PARIS — European Space Agency (ESA) officials said June 15 that their $1.4 billion Rosetta comet-chaser spacecraft has been put into a slow spin and switched off for a 2.5-year hibernation period that they hope will give them enough time to overcome attitude-control and propulsion system problems.
Mission managers said the hibernation will permit Rosetta to rest its four reaction wheels, two of which have shown signs of degradation. The satellite needs three to function, and one of the two problem wheels will be used only as a spare when the satellite is awakened in January 2014 in preparation for its approach to a comet.
Paolo Ferri, head of the planetary missions division at ESA’s Esoc space operations center in Darmstadt, Germany, said Rosetta operations teams have the luxury of the 2.5-year hibernation to develop software to operate the satellite with three and, if necessary, just two reaction wheels.
“Obviously if we had to operate the satellite and work around this problem at the same time, we would have a more serious situation,” Ferri said in an interview. “The hibernation period means we do not have to rush to a solution.”
The propulsion problem aboard Rosetta is in the form of a leak in a helium-pressurization system that enables the propellant reservoir to direct fuel to the probe’s on-board thruster engines.
Ferri said ESA had planned to repressurize Rosetta for future operations, allowing the satellite to maximize fuel efficiency. They have now decided against that because of the risk of aggravating the leak. The resulting operations will mean Rosetta will use more fuel than it would otherwise and will fly a less-efficient route. But Ferri said the consensus is that it will still have enough fuel to complete its comet rendezvous in mid-2014.
Rosetta was launched in March 2004 for a planned rendezvous with the 67/P Churyumov-Gerasimenko comet. At the furthest point in its mission Rosetta will be some 790 million kilometers from the sun, meaning conventional solar panels would be incapable of delivering the necessary power.
With ESA governments still hesitant to develop nuclear-powered satellites, Rosetta is fitted with two 14-meter-long solar arrays. Already at the limit of what they can do owing to their distance from the sun, the arrays as of June 8 — the day the satellite was put into hibernation — are now even more challenged because Rosetta has been put into a slow spin to steady its course, turning over on itself every 90 seconds. The satellite has been oriented to maximize solar exposure to provide some 500 watts of power — just enough to keep the thermal-control system and the on-board computer functioning.
Rosetta’s 2.2-meter-diameter high-gain communications antenna is oriented toward the Earth for just four seconds of each rotation, giving ground controllers limited data on the spacecraft’s health. It takes about an hour for the signal to make the round trip between Rosetta Earth.
Ferri said the Rosetta operations team had a few tense moments June 8 as they set the satellite into spin mode and then waited to see how it reacted. It may be the first time a three-axis-stabilized satellite with such large solar arrays, which are not designed to be spun, has performed such a maneuver.
During a series of presentations at Esoc on June 15 to celebrate both Rosetta’s successful hibernation and the 25 years since Europe’s Giotto satellite performed a close encounter with the Comet Halley, former ESA officials said both missions are demonstrations of what Europe can do on its own in space science.
Roger M. Bonnet, ESA’s former science director and now director of the Berne, Switzerland-based International Space Science Institute, said Giotto was originally intended to launch aboard the U.S. space shuttle as part of a larger ESA-NASA mission.
NASA bowed out of the program due to budget issues — not unlike what the U.S. agency did earlier this year with respect to a planned collaboration with ESA on a future science mission. ESA is still trying to figure out how to recover the mission.
“We had been totally dependent on the U.S. for the launch,” Bonnet said. “ESA reacted, with anger and vision,” to NASA’s decision by deciding to launch Giotto aboard a European-built Ariane rocket, which was only just debuting when Giotto was in development. “We went from being a junior partner to being a full partner,” he said.
David Southwood, who succeeded Bonnet as ESA’s science director and retired in May, said Rosetta is an example of ESA matching its ambition with its available budget. “Rosetta was a compromise given the budget so that we could do it on our own if necessary,” Southwood said. The lesson, he said is that the agency should “take on a target, but a target you know you can hit.”
Attached to Rosetta is the Philae lander, which will be propelled to the comet and land on it with the help of a spear-type attachment to keep it in place on the surface in one of the mission’s most challenging goals.
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