Rosetta changes trajectory, continues search for Philae

Chuck Bednar for redOrbit.com – @BednarChuck

The ESA’s Rosetta orbiter is continuing its search for the Philae lander, despite experiencing issues during a recent flyby of Comet 67P/Churyumov–Gerasimenko that forced the spacecraft to enter safe mode and change its trajectory, the space agency has announced.

Rosetta, which entered orbit around Comet 67P/C-G last September, is now once again using some of its science instruments, and more are expected to be re-activated before the end of the week. It was forced to enter safe mode after experiencing “significant difficulties in navigation” that caused its large solar arrays to be exposed to too much drag.

Rosetta getting confused

On March 28, the ESA spacecraft performed a 16 km flyby (about 14 km from the surface) over the comet’s large lobe, and its star trackers which are used for navigation began to mistake comet debris for stars. In addition, its high gain antenna began to drift away from pointing at the Earth, impacting communications and forcing Rosetta to be put into safe mode.

The agency said that the orbiter has been successfully recovered, and had started using some of its science instruments again, but had been moved into an “escape trajectory” that took it roughly 400km from Comet 67P/C-G. An orbital correction maneuver was performed on April 1 to bring it back, followed by a second that allowed it to reach its target distance of 140km on April 8.

Due to those navigational difficulties, ESA officials said that they plan to exercise caution while bringing Rosetta closer to the comet. Specifically, they said that they will have to assess how the star trackers behave in the environment of the increasingly active comet, and whether or not the earlier confusion by comet particles responsible for those issues can be avoided.

Rosetta spacecraft operations manager Sylvain Lodiot said that the problems have required “a complete replanning of the upcoming flyby trajectories,” and that the team plans to move first to “a terminator orbit” of 140km before closing to a distance of 100km. Then they will start flying “pyramid” trajectories, monitoring the spacecraft’s performance before moving closer.

“We’re now assessing the impact of the new trajectory scheme on the planned science observations for the months ahead, including those which anticipated close flybys,” added Rosetta project scientist Matt Taylor. “Our science operations team… is extremely busy working with the instrument teams to optimize science observations and associated spacecraft pointing for this new scheme. As we move forward, we will analyze what can be modified and improved in order to maximize science return within the capabilities of the spacecraft.”

The search is still on for Philae

Despite those changes to Rosetta’s trajectory, the orbiter’s search for the Philae lander will not be affected, according to the ESA. A new listening window for the lander will begin on April 12, and Dr. Stephan Ulamec of the German Aerospace Center (DLR) said that the communication unit on Rosetta will be turned on “around the clock” during this period.

Dr. Ulamec said that Philae “most likely” will not wake up until May or June, but added that the agency scientists “don’t want to miss the moment” in case it has enough energy and an operating temperature that is high enough to reactivate earlier. In order to wake up and listen for signals from Rosetta, the lander must have at least 5.5 watts of power and an operating temperature of at least -45 degrees Celsius, and it needs 19 watts of energy to respond.

“As we did already last month, we will send blind commands to the lander that will help it make optimum use of its energy for heating and communication,” he added. If Philae is already awake, it may be able to receive and run commands, even if it lacks the power to report that it has done so. Once it can transmit data again, it will report on its health, its temperature, and the amount of energy that its solar panels are receiving, according to the ESA’s report.

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Source: http://www.redorbit.com/news/space/1113371652/rosetta-changes-trajectory-continues-search-for-philae-041515/