Comet Gets Visit from NASA's Deep Impact Spacecraft

After a cosmic chase lasting months and covering millions of miles, a comet-hunting spacecraft finally caught its icy quarry.

NASA's Deep Impact probe zoomed to within 435 miles (700 kilometers) of Comet Hartley 2 at 10:01 a.m. EDT (1401 GMT) today, taking pictures all the while. The close encounter marked just the fifth time that a spacecraft has ever visited a comet. [Brief History of Comet Close Encounters.]

Mission scientists hope the rendezvous reveals what Hartley 2's icy nucleus is made of. By comparing Hartley 2 to the four other comets spacecraft have visited, they're hoping to gain a better understanding of comet structure and behavior, and perhaps of the solar system's formation.

"This comet is unlike any we've visited before, and we don't know what we're going to find," Mike A'Hearn of the University of Maryland, principal investigator of Deep Impact's mission, said before the encounter.

Researchers are eagerly anticipating sifting through the wealth of comet observations Deep Impact is expected to beam back to Earth. During the encounter, the spacecraft is expected to snap about 118,000 images, NASA officials said.

A long road to Hartley 2

The $252 million Deep Impact spacecraft took a circuitous route to Comet Hartley 2.

NASA launched the current spacecraft in 2005 to serve as a mother ship for the Deep Impact mission, which intentionally sent an impactor probe crashing into the comet Tempel 1 in July 2005 to study the object's composition.

After that mission ended, NASA decided to squeeze some more life out of the Deep Impact observer spacecraft. They planned to send it after a comet named Boethin, aiming for a close flyby in December 2008. But that didn't pan out because Boethin vanished, likely breaking up into many tiny pieces.

So researchers settled instead on Comet Hartley 2, a small ice ball that makes a long, looping trip around the sun once every 6 ½ years.  The comet was discovered in 1986 by Australian astronomer Malcolm Hartley, who planned to be in the Deep Impact mission control center at JPL during the rendezvous.

On June 27 of this year, Deep Impact whipped past Earth, using our planet's gravity to set it on a course for Hartley 2. The extended mission to rendezvous with Comet Hartley 2 costs about $42 million, NASA officials have said.

In September, Deep Impact went into approach mode as it neared its icy target, taking pictures and gathering data to prepare for the flyby.

Yesterday, at about 4  p.m. EDT (2200 GMT), it switched to encounter mode. Deep Impact locked its instruments — two telescopes with digital color cameras and an infrared spectrometer — on Hartley 2, and the data started pouring in.

Deep Impact will continue photographing Comet Hartley 2 for about three weeks as the comet speeds off into the dark reaches of space. After that point, the spacecraft's comet-watching mission will be basically over, and Deep Impact will be decommissioned after a final calibration run, NASA officials said.

The spacecraft can retire with its head held high, having delivered on two separate comet-hunting missions, mission managers said.

"This is going to give us the most extensive observation of a comet to date," said Tim Larson of NASA's Jet Propulsion Laboratory. Larson is project manager of Deep Impact's mission to Hartley 2, which NASA calls EPOXI.

Waiting for the data deluge

The first photos should start flowing into researchers' computers within an hour or so after the rendezvous, scientists said. But the complete data dump will take awhile.

"We will be waiting,"  A'Hearn said. "The best images won't reach Earth until many hours after the actual encounter." [The Best Comet Photos of All Time.]

Data from the close approach will continue to download through Saturday (Nov. 6), but NASA will release preliminary results sooner than that.

A news conference is scheduled for 4 p.m. EDT (2200 GMT) today (Nov. 4), agency officials said.

NASA's broad EPOXI mission has been using the recycled and repurposed Deep Impact spacecraft to track and study various celestial objects. The name "EPOXI" is derived from the mission's dual science investigations — the Extrasolar Planet Observation and Characterization (EPOCh) and Deep Impact Extended Investigations (DIXI).

* Deep Impact's Top 10 Comet Crash Images

* The Best Comet Photos of All Time

* Comet Hartley 2 Fires Out Poison Gas As NASA Probe Approaches

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