Space tractor.

TECHNOLOGY RESEARCH NEWS

More than a few Hollywood adventures have featured asteroids smashing into the earth and wiping out cities or generating monstrous tsunamis. The notion of taking defensive action against an earth-bound asteroid is starting to gain traction in the real world.

Two NASA astronauts have proposed a gravitational tractor spacecraft that could tow an asteroid to a safe trajectory. Existing asteroid rescue plans call for spacecraft to nudge asteroids aside, but have practical challenges: it would be difficult for a spacecraft to maintain both position and contact with an asteroid because most asteroids rotate and have unstable surfaces.

The NASA plan sidesteps contact by using gravity as an invisible tow line. This requires positioning a spacecraft of sufficient mass close to the asteroid's surface and angling its thrusters to just miss the asteroid. The plan also calls for patience and foresight. Given a lead time of 20 years, a 20-ton spacecraft would require a year to tow a a 200 meter asteroid to a safe trajectory.

The European Space Agency (ESA) is preparing to test another alternative asteroid deflection plan. The plan, dubbed the Don Quijote project, involves smashing a spacecraft into an asteroid in order to deflect it. The plan calls for two spacecraft -- one to smash into the asteroid and the other to record what happens. The agency recently narrowed the choice of asteroids for the initial test to two.

NASA tracks near earth objects and assesses the risks they pose. The site uses the Torino scale to indicate the chance of impact and the degree of damage.

(Gravitational Tractor for Towing Asteroids, Nature, November 10, 2005)

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