NASA to blast hole in comet

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The Guardian May 26, 2001

Nasa to blast hole in comet

Scientists to study results of Deep Impact mission

James Meek, science correspondent It may go down in history as the earliest act of deliberate cosmic vandalism, mankind's first halting steps at preventing its own extinction, or - as the scientists involved modestly hope - an explosive, useful experiment in the wilds of space.

Whatever it is, the Deep Impact mission, just given the go-ahead by Nasa, will be unlike any previous space venture in the scale of the planned damage to a celestial object, a comet called Tempel 1.

Scientists will fire a 350kg bullet into the heart of the comet as it passes between Earth and Mars, gouging out a crater seven storeys deep and 100 metres wide.

The blast, to be broadcast on television and the internet from cameras mounted on the mother ship, is scheduled for July 4 2005. With a mission price tag of about £200m, it will be the most expensive Independence Day fireworks.

The "bullet" itself, known as an impactor, will have a camera to record its last moments as it slams into the comet at 10km a second (22,300mph).

The scientific goal is to gain the first glimpse of a comet's interior by studying the walls of the crater and the debris thrown out by the impact.

That is if there is a crater. Astronomers' best guess is that the nuclei of comets are a cocktail of ice, frozen alcohol and methane, with a sprinkling of rock and dust.

But they cannot be sure. Tempel 1, discovered in 1867, orbits the sun every five-and-a-half years and is only about 1km wide. The impactor could shoot straight through or break the comet into large pieces.

"I wouldn't be at all surprised if it broke into several chunks - comets are pretty fragile," said Duncan Steel, a comet specialist at the University of Salford, Greater Manchester. "We see several of them of them breaking apart every year."

This would pose no danger to Earth, he added. "The chunks would still follow the comet's orbit, which is outside Earth's orbit," said Dr Steel.

Around 40,000 tonnes of space debris rain down on Earth each year, and the destruction of a small comet close to the planet would risk doubling that. Dr Steel said Nasa had sensibly gone for a comet far enough away to avoid that problem. "The chances of being hit by a big lump are essentially zero. We've got much worse things to worry about, like all the asteroids we haven't found yet."

He dismissed any aesthetic or moral objections that space environmentalists, a largely unformed body as yet, might have to gouging holes in pristine parts of the solar system.

"As soon as we landed on the moon and made footprints you could say we were polluting the moon," he said. "A lot of these airy fairy ideas are pretty absurd, aren't they."

Deep Impact is due to be launched in January 2004 and orbit the sun for a year before swinging out to intercept Tempel 1 in July 2005. It will release the cylindrical impactor into the comet's path before moving to watch the crash from a safe distance.

The US scientists involved claimed they came up with the title Deep Impact long before the writers of the 1998 film of the same name. In the movie, astronomers discover a comet on a collision course with Earth. A manned spacecraft is sent to intercept it, plant nuclear charges and destroy or deflect it before it can wipe out human life. Nasa's Deep Impact is not a trial run for such an emergency, but the knowledge gained will inevitably be useful if a planetary defence programme is set up.

Nasa recently landed a space probe on a small, peanut-shaped asteroid, Eros, and in 2011 the European space agency will try to land its unmanned Rosetta craft on a comet. Deep Impact will go further than either of these missions in altering the course of a comet, or breaking it up.

There is thought to be a one in a thousand chance of an undiscovered asteroid or comet 1km or more across hitting earth in the next century.

In the 1990s Nasa planned a mission called Clementine-2, a mother ship carrying three 1.5-metre harpoons to be fired into three asteroids at 10km a second, to test the technology.

But President Bill Clinton vetoed the project in 1997, on the grounds that it was an overly aggressive hangover from the Star Wars era which would needlessly alarm the Russians and Chinese.



-- (Paracelsus@Pb.Au), May 26, 2001

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