Despite an errant bounce that left Europe's Philae comet lander in a precarious sideways position, scientists working with the probe have said they will deploy a key instrument that can tell them what the comet is made of.
The Alpha Particle X-ray Spectrometer (APXS) is a coffee-cup size device that sits on an extendable robot arm and works by measuring the response of comet material to a radioactive source.
The readings can be used to infer what chemical elements are present in the comet.
The decision to use APXS comes as good news for Ralf Gellert, a physicist at the University of Guelph who helped build the device 15 years ago when he was a postdoctoral researcher at the Max Planck Institute in Mainz, Germany.
"If everything goes well, there could be data [on Friday]," said Prof. Gellert, who was at the European Space Operations Centre following Philae's dramatic landing on Comet 67P/Churyumov-Gerasimenko on Wednesday.
If APXS is successful, it will increase the growing scientific return from the lander, which could soon be curtailed by a loss of electrical power.
On Thursday, mission scientists confirmed that when the lander first touched down it did not fire two harpoons that could have anchored it to the comet. Instead, the lander bounced up to the height of one kilometre and was free-floating for two hours before it set down again. This was followed by a much shorter bounce that put the lander up against a steep cliff face with one of its three metal legs tipped skyward.
While scientists are elated that the lander survived at all, the result puts Philae in a serious predicament. The comet rotates every 12.4 hours, and scientists were expecting the lander would be in daylight for at least half of that period. Instead, indications are that sunlight is only reaching the lander's solar panels 1.5 hours during each rotation.
Philae arrived with about 60 hours of battery power and was designed to recharge its batteries with the sun. Scientists say Philae can't last long with so little energy available for recharging.
"I think we're in a scenario where we're near the end of the lander," said the mission's project scientist, Matt Taylor.
"The long-term science is the most at risk for the moment," said Jean-Pierre Bibring, lead scientist for the lander.
The short timeline puts a premium on getting as much done as quickly as possible, but there is another factor: Some instruments, including APXS, have moving parts.
In the low-gravity environment on the comet, whenever something on the untethered lander moves there is nothing to stop the lander from recoiling in the opposite direction, which could worsen its situation.
"You need to know which movements will send the lander in which direction," Prof. Gellert said.
It's also possible that by moving some parts on the lander, scientists can improve its exposure to sunlight – but such chances will have to be weighed against the desire to eke out as much data as possible with the battery power available.
Dr. Bibring said his team was working with the expectation that all the science instruments on the lander would be used. That could be a tall order if different groups from the various European nations that provided the instruments have to compete for limited power – but it also explains why APXS has been given the green light.
Photos from the lander show it is resting on a rock-like surface with bare, angular surfaces rather that the granular or powdery terrain expected at the intended landing site.
APXS performs best when used on hard, sheer surfaces.
Notwithstanding the challenges, mission scientists continued to voice amazement on Thursday that the lander is alive and returning data from the surface of a comet, a never-before-achieved feat.
"Every bit of data that we can get right now is really the icing on the cake," Prof. Gellert said. "It can only get better."