Comet mission not just spacedust

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NASA's Deep Impact spacecraft will release a washing-machine sized piece of copper to collide with a comet the area of Washington, D.C. The comet, Tempel 1, is more than 268 million miles from Earth. The goal is to blast a crater some seven stories deep and gather information about the comet's internal structure from the debris that flies off after impact.

Why the distant collision? Astronomers hope it will yield answers to some key questions about planet formation at the dawn of our solar system.

In a welcome international effort, other countries are seeking the same answers. Two months after the Tempel I impact, Japan's most ambitious space mission to date, Stardust, will attempt a more nimble feat. The spacecraft Hayabusa - from the Japanese word for peregrine falcon - will fly down to a small asteroid, and, like a "bird of prey" snatch samples of dirt. In 2014, the European Space Agency's Rosetta lander will reach an ice-caked comet, go into orbit around it, then attempt to touch down on the surface.

But there's another useful purpose from these missions that's not limited to gathering physical samples. They also provide navigational knowledge on how to identify, plot a trajectory toward, and then rendezvous with a potential "rogue" space object that might be on a collision course with Earth, and cause serious damage to the planet.

Given enough warning, a spacecraft could be launched with an impact device loaded with explosives to destroy or deflect one of these objects. That adds a whole new dimension to the study of space dust.