Deep Impact Pelted by Ice During Nov. 4 Comet Flyby

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NASA’s Deep Impact probe was pelted with ice particles as it flew past Comet Hartley 2 on Nov. 4, but still managed to beam back photos of never-before-seen comet features and behavior, scientists announced Nov. 18.

The probe probably got hit at least nine times by ice particles during its flyby but did not sustain any noticeable damage, the scientists said.

The spacecraft flew to within 700 kilometers of the small, active comet and witnessed a blizzard of water ice around its nucleus — something never observed before, researchers said.

“When we first saw this, our mouths just dropped,” Brown University-based mission scientist Pete Schultz told reporters. “To me, this whole thing looks like a snow globe that you’ve shaken.”

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, the jets of which 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. 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 are not hard like hailstones, researchers said. Deep Impact measurements revealed that they are 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 did not do any apparent damage to the $252 million Deep Impact probe.

Since Deep Impact was moving at 43,450 kilometers per hour at the time, even tiny objects could have damaged the craft — if they had been hard enough.

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

For example, 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 is just speculation.

The flyby has given researchers a lot to think about.

“It has emphasized how different comets are from one another,” said A’Hearn. “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.