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Rosetta mission: Philae tight landing spot on comet prompts
tough decisions for Esa
Rosetta mission controllers must decide whether to risk
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 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.
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.
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.