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How to deflect an asteroid

February 13, 2002

STRANGE as it may seem, averting Armageddon isn`t the top priority for most asteroid hunters. They`d be happy just to know where the rock that could wipe out life on Earth will come from. But an astronomer in Italy thinks he can save the world-with space-based missiles.
        By the end of the decade, astronomers will have located over 90 per cent of the near-Earth asteroids capable of causing a global catastrophe. Last week, NASA`s Spaceguard Survey reported that over 100 rocks more than a kilometre across were discovered in 2001, bringing the total known to 587 of the estimated 1743 whoppers out there.
        But what good is all this observing if there are no plans for averting disaster? Claudio Maccone at the Centre for Astrodynamics in Turin claims to have worked out the best way to deflect an asteroid. In the past, studies have assumed missiles will be launched from Earth, but Maccone says space-based missiles would be far more effective. "You are in the worst possible situation to deflect a body when it`s pointing at you," he says.
        An incoming asteroid would approach the Earth on a curved, hyperbolic path. Simple mathematics shows that for every such hyperbola, there`s an elliptical orbit around the Earth which intersects it at 90 degrees-the ideal angle for a missile strike because even a small impact should deflect the asteroid from its collision course (see Graphic).
        To get a missile into the right elliptical orbit would require several long burns from Earth, but just one tiny push from certain orbits in space, says Maccone. So he recommends putting space-based missile launchers at "Lagrangian points". These are locations in space where the gravity of the Earth and Moon balances out in such a way that a satellite maintains its position relative to each body with minimal correction from its thrusters.
        It will be hard to convince governments to prepare planetary defences that might never be needed, but Maccone feels it`s not too soon to start discussing what form they might take. Although we`re likely to become aware of an impending collision with one of the rare, large asteroids many decades in advance, numerous asteroids smaller than one kilometre across could still cause a local disaster, and those are thought to hit Earth once every couple of centuries.
        That makes surveillance all the more of a priority, says Brian Marsden of the Minor Planet Center in Cambridge, Massachusetts, especially in the southern hemisphere where none is currently undertaken. "The important thing in any military operation is to know your enemy."

Author: Eugenie Samuel
More at: Acta Astronautica (vol 50, p 185)