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Small asteroids 'just lumps of gravel'
The galaxy is littered with small, fast-spinning asteroids that are little more than piles of gravel held together by weak physical forces, say researchers.

The study, led by US asteroid scientist Professor Daniel Scheeres from the University of Colorado, Boulder, has been submitted to the journal Icarus and is posted on the arXiv.org website.

The smallest asteroids we know of are just tens of metres across. Previously, scientists believed these small asteroids would simply be monolithic lumps of rock.

"Our model indicates that these bodies don't have to be monoliths and can, instead, be gravel piles," says Scheeres.

Scheeres and colleagues modelled the effects of gravity, friction, electrostatic forces, and pressure from solar radiation on asteroids less than 100 metres in diameter.

Asteroids slowly build up their spin rate owing to the slight pressure of solar radiation forcing them to "spin up" like a propeller, a process called the YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect.

They found that as they spin faster, material in the asteroids is 'flung out' to the edges and lost.

"As the body spins faster, the largest boulders on the surface will generally go into orbit first, as these lie further from the centre of mass of the asteroid and are less affected by cohesive forces. Thus, over time, one may just have the finer material left on the body," Scheeres says.
Keeping it together

What keeps the remaining dust together is not gravity but the attraction between the molecules themselves, the researchers say.

These van der Waals forces on Earth can bond together the polar ends of water molecules, but are much weaker than chemical bonds.

In the low pressure environment of space, particle surfaces can come in extremely close contact with each other, the researchers write.

"In these situations the strength of van der Waals forces can become stronger than are experienced between similar particles on Earth."

If the model is correct it means that studies of similar-sized powders on Earth could help to better understand asteroids in space, says Scheeres.

"Essentially, our work shows that the properties of millimetre to centimetre sized gravel on asteroids should be similar to the properties of bread flour on the Earth."

Australian earth scientist Professor Trevor Ireland, from the Australian National University in Canberra, who researchers the properties of asteroids, says the research is "very interesting" and a "great theoretical model".

He says that while on Earth high gravity is the main force that helps things stick together, in space, weaker physical forces like van der Waals forces become the dominant force.

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