Asteroid 2008 BT18 is gliding past Earth this weekend and astronomers have just discovered that it is a binary system. "The sizes of the two components are 600 m for the primary and >200 m for the secondary," says Lance Benner of JPL. "The primary looks spheroidal, but we don't yet know about the shape of the secondary." Benner and others using a giant radar in Arecibo, Puerto Rico, obtained this "delay-doppler" image of the pair on July 7th:

Asteroid 2008 BT18
©Arecibo


"We're also getting images from NASA's Goldstone radar in the Mojave desert of California," he adds. That radar is smaller than Arecibo but it is still registering a strong echo that should reveal much about the target, including the binary orbit, masses and bulk density of the components.

About 16% of all near-Earth asteroids are binaries, but only a handful have come this close. "2008 BT18 is giving us a good look at a double asteroid," says Benner. Studying the make-up and dynamics of these systems may help researchers figure out how to deflect binaries on a collision course with Earth. 2008 BT18 poses no threat, but some undiscovered binary asteroid, one day, might. "The Arecibo observatory, where 53% of all near-Earth binaries have been discovered, is crucial to these studies."

Southern hemisphere readers, you may be able to observe this double-rock using your own backyard telescope and CCD camera. At closest approach (1.4 million miles) on July 14th, 2008 BT18 will flit through Canis Major heading south and glowing like a 13th magnitude star: ephemeris, 3D orbit.