NASA’s Second Close Encounter With a Comet

2.13.11 Repost Courtesy of the New York Times:

See http://www.nytimes.com/2011/02/14/science/space/14comet.html

Article By KENNETH CHANG
Published: February 13, 2011

The last time NASA visited the Tempel 1 comet, it was with fireworks, on July 4, 2005. On that day, the Deep Impact spacecraft slammed an 820-pound projectile into Tempel 1, excavating a plume of ice and dust.

NASA

The Tempel 1 comet was photographed by NASA’s Deep Impact spacecraft in 2005.

On Monday nightValentine’s Day — NASA will return to Tempel 1 but will not bombard it. This time, a different spacecraft, Stardust, will zip past at more than 24,000 miles per hour, taking 72 high-resolution pictures of the comet’s surface.

Stardust will make its closest approach, within 125 miles, at 11:37 p.m. Eastern time.

Tim Larson, the mission’s project manager, said NASA was not deliberately scheduling its missions to coincide with holidays. “That’s just how the orbital mechanics worked out on these,” he said, “although it makes for great P.R.”

Tempel 1 will be the first comet to be seen at close range twice, and scientists will make a then-and-now comparison — one that they expect will reveal a change in topography and tell them more about the inner workings of comets.

“Here’s a chance where we can see what has changed, how much has changed,” said Joseph Veverka, a professor of astronomy at Cornell and the mission’s principal investigator, “so we’ll start unraveling the history of a comet’s surface.”

For example, photographs taken by Deep Impact in 2005 showed areas that looked old and others that seemed much younger. But the snapshots did not tell the ages of any of them. “We have no idea whether we’re talking about things that have been there for a hundred years, a thousand years, a million years,” Dr. Veverka said.

In the five and a half years since Deep Impact’s visit, Tempel 1 — whose orbit brings it as close to the Sun as Mars and as far away as Jupiter — has completed a full orbit.

Stardust was launched in 1999 and arrived five years later at its primary destination, a comet named Wild 2, where it collected particles of dust. Stardust then looped back to Earth and released a canister containing the comet dust, which parachuted back to the ground.

The spacecraft, still operating well, continued onward, and NASA decided to use it for a return visit to Tempel 1. (Deep Impact, meanwhile, also extended its scientific journey, visiting another comet last November.)

One more puzzle that scientists may be able to solve with the second look at Tempel 1 involves depressions that look like the type of craters caused by impacts. The depressions, though, could have been caused by explosions that were a result of underground ice that converted to gas.

The scientists will now be able to compare the depressions with something they know is definitely a crater — the scar left by Deep Impact. “Simple question,” Dr. Veverka said, “direct answer.”

A version of this article appeared in print on February 14, 2011, on page A16 of the National edition.
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