Nasa plans to crash into comet

For the first time in history, Nasa is setting off on a collision course with a comet, in hopes of blasting a huge hole in the celestial snowball and gazing upon the original ingredients of the solar system preserved inside.

It all begins with a planned Wednesday launch of Deep Impact, a copper-fortified, comet-busting spacecraft.

Nasa has a single second—at precisely eight seconds past 1.47pm (6.47pm GMT)—to send Deep Impact on a 431-million-kilometre, six-month voyage to Comet Tempel 1. Good weather is forecast, cheering scientists who are up against a firm, two-week deadline for launching the probe.

Scientists have no idea what Comet Tempel 1 looks like. They don’t know whether the spacecraft will have to punch through a crust as hard as a concrete sidewalk or as flimsy as corn flakes.

All they know with certainty is that the nucleus, or core, of the comet is three times as long as it is wide, and that they must crash into the sunlit side to capture pictures of the resulting crater and all the ice, dust and other primordial matter shooting out of the hole on July 4.

Uncertainty over the comet’s shape “has caused us some concern about, can we hit the comet”, said Jay Melosh, a planetary geologist at the University of Arizona.
Officially, Nasa puts the odds of a bullseye at better than 99%.

“We hope we’ll make a crater ... perhaps 300 feet [90m] in diameter, 100 feet [30m] deep, that we’ll get through that crust and reveal the interior of the comet,” Melosh said on Tuesday. “But we don’t know what comets are made of. We don’t know how strong they are.”

This $330-million mission should provide those answers, along with clues to the origin of the solar system 4,5-billion years ago.

Because of the relative speed at the moment of impact—37 000kph—no explosives are needed for the job. The force of the smash-up will be the equivalent of 4,5 tonnes of TNT going off.

“The amount of energy of the spacecraft is about 10 times larger than an equivalent mass of TNT hitting the comet, so we could pack it with explosives, it wouldn’t make much of a difference,” Melosh said. “Just by colliding with it, we’re going to blast a big hole.”

Deep Impact actually consists of two spacecraft—the vehicle-sized mothership and the TV-sized impactor that will spring free one day before the July 4 strike.

The mothership is equipped with the largest telescope to date destined for deep space, to record the impact from a safe 483km away. Ground observatories also will record the event, as will amateur astronomers.

Both craft are shielded to protect against all the dust coming off the comet. Once on its own, the impactor will have to manoeuvre through this cloud of fine but potentially damaging particles to get out ahead of the comet and be run over by it.

Those last 24 hours will be the most perilous part of the journey.

“The last 20 minutes is when I’m going to really bite my fingernails,” Melosh said.

“I’d even go with the last one minute,” said the University of Maryland’s Michael A’Hearn, the principal scientist.

The impactor will vaporise instantly when it hits, as will its small payload, a compact disc containing more than 500 000 names of people who wanted to tag along vicariously.—Sapa-AP

On the net:

Deep Impact home page

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