Sun-Hugging Asteroid

Astronomers find the first minor planet that resides completely within Earth's orbit.

February 19, 2003
by Vanessa Thomas

The arrow points to the 16th-magnitude asteroid 2003 CP20.
Milos Tichy / Klet Observatory

The Lincoln Near Earth Asteroid Research (LINEAR) program has discovered the first known asteroid to have an orbit that fits entirely within Earth's. It is the first solar system object other than Mercury and Venus to have a confirmed aphelion distance that's less than one astronomical unit.

Asteroid 2003 CP20 was discovered on February 11 with the 1-meter LINEAR telescope in New Mexico. The LINEAR program has found hundreds of near-Earth asteroids in an effort to identify potentially hazardous objects that might someday collide with our planet.

The newfound asteroid will not strike Earth, though. According to the Minor Planet Electronic Circular issued last Thursday to announce the asteroid's discovery, 2003 CP20 is 0.978 AU from the sun at aphelion. This is about 186,000 miles (300,000 kilometers) closer to the sun than Earth gets at perihelion (0.98 AU). And thanks to its orbit, which is inclined 25 degrees from the ecliptic, 2003 CP20 doesn't come within 0.19 AU (more than 18 million miles or 28 million kilometers) from our planet. However, it does occasionally come within 0.05 AU (46.5 million miles or 7.5 million kilometers) of Venus.

Astronomers know of more than a thousand asteroids that pass within Earth's orbit as they travel around the sun. However, all of the others have aphelion distances greater than 1 AU. Five years ago, University of Hawaii astronomer David Tholen and student Robert Whiteley discovered an asteroid designated 1998 DK36 that appeared to remain completely within Earth's orbit. However, they lost sight of the object before they had enough data to confirm its orbit, and the asteroid has yet to be seen again.

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