19-km-long Gaspra - photographed by the Galileo spacecraft in 1991. Researchers now understand more about how the sun helps this asteroid and others spin on their way.

Credit: NASA

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PARIS: Tiny pushes caused by sunlight can be enough to change an asteroid's spin, a finding that helps to explain an enduring mystery about space rocks.

The new findings are the first direct confirmation of a long-suspected phenomenon - proving that forces other than gravity help to shape an asteroid's path, according to researchers.

The force is the so-called 'Yarkovsky-O'Keefe- Radzievskii-Paddack effect' - more conveniently known as YORP.

YORP describes a phenomenon where, as an asteroid turns, light from the Sun slightly warms each side, which in turn gradually spin away from the Sun and lose this heat to the chill of space. According to YORP, that loss of energy gives the asteroid a very tiny kick.

It has been known for several years that this minuscule push can take the form of propulsion, eventually changing the asteroid's orbit.

Now, three studies, released by the British journal Nature and Science in the United States, show that solar energy can affect the asteroid's rotation, too.

A team led by Finnish researcher Mikko Kaasalainen of the University of Helsinki, looked at detailed pictures of a large asteroid, 1862 Apollo, which is 1.4 km long, that were taken in 1980, 1982, 1998 and 2005.

By building a computer model of the asteroid, they found that the rock's rotation had very slightly speeded up - the equivalent to one extra spin over 40 years.

Stephen Lowry of Queen's University Belfast in Northern Ireland, led one of two other teams which took measurements of a smaller asteroid called (54507) 2000 PH5. This rock that was discovered in 2000 and has a close, zippy orbit around the Sun that brings it near Earth.

"We have never before had direct observation of YORP," said Lowry, explaining that in some cases, the phenomenon can take aeons, "sometimes millions of years", before having a visible effect.

YORP can speed up the spin - or conversely, slow it - and this explains a puzzle that asteroid-spotters have tried to solve for decades: why asteroids that are under 10-km-long can have an extreme variety of spin rates.

The size, shape and topography of the rock, its colour and quite possibly its mineral content, are among the factors that determine how much solar radiation the asteroid absorbs and whether YORP will accelerate the spin or brake it, said Lowry.

Once the spin gets going seriously, it may create a centrifugal force that can cause the asteroid to break apart, Lowry believes. This would explain clusters of asteroids whose break-up cannot be explained by collision alone.

YORP may be also be a tiny force that in some cases helps to nudge asteroids out of their orbital paths and onto a track with our planet, becoming "Near Earth Objects" that from time to time spark fears of a collision.

Also this week, NASA officials said the U.S. space agency is capable of finding nearly all the asteroids that might pose a devastating hit to Earth - but there isn't enough money to pay for the task, so it won't get done.

The cost to find at least 90 per cent of the 20,000 potentially hazardous asteroids and comets by 2020 would be about US$1 billion (A$1.28 billion), according to a report released this week.

The agency is already tracking bigger objects - at least one kilometre in diameter - that could wipe out most life on Earth, much like what is theorised to have happened to dinosaurs 65 million years ago. The U.S. Congress had asked NASA to track objects down to 140 m - even these could create explosions equivalent to 100 million tons of dynamite and devastate a small country.

"We know what to do, we just don't have the money," said Simon "Pete" Worden, director of NASA's Ames Research Centre in California.