ASTEROID 2008BT18's CLOSE APPROACH...

In mid July 2008 asteroid 2008BT18 came close to Earth - this is not overly common, but the groovy thing is that 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:

"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,

From http://www.spaceweather.com/

I imaged 2008BT18 on Monday July 14.63-70 2008 (UT) with my 41cm (16") @f5.2 Meade Newtonian and STL11000M CCD (the FOV is 1.0 x 0.7 degrees, north is down, east is right). I took 10 second exposures at 3x3 binning for max sensitivity and download time. I then combined the 102 images in Maxim DL to create the AVI file below... The asteroid was mag 14 and travelling at 1.2 degrees an hour in a PA of 228. It's period of orbit is 3.31 years, it's perihelion distance is 0.89 AU and aphelion distance 3.54 AU. The distance from Earth was 0.01515483 AU (2,267,130 km). The elongation from Sun 63.79 degrees (morning sky).