Philae makes historic landing on a comet!

[Peter Lemkin]

Rosetta mission controllers must decide whether to risk making lander hop from shadow of cliff blocking sunlight to its solar panels

Rosetta mission's Philae lander: follow latest developments

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• Ian Sample and Stuart Clark in Darmstadt
  
  
• The Guardian, Thursday 13 November 2014 18.30 GMT
  

How Esa scientists believe Philae has landed on the comet on its side. Photograph: European Space Agency/Reuters

The robotic lander that touched down on a comet on Wednesday came to rest on its side in the shadow of a cliff, according to the first data beamed home from the probe.
Pictures from cameras on board the European Space Agency's Philae lander show the machine with one foot in the sky and lodged against a high cliff face that is blocking sunlight to its solar panels.
The precarious resting place means mission controllers are faced with some tough decisions over whether to try and nudge the spacecraft into a sunnier spot. If successful, that would allow Philae to fully recharge its batteries and do more science on the comet, but any sudden move could risk toppling the lander over, or worse, knock it off the comet completely.
The washing machine-sized lander was released by its Rosetta mother ship at 0835am GMT on Wednesday morning and touched down at a perfect spot on the comet's surface. But when anchoring harpoons failed to fire, the probe bounced back off into space. So weak is the gravitational pull of the comet that Philae soared 1km into the sky and did not come down again until two hours later. "We made quite a leap," said Stephan Ulamec, the Philae lander manager.
In the time it took the probe to land for the second time, the comet had rotated, bringing more treacherous terrain underneath. The spacecraft bounced a second time and finally came to a standstill on its side at what may be the rim of an enormous crater. Technically, the agency pulled off not only the first landing on a comet in history, but the second and third too.
"We bounced twice and stopped in a place we've not entirely located," said Jean-Pierre Bibring, Philae's lead scientist. Teams of scientists are now trying to work out where the probe is. What mission controllers do know is that they are not where they hoped to be. "We are exactly below a cliff, so we are in a shadow permanently," Bibring added.
Space lander sends first image of landing video With most of Philae in the dark, the lander will receive only a fraction of the solar energy that Esa had hoped for. The spacecraft needs six or seven hours of sunlight a day but is expected to receive just one and a half. Though it can operate for 60 hours on primary batteries, the probe must then switch to its main batteries which need to be recharged through its solar arrays. If Philae's batteries run out it will go into a hibernation mode until they have more power.
The spacecraft was designed with landing gear that could hop the probe around, but from its awkward position on its side the option is considered too risky.
Though caught in a tight spot, the Philae lander's systems appear to be working well. The Rosetta spacecraft picked up the lander's signal on Thursday morning and received the first images and more instrument data from the surface of the comet.
One of Philae's major scientific goals is to analyse the comet for organic molecules. To do that, the lander must get samples from the comet into several different instruments, named Ptolemy, Cosac and Civa. There are two ways to do this: sniffing and drilling. Sniffing involves opening the instruments to allow molecules from the surface to drift inside. The instruments are already doing this and returning data.
Panoramic view around the point of Philae's final touchdown on the surface of comet 67P, taken when Rosetta was about 18km from centre of comet. Parts of Philae's landing gear can be seen in this picture. Photograph: European Space Agency/AFP/Getty Images Drilling is much riskier because it could make the lander topple over. Newton's third law of motion says that for every action there is an equal and opposite reaction. In the minuscule gravity of the comet, any movement on Philae will cause motion. The drill turning one way will make Philae want to turn the other. Pushing down into the surface will push the lander off again. "We don't want to start drilling and end the mission," said Bibring.
But the team has decided to operate another moving instrument, named Mupus, on Thursday evening. This could cause Philae to shift, but calculations show that it would be in a direction that could improve the amount of sunlight falling on the probe. A change in angle of only a few degrees could help. A new panoramic image will be taken after the Mupus deployment to see if there has been any movement.
Meanwhile, the Rosetta orbiter team will continue to try to pinpoint Philae's position. This will be done with images from the mother ship's Osiris wide-angle camera and its Concert radar instrument. The Concert instrument communicates with Philae and is designed to beam radar through the interior of the comet to reveal the internal structure. When Rosetta is directly overhead, Concert can also help to pinpoint the lander's position. "When we see where we are, we can decide what to do next," said Bibring.
But time is tight, the first 24 hours of the battery life will soon be gone. It is expected to be completely drained sometime on Saturday. "We are running against the clock," admits Bibring, "Don't put the emphasis on failure, it is gorgeous where we are."