Rosetta Survives Close Encounter with Comet Debris

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PARIS — Europe’s Rosetta comet-chaser satellite has apparently survived a dangerously close flyby of Comet 67P, which it is accompanying toward the sun, despite a dazzling of its star trackers by cometary debris and a subsequent shutdown of its science instruments.

The European Space Agency on April 2 said Rosetta, which following the March 29 automatic emergency shutdown had been moved 400 kilometers away from the comet, had been cleared for a new, albeit less-close, approach to within 140 kilometers.

Comet-67P's orbit
Comet 67P/Churyumov-Gerasimenko is a Jupiter-family comet. Its 6.5 year journey around the Sun takes it from just beyond the orbit of Jupiter at its most distant, to between the orbits of Earth and Mars at its closest. The comet hails from the Kuiper Belt, but gravitational perturbations knocked it towards the Sun where interactions with Jupiter’s gravity set it on its present-day orbit. Credit: ESA

“Limited science operations will be resumed in the coming days and weeks,” ESA said, adding that a revised flight plan might be needed to assure that operations are preserved as the comet’s orbit takes it closer to the sun before moving away again after Aug. 13.

Rosetta’s prime mission now is to follow Comet 67P as closely as possible to examine the increasing amount of dust and gas it throws off as its surface temperature increases.

Its secondary mission is to watch for any signals from the Philae lander, whose batteries drained in November. Rosetta and Philae ground teams hope that as the shaded part of Philae’s landing area — far from the planned touchdown area — receives more sunlight, the lander will reawaken sometime in April.

ESA’s European Space Operations Center in Darmstadt, Germany, has been performing what may be an unprecedented series of satellite flight maneuvers to capture multiple views of Comet 67P as well as to orient Rosetta’s antennas to pick up any signs of life from Philae.

The maneuvers included a March 28 flyby that closed to within 14 kilometers from the surface of the comet’s larger lobe.

Each close encounter becomes more risky with the ever-increasing dust and gas emissions, and creates a drag effect on Rosetta’s solar arrays. The closer it approaches, the more its positioning sensors lose track of any reference points as their field of view is filled with the comet.

During the March 28 flyby the spacecraft’s star trackers mistook cometary dust particles for stars — “hundreds of false stars,” ESA said — causing Rosetta to lose its orientation and its high-gain antenna to lose its fix on Earth.

That caused a sharp drop in the radio signals received by the Rosetta ground team. The agency said the star trackers were returned to full operations nearly 24 hours later as the craft moved away from the comet, but that the false-star phenomenon was still occurring. During a reconfiguration of the navigation systems, Rosetta went into “safe mode,” which is a protective configuration that includes shutting down the science instruments.

By March 30 Rosetta was brought out of safe mode and moved to a distance of about 400 kilometers from Comet 67P. On April 1 mission control began moving Rosetta to a distance of 140 kilometers, where it was expected to be by April 8.

“Science and operations teams are currently discussing the impact of the recent navigation difficulties on the current planned trajectories, possibly resulting in further replanning in order to ensure that the spacecraft can operate safely as the comet activity continues to increase towards perihelion [its closest distance from the sun] in August,” ESA said.