Evastar
Inactive User
A Nasa probe is about to sweep past Comet Hartley 2, acquiring a swathe of pictures to send back to Earth.
The Deep Impact spacecraft will get as close as 700km (430 miles) to the colossal block of ice and dust.
Its two visible-light and one infrared imager will endeavour to pick out features not seen on the four previous cometary encounters made by spacecraft.
This should give scientists further insight into the diverse properties and behaviours of these remarkable objects.
"The nuclei of the four comets that we have seen up close are very different from one another - both the overall shape and the kinds of features seen on the surface," says principal investigator Mike A'Hearn from the University of Maryland, College Park.
"What we want to understand is why these differences occur when we don't see obvious differences in the processes that should shape what we see."
The closest approach to Hartley 2 - a roughly 2km-long, gherkin-shaped object - should occur at 1402 GMT. The probe will whiz by at a relative speed of 12.5km/s. The event is occurring just over 20 million km from Earth.
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.
WISE image of Comet Hartley 2 (Nasa) Comet Hartley 2 pictured by the Wise telescope
The new venture is known by the name Epoxi. It has 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.
The "recycled" probe is not perfectly configured for the latest rendezvous but Nasa managers say the decision to re-use it is a very efficient way to maximise the science return from a space mission.
Deep Impact began imaging Hartley on 5 September and continued to return some 2,000 pictures a day of the object, right up to about 18 hours before closest approach.
At that point, the probe swung itself around to get its instruments in the best position to view Hartley. In doing so, the spacecraft stopped transmitting through its high-gain antenna and started using a low-gain connection to send only essential engineering information back to Earth.
That will continue until just after closest approach when the probe will be able to swivel once more to use its high data-rate link to download all the pictures.
"The closer we get to the comet, the better our image resolution; so we want to get as close as we can," commented Tim Larson, the Epoxi project manager from Nasa's Jet Propulsion Laboratory in Pasadena.
"But we are limited by the turn rate of the spacecraft - as the comet zooms by us we have to be able to turn fast enough to track it all the way by. And then if we get too much closer than 700km, we also start getting into a denser portion of the coma that poses more threats of damage to the spacecraft due to the particles that are there."
Deep Impact carries both medium resolution and high resolution cameras. It also has an infrared spectrometer that will give clues to the composition of the comet.
THE FOUR COMETS PASSED BY SPACECRAFT
Comets (Nasa/Esa)
* Halley's nucleus was by far the biggest seen - 15km in length
* Comet Borrelly was about 8km in its longest dimension
* Wild 2's dusty shroud (coma) was sampled by the Stardust probe
* Tempel 1 was Deep Impact's primary mission "target"
The giant balls of ice, rock and dust like Hartley 2 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 have previously visited comets Borrelly, Wild 2, and Halley. All are considerably bigger than Hartley. 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 have seen a lot of short-term changes on the object, and it ejects twice as much gas every minute as Tempel 1.
"We're trying to find out if all the new phenomena we saw at Tempel 1 are universal across all comets or are they special to Tempel 1," said Dr A'Hearn.
"The other key goal is to separate out the primordial features which we think we saw on Tempel 1 in the layering of the cometary nucleus and see what that can tell us about the formation of the Solar System 4.5 billion years ago."
Deep Impact will keep imaging Hartley for more than 20 days after the close pass. Nasa says the best pictures from the flyby should all be back on Earth by Friday.
BBC News - 'Recycled probe' to flyby comet
The Deep Impact spacecraft will get as close as 700km (430 miles) to the colossal block of ice and dust.
Its two visible-light and one infrared imager will endeavour to pick out features not seen on the four previous cometary encounters made by spacecraft.
This should give scientists further insight into the diverse properties and behaviours of these remarkable objects.
"The nuclei of the four comets that we have seen up close are very different from one another - both the overall shape and the kinds of features seen on the surface," says principal investigator Mike A'Hearn from the University of Maryland, College Park.
"What we want to understand is why these differences occur when we don't see obvious differences in the processes that should shape what we see."
The closest approach to Hartley 2 - a roughly 2km-long, gherkin-shaped object - should occur at 1402 GMT. The probe will whiz by at a relative speed of 12.5km/s. The event is occurring just over 20 million km from Earth.
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.
WISE image of Comet Hartley 2 (Nasa) Comet Hartley 2 pictured by the Wise telescope
The new venture is known by the name Epoxi. It has 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.
The "recycled" probe is not perfectly configured for the latest rendezvous but Nasa managers say the decision to re-use it is a very efficient way to maximise the science return from a space mission.
Deep Impact began imaging Hartley on 5 September and continued to return some 2,000 pictures a day of the object, right up to about 18 hours before closest approach.
At that point, the probe swung itself around to get its instruments in the best position to view Hartley. In doing so, the spacecraft stopped transmitting through its high-gain antenna and started using a low-gain connection to send only essential engineering information back to Earth.
That will continue until just after closest approach when the probe will be able to swivel once more to use its high data-rate link to download all the pictures.
"The closer we get to the comet, the better our image resolution; so we want to get as close as we can," commented Tim Larson, the Epoxi project manager from Nasa's Jet Propulsion Laboratory in Pasadena.
"But we are limited by the turn rate of the spacecraft - as the comet zooms by us we have to be able to turn fast enough to track it all the way by. And then if we get too much closer than 700km, we also start getting into a denser portion of the coma that poses more threats of damage to the spacecraft due to the particles that are there."
Deep Impact carries both medium resolution and high resolution cameras. It also has an infrared spectrometer that will give clues to the composition of the comet.
THE FOUR COMETS PASSED BY SPACECRAFT
Comets (Nasa/Esa)
* Halley's nucleus was by far the biggest seen - 15km in length
* Comet Borrelly was about 8km in its longest dimension
* Wild 2's dusty shroud (coma) was sampled by the Stardust probe
* Tempel 1 was Deep Impact's primary mission "target"
The giant balls of ice, rock and dust like Hartley 2 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 have previously visited comets Borrelly, Wild 2, and Halley. All are considerably bigger than Hartley. 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 have seen a lot of short-term changes on the object, and it ejects twice as much gas every minute as Tempel 1.
"We're trying to find out if all the new phenomena we saw at Tempel 1 are universal across all comets or are they special to Tempel 1," said Dr A'Hearn.
"The other key goal is to separate out the primordial features which we think we saw on Tempel 1 in the layering of the cometary nucleus and see what that can tell us about the formation of the Solar System 4.5 billion years ago."
Deep Impact will keep imaging Hartley for more than 20 days after the close pass. Nasa says the best pictures from the flyby should all be back on Earth by Friday.
BBC News - 'Recycled probe' to flyby comet
