As recently as the 1970s, dynamicists scoffed at the notion that some asteroids have moons, even though observers had reported seeing occasional double blink-outs when asteroids passed in front of stars.
They aren't scoffing any more. More than 100 bodies in the asteroid and Kuiper belts are now known to be binaries. In fact, by some estimates roughly 15% of small objects in the asteroid belt and in near-Earth orbits are paired. It's an embarrassment of riches: far too many exist to have been formed by bodies simply crashing into one another.
A new study, published by a trio of theorists in the July 10th issue of Nature, finds that some asteroids can spin so rapidly that they literally fly apart. The researchers invoked a process known as the YORP effect that causes small bodies to spin up or slow down when exposed to sunlight.
YORP stands for Yarkovsky, O'Keefe, Radzievskii, and Paddack — the four scientists who identified the effect. The phenomenon occurs when a body absorbs sunlight and reradiates it as heat. For a spherical object, this is a zero-sum exchange of the two forces. But if the asteroid is faceted, as most small ones are, then some heat gets radiated at an angle instead of radially, causing a tiny torque that can either speed up its rotation or slow it down.
The Nature authors focused on different types of "rubble-pile" asteroids, chunky collections held loosely together by gravity. In their computer simulations, rubbly asteroids behave much like a fluid. As their spin rates quicken to just a few hours, the asteroids first become elongated footballs as mass slides from their poles to their equators. Eventually chunks of matter fly off these whirling dervishes — sometimes reaccumulating onto the main bodies and sometimes creating moons close by.
These YORP-driven cleavings work so well, say the authors, that the process likely is the source of many, if not most, binary asteroids. The University of Maryland's press release about the new finding includes an animation showing how a rubble pile can spin itself to pieces.