Trapped in a forbidding jumble of sun-blocking cliffs and rocky debris, the Philae comet lander, its batteries nearly depleted, somehow managed to contact the Rosetta mothership Friday in true cliffhanger fashion, relaying stored science data back to Earth and receiving commands to turn in place in a last-ditch bid to bring a larger set of solar cells into sunlight.
It was not immediately clear whether the maneuver could forestall a shutdown, allowing the lander to continue its historic observations of Comet 67P/Churyumov-Gerasimenko, or whether it would simply improve the chances the spacecraft might "wake up" down the road when the comet gets closer to the sun and the light falling on the solar cells intensifies.
Either way, engineers at the European Space Operations Center in Darmstadt, Germany, were relieved Philae had survived another day caught between a rock and a dark place, as it were, and at least had a chance to bring stored data back to Earth, possibly including the results of an attempt to drill into the crust of the nucleus to collect pristine material for analysis.
"Rather intense activities in ESOC's Main Control Room tonight," the European Space Agency said in an understated Tweet. "We're talking with a comet."
The lander was dutifully executing commands uplinked earlier in the day when the Rosetta orbiter, serving as a relay between Philae and the flight control team in Germany, moved out of view in its convoluted path around Comet 67P/Churyumov-Gerasimenko. Engineers were unsure if the lander would have enough stored power to re-establish contact during the next opportunity, but right on schedule, just before 5:30 p.m. EST (GMT-5), Rosetta locked onto the lander's signal.
Commands had already been uplinked to Rosetta ordering the lander to lift itself slightly on its three landing legs and to rotate its body, Lazy Susan fashion, to bring a larger set of solar cells into the small amount of sunlight reaching the spacecraft. The maneuver could help Philae extend its lifetime, although engineers were not optimistic earlier in the day.
"For the short term, I'm afraid there's little hope," Stephan Ulamec, lander operations director, said earlier in the day. "For long term, we have one solar generator that is illuminated every cometary day, (for a) short (period) but it is illuminated. So there is some hope that at some stage when we're closer to the sun that Philae wakes up again and talks to us. But we need to be very lucky that this happens."
Rosetta released Philae early Wednesday for a seven-hour descent to the surface of 67P/Churyumov-Gerasimenko. The lander was equipped with harpoons and ice screws to anchor itself to the nucleus at the moment of touchdown, but the systems failed to operate and Philae bounced back up into space, reaching an altitude of more than 3,000 feet before coming back down again two hours later.
The lander then bounced away again, finally coming to rest more than half a mile from its original touchdown point. Or so the telemetry indicates. Engineers do not yet know precisely where the spacecraft ended up, whether the initial two-hour bounce was mostly vertical and how that played in with the comet's 12.4-hour rotation.
Wherever it is, frames from a panorama taken after it came to rest revealed nearby cliffs and a chaotic jumble of dark rock-like debris casting long shadows and allowing only minimal sunlight to reach Philae's solar cells.
"We are not toppled over," Valentina Lommatsch said from the lander control center. "It looks like we're kind of surrounded by rocks. ... But pictures show all three legs on the ground, and I can confirm from the solar data we have not moved at all since the first set of panorama images after the third landing. So we are on the ground, we just have really (been) unlucky in a corner surrounded by rocks."
Philae was launched with 50 to 60 hours of charge in its primary and secondary batteries. By Friday morning, most of that was depleted.
In the lander's original orientation sunlight reached the solar panels during a roughly 90-minute period each 12.4-hour "day." For most of that time, less than 1 watt is available, but power output climbs to 3 or 4 watts for about 20 minutes.
"The lander needs 5.1 watts to boot, so we have to at least get that," Lommatsch said. "After we have that, in order to charge the batteries we have to heat it up to zero degrees Celsius. In the simulations that we've run, that would mean that we'd need about 50 to 60 watt-hours a day in order to reach zero degrees and still have some of the daylight left to charge the battery. So it doesn't look that great."
But even if Philae batteries drain and the spacecraft drops off line, all may not be lost. Echoing Ulamec, Lommatsch said as 67P/Churyumov-Gerasimenko moves closer to the sun and warms up, the amount of sunlight hitting the spacecraft will intensify enough to possibly rouse the lander from its slumber.
But it would take a considerable amount of luck. Rosetta would need go be within line of sight and the lander would need enough power to drive is computer and radio gear.
"Having a link would need additional power again, so we'd have to have something in the battery in order to have a link or be extremely lucky that Rosetta is looking for us in the moment that sunlight reaches the solar panels," Lommatsch said.
While "it looks a bit bad," she said, "we can only hope that as we approach the sun, maybe in August if we don't have too much dust or a huge coma blocking the sun, perhaps there would be a chance that at some point we could come back and at least see how the lander's doing. "So cross your fingers, or press your thumbs if you're German, perhaps we'll hear something from the lander again."