- The comet lander Philae fell silent when its batteries drained
- Scientists are confident communication can resume when sun warms the spacecraft
- Rosetta will continue to orbit Comet 67P into 2015 as it tracks around the sun
- Project members are trying to find Philae after it bounced across comet's surface
London (CNN) -- It flew for 10 years, crossed millions of miles of space, bounced over the surface of a comet and returned heaps of data ... and then quietly faded away.
The little spacecraft Philae that has captured the imagination of thousands with endearing tweets to the comet-chasing mother ship Rosetta stopped transmitting when its batteries drained.
"@ESA_Rosetta I'm feeling a bit tired did you get all my data? I might take a nap ..." The forlorn message was picked up by followers of @Philae2014 shortly before it fell silent.
Rosetta and Philae's love affair
The Rosetta orbiter mission will continue to track Comet 67P on its journey around the sun in the coming months but after the final transmission from Philae, the lander mission appeared to be over.
But is it?
Scientists from the European Space Agency (ESA), which is leading a consortium that includes NASA to find out more about the composition of comets and how they interact with the sun, say there's still a good chance that Philae will revive.
Rosetta's lander Philae is on the surface of Comet 67P/Churyumov-Gerasimenko on Thursday, November 13, and sending back images. One of the lander's three feet can be seen in the foreground. While Philae is the first probe to land on a comet, Rosetta is the first to rendezvous with a comet and follow it around the sun. The information collected by Philae at one location on the surface will complement that collected by the Rosetta orbiter for the entire comet. 
Rosetta's lander Philae captured this image during its descent to the comet from a distance of 3 kilometers (1.8 miles) from the surface on Wednesday, November 12. 
Philae took this parting shot of its mother ship shortly after separation on November 12, as Philae headed for a landing on Comet 67P. The lander touched down on the comet seven hours after separating from the orbiter. 
Rosetta's OSIRIS camera captured this parting shot of the Philae lander after separation. 
This mosaic is made of four individual images taken 31.8 kilometers (about 20 miles) from the center of Comet 67P/Churyumov-Gerasimenko on November 4. 
Rosetta took this image of comet 67P/Churyumov-Gerasimenko on September 15. The box on the right shows where the lander will touch down. 
The spacecraft sent this image as it approached the comet on August 6. From a distance of 130 kilometers (nearly 81 miles), it reveals detail of the smooth region on the comet's "body" section. 
This image, captured August 7, shows the diversity of surface structures on the comet's nucleus. 
The comet's "head" can be seen in the left of the frame as it casts a shadow over the "body" in this image released August 6. 
This image of the comet was taken on August 1. 
Rosetta's mission started on March 2, 2004, when it was launched on a European Ariane 5 rocket from Kourou, French Guiana. 
Rosetta is named after the Rosetta Stone, the black basalt that provided the key to deciphering Egyptian hieroglyphs. Scientists think the mission will give them new clues about the origins of the solar system and life on Earth. The mission is spearheaded by the European Space Agency with key support from NASA. 
This photo shows Rosetta being tested before it was wrapped in insulating blankets and loaded on a rocket for launch. 
Rosetta has massive solar wings to power the spacecraft. They were unfurled and checked out at the European Space Agency's test facilities before being packed up for liftoff. 
After its closest approach to Earth in November 2007, Rosetta captured this image of the planet. 
Rosetta snapped this image of Earth in November 2009. The spacecraft was 393,328 miles from Earth. 
Rosetta passed asteroid Steins in September 2008, giving scientists amazing close-ups of the asteroid's huge crater. The asteroid is about 3 miles in diameter. 
Rosetta took this image of Mars as it looped through the solar system. 
This image was taken by an instrument on Rosetta's Philae lander just minutes before the spacecraft made its closest approach to Mars. Part of Rosetta and its solar arrays are visible. 
On July 10, 2010, Rosetta flew about 1,864 miles from asteroid Lutetia, which is 10 times larger than asteroid Steins. 
Look closely at the top of this picture. See that dot? That's Saturn. Rosetta snapped the picture of asteroid Lutetia and captured Saturn in the background. 
After taking pictures of Earth, Mars and asteroids, Rosetta was put into hibernation in May 2011 after it reached the outer part of the solar system. Mission managers woke it January 20, 2014. 
In November, Rosetta will become the first spacecraft to deploy a robot for a soft landing on a comet. It also will be the first probe to escort a comet into our inner solar system. This drawing shows how Rosetta will drop its robot lander, Philae, onto the comet. 
The robot lander will fire harpoons to anchor itself to the comet. Scientists expect the lander to send back data to Earth for at least a week and possibly for many months as the comet heads toward the sun. 
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Rosetta: The comet chaser 
A touchdown 10 years in the making 
Rosetta scientist inspires next generation 
Sheer breath-taking human ingenuity And the malfunction that caused the probe to bounce in the very weak gravity might actually turn out to be a blessing.
"It's good luck through bad luck," said lander system engineer Laurence O'Rourke.
"Yep, we're stuck against a wall. But when you look at the original location of where we were supposed to land -- it's a beautifully flat area -- we would never have seen the images and the structures of the comet wall unless we landed where we did. We'd have been in a dust field," he told CNN.
"We've had a number of happy accidents. It's bad luck that we bounced but the good luck is that all the instruments were on ... so what you have is a major scientific benefit -- results from two different locations when we were only expecting one."
Lander problems started when a thruster and harpoons designed to anchor the small probe to the comet failed. Philae bounced away from the original target touchdown area and appears to have settled under the face of a cliff. Project scientists are still not exactly sure where, even though the lander performed its experiments and sent back data.
O'Rourke said the team are still searching for it and that the location will be key to estimating when a new signal might come from the spacecraft.
He explained that the final resting place of Philae did not allow enough sunlight to fall on the solar panels so the probe ran out of power. It was able to complete its original mission, operating on the comet from the initial battery charge.
But before Philae closed down, engineers managed to rotate the probe so a larger solar panel would be exposed to sunlight and they believe this will be enough to automatically restart the spacecraft when the comet's orbit takes it closer to the sun.
Again, scientists think this shadowy spot on the comet might be another stroke of luck. The original landing zone would have exposed Philae to temperatures that would have burned out the electronics -- probably after about three months.
Philae lander system engineer Laurence O'Rourke
"The lander has the ability of keeping itself warm but doesn't have the capability to cool itself down," O'Rourke told CNN.
"The current environment benefits the lander because we're not worried about it overheating any more. We can keep it warm if we get enough power," he said.
O'Rourke believes that if the lander comes back online there's a high chance it will survive until the comet's closest approach to the sun in August next year.
So how does Philae come back from the dead?
O'Rourke explained the sequence:
- If enough sunlight falls on the solar panel, Philae will reboot
- It then needs more energy to warm the batteries
- Once warmed, the batteries will start recharging
- When it has enough power Philae will try to contact the orbiting Rosetta -- for two minutes every 30 minutes.
- If it fails to make contact, Philae will power down the transmitter and try again 30 minutes later
The power it needs is tiny -- roughly equivalent to the output of two or three domestic AA batteries -- but it will be enough to restart the lander mission.
When the Rosetta team find Philae they will then be able to estimate the point at which enough sunlight falls on the solar panel to expect a transmission from the probe, announcing that it's still alive. As long as it doesn't get too cold there's still hope.
In a statement, lander project manager Stephan Ulamec said he was confident that contact could be resumed -- probably in the spring of 2015.
In the meantime, scientists are sifting through the data from Philae's 10 instruments to see what they may have already discovered.
"When you look at what Philae's done -- it's been quite extraordinary," said O'Rourke. "A little satellite placed on a comet so far away -- you're always rooting for it. To give it that extra chance to have more power felt really good," he said.
Watch Rosetta's 10-year chase across the solar system
Interactive: Rosetta and its mission
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