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Mini-motors touch down on comet’s surface

17 November, 2014

The Philae craft that landed on the 67P / Churyumo Gerasimenko comet recently was carrying tiny motors supplied by two Swiss manufacturers.

However, two of the motors did not get a chance to operate because they were part of the harpoon system designed to secure the craft to the comet’s surface. When the Philae landed, a pair of barbed harpoons, connected by cables to the lander, were meant to be fires into the surface using explosive charges, but they failed to operate.

Two winches, powered by 11W gear-motors, had been due to pull the cables taut to hold the lander in place. The 16mm-diameter motors, made by Faulhaber, were designed to deliver 300–450mNm of torque in both directions of rotation.

Because the lander was not secured in place, it bounced back into space twice before coming to rest in a shaded location where its solar panels were not able to gather as much sunlight as had been hoped. Other Faulhaber motors were then deployed to rotate part of the craft through 35 degrees to allow the panels to capture more light.

Two 13mm-diameter DC motors supplied by maxon motor were used to lower an alpha x-ray spectrometer called APXS onto the comet’s surface. The instrument was designed to record the comet’s chemical composition and provide data on the presence of elements such as carbon and oxygen.

The motors used on the Philae lander were slightly modified versions of standard designs. One of the modifications involved their lubrication. Grease and oil cannot be used in outer space because they would either solidify at the low temperatures or vaporise in the vacuum.

If the Philae lander’s harpoon system had fired correctly, a gear-motor-powered winch would have drawn in cables to keep the craft on the comet

Faulhaber therefore turned to solid lubricants, but it could not use graphite because the material lubricates properly only if it can take up gases such as water vapour or nitrogen between its layers. Because these gases are absent in space, graphite would act like chalk.

So, the winch motors were lubricated using molybdenum disulphide which works effectively in a vacuum as well as at extreme temperatures.

Because of the low temperatures, the nickel-plated brass casings of the winch motor gearboxes had to be replaced by steel casings that matched the thermal expansion of the cogs.

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