Land and oceans
White roofs could cool cities: study
Enzyme crystal helps crack HIV puzzle
Sugar sweetens decision making
Twilight zone secrets revealed
Astronomers spot asteroid collision
Algae master quantum mechanics
Protein 'ushers' key to beating malaria
Researchers spin artificial bee silk
New view of Pluto increases mystery
Cell's power packs came from within
Antarctic snow linked to WA dry
Termites inspire hydrophobic materials
Study shows why it's scary to lose money
Soil impact underestimated: climate study
Lack of oxygen forced fish's first breath
Harder Sudoku puzzles on the way?
Weed genes could help feed the world
Logging makes forests more flammable: study
Food crisis looms warn scientists
Tiny sensors track 'lost' objects
'Climategate' university orders review
'Plumbing' key to flowering success
New twist on solar cell design
Scientists set new temperature record
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 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.

Horny mother beetles fight for dung
Light-speed computing one step closer
Small asteroids 'just lumps of gravel'
Gene study reveals diverse gut zoo
Dinosaur extinction caused by asteroid: study
Study finds methane bubbling from Arctic
New view reveals Mars' icy history
Some nano-sunscreens 'come at a cost'
Dust bunnies could harbour toxic load
Aphid genome reveals its 'Achilles heel'
Tailored diet may slow down DNA damage
Scientist probe ballistic chameleon tongue
Moa eggshells yield ancient DNA
Toothbrush tech helps buses go green
Gene protects some Tassie devils from tumour
Smaller fish cope better with acidic water
Lunar mirror mystery solved
Parents give fewer bad genes than thought
Women on pill may live longer
Antarctic winds affect key ocean layer
Researchers uncover thalidomide mystery
Boost for evidence of early ocean
Ocean geoengineering may prove lethal
People leave unique 'germ print'
Rogue star on collision course
Butterflies 'fly early as planet warms'
Glaucoma may start in the brain
Tools push back dates for humans on Flores
Stem cell capsules to target broken bones
Ecstasy damages complex memory: study
Earliest animals flexed their muscles
Insomnia may shrink the brain: study
Experts call for 'resilience thinking'
Tutu's DNA could point to medical cures
Humble algae key to whale evolution
Happiness linked to healthy heart
Fewer cyclones, but more intense: study
Cosmic candles result of colliding stars
Flightless mosquitoes may curb dengue
Childhood poverty may leave its mark
Cautious response to technology strategy
Nanowire RAM to make ever-ready computers
Are non-smokers smarter than smokers?
There's iron in them thar Martian hills
'Shell Crusher' shark swam ancient oceans
Nanotechnology may tap into your mind
Small dogs originated in the Middle East
Brain 'hears' sound of silence
Swimmers 'may not understand' tsunami risk
Altruism surfaces on slow-sinking ship
Chile quake tops Haiti, but less deadly
Weedkiller 'makes boy frogs lay eggs'
Visit Statistics