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Scientists set for comet-smashing space mission NASA and university astronomers are eagerly awaiting the launch of a space probe bound to collide with a comet and give researchers a glimpse inside the solar system’s icy wanderers.
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NASA and university astronomers are eagerly awaiting the launch of a space probe bound to collide with a comet and give researchers a glimpse inside the solar system’s icy wanderers.

The Deep Impact mission, now set to launch on Jan. 12, will send two spacecraft on brief spaceflight to Comet Tempel 1, where an "Impactor" payload will slam into the object while its mothership, "Flyby," looks on.

The entire mission should last only a few months, but researchers are hopeful they will finally be able to break through the outer surface of a comet.

"Only the internal material of a comet is unchanged from the beginning of the solar system," said Deep Impact principal investigator Michael A'Hearn, of the University of Maryland, during a press briefing today at NASA headquarters in Washington D.C. "But there are no data on the interior, and that's what we're hoping to solve with Deep Impact."

The $330 million mission has been delayed twice from its original Dec. 30 launch date, first to allow engineers to rerun ground tests, then once more to replace a launch component. Deep Impact has a window running through Jan. 28, 2005, during which time the mission can launch anytime and still make its July 4, 2005 rendezvous with Comet Tempel 1.

"I have every confidence that we're going to make it by the 28th," said Deep Impact project manager Rick Grammier, of NASA's Jet Propulsion Laboratory in Pasadena, California, during the briefing. "We're good to go."

If successful, Deep Impact will be NASA's second probe to visit a comet in as many years. On Jan. 2, 2004, the Stardust spacecraft flew through the comet 81P/Wild 2, taking images and collecting samples that will be returned to Earth for study in 2006. The European Space Agency (ESA) also launched its comet probe Rosetta on March 2 to visit comet 67P/Churyumov-Gerasimenko, where it is expected to drop its lander Philae on the comet's surface in 2014.

A quick comet cruise
Deep Impact is unique among NASA missions in its brevity. From launch to impact, the mission spans just six months of spaceflight, giving mission controllers little time to shake out any bugs that may pop during the cruise.

To that end, spacecraft engineers have been continually testing their flight hardware and software to ensure the mission will go smoothly. At least twice during that testing they found need to recheck their work, hence the launch delays.

"Our checks are in place to detect these things," Grammier said. "That's why they're there."

Flyby and Impactor won't be alone on their cruise either. A host of ground- and space-based telescopes, including the Hubble and Spitzer space telescopes and the Chandra X-ray observatory, will be watching Comet Tempel 1 before, during and after Deep Impact's encounter.

"There will be lots of exciting science at many wavelengths from many observatories around the world," said Deep Impact co-investigator Karen Meech, of the Institute of Astronomy in Hilo, Hawaii, during the briefing. "We're hoping to see a change in the comet's chemistry

Looking inside a comet
Once Deep Impact arrives at Comet Tempel 1, much of what is expected to occur should do so automatically.

The Flyby mothership is expected to release the Impactor probe, an 820-pound (372-kilogram) spacecraft with its own camera to record the impact, in Tempel 1's path.

The comet should then overtake Impactor in a collision that is anticipated to occur at about 23,000 miles (37,014 kilometers) an hour. Moments before the crash, Impactor will snap the closest images - ranging between 20 to 300 kilometers depending on dust levels - ever obtained of a comet nucleus, while Flyby and Earth instruments will monitor the collision from afar.

By studying the collision's ejecta and resulting crater, researchers should be able to determine the physical properties of Tempel 1 as well as get a front row seat to crater formation.

"It's less than a 1 percent chance of missing the target," Grammier said. "But we're doing something we haven't done before, so we're just going to keep testing."

Flyby is equipped with both high and low-resolution cameras to monitor the collision, researchers said.

"While a technically challenging, this is an utterly simple experiment," A'Hearn said.

Launch ahead
But before Deep Impact can lay the smackdown on its cometary target, it has to get off the ground first.

The current Jan. 12 launch target lies near the middle of the current launch window, and while NASA officials are confident the mission will fly before Jan. 28, mission planners are making contingency plans.

They have selected a short list naming about six potential replacement comets that Deep Impact could swing by should it miss its Comet Tempel 1 window, mission planners said. The earliest available comet alternate would require a launch in about two months or so after the initial target, though that comet has a smaller nucleus and sheds a larger amount of dust which could make spacecraft targeting difficult, they added.

Deep Impact scientists said Tempel 1 need not be the only comet their mission visits during its lifetime. Although Impactor will be destroyed in the encounter, Flyby will swing past the comet and could encounter at least one other icy wanderer should researchers choose to extend their mission.

"We've identified at least two other comets that Flyby could visit in an extended mission," A'Hearn said. "But we have not yet chosen which [comet] we'd propose for that."

An earlier version of this report listed incorrect weights for the Impactor probe.