Rosetta STM feels hot and cold all over
3 May 2000After more than two weeks of being boiled and then frozen, the Rosetta spacecraft has shown that it can withstand the extreme temperatures to be encountered on its 11 year odyssey to Comet Wirtanen.On 16 April, the Rosetta Structural Thermal Model (STM) completed a prolonged series of tests in the Large Space Simulator, a giant thermal-vacuum chamber at the European Space Research and Technology Centre (ESTEC) at Noordwijk in TheNetherlands. Preliminary results show that the spacecraft behaved as expected, and engineers are confident that ESA's comet chaser will be ableto survive successfully its journey beyond the asteroid belt and back.
Rosetta's ordeal took place inside a huge sealed cylinder which is 10 metres across and 16 metres high - the largest thermal-vacuum chamber in Europe. The intention was to subject the spacecraft to the same cold, airless conditions and variable solar illumination that it will have to endure in deep space. This meant that the Rosetta STM had to be mounted so that it could be moved and different parts of the satellite could be illuminated according to its mission profile.
In the absence of the spacecraft's two giant solar arrays, electricity was provided to different parts of the spacecraft by about 100 external power sources. These were attached to the STM by several kilometres of cable. 450 temperature sensors located all over the spacecraft structure were required in order to obtain the most accurate information possible on its thermal behaviour.
Once all of the air was pumped out of the test chamber on 30 March, its black walls were flushed with liquid nitrogen so that their temperature plummeted to -180°C. In place of the Sun, the array of xenon arc lamps of the facility was used.
Light from these was directed and collimated by the mirrors of the facility onto the STM so that it would simulate the variable exposure to sunlight and shadow that Rosetta will experience at the Earth's distance from the Sun (1 AU). In this 'hot case', all of the spacecraft's louvers were kept fully open to expel as much excess heat as possible.
A week later, the lamps were switched off to simulate conditions beyond 5 AU, where levels of sunlight are only 4% those at the Earth. In this case, the louvers automatically went in their closed position in order to block the heat radiated from the radiators and to conserve as much heat as possible on the powered-down spacecraft. In addition to the flight model louvers procured from US, the spacecraft was also equipped with a qualification louver entirely designed and built in Europe. Both types worked satisfactorily.
"We are delighted with the results of these tests," said Daniele Stramaccioni, Rosetta Thermal Control Engineer. "The preliminary results confirm that the temperatures on Rosetta are comparable to those predicted by our models. This demonstrates that the spacecraft will be able to survive the hostile space environment with the very limited power resources that are available at far distance from Sun."
With this last major hurdle safely negotiated, the STM is now being stripped down in preparation for the next stage of the assembly and test programme. Once the lander separation test is completed in early May, the STM will be transported to the Alenia plant in Turin, where it will be modified and used as the Rosetta Electrical Qualification Model (EQM).