In the great list of physics conundrums, the question of small astronomical body mechanics doesn’t come near the top. But it is one of those questions that always seemed puzzling.

Most models for planetary formation assume that a process of accretion creates large bodies like the planetesimals from microscopic dust in the pre-stellar nebula. The primary mechanism driving the accretion is thought to be gravity. And while a small object the size of snowball might not have much gravity, in the micro-gravity environment of the nebula, it’s probably sufficient for the task.

The problem though is that the *self-gravitational* attraction of larger objects (like a boulder) isn’t strong enough to hold them together if they’re doing something as likely as spinning a bit. Self-gravitation is what holds the sun, the earth and all of the planets together. The problem has been with the small objects like moonlets, planetesimals and smallish asteroids.

Now there’s a new mechanism suggested that holds things together – and it’s the same one that lets bugs walk on water and creates the meniscus disks in a graduated cylinder:

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.

[…]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.”

Read the full article here.