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Spacecraft Flies Past Snowstorm From Comet

A NASA spacecraft had to survive a cosmic snowstorm once it reached a distant comet two weeks ago, but still managed to beam back photos of never-before-seen cometary features and behavior, scientists announced Thursday.

The Deep Impact probe probably got hit at least nine times by ice particles during its flyby of Comet Hartley 2 on Nov. 4 but did not sustain any noticeable damage, the scientists said. 

The spacecraft flew to within 435 miles (700 kilometers) of the small, active comet and witnessed a blizzard of water ice around its nucleus -- something never observed before, researchers said. [New photo of the comet blizzard]

"When we first saw this, our mouths just dropped," Pete Schultz of Brown University, a scientist on the flyby mission team, told reporters. "To me, this whole thing looks like a snow globe that you've shaken."

Fuzzy flakes

The snowstorm was driven mainly by powerful jets of carbon dioxide at the ends of the elongated comet -- features not seen on any other comets, whose jets appear to be fueled by water vapor, researchers said. 

Hartley 2's jets drag water ice out from the comet's interior, forming a diffuse, snowy cloud around the chicken-drumstick-shaped comet, scientists said. [Gallery: Photos of Comet Hartley 2 Flyby]

Most of the particles are small, but some of them may be larger than any hailstones on Earth.

"The biggest ones are at least the size of a golf ball, and possibly as big as a basketball," said mission principal investigator Mike A'Hearn of the University of Maryland.

But the ice chunks aren't hard like hailstones, researchers said. Deep Impact measurements revealed that they're fluffy aggregates of tiny crystals.

"They're akin more to a dandelion puff," said science team member Jessica Sunshine, also of the University of Maryland.

This fluffiness explains why the ice particles didn't do any apparent damage to the $252 million Deep Impact probe. 

Since Deep Impact was moving at 27,000 mph (43,450 kph) at the time, even tiny objects could have put some serious dents in the craft -- if they'd been hard enough.

Learning about comets

The new observations are revealing a great deal about what makes Comet Hartley 2 tick, researchers said. The early indications are it's quite a bit different than the other four comets spacecraft have investigated up close.

For one thing, Hartley 2 seems to have much more carbon dioxide ice than the other comets, A'Hearn said. This could explain why its intriguing jets are mostly carbon-dioxide-powered. 

But why Hartley 2 should have more carbon dioxide in the first place is not clear. Hartley 2 may have formed farther away from the sun, A'Hearn added, though that's just speculation. 

The flyby has given researchers a lot to think about.

"It has emphasized how different comets are from one another," A'Hearn said. "Things have just gotten much more complex."

Deep Impact will be watching Hartley 2 recede into the depths of space until around Nov. 25, sending home about 3,000 new images every day, NASA officials said. 

This is the second comet the spacecraft has observed. In 2005, Deep Impact served as the mothership for a mission that successfully smashed a probe into Comet Tempel 1, to see what it's made of.

 * Gallery: Photos of Comet Hartley 2 Flyby, Best Comet Photos

 * Video: Comet Hartley 2's Jets of Poisonous Cyanide

 * Fumes of Dry Ice, Not Water, Are Blasting from Comet

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