PARIS — The European Philae comet lander’s accidental touchdown location nearly froze it to death during seven months of cometary winter but now will keep it functioning as the comet moves to its closest distance from the sun in mid-August, European space scientists said June 17.
If Philae’s November landing had been in the exposed area where scientists wanted it, the probe would have operated longer before its batteries shut down but it would not have survived beyond March given the rise in temperature, they said.
The lander’s reawakening was first measured June 13 with some 85 seconds of communications, with another four minutes of intermittent links June 14.
Barbara Cozzoni, a member of the German Aerospace Center’s lander control center in Cologne, Germany, said the data suggest that Philae had been awake for several days before June 13 but had been unable to communicate with its Rosetta mothership in orbit around the comet.
In a briefing during the Paris Air Show, Cozzoni and other Rosetta and Philae managers said one of their top priorities now is to increase the duration and predictability of communications links between Rosetta and Philae.
Elsa Montagnon, deputy Rosetta flight director at the European Space Agency’s European Space Operations Centre (ESOC) in Darmstadt, Germany, said Rosetta’s orbit around Comet 67P would be lowered to around 180 kilometers to secure better communications.
Rosetta had come as close as 8 kilometers from Comet 67P’s surface in February, with a 15-kilometer pass in March, before retreating to a safer distance as cometary dust risked blinding the satellite’s star trackers and forcing Rosetta into safe mode.
Rosetta and Philae are accompanying Comet 67P on its voyage toward the sun, a 6.5-year orbit that will bring it to a point between Earth and Mars on Aug. 13, its closest distance from the sun.
The trick for ESOC Rosetta flight teams is to get as close as possible to communicate with Philae and to continue attempts to identify exactly where it is on Comet 67P.
But as the comet approaches the sun it is throwing off greater quantities of dust that could blind the star trackers, which are essential navigation aides for Rosetta. “Imagine driving in a snowstorm,” Montagnon said. “You don’t see very much, and it’s not very safe.”
Montagnon said Rosetta’s flight team decided on June 15, after Philae’s awakening, to permit Rosetta to close to within 180 kilometers. But as soon as outgassing becomes an issue, she said, the orbiter will again be moved back to a safer distance.
Cozzoni said the telemetry sent from Philae as of June 17 suggests the lander is in good shape, with its software performing well and its solar arrays collecting sufficient power to keep internal temperatures sufficiently high.
“The minimal temperature [for operations] is minus 45 degrees Celsius. We are now at minus 36 degrees, so we don’t need to spend time warming up,” Cozzoni said. “But we need much more telemetry to assess the lander’s status and understand the day/night cycle duration. We need to know how long the day is.”
Philae teams hope Rosetta will be able to establish predictable communications intervals with Philae. The lander’s software senses the approach of night by the fall in temperature, but planning science operations will require that ground teams have an understanding of how much sunlight is available at a given time.
Philae’s secondary battery will also need to be recharged, Cozzoni said.
Philippe Gaudon, Philae project manager at the French space agency, CNES, said eight of Philae’s 10 instruments functioned before the November shutdown. The two that did not — the SD2 drill and the Alpha Proton X-ray Spectrometer to analyze the comet’s surface — are high priority for the coming Philae operations campaign.
Gaudon said Philae operations would begin with the least power-hungry and those that require the least mechanical movement. As the weeks go on, the spectrometer and drill will be deployed.
Jean-Pierre Bibring, lead Philae scientist from the Institute of Space Astrophysics in Orsay, France, said he was “amazed” at how much Comet 67P seemed to be “cooperating” with the Philae mission.
During the seven-month forced hibernation, he said, Philae’s temperature dropped to as low as minus 150 degrees Celsius. “We weren’t sure if it would survive,” Bibring said, “and we still don’t know about all the instruments.”
He said the intended landing site would see temperatures above 40 degrees Celsius and that in these conditions the lander “would be dead by March.”
Bibring noted that Comet 67P appears to contain much less ice than had been predicted of comets in general, and that the seven months of inactivity had apparently not covered Philae’s four solar panels with dust. “We’re getting more than 25 watts of power on the panels,” he said. “We are now getting several tens of seconds, and up to a few minutes, of communications with Rosetta. We want tens of minutes of connection.”
The Philae-Rosetta mission is budgeted to the end of the year. Montagnon said ESA’s science division has been asked to approve continued operations through September 2016. After that, she said, Rosetta could join Philae on the comet’s surface in what would be a fitting end to the mission.
