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Rosetta to keep listening for signs of life from comet lander Philae

Paul Sutherland, Feature writer
Nov 16, 2014, 22:48 UTC

Sen—Rosetta space scientists are thrilled that its tiny comet lander Philae beat all the odds to complete its primary mission, returning data from all its on-board experiments before its main battery failed.

The plucky probe did all that was expected of it in the initial phase of the mission during 57 hours sitting on the surface of Comet 67P/Churyumov–Gerasimenko. 

As we have previously reported, Philae itself has not died but is now in a state of hibernation and could be re-awoken in the weeks or months ahead if it receives enough sunlight.

The European Space Agency mission team are still uncertain whether this will happen, mainly because they still do not know exactly where Philae ended up on the comet. 

But they revealed they had imaged the probe and its shadow during its first bounce. 

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New analysis of an image of the comet shows Philae and its shadow. Image credit: ESA/Rosetta/NAVCAM

After its harpoons failed to fire, the washing machine-sized probe bounced twice before becoming lodged against a cliff that is blocking the Sun’s rays. It was getting just 1.5 hours of sunlight every 12 hours when it needed six or seven hours for the secondary batteries to charge up and keep Philae awake. 

During the coming days and weeks, mission controllers will continue to listen for any signals from the lander as its Rosetta mothership flies overhead in orbit around the comet.

In Philae’s favour, it is being carried closer to the Sun and will also experience a seasonal change that may deliver more solar power. However the comet will also become more active and produce more “sublimation”—the process where its ice turns to jets of vapour, which could surround Philae with clouds of gas and dust.

Asked if Philae could wake up, Comet expert Professor Alan Fitzsimmons, of Queen’s University Belfast, said today: “In terms of sunlight there are two positives. First, as the comet approaches the Sun, the inverse square law means that the flux (amount of sunlight) on the solar arrays will increase. By late March next year this will have doubled, so in theory twice as much power can be delivered by the solar arrays. 

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A labelled trajectory of Rosetta’s orbit, following the comet landing on 12 November. Image credit: ESA 

“Second, the orientation of the spin axis means right now the comet is experiencing “Northern” summer. Next spring/summer the Sun will be near equatorial. This might shorten the maximum length of the shadows on the comet near Philae and increase the exposure time on the solar panels.”

But Professor Fitzsimmons added that there were two negatives. He said:  “The local geology may mean that Philae does not get an increased amount of sunlight as the comet moves from Northern summer. More importantly, the comet is active with increasing sublimation of surface/subsurface ice, increasing the dust environment around the nucleus. 

“Also the surface undergoes active erosion with landslides and subsidence clearly visible in the Rosetta images. Ground-based data from the last orbit showed a major sublimation site may start up next year near the equator. So in summary, no-one knows right now.”

Mision controllers hope that they have stacked the odds in Philae’s favour by lifting the lander’s main body by 4cm and rotating it about 35°, along with its fixed solar panels before power was lost. 

Stephan Ulamec, who was lander manager at ESA’s Space Operations Centre (ESOC) in Darmstadt, Germany, this week said: “It has been a huge success, the whole team is delighted.

“Despite the unplanned series of three touchdowns, all of our instruments could be operated and now it’s time to see what we’ve got.”

During coming months, scientists around the world will be poring over the mass of data returned from Philae’s ten experiments, including its drill which bit into the crumbly surface before hitting something more solid.

In the meantime, mothership Rosetta is still performing superbly. Having despatched Philae, it is now moving out to an orbit 30km from the comet, but will return to a 20km orbit on 6 December as it watches the comet warm and become active on its journey into the Solar System.

In a statement, Matt Taylor, ESA’s Rosetta project scientist, said: “The data collected by Philae and Rosetta is set to make this mission a game-changer in cometary science.”

Fred Jansen, ESA’s Rosetta mission manager, said: “At the end of this amazing rollercoaster week, we look back on a successful first-ever soft-landing on a comet. This was a truly historic moment for ESA and its partners. 

“We now look forward to many more months of exciting Rosetta science and possibly a return of Philae from hibernation at some point in time.”