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STRANGE as it may seem, averting Armageddon isn鈥檛 the top priority for most
asteroid hunters. They鈥檇 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鈥攚ith 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鈥檚 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. 鈥淵ou are in the worst possible situation
to deflect a body when it鈥檚 pointing at you,鈥� he says.

An incoming asteroid would approach the Earth on a curved, hyperbolic path.
Simple mathematics shows that for every such hyperbola, there鈥檚 an elliptical
orbit around the Earth which intersects it at 90 degrees鈥攖he 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 鈥淟agrangian
points鈥�. 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鈥檚 not too soon to start discussing
what form they might take. Although we鈥檙e 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. 鈥淭he important thing in any
military operation is to know your enemy.鈥�