January 30, 2015

Philae lander – waiting for a sign of life

The Lander Control Center (LCC) at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is quiet. While the Philae lander is hibernating on the surface of Comet 67P/Churyumov-Gerasimenko, the control room team are able to take a break. Philae' s battery finally ran out at 01:36 CET on 15 November 2014, following a triple landing and more than 56 hours of scientific work. Now, the team is waiting for the lander to generate sufficient energy to 'phone home' as the comet continues its journey towards the Sun. "The greatest likelihood of making contact with Philae is in May," says Philae Project Manager Stephan Ulamec from DLR. "But we will start looking to see whether Philae has been exposed to enough sunlight and been able to acquire sufficient energy as early as the end of March." The temperature at Philae’s icy location is likely to be more favourable by then. Unfortunately, the scientists have not been able to pinpoint the lander’s precise location, and so far it has not been found on any of the images acquired by the Rosetta orbiter’s Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). The flight manoeuvres to be undertaken in the near future are unlikely to change this situation.

Multiple landings

Philae touched down on Comet 67P right on time at 16:34 CET on 12 November 2014, after slowly descending towards the surface of the comet from an altitude of about 22 kilometres. But the data received at the LCC quickly showed the DLR team that Philae was on the move again. "The output of the solar panels installed on the outside of the lander showed substantial fluctuations in the energy curves – which meant that Philae was still rotating and could not be standing firmly on the ground," explains Ulamec. Scientists on the various teams evaluated the data, including measurements made by the Multi-Purpose Sensors for Surface and Subsurface Science (MUPUS) and the Rosetta Lander Magnetometer and Plasma Monitor (ROMAP) , and were finally able to confirm that the lander was no longer on the surface of the comet and was indeed in flight again. This meant that Philae, at a distance of 500 million kilometres from Earth, had alighted on its carefully selected landing site, Agilkia, only briefly before departing again.

Shadowed position near a crater rim

Philae did not reach its final landing site until 18:32 CET, after another brief touchdown. Since then, it has been determined that the lander is located in a shadowed position close to the rim of a crater on the 'head' of the comet, adjacent to its equator. Philae's final landing site has now been named after the ancient Egyptian city of Abydos. Originally, the Lander Steering Committee had picked 'Agilkia' from the suggestions submitted during an international competition held by DLR, the European Space Agency (ESA) the French space agency (Centre National d'Études Spatiales; CNES) and the Italian space agency (Agenzia Spaziale Italiana; ASI) to name the landing site. At that time, Abydos came in second. Now, though, the multiple landings have promoted this proposal to first place. Abydos is not known merely as an ancient necropolis, but also for its unusual hieroglyphics, which prompted a spate of entertaining speculation; an inscription from the city shows characters that resemble a helicopter and a submarine.

A needle in a haystack

The Comet Nucleus Sounding Experiment by Radio wave Transmission (CONSERT) instrument allowed the scientists to narrow down Philae’s final location to an area of around 350 by 30 metres, but it was not possible to identify its exact position. It is still not possible to see the lander on the images acquired by OSIRIS from distances of 20 and 30 kilometres. The scientists trained the camera repeatedly on the area of the landing site, especially during the comet daytime, during which the landing craft was exposed to 1.3 hours of sunlight. However, Philae, no bigger than a washing machine, would have a diameter of just three pixels in these images – turning their efforts into a search for a needle in a haystack.

The Rosetta orbiter will pass over the comet at a distance of just six kilometres on 14 February 2015, but its planned path will take it to the opposite side from the lander. There are no plans to change the orbiter trajectory to search for Philae. Instead, the scientists will re-orient OSIRIS when the landing site comes into view; re-orienting OSIRIS will not interrupt the rest of the science programme.

Energy boost for communication

The team at the DLR LCC does not expect the first opportunity to contact with Philae to come until the end of March. The lander requires additional energy, which it can only generate as 67P/Churyumov-Gerasimenko approaches the Sun. The orbiter must also pass over Abydos within exactly the right window of opportunity to enable communication with Philae. Furthermore, the communication unit installed in the orbiter, designed to make contact with the lander, will not be activated until these conditions are satisfied. If Philae does wake up, it is programmed to listen for the orbiter and to transmit a signal at regular intervals. The lander requires 17 watts to wake up and transmit a sign of life.

The lander's electronic systems require an operating temperature higher than minus 40 degrees Celsius to function. "We hope that, until then, Philae will survive its hibernation at low temperatures," says Ulamec. At its original landing site, Philae would have been illuminated by 6.5 hours of sunlight per comet day, allowing for its battery to recharge much more quickly. But it would most likely have overheated by March and would have become inoperable. Now, though, proximity to the Sun is good for Philae at its shadowy location, as the intensity of solar radiation is increasing and the temperatures are more amenable to operation, allowing the solar cells fitted to the lander to generate a greater amount of power.

Waking up in several stages

The mission team will still be called on to exercise patience, even if Philae does respond. After the initial awakening, it could take several weeks for the lander to generate sufficient power to execute the first commands received from the LCC and then start charging its battery. By summer, Philae will probably have stored sufficient energy to use its instruments for several hours. Naturally, though, the length of time for which this is possible depends on how much energy the experiments require. The lander's team of engineers and scientists is already busy discussing and planning which instruments to use for which measurements. Temperature measurements, image acquisition or the Cometary Sampling and Composition (COSAC) instrument in sniffer mode do not use that much energy, while deploying the drill needs a lot more.

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Manuela Braun

Editor HR
German Aerospace Center (DLR)
Central HR Marketing
Münchener Straße 20, 82234 Weßling

Stephan Ulamec

MMX rover scientific manager
German Aerospace Center (DLR)
Space Operations and Astronaut Training
Microgravity User Support Center (MUSC)
Münchener Straße 20, 82234 Weßling