Probe sweeps past 'space peanut'
Nasa's Deep Impact probe has flown by Comet Hartley 2.
The first pictures revealed a roughly 1.5km-long, peanut-shaped object with jets of gas streaming from its surface.
The pass, which went to about 700km from Hartley, was only the sixth time a spacecraft had made a close approach to a comet.
Nasa said it would take many hours to retrieve all of the data recorded by Deep Impact's two visible-light imagers and one infrared sensor.
But the initial pictures to get to ground gave a fascinating view of the comet's icy body, or nucleus.
"The dominant signature is [the] two rough ends and a smooth middle," said Dr Jessica Sunshine, the mission's deputy principal investigator from the University of Maryland, College Park.
"What we see is that where the activity is, where the jets are, is the rough areas. And the middle - in our best current interpretation - we think is fine grained material that has been re-distributed across the comet and collected in a topographic low."
The information from the flyby should give scientists further insight into the diverse properties and behaviours of what are some of the Solar System's most remarkable objects.
"Every time we go to comets, they're full of surprises," said principal investigator Dr Mike A'Hearn, also from the University of Maryland.
The closest approach to Hartley 2 - a highly elongated object - occurred just before 1400 GMT. The probe whizzed by at a relative speed of 12.5km/s. The comet was about 23 million km from Earth at the time.
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.
The new venture is known by the name Epoxi. It required a series of deep-space manoeuvres, including three gravitational slingshots around Earth, to put the spacecraft in the right part of the sky to meet up with Hartley.
Comet Hartley is named after the Australian astronomer Malcolm Hartley who first identified the body in a photographic plate from a sky survey undertaken in 1986.
He was at Nasa's mission control at the Jet Propulsion Laboratory (JPL) in California to see the images of his comet come back from Deep Impact.
"It's awesome," he said. "I've been overwhelmed by everything that's happened in the last two weeks.
"There'll be enough data downloaded to keep researchers busy for the next five, 10, 15 years probably. It's proving to be very interesting."
Tim Larson, the Epoxi project manager at JPL, said: "The mission team and scientists have worked hard for this day. It's good to see Hartley 2 up close."
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.
To date, spacecraft have flown close by six other comets - Tempel 1, 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, several considerably so. But Hartley - it was discovered in 1986 by the Australian astronomer Malcolm Hartley - has already proved itself to be a fascinating target.
Even from a distance, scientists saw a lot of short-term changes on the object - huge outbursts of dry ice, or carbon dioxide, pulling copious quantities of dust from the comet. Those outbursts would appear to come from the active areas seen in the new images, the researchers told a press conference following the flyby.
"The reason we wanted to go to Hartley 2 specifically was that it was a very small, very active comet, and was therefore different from the other comets we had studied in detail," said Dr A'Hearn.
"What we hoped to do was to use the difference between a small active comet and a large, relatively inactive comet like Tempel 1 or Borrelly to address the question of what parts of comets are due to recent processing and what parts tell us about the formation of the Solar System 4.5 billion years ago."
Deep Impact will keep imaging Hartley for a further 20 days. Nasa says it is examining requests to use the spacecraft and its instruments for further science observations. The limited fuel reserves now on Deep Impact mean it cannot be manoeuvred to flyby a third comet, however.