The Electrical Support System Processor Unit on Rosetta is being switched off
After almost two years of extreme highs and devastating lows, the Philae lander has bid its final farewell.
At 10am BST on July 27, the Electrical Support System Processor Unit (ESS) on Rosetta was switched off. The ESS is used to communicate between Rosetta and its lander.
Philae has been silent since July 9 2015. In a series of heartbreaking tweets, the mission team (posing as the lander) wrote: “It’s time for me to say goodbye. Tomorrow, the unit on @ESA_Rosetta for communication with me will be switched off forever…”
Switching off the ESS is part of the preparations for Rosetta’s end of mission. By the end of July, the spacecraft will be 520 million km from the Sun and will start seeing a significant loss of power – about 4W per day.
To continue the mission’s scientific operations for as long as possible, the team took the decision to power down the ESS to reduce how much energy is used by non-essential instruments.
There has been no contact with Philae since this time last year, and in January the lander was officially classified as being in “a state of eternal hibernation.” A month later, Philae tweeted: “It’s cold & dark on #67P & chances of communicating with @ESA_Rosetta are decreasing, but I won’t give up just yet.” Scientists had hoped Philae would regain contact, and they left the ESS on just in case.
During this time Rosetta reached altitudes well below 10km from the surface of Comet 67P/Churyumov-Gerasimenko but no signal from the lander was received. Conditions on the comet have now also said to have become “hostile.”
In particular, Philae is thought to now be 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 icy environment.
“Unfortunately, the probability of Philae re-establishing contact with our team at the DLR Lander Control Centre (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 Centre.
“The Philae mission was one-of-a-kind – it was not only the first time a lander was ever placed on a comet’s surface, but we also received fascinating data,” added Pascale Ehrenfreund, chair of the DLR executive board.
“Rosetta and Philae have shown how aerospace research can expand humankind’s horizon and make the public a part of what we do.”
The decision to shut down ESS was taken by the mission manager and will be carried out by the Rosetta Mission Operations Centre, with support from the DLR Lander Control Centre and the Rosetta Science Ground Segment.
Rosetta will continue to orbit the comet and carry out measurements 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 is also switched off to conserve power.
The last images of Philae will likely be taken over the next month or as the Rosetta spacecraft photographs the lander during close fly-bys.
Philae landed on Comet 67P on November 2014 following a 10-year journey. After its harpoon system failed, the lander “hopped” several times on the surface before landing on its side in an unknown location, partly in the shade.
After reaching the point nearest to the Sun on August 13 2015, the comet, Rosetta, and solar-powered Philae began making their way out of the interior of the Solar System.
If Philae had come to rest on its original landing site and had anchored itself there, it would have had more sunlight available to use as an energy supply, but would likely overheated in March 2015 as the comet approached the Sun.
Philae last made contact on June 13 2015 and sent data on its ‘health’. Overall, contact with the ground team was established seven more times, but these were erratic and unpredictable.
It then sent its last batch of readings on July 9 2015. The project engineers believe that the reason for the irregular contact and subsequent silence could have been a failure in the lander’s transmitter.
“Although some measurements could not be carried out, overall, Philae was a success,” Ekkehard Kührt, a planetary scientist at DLR said. “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.”
During the mission, Philae sent back high-resolution images of the comet. Its 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. Meanwhile, the comet nucleus was examined using radio signals transmitted from the lander to the orbiter, which provided information about the comet’s structure.