[meteorite-list] 'Gravity Tractors' Can Deflect Asteroids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Nov 9 14:13:38 2005
Message-ID: <200511091912.jA9JCCY07985_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2005/051107/full/051107-7.html

Gravity tractors beat bombs

Any threatening asteroids just need a nudge, say astronauts.

Mark Peplow
nature.com
November 9, 2005

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 Nature[1] , 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. This will alter its orbit
such that it has a slim chance, about 1 in 15,000, 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.

References

   1. Lu E. T.& Love S. G.. Nature, 438. 117 - 178 (2005).
Received on Wed 09 Nov 2005 02:12:11 PM PST