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Space probe swings by Earth for rendezvous with comet
BY JUSTIN RAY
SPACEFLIGHT NOW

Posted: June 28, 2010


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NASA's intrepid comet explorer used Earth as a gravitational slingshot Sunday, zipping by the planet to reshape its interplanetary trajectory and take aim toward its next target.


An artist's concept of the spacecraft. Credit: NASA/JPL-Caltech
 
The Deep Impact spacecraft's precisely choreographed Earth flyby was designed to provide the necessary targeting to reach Comet Hartley 2 for a dramatic encounter on November 4.

"Earth is a great place to pick up orbital velocity," said Tim Larson, the project manager at NASA's Jet Propulsion Laboratory. "This flyby will give our spacecraft a 1.5-kilometer-per-second (3,355 mph) boost, setting us up to get up close and personal with Comet Hartley 2."

The probe planned to soar just 18,890 miles above the South Atlantic with a relative speed of 12,750 mph at 6:03 p.m. EDT (2203 GMT), Larson said. That altitude is closer to Earth than where geostationary satellites fly.

"The speed and orbital track of the spacecraft can be changed by changing aspects of its flyby of Earth, such as how close it comes to the planet," explained University of Maryland astronomer Michael A'Hearn, principal investigator for the mission.

"There is always some gravity boost at a flyby and in some cases, like this one, it is the main reason for a flyby. The last Earth flyby was used primarily to change the tilt of the spacecraft's orbit to match that of comet Hartley 2, and we are using Sunday's flyby to also change the shape of the orbit to get us to the comet."

Only four comets have been examined in detail by visiting spacecraft, so scientists are eager to explore another by reusing the Deep Impact vehicle. The craft performed its primary mission to fire an instrument-laden projectile into Comet Tempel 1 five years ago, then engineers devised a strategy to redirect the mothership to rendezvous with Hartley 2.

"What makes this opportunity really exciting is it's the first time that we've been able to use the same suite of instruments to observe two different comets. That gives you a really good opportunity to do some very good comparative studies," Larson said.

Previous spacecraft results from encountering Halley, Borrelly, Wild 2 and Tempel 1 showed similar features among the comets, yet also some very different characteristics.

"One of the questions that we're trying to answer is what is common to comets versus what's unique from one comet to the next? Are there special cases? And out of those features then might indicate which are primordial features to the early formation of these comets and which ones are driven by evolution of the comets," Larson said.


EPOXI's mission logo. Credit: NASA/JPL-Caltech/UMD/GSFC
 
Imaging of Comet Hartley 2 by the Deep Impact spacecraft begins on September 5. "From there on in, we spend about two months watching the comet very frequently," Larson said.

The probe and comet are destined to pass by each other only 435 miles apart at 9:50 a.m. EST (1450 GMT) November 4 at a relative velocity of 27,500 mph.

"We'll be flying between the comet and the Sun and just south of the comet nucleus. So that will give us really good viewing geometries in terms of lit areas on the surface of the nucleus," Larson said.

Given the incredible speed of the encounter, the opportunity to capture the sharpest pictures will be brief. But engineers expect to get a resolution of 6.5 feet per pixel.

"The period when we are close enough to the comet to really be able to image in high detail will be relatively short. Basically plus or minus two hours around closest approach will be the range when we get our best, highest resolution images," Larson said.

"Outside of that, the resolution of the nucleus isn't quite as good, but we still get a lot of quality and usable information in terms of seeing jets, outbursts, structures within the comet, the coma of the comet, getting a better understanding of the rotation."

The spacecraft carries 4.7-inch and 11.8-inch telescopes with medium- and high-resolution cameras for imaging the comet's frozen heart, plus an infrared spectrometer to obtain temperature and composition data of the spewing dust, ices and gases.

Comets are wandering cosmic time capsules preserving 4.5-billion-year-old primordial material from the solar system's creation. The pristine building blocks in these rocky snowballs could tell astronomers what conditions were like when the solar system was spawning planets.

Deciphering the chemical fingerprints of comets has become a priority for scientists because these objects peppered the young Earth, possibly delivering the organic materials needed for the rise of life, the water for our oceans and even playing a role in generating the atmosphere.

Hartley 2, which was discovered in 1986 by Malcolm Hartley, is a Jupiter-family comet with an orbital period of 6.4 years.

"Based on the observations that we have of this comet, we think the diameter is somewhere in the vicinity of about 1.2 km (0.75 miles), which makes it significantly smaller than Tempel 1. Tempel 1's largest diameter was around 6 km (3.7 miles). So it's a quite a bit smaller, although the observations indicate it is a very active comet," Larson said.

Distant looks to date show this comet is a dust-producing machine.

"It implies that probably the vast majority surface is active and involved in dust production. We don't know a lot about this comet, but that's what we've been able to find out so far," Larson said.

Despite Hartley 2 producing more dust as compared to Tempel 1, the spacecraft team doesn't expect the upcoming encounter to be riskier.

"Based on some of the (infrared) signatures, it would seem to indicate it tends to be smaller dust grains rather than larger ones. The environment in terms of hazard to the spacecraft, we think, is probably a more benign environment at the flyby altitude."

After the encounter day, the spacecraft will spend another three weeks looking back at the comet as they part company.

For more on the history of the Deep Impact mission, check out our launch coverage from January 2005 and the impact coverage from July 2005.

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