NASA's Deep Impact Spacecraft on Target for Comet Crash
A jet of dust spews jet extends about 2,200 kilometers from Comet Tempel 1 in this earlier image taken by the Hubble Space Telescope on June 14, 2005.
Credit: NASA/ESA/P. Feldman/H. Weaver.

PASADENA -- With a little more than two days left in its six-month journey, managers for NASA's Deep Impact mission said the spacecraft is on course to make its historic encounter with a comet late Sunday evening.

The mission is slated to crash an 820-pound (371-kilogram) Impactor probe into Comet Tempel 1 and record the event via a Flyby mothership. The impact is expected to take place at 1:52 a.m. EDT (0552 GMT) on July 4.

"I'm pleased to report that both the Flyby and the Impactor spacecraft are ready for encounter operations," Dave Spencer, Deep Impact's mission manager at NASA's Jet Propulsion Laboratory (JPL), announced today at a mission briefing.

The announcement follows several major mission milestones that were achieved over the past two weeks.

On June 23, the spacecraft successfully completed its third trajectory correction. The burn of the spacecraft's engines changed Deep Impact's speed by 13 miles per hour (about 21 kilometers per hour). Another trajectory correction for final targeting before Impactor release is scheduled for 8:00 p.m. EDT July 2 (5:00 p.m. PDT).

Mission planners separated the spacecraft's flight operations into six mission phases; launch, commissioning, cruise, approach, encounter and playback. The five-day encounter phase incorporates the final approach to the comet and transmission to Earth of collected data.

Fresh comet science

Although Deep Impact is still about 1.7 million miles (2.7 million kilometers) from Tempel 1, it is already giving astronomers an eyeful. Onboard cameras have picked up several outbursts of material jetting out of the comet, most recently on June 30, and allowed researchers a chance front row seat to the phenomenon.

"We've known for a long time that comets have outbursts, but we don't know what drives them," said Michael A'Hearn, Deep Impact's principal investigator at the University of Maryland, during the briefing. "We are getting great science now."

One such outburst was so strong it nearly doubled the amount of water in hazy cloud, called a coma, surrounding Tempel 1's nucleus, mission scientists said. In addition to water, Deep Impact's onboard spectrometer has also picked up traces of complex hydrocarbons, carbon dioxide and carbon monoxide, they added.

Deep Impact researchers said that past probes to comet Halley in 1986, comet Borrelly in 2001 and comet Wild-2 in 2004 recorded images at resolutions that ranged between 328 feet (100 meters) - or just over an entire football field - and about 75 feet (about 23 meters).

"Our spacecraft is going to resolve items about the size of the football," said Don Yeomans, co-investigator for Deep Impact at JPL. "So that's good."

But it's the main event astronomers are waiting for. Mission scientists hope Deep Impact's Impactor probe will punch through Tempel 1's outer surface and reveal inner material that has lain unchanged since the formation of our solar system.

"Literally, these materials have not seen the light of day in 4.6 billion years," said Jessica Sunshine, a mission co-investigator at Science Applications International Corp. (SAIC).

Impactor separation

On late Saturday evening, 24 hours prior to its estimated collision with Tempel 1, the Impactor will separate from the Flyby mothership and begin its final approach on the comet. The mothership will then adjust its course to monitor the Impactor's mission, Spencer explained.

"Twelve minutes later the flyby spacecraft--now flying on its own--will turn and perform its largest maneuver of the entire mission," Spencer said. "It will slow itself down by about 220 miles per hour relative to the Impactor ... this slowdown will allow it to witness the impact events itself and subsequent crater formation for about 13 minutes after impact."

Should something go wrong and the Flyby craft fail to break away from Tempel 1 after Impactor's release, there is a contingency plan to ram both vehicles into the comet's surface, Spencer added.

During the entire encounter, both the Impactor and Flyby will use medium and high resolution imagers and an infrared spectrometer to collect and send to Earth pictures and spectra of the event.

The plan is not without technological and environmental risks: Following impact and the initial observations, the Flyby spacecraft will adjust its position, using its shielded solar array to protect its major hardware from any debris thrown up from the comet's surface by the collision. During this time the mission team will be working at full speed to download as much information as possible from the spacecraft, systems engineer Jennifer Rocca said.

"We will try our best to transmit the highest priority images from both Flyby and Impactor before Flyby reaches its closest approach to the comet," Rocca explained. "This allows us to capture our very best data even if Flyby suffers damage as it flies close to Tempel 1."

The spacecraft will be assisted with information gathered by a variety of spaceborne science platforms including the Hubble and Spitzer space telescopes, the Chandra X-ray Observatory, the Swift and Submillimeter Wave Astronomy satellites, the European Space Agency's XMM-Newton X-ray observatory and the Rosetta spacecraft. Observatories on Earth will view the impact and its aftermath.

All information transmitted from Deep Impact will be collected on Earth for mission managers through NASA's Deep Space Network (DSN). The DSN consists of three deep-space communications facilities placed approximately 120 degrees apart around the globe: one at Goldstone, in California's Mojave Desert; a station outside Madrid, Spain; and the last near Canberra, Australia.

"Our antenna coverage for our short encounter event is absolutely critical," said Rocca. "So we planned our impact to occur in an area of overlap between a 70-meter antenna at Goldstone and a similar one in Australia." Staff Writer Tariq Malik contributed to this story from New York City.

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