Philae was more than 500 million kilometres from Earth when it touched down on Comet 67P/Churyumov-Gerasimenko one year ago, on 12 November 2014. "We looked after and planned this mission for almost 20 years and launched the Rosetta orbiter and Philae lander on their journey through space – so landing day really was quite special," says Philae Project Manager Stephan Ulamec from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) to sum up the mood on the day. The team at the DLR Lander Control Center (LCC) worked with Philae and its 10 on-board instruments for approximately 60 hours before the power in the lander's primary battery ran out. Recordings made by the acoustic SESAME experiment now confirm that the lander has all three 'feet' on the ground at the rugged site where it landed. Now, the ESA Rosetta orbiter is closing in on the comet and its communication unit has been reactivated to 'listen' for signals from the lander. "It is difficult to predict if and when we will make contact," says Ulamec.
The comet and the lander are currently located 245 million kilometres from the Sun. "One comet day lasts 12.6 hours, so Philae is exposed to sunlight for approximately four hours per comet day," says Koen Geurts, the Technical Manager for the lander at DLR. "That is enough to power its systems." It is important to bear in mind, though, that the conditions on the comet – and therefore the likelihood that Philae will contact its team on the ground – will worsen over the coming weeks. "The comet is currently moving further away from the Sun, so the available energy is decreasing. It is also getting colder inside the lander. “Philae can cope with an internal temperature of minus 51 degrees Celsius. If the temperature falls below this mark, the lander will no longer be able to activate its systems and will cease to be operational. This is likely to happen in January 2016 at the latest, bringing Philae’s mission to an end.
The lander established contact seven times between 13 June and 9 July 2015 to report on its 'state of health', but Philae has remained silent ever since. "Actually, its memory should be full of scientific data, provided it received and executed our commands," says Geurts. The team at DLR has conducted detailed analyses of the data received from the lander, and now predicts that one of Philae's two receivers and one of its two transmitters have failed. Furthermore, one of the data packets that Philae sent contains information relating to a comet day in which the landing craft was not in contact with Earth, which appears to suggest that the second transmitter is not fully functional. "This means that the craft was in operation and had received signals from the orbiter, but did not switch on its transmitter." This would explain why contact with Philae has been sporadic and quite rare.
Philae certainly survived 13 August 2015, the day it came closest to the Sun, without overheating. This is due to the environment of its final landing location. First, it touched down at a very sunny landing site. But it bounced from there and eventually came to rest at a location shrouded in deep shadow, which protect the craft from excessive temperatures. It is likely that even during the comet’s closest approach to the Sun – a mere 186 million kilometres away – it did not heat up to more than 40 degrees Celsius, a temperature that the lander can easily withstand.
The DLR team is implementing a variety of strategies to help the lander wake up. Amongst other things, it has activated the previously unused redundant power subsystem in the hopes that it will persuade the landing craft's last, somewhat temperamental transmitter, to switch on. The team is also working on a sequence of commands that repeatedly activates and deactivates the transmitter in the hope that this will get it up and running.
The DLR engineers and scientists are ready to proceed in the event that Philae wakes up and establishes stable contact. "We will very quickly be able to take photographs and conduct measurements using the MUPUS, SESAME, Ptolemy, ROMAP and COSAC instruments as soon as the lander has sufficient energy," explains Ulamec. "These commands are already preconfigured and are stored in Philae's memory." All the same, it would take more reliable and predictable contact with Philae to acquire a soil sample – something that is greatly desired; an attempt to do this was unsuccessful in November 2014.
Reflecting on the harpoon system
The Philae team has also looked into the failure of the harpoon system designed to allow Philae to anchor itself to the comet's surface. The harpoons did not fire, so the lander lifted off again and bounced several times before settling at its current landing site, Abydos. "The filaments carrying electricity to detonate the explosive charge that fires the harpoons may have caused the malfunction," explains Ulamec. "Perhaps the explosives degraded during the long journey through space. It is also possible that none of the four filaments managed to trigger a detonation, or that they burned out before ignition could take place." It would be possible to repeat the attempt to fire the harpoons if Philae gets in touch. The anchors are fitted with temperature sensors and accelerometers that would help to acquire additional scientific data.
Listening with its feet
One thing is certain – at the final landing site, Philae has not ended up with one leg sticking up in the air. "The SESAME instrument, which has sensors fitted in all three of the lander's feet, allowed us to listen in on MUPUS attempting to hammer its penetrator into the comet – and we detected signals in each of the legs," explains DLR planetary researcher Martin Knapmeyer. The scientists use the MUPUS hammering probe as a loud sound source to analyse the surface material properties. SESAME recorded the vibrations that MUPUS produced, which passed through the comet. This is the first time since the Apollo 17 Mission in the 1970s that active seismic measurements have been conducted on a celestial body.
The seismic instrument SESAME remained active for over two hours while MUPUS hammered with a variety of different impact forces in order to penetrate the surface. It was surprising how slowly the waves spread. "The propagation speed in solid ice without bubbles is approximately two kilometres per second. But we measured a speed of roughly 100 metres per second." This may indicate that the comet material is extremely porous and somewhat loose. But MUPUS only succeeded in penetrating the subsoil by a few centimetres before encountering an unexpectedly hard surface. "This could mean that the uppermost layer is particularly hard and sintered, and that a stratum of loose material lies just beneath," says DLR planetary researcher Tilman Spohn, Principal Investigator for the MUPUS instrument.
Even if conducting all experiments exactly as planned was not possible, "the mission is certainly a major success. We managed to land on a comet nucleus for the very first time – and that is a huge leap forward," say the DLR planetary researchers. Until now, images acquired directly from the surface of a comet and thermal measurements taken on the ground were just a dream. "Fate has been kind to us in some ways – we encountered a very special surface morphology on Churyumov-Gerasimenko, one that is rich in structures such as cliffs and craters, most likely caused by outgassing. But we also found formations similar to sand dunes alongside rugged landscapes. These are really quite unexpected discoveries."