Philae Mission Winds Down As Rosetta Orbiter Enters New Science Phase

PARIS - The Rosetta mission's Philae lander completed its primary science mission Nov. 15 after nearly 57 hr. on Comet 67P/Churyumov–Gerasimenko, but the robotic probe has ceased contact with ground controllers and has likely gone into hibernation.

After being out of communication visibility with Philae since 5 a.m. EST Nov. 14, Rosetta regained contact with Philae at 5:19 p.m. that night and remained strong until 7:36 p.m. Nov. 15.

During that time, the European Space Agency (ESA) says the lander returned all its so-called “housekeeping” data, as well as science data from “targeted” instruments, including ROLIS (ROsetta Lander Imaging System), COSAC (Cometary Sampling and Composition), Ptolemy, the SD2 (Sampling, Drilling and Distribution) device, and CONSERT (COmet Nucleus Sounding Experiment by Radio wave Transmission).

“This completed the measurements planned for the final block of experiments on the surface,” the agency said in a Nov. 15 blog post.

In addition, the lander’s body was lifted 4 cm and rotated 35 deg. in an attempt to put more sunlight on Philae's solar panels. But as the last science data was relayed to Earth, Philae’s power rapidly depleted.

“It has been a huge success. The whole team is delighted,” says Stephan Ulamec, lander manager at the German Aerospace Center DLR, who monitored Philae’s progress from ESA’s Space Operations Center in Darmstadt, Germany, over the past week. “Despite the unplanned series of three touchdowns, all of our instruments could be operated and now it’s time to see what we’ve got.”

ESA says images taken during Philae's Nov. 12 descent from the main Rosetta spacecraft show the surface of the comet is covered by dust and debris ranging in size from millimeters to meters. In addition, panoramic images show layered walls of harder-looking material.

Philae science teams are now studying the data to see if any of this material was sampled by SD2, which was deployed 25 cm below the lander's base plate on Nov. 13. The device is designed to analyze ice and dust grains on the surface, and dig up to 23 cm into the comet and retrieve samples for other onboard experiments to study in detail. For example, SD2 was to interface with COSAC and Ptolemy to study the geochemistry of the comet, including hunting for complex organic molecules, and ÇIVA, the micro-camera instrument, which was to image and study the composition, texture and albedo (reflectivity) of the samples collected.

“We still hope that at a later stage of the mission, perhaps when we are nearer to the Sun, we might have enough solar illumination to wake up the lander and reestablish communication, ” Ulamec says.

From now on, no contact will be possible unless sufficient sunlight falls on the solar panels to generate enough power. Because mission managers were able to rotate Philae's solar panels to afford more Sun exposure, it is possible that as the comet moves closer to the Sun, enough sunlight will fall on the solar generators to pull Philae from hibernation.

ESA says the Rosetta orbiter will continue listening for a signal from Philae each time its orbit brings it into line-of-sight visibility with the lander. However, given the low recharge current coming from the solar panels, it is unlikely that contact will be reestablished with the lander anytime soon, the agency says on its blog.

“At the end of this amazing rollercoaster week, we look back on a successful first-ever soft-landing on a comet. This was a truly historic moment for ESA and its partners,” says Fred Jansen, ESA Rosetta mission manager. “We now look forward to many more months of exciting Rosetta science and possibly a return of Philae from hibernation at some point in time.”

As Philae's science mission winds down, ESA says the Rosetta spacecraft is moving back into its pre-lander-deployment orbit at 30 km altitude from the comet's nucleus.

It will return to a 20-km orbit Dec. 6 and continue its mission to study the increasingly active body in detail as it orbits toward the Sun. ESA says Rosetta's closest encounter with our brightest star will take place Aug. 13, 2015—almost one year after Rosetta's Aug. 6 rendezvous with 67P.

In the meantime, Rosetta flight engineers will execute more “unbound” orbits around the comet and perform a series of daring flybys, including some as close as 8 km from 67P's center.

Rosetta's data will allow scientists to watch near- and long-term changes that take place on the comet, potentially helping to answer key questions regarding the history of the Solar System.