ESA Managers Face Choice on Rosetta Priorities

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PARIS — Europe’s Rosetta spacecraft, which for 12 months has been riding alongside a comet making its way toward the sun, on Aug. 13 witnessed the object’s closest solar approach from a distance far enough away to avoid being damaged by increasing amounts gas and dust being kicked off by the comet.

In a delayed effect, the comet’s surface activity is expected to continue to rise for a few weeks even as it moves away from the sun, Rosetta program managers said. The spacecraft is unlikely to be able to return for a close-in examination until mid-September, when the comet is expected to have cooled enough to reduce outgassing.

Rosetta’s near-term assignment is a subject of debate. Scientists affiliated with Rosetta’s Philae lander, last heard from on the comet’s surface July 9, would like Rosetta to move in for a fresh attempt at communicating with the 100-kilogram Philae as soon as possible.

But doing so would risk confounding Rosetta’s instruments at a time when other scientists want to study the composition of the comet’s tail, an objective that requires the probe to be about 1,000 kilometers from the comet’s nucleus.

Philae scientists say the lander likely will run out of solar power sometime in November, making it crucial for them to secure Rosetta search time as soon as the comet’s surface cooling permits.

“The month of October will be a race against the clock to re-establish contact,” said Philippe Gaudon, head of the Rosetta mission at the French space agency, CNES. In an Aug. 12 statement, Gaudon said Philae backers understand that they cannot reasonably ask Rosetta managers to risk the satellite’s health in an early close-quarters attempt at harvesting a fresh crop of Philae surface data.

Rosetta’s star trackers have already been temporarily blinded by earlier Rosetta flybys that were optimized to receive Philae communications but caused the instruments to lose their orientation as they confused the comet’s gas for stars.

Rosetta flight controllers at the European Space Agency’s ESOC operations center in Darmstadt, Germany, moved Rosetta away from the comet in the weeks leading to perihelion — or closest approach to the sun — to a distance of nearly 300 kilometers. Communications with Philae are generally considered unfeasible if Rosetta is more than 150 kilometers from the comet.

Gaudon said he also understood that making Philae a priority starting in mid-September would mean sacrificing a one-time-only chance at examining the effects of perihelion on the comet’s tail.

“The tail by then would no longer be as dense,” he said.

“We don’t want to endanger the orbiter,” Gaudon said. “We understand the requests of the other instrument teams.”

Gaudon added that one possible scenario is to have Rosetta’s navigation system rely only on the satellite’s gyroscopes during a close flyby, switching off the star trackers to avoid the risk of the probe losing its bearings.

Comet 67P’s 6.5-year orbit takes it as far away as beyond’s Jupiter orbit, and as close as between the orbits of Earth and Mars. It reaches perihelion at 185 million kilometers from the sun – further away than Earth’s 150-million-kilometer distance.

But lacking an atmosphere, the comet heats up dramatically as it nears the sun, reaching temperatures of about 80 degrees Celsius in its southern latitudes. The highest temperature recorded on Earth – in Death Valley, California – was 56.7 degrees Celsius. In the days preceding perihelion, Rosetta researchers estimated that Comet 67P was throwing off 100 kilograms per second of gas and dust as its surface temperature climbed.

Philae is designed to function at up to 50 degrees Celsius. Given its unintended landing spot – somewhere on the comet’s northern hemisphere — the lander is shaded from the worst of the heat and will not be exposed to temperatures higher than that, said Koen Geurts, Philae technical project manager at the German Aerospace Center, DLR.

Whatever the ultimate tradeoff decided between Rosetta and Philae data, Rosetta is scheduled to continue following Comet 67P until September 2016, after which it will be guided down to the comet’s surface to end the mission launched in March 2004.