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Gravity tractors beat bombs

November 9, 2005 By Mark Peplow This article courtesy of Nature News.

Any threatening asteroids just need a nudge, say astronauts.

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The next time an asteroid threatens to crash into the Earth, forget about calling Bruce Willis; send for a tractor driver instead. A pair of NASA astronauts has unveiled a design for an innovative space tug that could one day save the world.

Even a relatively small asteroid some 200 metres wide could be nasty if it hit the Earth, potentially taking out a small country. Researchers have taken this threat very seriously, and are looking at ways to predict future collisions and prevent them.

Just as in the film Armageddon, in which Bruce Willis helps to destroy an incoming asteroid, scientists have considered detonating a large nuclear device to split a rock into less threatening chunks. Although that may be a spectacular solution, Edward Lu of NASA's Johnson Space Centre in Houston, Texas, dismisses that as "the blast and hope method".

"The reason it's not a good idea is that you don't know the outcome," he explains. The internal structure of asteroids is poorly understood, and blasting the spinning rock at the wrong instant could even make the situation worse, he says.

Big tow

"What you really need is something controllable," Lu argues. And that means a space tug.

The difficulty with this is that an asteroid could be little more than a pile of rubble, making it difficult for the tug to hitch up. And as most asteroids rotate, an engine anchored to the surface would have to be fired intermittently to push the rock in the right direction, wasting precious time.

So Lu and his fellow astronaut Stanley Love propose a spacecraft that simply hovers over the surface of the asteroid, using gravity as a towline. In this week's Nature1, they calculate that a 20-tonne craft could safely deflect a typical 200-metre-wide asteroid in about a year, assuming there is 20 years of warning to launch and get into position.

Space invaders

"I think it's a fascinating idea," says David Morrison, a researcher at NASA's Ames Research Center in Moffett Field, California. Morrison chairs the working group on near-Earth objects for the International Astronomical Union.

Morrison agrees that the threat of near-Earth asteroids should be taken seriously. Observation programmes such as the Spaceguard Survey are now scanning the skies for rogue rocks. "But no one has spent significant money on the issue of how to deflect it if we found one," says Morrison.

"Those asteroids we have discovered already are not an immediate threat," says Morrison. But given their rate of discovery, Morrison estimates that there may still be a handful lurking in space that have Earth in their sights.

Lu, who is part of the B612 Foundation, a group of space scientists who want to see asteroid-deflecting technology in place by 2015, has high hopes that the gravity tractor will be built.

To get it up and running, Lu recommends that the US government invests in nuclear-powered ion-propulsion engines, which are more manoeuvrable than chemical ones and are less vulnerable to running out of fuel. NASA recently cut the programme dedicated to developing this kind of propulsion system in favour of more immediately applicable projects.

Wait and see

The B612 Foundation recently lobbied NASA to put a radio transmitter on the 320-metre-wide asteroid 99942 Apophis. The rock is expected to pass just 30,000 kilometres from the Earth in 2029. TThis will alter its orbit such that it has a slim chance, about 1 in 5,560, of hitting the Earth in 2036.

NASA responded with a 'wait and see' strategy, promising to track Apophis to see whether it passes through the tiny 'keyhole' of space that would put it on a collision course. This would still leave enough time to mount a mission to avert disaster, the agency said.

But Lu argues that a tiny deflection a few years before the close approach of 2029 would definitely prevent a later impact, and could be achieved with a one-tonne gravity tractor. "It's always easier to do advanced planning than clean up after a disaster," he cautions.


  1. Lu E. T.& Love S. G. Nature, 438. 177 - 178 (2005).


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