Rosetta Lander put through its paces
10 May 1999
No spacecraft has yet landed on the surface of a comet. But this will change in 2012 when the Rosetta lander will set down on the nucleus of Comet Wirtanen and return a flood of unique data about this primitive chunk of dirty ice.In order to ensure the success of such an ambitious project, a full-size structural-thermal model of the spacecraft has been undergoing anexhaustive series of tests at the Ottobrunn (Munich) facilities of IABG.The lander's trials began in a special soundproof chamber in mid-April. In an effort to simulate the noise from a rocket launch, it was suspended from the ceiling and subjected to a deafening range of sounds from huge loudspeakers - loud enough to cause instant death for anyone foolish enough to stray inside the chamber.
After passing these acoustic tests with flying colours, the lander was moved to another room and placed on a vibration table where it was shaken back and forth, up and down. Different types of vibration were applied to the spacecraft to see how it would stand up to the shaking. Although most lander systems were given the all-clear, some minor problems arose with locks used to hold the landing gear in place during launch. Engineers will now be asked to stiffen these on the flight model.
The latest torture for the lander to endure was a four-day spell in the thermal vacuum chamber - a giant 'oven' from which all of the air has been extracted. Here, the extreme temperatures the lander will experience in space were simulated.
The spacecraft will have to operate at three times the Earth's distance from the Sun, where the amount of sunlight reaching the comet is only one ninth that on Earth. At such distances, night temperatures can plummet to around -200°C. This deep-freeze is simulated at IABG by pumping extremely cold liquid nitrogen through a shroud which lines the walls of the vacuum chamber.
At the other extreme, despite its great distance from the Sun, daylight temperatures inside the lander may reach 60°C when all the instruments are switched on. Dark areas on top of the 'lid' absorb even more heat and may soar to 100°C. This hot scenario is simulated by subjecting the lander to radiation from an artificial Sun - powerful xenon lamps which are specially designed to replicate actual sunlight.
"We think the comet rotates quite quickly - once in about seven hours," said Stephan Ulamec, project manager for the Rosetta Lander. "However, we need to be sure the lander can survive if the comet nucleus rotates much more slowly, say once every 50 hours."
After completing this freeze-bake cycle on 8 May, the lander has moved on to its final trials. These involve checking its centre of mass by placing it a cage and seeing how it balances when turned on all three sides.
"We can compensate for a non-ideal position of the centre of gravity adjusting the nozzle of the cold gas system," said Stephan Ulamec. "There is little opportunity to shift the instrument boxes because they are packed so tightly into the lander. We could also apply some extra mass to balance it, but we try to minimise this because it's dead weight," he added.
If all goes well, the lander should be transferred back to the German Aerospace Research centre (DLR) in Cologne on 13/14 May.
