Goodbye, Philae

Philae, the European Space Agency's comet lander, made history and melted hearts when it touched down on Comet 67P/Churyumov-Gerasimenko as part of the Rosetta mission in 2014. It didn't just bring us the closest we'd ever come to a comet; it landed right on it.

In all likelihood, Philae is now at permanent rest. Nothing has been heard from the lander since July last year, and last-ditch attempts to communicate with it earlier this month yielded no response. When you're a comet lander, inability to communicate is pretty much as close as you get to death.

"I would say the chances to hear something from Philae are getting very, very low at the moment," Philae project manager Stephan Ulamec from the German Aerospace Center told me. "This is mainly due to the fact that the situation gets worse every day—the comet is moving away from the Sun so the situation is not improving—and if we do not hear something today, there is little reason why we should hear something tomorrow."

ESA puts Philae's likely inevitable demise at the end of January, by which point its comet steed will be too far from the Sun's heat to accommodate the lander's electronic components. If it's not already done for, it'll freeze into silence.

Exasperatingly, it's impossible to know Philae's current status. Matt Taylor, the Rosetta mission's project scientist, described it as "Schrödinger's lander." Like Erwin Schrödinger's paradoxical cat, it's impossible to know whether the lander is alive or dead now that we've stopped receiving signals.

"We just don't know what state it's in, because we haven't communicated with it," he said. "It could be that it's perfectly healthy and for some reason the communications aren't going through… Or, the lander is not functioning and that's why we haven't communicated with it."

Philae has had a rough ride from the moment it touched down on 67P. Ulamec, who has been working on Philae since the first proposals were being drawn together over 20 years ago, remembers initial concepts for a surface payload ranging from wild ideas like a giant corkscrew to much more modest surface packages. As more institutions got involved and put forward instruments and experiments for the lander, it evolved into the 100kg fridge-sized Rosetta lander and was named Philae after the obelisk that helped decode the Rosetta Stone.

Launched onboard the Rosetta orbiter in 2004, Philae had its starring moment on 12 November 2014, when it made its descent to the comet surface. Laurence O'Rourke, ESA's Rosetta science operations manager and Philae lander engineer, said the hardest part of preparing to land was the inability to choose a landing spot—they didn't know what 67P looked like before Rosetta got up there to check it out.

"We knew nothing more about the comet's surface than the fact it was more or less about four kilometres in size, but we hoped there were some flat areas," said O'Rourke. "You have no information about it."

With only months left before the landing, the scientists got a surprise: the comet was, in Ulamec's words, "a rubber ducky and not a potato." Rather than one ovoid rock, 67p had two lobes, the result of two objects colliding billions of years ago.

They selected a landing site, Agilkia, on the dusty plain of the ducky's head. "We had a few months to go from not knowing what this thing looks like to trying to land a fridge onto it," said Taylor.

And land they did, with Philae touching down within 100 metres of the selected site after its 20 km flight. But there was a problem: the harpoons that were meant to fire and anchor the lander to the comet malfunctioned, and the lander bounced—twice—before coming to a stop in an unplanned, rockier location known as Abydos.

"The harpoons did not fire, so that meant we weren't able to arrest or stick ourselves to the surface of the comet and we bounced about a kilometre across the surface and ended up actually in a region we would never have dreamed to have been in," said Taylor.

O'Rourke said the harpoons had shown problems before when they were tested after spending time in a vacuum chamber. "When they were tested, we had an issue with firing the harpoons," he said. So the engineers changed the way they were fired. "What happens? They didn't fire," said O'Rourke. "Was there anything we could do about it? Well, we did all we could; it's just he way it is."

In those few hours Philae was bouncing, O'Rourke, who was at the lander control center, said he wasn't worried. "There's an expression, 'Where there's life there's hope,'" he said. "While we were getting the signal from the lander, everything was fine in principle."

Bumpy landing or not, there was no questioning the historic moment: for the first time, a spacecraft had successfully landed on a comet. The news met with the kind of media frenzy usually reserved for human spaceflight.

Both Taylor and O'Rourke offer a glass-half-full take on the lander's unexpected relocation. "We ended up somewhere much more interesting," said Taylor, noting that the environment was less dusty than the planned site, giving better access to the materials just under the comet surface. "And also, because we did this traverse across the surface of the comet, we did more science than we would have been able to if we had stayed in one place."

Despite its precarious position, the lander was able to collect a ton of data in its first science sequence over two and a half days. While Rosetta observes the comet from orbit, Philae was intended to get a ground zero view by "scratching and sniffing" up close.

Taylor gave a few examples of observations Philae made: The Cometary Sampling and Composition (COSAC) instrument analysed samples of the comet's materials and found four organic compounds never before detected on a comet; the Comet Infrared and Visible Analyser (CIVA) panoramic camera imaged 67P's cracked surface up close, revealing how gas could escape from the comet to generate the outer coma; and a hammer tool that made up part of the Multi Purpose Sensors for Surface and Subsurface Science (MUPUS) instrument found that the surface at the lander's new location was significantly harder than expected for the landing. "It may have been that the harpoons wouldn't have gone through anyway," said Taylor, conceding that he liked to see the optimistic side of events.

The only instrument that didn't function properly was the drill. Researchers left this until last, concerned that it could dislodge Philae from its still largely unknown perch and tip it over. The drill deployed fine, but did not manage to collect a sample. Nevertheless, the science sequence was overall a resounding success: it collected 80 percent of the planned data.

But Philae's detour took its toll. The onboard battery was only designed to last for the first science sequence, after which the scientists hoped the lander's solar arrays would provide more power. Wedged next to a cliff, however, Philae wasn't in a good position for sunbathing. Three days after landing, it fell silent.

The way the researchers tell it, the mission was always mainly about the first science sequence, with any extra time granted by the solar panels a hoped-for bonus. After Philae went dark, they focused their efforts on trying to regain contact and at least recoup some of the data Philae hadn't been able to transmit before hibernating.

A window of opportunity appeared to open on 13 June last year: over 85 seconds, Philae sent signals via Rosetta to the ESA operations centre. "Our teams, mainly in the control centre here, were working out messages or command sequences that could help to establish the link," said Ulamec.

Philae made contact a total of eight times, the last of which was on 9 July—but the communication phased in and out.

O'Rourke said that in the last contact, the data revealed Philae was having some problems with its communications equipment. Philae has two receivers to input commands and two transmitters to respond. "On the 9 July, we saw that one of the receivers had died, or at least was no longer working, and one of the transmitters also had died," he said. "So you're on the redundant components, you're on the backup—but also we saw on the 9 July that the other transmitter, the one that was still working, had a short and had a problem."

Ulamec's team attempted to correspond with the lander without any feedback. "We tried blind commanding, called TC backup mode, where we send commands into the blind without having two-way communications, hoping the lander could receive them, do something, then when there is the next chance for a telemetry link we would get better data or we would enable the lander to send data," he explained.

The efforts didn't pay off; the eight packages Philae sent back during its brief revival all contained housekeeping data rather than more science observations.

Why Philae couldn't reconnect remains a mystery, but one likely scenario is that it simply got too cold already, tucked away in the shade as it was. "When it fell asleep in November 2014 we were able to see the outside temperature and we could see the temperature was pretty cold, over -100 degrees [Celsius]," said O'Rourke. The temperature of the lander would have slowly decreased down to around -100 or -150 before heating up in the sunlight, resulting in "thermal cycling" from cold to hot. O'Rourke suggests this could have put too much pressure on electronic components in the lander. "They're designed to do that, but not continuously for many many days," he said.

After the brief contact in July, comet 67P continued to move closer to the Sun, which meant Rosetta had to put more distance between itself and the comet surface owing to the risk posed by increased activity as the icy material warmed up to give off greater plumes of gas and dust. This would make communication between it and the lander more difficult. Since then, nothing has been heard from Philae.

"We don't know exactly what it is," said Taylor. "It is frustrating, and it was frustrating in July."

Rosetta is now closer to Philae than it was when signals were heard then, so the fact that it hasn't picked anything up doesn't bode well. "This is one of the things about doing deep space robotic exploration," Taylor added. "You don't know, because you can't go up there and have a tinker with it."

If Philae didn't get too cold then, and hasn't since, it probably will soon. But the operators are continuing to listen out just in case. "I have to say that while it is true that, from a simulations perspective, we believe Philae doesn't have much of a life after January, we haven't given up hope at all," said O'Rourke. "Philae has never stopped surprising us, and I would not be in the slightest bit surprised that we get a signal from Philae in February." He'd draw the line at March or April.

And Philae still has one major role to play in its slumber. The lander is just a small part of the overall Rosetta mission, and in summer the Rosetta orbiter will get closer to the comet surface—close enough to see Philae for itself.

These images, as well as perhaps clearing up some of the mysteries of Philae's position, will add value to the scientific data already collected; knowing exactly where the measurements were made will add crucial context. "Once you have the context of the wider area around it, with more precision, you can do better science—or more science," said Taylor.

After taking these images, Rosetta will finish its mission by touching down on the comet surface itself. Exact plans for this impromptu kamikaze finale haven't been confirmed, but Taylor said the science team recognised there'd be a certain poetry to bringing the orbiter down in the same area as the lander, effectively reuniting Rosetta and Philae in the spacecraft afterlife.

One of the more visible human characters on the mission, Taylor described the overall experience as "draining."

"It's been highly exciting and stimulating, but it's been a lot more stressful and a lot more draining than I could have imagined when I said, 'Yes, I will do the job,'" he said. "It's something I don't think I will experience again in my career in my life, because I don't think there's anything like Rosetta in existence. In a way, that's a relief for me personally."