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Small Asteroids Are Held Together by Van Der Waals Forces

Small spinning asteroids are piles of rubble and dust that ought to fly apart but don’t. Now astronomers have worked out why not.

What holds small asteroids together? Surely not gravity, they’re too small for that. Today, Daniel Scheeres and buddies at the University of Colorado enlighten us with a study of the forces at work in these small bodies.

In 2005, the Japanese Hayabusa mission circled and landed on the potato-shaped asteroid Itokawa, which measures just a few hundred metres in size. (It is due to return to Earth later this year with a sample of asteroid dust.)

Spin rate statistics suggest that Ikotawa and asteroids like it are piles of rubble held together by gravity on scales of 150 metres and larger. But smaller boulders should fly off into space at this rate of spin.

But that creates a puzzle. Images from Hayabusa show that on smaller scales, Ikotawa is little more than a collection of boulders and dust. But if gravity cannot beat the centripetal forces involved, what’s holding Ikotawa together?

Astronomers have known for some time that the forces involved do not need to be large: various simulations have shown that even small cohesive forces can make spinning piles of rubble stable in low gravity environments.

Of the various possibilities, the main ones that astronomers have studied are radiation pressure from the Sun, friction and electrostatic forces between ionised dust (which is responsible for dust levitation on the Moon and so more likely to push dust apart).

The goal of the latest work by Scheeres and company is to “perform a survey of the known relevant forces that act on grains and particles, state their analytical form and relevant constants for the space environment, and consider how these forces scale relative to each other.”

Scheeres and co show that none of the usual suspects is the likely culprit. Instead it looks as if small asteroids are held together by van der Waals forces.

That has two interesting implications. First, for asteroid evolution. Scheeres and co suggest that spinning asteroids gradually throw off larger boulders until they end up as rubble piles held together by van der Waals forces. That may help to explain the size distribution of asteorids.

Second, this process may also explain, at least in part, the formation of planetary rings such as those around Saturn which are made up exclusively of small bodies.

If Scheere and co are right, their conclusions will lead to a significant re-assessment of the surface properties of asteroids, not to mention of the structure and evolution of planetary rings. No small feat.

Ref:arxiv.org/abs/1002.2478: Scaling Forces To Asteroid Surfaces: The Role Of Cohesion

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