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Comet-bound probe needs Earth to shape its trajectory
BY JUSTIN RAY
SPACEFLIGHT NOW

Posted: June 24, 2010


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The NASA spacecraft made famous for firing a projectile into a comet five years ago is speeding back toward Earth to receive a critical boost Sunday that will send the satellite to another cometary rendezvous this November.


EPOXI's mission logo. Credit: NASA/JPL-Caltech/UMD/GSFC
 
The Deep Impact provided celestial fireworks on July 4, 2005 when it sent an instrumented smart bomb into Comet Tempel 1, excavating the ancient materials buried inside the nucleus for scientists to examine.

The mothership had a front-row seat for the collision, beaming back stunning imagery from its medium- and high-resolution cameras, plus collecting infrared spectrometer data about the characteristics of dust, ices and gases spewing from the comet.

After safely traveling away from Tempel 1, engineers began plotting plans to "recycle" the spacecraft and give it a new mission. Sufficient fuel reserves meant the craft could be dispatched to another rendezvous and use its sophisticated instruments to explore a second comet.

NASA approved the EPOXI mission, a dual-purpose extension of the Deep Impact spacecraft's life to observe stars with its telescope in the hunt for extrasolar planets and the flyby of one more comet.

The $43 million cost is roughly one-tenth the price tag for building and launching a whole new comet-visiting satellite.

"The spacecraft is in good shape. All of the equipment onboard is functioning well," said Tim Larson, the EPOXI project manager at NASA's Jet Propulsion Laboratory.

And so the Deep Impact mothership, instead of being turned off, remains alive and healthy, sailing through interplanetary space with a date to intercept Comet Hartley 2 on November 4.

But that encounter will be a gentle one, with the probe just passing by and observing the comet. There won't be any impacts this time.

Getting to Hartley 2 has required a series of Earth flybys to use the planet as a gravitational slingshot to bend the spacecraft's trajectory. One more boost is needed, and that will happen Sunday to put the satellite on the proper heading.

Soaring a mere 18,890 miles above the South Atlantic with a relative speed of 12,750 mph, the craft will make its closest approach at 6:03 p.m. EDT (2203 GMT), Larson said.

"It is a close flyby, so it gives us enough of a gravity assist that it changes our trajectory a little bit. It moves it out a little ways, outside of the current trajectory, and it raises it up above the ecliptic plane just a little. So that changes our trajectory enough so it gets us close to Comet Hartley 2 in November," Larson described.

"Out of our set of flybys, this is our closest and the one that gives us the most effect from Earth's gravity," he added.

Imaging of Comet Hartley 2 begins on September 5, commencing two months of intense monitoring leading up to the rendezvous.

Ground controllers have small trajectory correction maneuvers set aside over the next several months to slightly adjust the flight path using the craft's thrusters. The first burn on July 19 can erase tiny errors from Sunday's flyby, then further tweaks could be performed on September 29, October 27 and November 2 to fine-tune the course, if needed.

"Those maneuvers will be based on our radiometric tracking of the spacecraft, its trajectory we get from that, and then we'll also pull in the optical navigation that we get from watching the comet from the spacecraft. That will help refine our knowledge of the spacecraft trajectory relative to the comet. So those maneuvers will help refine our targeting based on all that data," Larson explained in a phone interview Thursday.

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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