Rare Close Encounter With Comet Hartley 2 Surprises and Thrills Scientists
This is another of the first images sent back to Earth from the NASA's EPOXI mission after it flew by comet Hartley 2 around 7 a.m. PDT (10 a.m. EDT) on Nov. 4, 2010.
Credit: NASA/JPL-Caltech/UMD

A spacecraft's close encounter with a comet today (Nov. 4) went off without a hitch, and the data it's beaming down is already surprising scientists.

NASA's Deep Impact probe zipped to within 435 miles (700 kilometers) of Comet Hartley 2 at 10:01 a.m. EDT (1401 GMT) today (Nov. 4), and it sent to Earth the first five close-up photos of the peanut-shaped comet about an hour later.

Scientists are already poring over these images, as well as thousands of others Deep Impact has taken of the comet since early September. The spacecraft's observations paint a picture of a strange comet that's tremendously active for its small size, with carbon-dioxide-fueled jets spouting voluminously from a rough, textured surface. [First close-up photos of Comet Hartley 2]

Researchers hope the flyby — one of just five missions that have photographed a comet's nucleus up-close — will help them gain a better understanding of comet structure and behavior. Since comets are leftovers from the solar system's early days, such knowledge could reveal a great deal about how our cosmic neighborhood came to be, scientists have said.

But researchers stressed that there's still a great deal of work to be done, as Deep Impact has already delivered a mountain of data and will keep pouring it on through late November.

"The engineers did a fantastic job of getting us data," said Mike A'Hearn of the University of Maryland, principal investigator of Deep Impact's comet flyby mission, which NASA calls EPOXI. "The scientific work is just beginning now."

Early scientific returns

Researchers have only just begun studying the first few images from the close flyby, and they've got a lot of work ahead of them. By the time Deep Impact takes its eyes off the fleeing Hartley 2 around the U.S. holiday of Thanksgiving (Nov. 25), the probe will have delivered about 120,000 comet images to researchers' computers, NASA officials said.

Yet, an intriguing picture of Hartley 2 is already emerging.

For one thing, the close flyby revealed that the comet is shaped like a chicken drumstick, and that it has a rough and heavily textured surface.

Hartley 2 is smaller than the other four comets that were previously imaged up-close — measuring just 1.25 miles (2 km) across — but it is incredibly active, with many jets spouting gas and dust.

The comet was discovered in 1986 by Australian astronomer Malcolm Hartley and orbits the sun once every 6 1/2 years.

Comets, cyanide and CO2 jets

Deep Impact observations have revealed that Hartley 2's jets are fueled primarily by carbon dioxide — a surprise, since most comet outgassing is thought to be driven by water, A'Hearn said.

While jets had been observed on other comets before, the close flyby pinpointed Hartley 2's jets to an unprecedented degree, revealing that many of them are coming from areas with rough terrain.

"This the first time that we can track these jets to individual topographical features on the nucleus," A'Hearn said.

The close encounter further revealed that the jets erupt from many different parts of Hartley 2, not just the areas heated most by sunlight — another surprise, and one that has researchers scratching their heads.

"There are jets in the nighttime, jets along the edge and jets in the sun," said Jessica Sunshine, an EPOXI scientist from the University of Maryland. "We have a lot of work to do to figure out what's going on."

In September, Deep Impact also observed huge jets of cyanide gas from Hartley 2, as the comet coughed up a few million tons of the stuff over the course of two weeks or so. And it puffed out the poisonous gas without dragging out any dust, A'Hearn said — yet another surprise, since such exhalations usually bring with them grainy materials from the interior.

"That's a phenomenon we haven't seen before," A'Hearn said. "We don't understand that yet."

Scientists aren't drawing many conclusions yet about what Hartley 2 is made of; they need to wait for more data to come home from Deep Impact's infrared spectrometer instrument, according to Sunshine. But researchers are ready for the unexpected.

"I think some surprises are yet to come on the compositional side," Sunshine said.

Going off without a hitch

Cheers erupted in the mission control room at NASA's Jet Propulsion Laboratory when the five close-encounter images of Hartley 2 flashed up on the big screen around 11:01 a.m. EDT (1501 GMT) today (Nov. 4). Scientists clapped, whooped and wandered about, shaking hands to celebrate a job well done.

"We couldn't have asked for a better performance from our spacecraft and our navigation team," EPOXI project manager Tim Larson of JPL told reporters today. "It was a huge moment for our team."

Deep Impact was on the ball, Larson said, making its rendezvous with Hartley 2 within 2 seconds of the planned time. When the probe began beaming images back to Earth just about on schedule, the team knew the mission was a success.

"That was hugely gratifying," Larson said.

Cracking the comet code

Scientists will continue digging into the data as it pours in. The ultimate goal of Deep Impact's mission, A'Hearn said, is to try to learn how comets have been shaped over the eons since the solar system's birth.

If they can learn what about comets — what aspects of their structure and behavior — dates back to 4.5 billion years ago, researchers can draw more conclusions about the solar system's birth and about planet formation, according to A'Hearn.

While scientists ponder such big questions, Comet Hartley 2 will continue to zoom around the sun — for a while. The sun appears to be cooking 3 to 5 feet (1 to 1.5 meters) of material off the comet's surface on each orbit, A'Hearn said. Hartley 2's smallest side measures just 1650 feet (500 m), so the comet's days are numbered.

"It won't be around very long," A'Hearn said.

Deep Impact's future foretold

Deep Impact, for its part, is in limbo after its comet observations end around Thanksgiving.

The $252 million spacecraft went the extra mile — 2.9 billion extra miles, actually — to chase down Hartley 2.

Deep Impact was originally designed to serve as mothership and observer on a 2005 mission to crash an 820-pound (371-kilogram) probe into Comet Tempel 1.

After Deep Impact completed this mission successfully, NASA scientists repurposed and recycled it to hunt down Hartley 2, sending it on a journey that covered 2.9 billion miles (4.6 billion km). The extended mission cost about $45 million.

Now, Deep Impact is almost out of fuel. It doesn't have enough to make another comet close encounter, but researchers could possibly use it as more of a stationary observing platform, according to Larson. But whether that will happen, or whether NASA will just decommission the probe — that's all up in the air.

"NASA's looking at future uses, but that won't be decided for a little while," Larson said.

Even if Deep Impact's days are numbered, it can go out with its head held high, according to NASA officials. The extended mission was relatively affordable compared to other missions, said Ed Weiler, associate administrator at NASA's Science Mission Directorate. Its $45 million price is about 10 percent of what it would have cost to launch a whole new mission.

"In these hard economic times, that's a really good deal," Weiler said.

NASA's broad EPOXI mission has been using the 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).