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

Excerpt from
theguardian.com

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

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 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 . 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 , 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 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 ,” said Stephan Ulamec, the Philae lander manager.

In the time it took the probe to land for the time, the comet had rotated, bringing more treacherous terrain underneath. The spacecraft bounced a time and finally came to a standstill on its side at what may be the rim of an enormous crater.

“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.

With most of Philae in , the lander will receive only a fraction of the 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 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.
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… 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 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.