Star Trek

Jan. 13, 2005. 01:00 AM

Liftoff for NASA's comet-smashing mission
Aims to puncture and record `heart' of Tempel 1 Collision will be equivalent to

4 1/2 tonnes of TNT


MARCIA DUNN
ASSOCIATED PRESS

CAPE CANAVERAL, Fla.A NASA spacecraft with a Hollywood name  Deep Impact  blasted off yesterday on a mission to smash a hole in a comet and give scientists a glimpse of the frozen primordial ingredients of the solar system.

With a launch window only one second long, Deep Impact rocketed away at the designated moment on a six-month, 430-million-kilometre journey to Comet Tempel 1. It will be a one-way trip that NASA hopes will reach a cataclysmic end on the Fourth of July.

"We are on our way," an excited Michael A'Hearn of the University of Maryland, the mission's chief scientist, said. Minutes later, the spacecraft shot out of Earth's orbit and on to its collision course.

"We'll be there July fourth," said NASA launch director Omar Baez.

It was not until later in the afternoon that scientists learned that Deep Impact's energy-producing solar panel had deployed properly. Although the spacecraft appeared to be healthy, it placed itself in a protective "sleep" mode because of an unknown problem, and flight controllers were reviewing strange sensor data, NASA said. The problem was not believed to be critical and flight controllers expected it to emerge within 24 hours, via recovery commands.



Scientists are counting on Deep Impact to carve out a crater in Comet Tempel 1 that could almost swallow the Roman Colosseum. It will be humans' first look into the "heart of a comet"  a celestial snowball still containing the original building blocks of the sun and the planets.

Because of the relative speed of the two objects at the moment of impact  37,000 km/h  no explosives are needed for the job. The force of the smashup will be equivalent to 4 1/2 tonnes of TNT, creating a flash that just might be visible in the dark sky by the naked eye in one spectacular Fourth of July fireworks display.

Nothing like this has ever been attempted before.


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`One of the scary things is that we won't actually know what it looks like until after we do the encounter.'

Jay Melosh, University of Arizona researcher

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Little is known about Comet Tempel 1, other than that it is an icy, rocky body about 14 kilometres long and five kilometres wide. Scientists do not even know whether the crust will be as hard as concrete or as flimsy as corn flakes.

"One of the scary things is that we won't actually know the shape and what it looks like until after we do the encounter,'' said Jay Melosh, a planetary geologist at the University of Arizona.

The comet will be more than 130 million kilometres from Earth when the collision takes place on the sunlit side of the comet, NASA hopes, in order to ensure good viewing by spacecraft cameras and observatories. The resulting crater is expected to be anywhere from two to 14 storeys deep and perhaps 90 metres in diameter.

Scientists stress that Deep Impact will barely alter the comet's orbital path around the sun and will not put either the comet or a chunk of it on a collision course with Earth.

In the 1998 movie Deep Impact, astronauts try to blow up a comet in hopes of saving the Earth, but the comet winds up being split in two and one section slams into the Atlantic, creating a huge tsunami on the East Coast.

A jagged, cratered comet like the one headed for Earth in the movie would be difficult if not impossible to hit because of all the shadows, Melosh said. Comet Tempel 1 is believed to be smoother and easier to strike, unlike that "Hollywood nightmare."

Deep Impact is carrying the most powerful telescope ever sent into deep space. It will remain with the mother ship when the impactor springs free the day before the comet strike, and will observe the event from a safe 500 kilometres away.

NASA space telescopes like the Hubble will also watch the collision, along with ground observatories and amateur astronomers.

The impactor will have a camera, too, that will snap pictures virtually all the way in.

The basic question that Deep Impact hopes to address is how Tempel 1 is put together. The size and shape of the projectile crater and the debris trail that streams from it will provide important clues.

"We've got a betting pool in the science team, and the smart money says it's a rubble pile held together mostly by gravity,'' said Don Yeomans, a senior research scientist at NASA's Jet Propulsion Laboratory and a member of the Deep Impact team. "If it is, the projectile will make a relatively shallow but spectacular gouge in the comet "about the size of the Rose Bowl," Yeomans said in a telephone interview with the Washington Post.

The entire mission costs $330 million (U.S.), all the way through the grand finale.

it would be ironic if the nasa explosion ended up changing the course of the comet putting it on a collision course for earth.