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Drama as dust 'stars' confuse comet probe Rosetta

Paul Sutherland, Feature writer
Apr 2, 2015, 1:32 UTC

Sen—European space scientists began to lose communications with their comet probe Rosetta at the weekend after it flew so close to 67P/Churyumov-Gerasimenko that starlike dust confused its navigation system.

On Saturday, March 28, the orbiting spacecraft dived to a height of just 14 km above the comet’s nucleus in its latest flyby to get close-up images of the surface plus other science data.

But the flyby resulted in what ESA’s Rosetta blog described as “significant difficulties in navigation”. Communications were affected as the probe’s high-gain antenna began to drift away from being pointed towards the Earth.

The probe had previously flown closer to the comet, flying just 6 km over the larger of its two lobes on Feb. 14 without serious incident. But the latest encounter shows how such manoeuvres are becoming more perilous as the comet becomes more active on its approach to the Sun.

The Rosetta team believe that flying their spacecraft close to the comet sent it through denser zones of gas and dust now being spewed from 67P. This had two effects. Firstly, the craft with its giant solar panels was exposed to greater drag, but also the navigation system became confused because it mistook bits of debris from the comet for stars that it was using to maintain its course.

This effect on Rosetta’s star trackers had been noticed during the earlier Feb. 14 encounter, but it had not been a problem. This time, they were sufficiently thrown by what they were seeing that the probe began to lose its correct orientation with respect to the stars, the Sun and Earth.

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Rosetta's star trackers are shown in red in this model, plus part of the high gain antenna can be seen in the background. Image credit: ESA/ATG medialab

The main star tracker reported that it was having trouble locking on to stars that it expected to see due to the presence of hundreds of false stars—specks of dust and debris that were reflecting sunlight. Full communication was lost for neary 24 hours before mission control at Darmstadt, Germany, was able to regain proper tracking.

ESA’s blogger Emily Baldwin reported: “In the meantime, a spacecraft attitude error had built up, resulting in the high gain antenna off-pointing from the Earth. Indeed, a significant drop in the radio signal received by ground stations on Earth was registered. 

“Following recovery of the star tracker system, the off-pointing was immediately automatically corrected and the operations team subsequently saw a return to a full strength signal from the spacecraft.”

But Rosetta’s problems were not yet over. The star trackers continued to have issues with starlike comet debris, affecting the spacecraft’s actions. The conflict with other navigation mechanisms led to the probe switching automatically to a safe mode, including the switching off of instruments. 

Normal status was resumed on Monday, March 30, but the mission team say it will take a while longer to resume normal scientific operations.

Rosetta is now back at a relatively safe distance of more than 200 km from Comet 67P, but the drama of the weekend flyby will force the space scientists to reexamine plans for any future close encounters with the nucleus. 

The team will be considering how to keep Rosetta safe as the comet becomes ever more active. Rosetta project scientist Matt Taylor told Sen: “That's all being discussed now . . . stay tuned!”

There are still four months to go before the comet reaches perihelion, its closest approach to the Sun, and by then it will be giving off significantly more gas and dust, putting the orbiter in greater peril.