For the mission, the spacecraft known as Deep Impact swooped through the halo of ice, dust, and gas surrounding comet 103P/Hartley 2, coming within 435 miles (700 kilometers) of the comet's nucleus.
Initial views of some of the roughly 32,000 pictures of Hartley 2 showed an odd, drumstick-like nucleus spewing jets of material from its knobby ends. (See pictures of comet Hartley 2.)
Now, a detailed analysis of the images is offering insight into the flurry of particles that surrounds the nucleus, which in turn is telling astronomers more about the unusual composition of this particular comet.
(Find out about comet Hartley 2's closest pass by Earth.)
"When we realized we had a cloud of snow around the nucleus [of Hartley 2], we were astounded," EPOXI principal investigator Michael A'Hearn, of the University of Maryland, College Park, said today during a press briefing.
Previous closeup pictures of the halo around another comet, Tempel 1, had shown a uniform haze of tiny particles of ice. But, based on brightness, astronomers estimate the ice chunks around Hartley 2 range in size from grains of sand to basketballs.
Overall, it appears that every 10-inch-wide (25-centimeter-wide) ice particle in Hartley 2's halo is enveloped by a swarm of thousands of 1-inch-wide (2.5-centimeter-wide) particles, added EPOXI scientist Peter Schultz, of Brown University in Rhode Island.
"It looks to me as if the nucleus has a posse of mini-comets surrounding it," Schultz said. And "the whole [comet] looks like a snow globe that you've simply shaken."
Comet Hartley 2 Full of Dry Ice
The chunks of ice are being blown off the comet by jets, which the new data show are driven by frozen carbon dioxide, aka dry ice, inside the nucleus.
The jets apparently form when the comet gets close to the sun. "Dust grains and ice are being pulled from the comet as dry ice sublimes"—or turns directly from a solid to a gas—said Jessica Sunshine, also an EPOXI scientist at the University of Maryland.
The strength of the jets suggests that "Hartley 2 probably has more dry ice than any other comet measured," added the University of Maryland's A'Hearn. (Related: "'Biggest' Comet Measured.")
Meanwhile, some of the dust blown off the ends of the nucleus appears to fall back onto the smooth "waist" of the body. This region has no jets, which hints that the covering of dust is blocking sunlight from warming deeper, carbon dioxide-rich areas inside the comet.
Instead, heat from the sun causes the waist area to release gentle puffs of water vapor as subsurface water ice sublimates. This action is akin to what scientists saw happening on comet Tempel 1—which the EPOXI spacecraft visited before Hartley 2.
(Related: "Frozen Water Discovered on 'Deep Impact' Comet.")
"It's incredible to have the same instruments and compare the data [from both comets] one-to-one," Sunshine said. Comets are thought to be leftover pieces from planet formation, so such comparisons can help astronomers better understand how materials were distributed in the early solar system.
A'Hearn added that Hartley 2 "is certainly producing more water vapor per second than Temple 1—roughly 300 tons per second. ... That says Hartley 2 is a very active comet for its size."
NASA Probe Took Some Hits During Comet Flyby
As for the EPOXI spacecraft, its mad dash through a cometary blizzard resulted in a few minor collisions.
The probe sped through the comet's halo at about 27,000 miles (43,000 kilometers) an hour, said EPOXI project manager Tim Larson, who's based at NASA's Jet Propulsion Laboratory in California.
Impacts would knock a spacecraft slightly off course, so by looking for unplanned deviations in the craft's trajectory, "we identified nine events in the ten minutes around closest approach that could have been dust hits," Larson said.
"We estimate that ... the particles that hit us ranged from 0.02 to 0.3 milligram. [That's] about the weight of an eyelash, but such particles can affect a spacecraft at those speeds."
(Related: "Comet Impact Triggered Saturn Moon's Huge Jets?")
So far it seems that the collisions didn't damage the EPOXI craft, which will continue to take pictures of Hartley 2 through Thanksgiving, albeit from farther away.
NASA next plans to use the probe to hunt for planets beyond our solar system, although officials are reviewing data before making an official announcement of the probe's fate.