PASADENA, Calif. - A NASA spacecraft was speedily closing in on its target Friday, a comet scientists hope to smash open this weekend, producing celestial fireworks for the Independence Day weekend. But the real purpose is to study the comet's primordial core.Mission scientists said the Deep Impact spacecraft was 1 1/2 million miles away from Tempel 1, a pickle-shaped comet half the size of Manhattan.
"We're closing in very rapidly, but we're still very far away," said Michael A'Hearn, an astronomer at the University of Maryland and principal investigator of the $333 million project.
The cosmic fireworks will not be visible to the naked eye. But skygazers with telescopes can view the collision 83 million miles up from parts of the Western Hemisphere in the United States, west of a line from Chicago to Atlanta, around 2 a.m. EDT Monday if all goes as planned.
Launched from Cape Canaveral, Fla., Deep Impact began a six-month, 268-million-mile voyage Jan. 12 toward Tempel 1. If all goes well, it will be the first time that scientists have ever peered into the heart of a comet.
The collision will not significantly alter the comet's path around the sun and scientists say the experiment poses no danger to Earth.
On Saturday, the spacecraft will spring free an 820-pound copper "impactor," which will begin a 500,000-mile dive toward the sunlit side of the comet. The impactor will have three chances to correct its flight path to ensure a collision, which is expected around 1:52 a.m. EDT Monday.
As the comet races toward it at 23,000 mph, the camera-equipped probe will shoot pictures as it awaits its fate.
Rick Grammier, project manager at NASA's Jet Propulsion Laboratory in Pasadena, said both the spacecraft and the impactor were operating normally Friday and he expects a successful mission.
Scientists expect the collision to blast a crater in the comet and hurl the pristine subsurface material out from the pit. Comets are considered remnants of the solar system's building blocks, and studying them could provide clues to how the sun and planets formed 4 1/2 billion years ago.
The 1,300-pound flyby spacecraft, carrying two cameras and an infrared spectrometer, will witness the impact from a distance of 5,000 miles. After the impact, the spacecraft will approach the comet, flying 310 miles beneath it, to get images of the aftermath.
Last month, the Deep Impact spacecraft detected the comet nucleus for the first time through a hazy cloud of dust and gas surrounding the icy body. The images taken from 20 million miles away should help the spacecraft zero in on its target.
Scientists also observed several short-lived outbursts of ice from Tempel 1 that dramatically brightened the comet. Many comets experience flare-ups although scientists are not exactly sure why. Grammier said the outbursts should have little impact on the spacecraft and probe.
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On the Net:
Deep Impact mission: http://www.nasa.gov/deepimpact


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