Nasa probe flew by 'snow globe' comet

Deep Impact's high resolution camera picks out the storm of particles surrounding Hartley

Analysis of the data gathered by Nasa's Deep Impact probe at Comet Hartley reveals the object is surrounded by a huge cloud of fluffy ice particles.

The space mission's chief scientist Dr Mike A'Hearn told reporters some of these "snowballs" were very large.

"We think the biggest ones are at least the size of a golf-ball and possibly up to the size of a basketball," he said.

Deep Impact swept past the comet on 4 November, getting as close as 700km to the 1.5km-long, peanut-shaped object.

The probe's visible wavelength and infrared instruments returned a wealth of pictures and other data that should give scientists further insight into the diverse properties and behaviours of what are some of the Solar System's most remarkable objects.

The assessment of the cloud of material surrounding Hartley suggests the presence of a wide range of particle sizes. For every 25cm particle, there might be a thousand 2.5cm-sized particles, said Dr Pete Schultz, a mission scientist from Brown University.

"To me this whole thing looks like a 'snow globe' that you've just simply shaken," was how he described the environment around the comet's nucleus.

But the team stressed these particles were not solid chunks of ice in the sense most people might understand them. Rather, they are collections of small grains.

"We know that the ice [grains] on a fundamental level can't be bigger than somewhere between one and 10 microns in size," explained Dr Jessica Sunshine, the mission's deputy principal investigator.

"That's about the thickness of our hair. What that means is that the snowballs are not what we thought to begin with - we're not seeing hail-sized particles. What we're seeing are fluffy aggregates of very small pieces of ice. They're akin more to a dandelion puff than an ice cube."

Deep Impact's rendezvous with Comet Hartley occurred about 23 million km from Earth

Since 4 November, the science team has had a chance to consider the different look and activity occurring at the rough ends of the comet compared with its smooth middle.

Data shows the flat terrain is where water is evaporating below the surface and percolating out through the comet's dust covering. The jagged regions, on the other hand, are where carbon dioxide jets are ripping ice and dust particles out of the comet.

Deep Impact is on an extended mission, having been re-tasked to visit Hartley following its successful flyby of Comet Tempel 1 in 2005.

On that primary mission, the spacecraft released an impactor that crashed into Tempel's nucleus kicking up thousands of tonnes of icy debris.

Comets are thought to contain materials that have remained largely unchanged since the formation of the Solar System. They incorporate compounds that are rich in carbon, hydrogen, oxygen and nitrogen.

Intriguingly these are the elements that make up nucleic and amino acids, the essential ingredients for life as we know it; and there are some who believe comet impacts in the early years of the Solar System could have seeded the Earth with the right chemical precursors for biology.

In addition to Hartley and Tempel 1, spacecraft have flown close by five other comets - Borrelly, Wild 2, Halley, Giacobini-Zinner, and Grigg-Skjellerup (the missions to Giacobini-Zinner and Grigg-Skjellerup did not return images). All are bigger than Hartley.

It is carbon dioxide which is driving the jets seen at the rough ends of the comet

Deep Impact's rendezvous with Comet Hartley occurred about 23 million km from Earth. The pair are now rapidly retreating from each other, although the probe continues to image Hartley.

The observation campaign will continue until late next week, by which time Deep Impact will have acquired some 122,000 pictures in total.

"That represents about 22GB of data, so this undoubtedly gives us an exhaustive view of this comet - more than we've been able to return from any other comet," said Tim Larson, the mission's project manager.

"After that, we'll do a final calibration on the instruments and the spacecraft will be [put] in a fairly quiet mode in December awaiting further instructions."

Nasa has requested ideas for what to do with Deep Impact next. Whatever that might be, it will not include another comet flyby. There is now insufficient fuel onboard to make major corrections to its trajectory.

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