As ISON irritatingly demonstrated, comets are known to fall apart. This is because they are made up of materials that are easily volatilized, and when they pass too close to the sun they are not only disturbed by gravitational tides, they are cooked enough that large fractions of them simply cook. But asteroids, being just rock, are supposed to be better behaved. Well… so much for that.

Hubble Witnesses an Asteroid Mysteriously Disintegrating

The Hubble data showed that the fragments are drifting away from each other at a leisurely one mile per hour — slower than the speed of a strolling human. The asteroid began coming apart early last year, but new pieces continue to emerge in the most recent images.

This makes it unlikely that the asteroid is disintegrating because of a collision with another asteroid, which would be instantaneous and violent by comparison to what has been observed. Some of the debris from such a high-velocity smashup would also be expected to travel much faster than observed.

Nor is the asteroid coming unglued due to the pressure of interior ices warming and vaporizing. The asteroid is too cold for ices to significantly sublimate, and it has presumably maintained its nearly 300-million-mile distance from the Sun for much of the age of the solar system.

This leaves a scenario in which the asteroid is disintegrating due to a subtle effect of sunlight, which causes the rotation rate to slowly increase. Eventually, its component pieces, like grapes on a stem, gently pull apart due to centrifugal force. The possibility of disruption by this so-called YORP torque has been discussed by scientists for several years but, so far, never reliably observed.

Huh.

This sort of thing could have implications for both asteroid mining and planetary defense. Asteroids may be easier to break up than might have been thought, which would make mining easier… and deflection harder. Of course, this is a single example, so it’s not good logic to assume it’s a universal fact that asteroids fall apart this easily. But still, it’s an interesting event, and if the asteroid was accessible it’d make an interesting target for exploration.