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Why hasn't this asteroid disintegrated?

Planetary scientists have found an asteroid spinning too fast for its own good. The object, known as 1950 DA, whips around every 2.1 hours, which means that rocks on its surface should fly off into space. So apart from gravity, some other sticky force—identified in a new study—must help to hold the asteroid together.

Astronomers have known that the vast majority of asteroids do not revolve faster than once every 2.2 hours. Beyond this limit, outward centrifugal forces exceed the gravitational pull the asteroid exerts on surface rocks, and the asteroid falls apart. But there are dozens of asteroids that spin faster than this theoretical cutoff. One idea is that these outliers are solid, metallic bodies with a tensile strength that would allow spins of any speed. But scientists tend to favor a “rubble pile” model—clumps of gravel and grit held together loosely—and these porous objects would not be able to resist a spinning self-destruction.

To figure out why that doesn’t happen, Ben Rozitis, a planetary scientist at the University of Tennessee, Knoxville, and colleagues took a closer look at 1950 DA, a near-Earth asteroid 1.3 kilometers across that orbits the sun every 2.2 years. The asteroid achieved notoriety in 2002, when astronomers announced that it had a one in 300 chance of hitting Earth in the year 2880, though that probability has since diminished to one in 19,800. The team knew the shape and size of the asteroid from abundant astronomical observations. Calculating its mass and density was more challenging.

Sunlight falls on parts of the asteroid and is retained as heat that provides a tiny propulsive force. This retained heat—and the resulting force—can be modeled by observing the asteroid in the infrared. By comparing these forces to small drifts in the asteroid’s measured orbit, Rozitis and his colleagues were able to calculate its mass, which was 2.1 trillion kilograms.

With the mass in hand, the researchers could then calculate gravity’s strength over the entire asteroid. They found that on about half of the asteroid’s surface—mostly near the equator where things spin the fastest—rocks should be flying off. Indeed, observations suggest that the asteroid’s surface is relatively smooth, and Rozitis and his colleagues calculate that any rocks larger than 6 centimeters across would have been lost already.

What’s keeping the remaining small rocks and dust on the surface? The researchers suggest van der Waals forces, weak forces caused by the attraction of polar molecules, which have slightly different charges on different sides of the molecule. For example, water molecules exhibit surface tension because of van der Waals forces, because the negative charge of one water molecule’s oxygen atom is attracted to nearby water molecules’ hydrogen atoms, which have a positive charge at their surfaces. Similar attractions could be occurring between molecules on the surfaces of different pieces of dust and rock. Rozitis says that the forces would be comparable to those that caused lunar dust to stick to astronauts’ space suits.

The paper, published online today in Nature, is a nice demonstration for a particular asteroid of a phenomenon that has been suspected for a long time, says Keith Holsapple, a planetary scientist at the University of Washington, Seattle, who was not involved with the work. “They look at it and say there must be some cohesion.” He’s not sure if van der Waals forces are sufficient to keep the asteroid together, however. Most laboratory estimates of these forces have been made with uniformly sized spherical particles, and he would like to see a variety of shapes and sizes tested in order to better approximate the textures of an asteroid.

Regardless, Holsapple says, it’s clear that small, fast-spinning asteroids like 1950 DA are fragile—and that they could easily be surrounded by a halo of rocks and particles that have been flung off. With NASA’s human exploration program targeting asteroids—and with some companies planning to mine asteroids in the distant future—asteroids like 1950 DA might best be left off the shortlist of targets, he says.