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 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 particles, said Dr Pete Schulz, 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.
Carbon dioxide is driving the jets seen at the ends of the comet But the team stressed these particles were not solid chunks of ice in the sense most people might understand. 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 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.
As well as Tempel 1, spacecraft had previously visited comets Borrelly, Wild 2, Halley and Grigg-Skjellerup (although no close pictures were taken of Grigg-Skjellerup). All are bigger than Hartley, several considerably so.
Halley’s nucleus was by far the biggest seen – 15km in lengthComet Borrelly was about 8km in its longest dimensionWild 2’s dusty shroud (coma) was sampled by the Stardust probeTempel 1 was Deep Impact’s primary mission “target”Comet Grigg-Skjellerup was passed by the Giotto probe but not imaged
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