Comet lander’s exploration cut short

On November 12, a robot called Philae fell from its mother ship Rosetta onto comet 67P/Churyumov–Gerasimenko, touching down not once but three times. Philae’s final resting place, however, put it in a tough spot — in a crevice, in the shadow of a cliff, with two legs on the ground and one in space.

The precarious position wasn’t exactly what mission scientists from the European Space Agency had planned for with the first ever comet landing (SN: 11/1/14, p. 22). Resting on two legs, rather than three, raised concerns about whether all of Philae’s instruments would work correctly. Worse, the cliff’s shadow also blocked solar panels from harvesting enough sunlight to recharge Philae’s batteries, leaving the lander with only about 50 hours of power to explore its new home. Mission scientists were left scrambling to wring as many details as possible about the comet before the lander and its 10 instruments went into hibernation on November 15.

One of Philae’s instruments revealed that it had gotten a whiff of carbon- and hydrogen-rich organic compounds. Another instrument on the lander provided the first high-resolution image of the comet’s surface, released within hours of Philae’s settling onto 67P. Taken from a height of only 3 kilometers, the photo showed that the dusty surface is covered with steep cliffs, boulders and other debris.

In the final hours of Philae’s battery life, the scientists took risks, programming the lander to hammer into the comet’s surface. The hammer penetrated only 10 to 20 centimeters before hitting a hard wall of material, hinting that 67P’s surface is a rigid, possibly icy, layer beneath just centimeters of dust.

Another instrument recorded the vibrations of Philae’s initial landing, which scientists interpreted as a short, significant thud. The two-second sound clip, released November 20, along with Philae’s bouncy landings, corroborates the discovery of a potentially icy layer under the comet’s dusty coat, says Martin Knapmeyer, a planetary scientist at the German Aerospace Center in Cologne and the lead scientist on Philae’s Cometary Acoustic Surface Sounding Experiment.

Philae drilled into 67P to collect surface samples and deliver them to the lander’s ovens for chemical analysis. An X-ray spectrometer also took data to identify elements and molecules at Philae’s landing spot. But there are questions about how well these instruments worked.

“A primary question was, Did we have enough data to complete some of the top questions we went there to address?” says Claudia Alexander, who works on the Rosetta mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. NASA contributed instruments and electronics to the mission.

Philae beamed back a lot of data that will change the way scientists think about comets, Alexander says. “But we’re still sorting out the completeness of what we got.” Mission scientists will need to combine even more data from other instruments before interpreting how the comet is structured and what kinds of layers it may have within its core.

Mission scientists are still hoping that Philae may provide data to answer these questions. In a last-ditch effort to extend its life, the team rotated Philae on November 14 to align a solar panel with the sun. That didn’t recharge the robot’s batteries. But the team is confident that as Philae rides on 67P into the inner solar system, the lander will eventually receive enough sunlight to recharge its batteries and possibly even restart its exploration. And the Rosetta orbiter is still studying 67P and will remain with the comet until December 2015. Already, according to one report, Rosetta has found unusual levels of heavy hydrogen, indicating that primordial comets didn’t provide the Earth’s oceans with water.