asteroids with the smallest aphelion distances, including the 7 smallest aphelion distance asteroids. Despite the poor solar phase angles (2003 CP20 had a phase angle of 83°), increased atmospheres, and generally poorer seeing conditions associated with searching at low solar elongations, LINEAR has found that trading depth of coverage near opposition for increased sky coverage in these areas has been a good trade. As long as survey programs like LINEAR continue to search regions at low solar elongations, more discoveries of inner Earth orbit asteroids will likely be made.

Simulations of the orbit of 2003 CP20 Orbit at the date of the discovery (10/02/2003) made with the JPL orbit simulator.
Copyright: JPL