The last prolonged silence had already indicated that contact with the Philae lander will be increasingly unlikely, and the conditions on Comet 67P/Churyumov-Gerasimenko have become more hostile. "Unfortunately, the probability of Philae re-establishing contact with our team at the DLR Lander Control Center (LCC) is almost zero, and we will no longer be sending any commands; it would be very surprising if we received a signal now," said Stephan Ulamec Philae Project Manager of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). For Philae, this means that, although ice-free, it is probably covered with dust in its shaded location on the comet and will go into permanent hibernation – no longer able to activate its systems in its cold environment. The European Space Agency's Rosetta spacecraft will continue to orbit the comet and carry out measurements with its scientific instruments until September 2016. The communication unit on board Rosetta will not be switched off yet – it will continue to listen for signals from the lander in the coming months until it will be switched off due to power constraints.
"The Philae mission was one-of-a-kind – it was not only the first time that a lander was ever placed on a comet's surface, but we also received fascinating data," says Pascale Ehrenfreund, Chair of the DLR Executive Board and a participating scientist on the mission. "Rosetta and Philae have shown how aerospace research can expand humankind’s horizon and make the public a part of what we do."
A world-renowned lander
On 12 November 2014, Philae accomplished its spectacular comet landing, despite a harpoon system that did not function after the 10-year journey through space. The lander endured multiple 'hops' on the comet's surface, and arrived at a final location that nobody on the team had foreseen. Across the globe, people followed its progress, curious to see whether a never before attempted landing on a comet would be successful. Finally, at 18:31 CET, DLR engineers and scientists were able to announce that Philae was on the surface of Comet 67P, 510 million kilometres from Earth, and was communicating with its ground stations. Google's search engine dedicated its start-up screen to the lander, and instead of the second letter O in its logo, depicted Philae extending its three legs. Newspapers from Africa to South America, from the United States to Asia and Australia, reported on the first comet landing. News in multiple languages informed that Philae had indeed accomplished its goal. Meanwhile, the team at the DLR LCC in Cologne worked around the clock to adapt their carefully prepared plans to the new situation, and to work with Philae in its unexpected location. "I had expected some interest," says Ulamec, "but this world-wide, immense and continued enthusiasm surprised me in an extremely positive way."
Hibernation at low temperatures
Scientists carried out over 60 hours of research with Philae's instruments, acquiring images, sensing molecules and attempting to hammer the unexpectedly hard surface. With its primary batteries, the lander was able to work at this site with minimal sunlight. Philae was able to send all the data it acquired to Earth. After reaching the point nearest to the Sun on 13 August 2015, the comet, Rosetta and Philae started making their way out of the interior of the Solar System: "Now, Comet 67P/Churyumov-Gerasimenko is over 350 million kilometres from the Sun," explained Ekkehard Kührt, a planetary scientist at DLR and responsible for DLR's scientific contribution to the Rosetta mission. "Night temperatures on the comet can fall below minus 180 degrees Celsius. Even during the day, the entire comet remains frozen." For a lander that is designed for temperatures as low as minus 50 degrees Celsius, this is an environment in which it can no longer function. If it had come to rest on its original landing site and had anchored itself there, it would have had significantly more sunlight available to use as an energy supply, but would probably have overheated in March 2015 as the comet approached the Sun.
Contact difficulties with breaks in communication
"Philae will, most likely, no longer transmit any signals due to the fact that it does not have sufficient power and the electronics are too cold," says Ulamec. In the last few months, there have been no radio signals from Philae. Its silence in August 2015, however, was due to a different reason. While the comet was at the point nearest the Sun, the Rosetta spacecraft was too far away to be able to receive signals from the lander and forward them to Earth. "There were also times last year when we did not understand why Philae had made no contact with us." Philae made contact on 13 June 2015 and sent data on its 'health'. Overall, contact with the ground team was established seven more times, but these remained erratic and unpredictable. On 9 July 2015, it sent information for the last time. "We repeatedly sent various commands to try to stabilise contact with Philae and conduct measurements with its instruments, but unfortunately it was not possible." The project engineers believe that the reason for the irregular contact and subsequent silence could have been a failure in the lander's transmitter.
Positive results for a premiere
With the Philae lander, engineers and scientists have drawn a mostly positive experience overall. "Although some measurements could not be carried out, overall, Philae was a success," Kührt stressed. "We ended up in an unknown environment and for the first time ever, gathered scientific data from a comet's surface, which we were able to complement with measurements from the orbiter." The Rosetta mission has demonstrated that cometary activity is significantly more complex than previously thought. "We have acquired a great deal of new information, but we are still far from a final understanding."
Even if the work with Philae was not all completed, for example, the chemical analysis of a soil sample or needing more time for scientific measurements: "We will not get such high-resolution and spectacular images as those that were acquired by the ROLIS camera underneath the lander and from the panoramic CIVA camera for a long time." In addition, the mass spectrometer found organic molecules on the surface and the MUPUS thermal probe and SESAME seismometer were able to determine the physical properties of the comet's surface. The comet nucleus was examined using radio signals transmitted from the lander to the orbiter, which provided information about the comet's structure. The comet did not have a measurable magnetic field. Meanwhile, many of the results have been published in scientific journals. "The analysis of the data will continue for several years," emphasised Kührt.
Knowledge for future missions
Several firsts in space were celebrated with the Rosetta mission. Never before had a spacecraft accompanied a comet on its path around the Sun; never before has a robot landed on a comet's surface and carried out measurements there. "If you are looking for a comparison with other historic missions, these would be the Viking missions, which sent detailed images of Mars for the first time, or the Voyager probes, which allowed a glimpse of the large planets of the Solar System," says Ulamec. The Philae landing was a useful lesson: "We can better adapt future missions to conditions on a comet."
The last images of Philae will probably be acquired in the summer of 2016, when the Rosetta spacecraft images the lander during close fly-bys. "When we see how Philae is positioned, we will be able to better interpret certain data, such as the measurements of the CONSERT radar experiment." In approximately six years, Philae and Rosetta, which will be landed on the comet in September 2016 at the end of its mission, will be closer to Earth – and Comet 67/P Churyumov-Gerasimenko will have circled the Sun once again.
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium headed by DLR, the Max Planck Institute for Solar System Research (MPS), the French Space Agency (CNES) and the Italian Space Agency (ASI).