Friday, Oct. 13, 2006Cool Asteroid Movie

NASA/JPL IMAGE COURTESEY OF DAN SCHEERES AND STEVE OSTRO
Frame from a movie showing a radar-derived computer model of near-Earth binary asteroid (66391) 1999 KW4. The larger component is about 1.5 kilometers in diameter and rotates once every 2.8 hours, and the components' orbital period is 17.4 hours

Back in 2001, the double asteroid known as KW4 passed within a couple of million miles of Earth--not nearly close enough to be a danger, but close enough that planetary scientists could bounce radar signals off the two mutually orbiting objects that make up the system. The result, presented in the online journal Science Express, is a series of incredibly detailed images of a couple of truly bizarre objects. The larger of the two, known as Alpha, is about a mile across, and spins once every 2.8 hours. The smaller object, Beta, is about a third as big, and denser, and orbits Alpha once every 17 or so hours.

What's surprising here is that Alpha is spinning fast enough to make its equator bulge way out--which wouldn't be possible if it were really a solid chunk of rock. Instead, it's clearly more of a "rubble pile" of smaller rocks held together--but just barely--by their mutual gravity.

This is important information because sooner or later a near-earth asteroid is going to crash into us (it won't be this one, though). If we get enough advance warning, we can try and do something about it. But one popular idea--detonating a huge bomb right next to the asteroid to knock it into a different orbit--might not work in this case. Instead, it could simply knock the asteroid apart, leaving most of the pieces on the same orbit as before--so we'd get multiple big impacts instead of one huge one. Oops.

Oh, and I did mention a movie. Here it is.

—M.L.