With less than 11 months to launch, the most advanced spacecraft ever to visit a comet is about to begin a critical series of thermal tests at the European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands. These tests will play a vital role in ensuring the success of ESA’s Rosetta mission to unravel the mysteries of Comet Wirtanen.
During its 10-year, 5 billion kilometre mission of exploration, the Rosetta spacecraft will be subjected to extreme temperature changes as it flies from the benign environment of near-Earth space to the dark depths of the Solar System beyond the asteroid belt.
In order to ensure that Rosetta will survive this hazardous trek – the cosmic equivalent of travelling from the sizzling deserts of North Africa to the frozen wastes of Antarctica – the spacecraft has now been installed in the largest thermal vacuum chamber in Europe, where every part of the Orbiter and Lander will be alternately baked and frozen inside an airless room.
“These are the most critical tests in our entire pre-launch programme because they reproduce the conditions that Rosetta will experience in flight,” said Rosetta Payload and Operations Manager, Claude Berner.
The three-week-long series of thermal tortures began on 20 February, when engineers started the lengthy process of removing air from the giant chamber. In order to create a vacuum equivalent to that of deep space, this ‘pumping down’ was expected to take more than one day.
Once the depressurisation is completed, the so-called ‘thermal balance – thermal vacuum’ tests can begin. Perched on a gimbal system – a table that can both tilt and rotate – the position of the spacecraft will then be adjusted so that every phase of its complex flight plan can be simulated.
“We will be working in three shifts, around the clock, to examine the condition of the spacecraft during simulations of 15 different mission phases,” said Claude Berner. “This means we will have to manoeuvre it into specific attitudes that represent critical parts of the mission in terms of exposure to heat or cold.”
During three circuits of the inner Solar System, the amount of solar radiation reaching Rosetta will vary by as much as 25 times. The period of maximum heating, which will take place near the Earth, is simulated by using mirrors and powerful lamps. These will expose different parts of the spacecraft to temperatures exceeding 150°C.
In order to simulate the cold of deep space, where the temperature drops well below -100°C, liquid nitrogen will be pumped through pipes in the chamber walls. More than 100 active sensors will be monitoring the responses of the spacecraft’s systems and instruments during this punishing programme.
“We want to make the tests as realistic as possible, so we are testing the full flight configuration of Rosetta in the chamber,” explained Claude Berner. “The spacecraft is blanketed by layers of aluminised kapton that provide insulation against extreme cold, while onboard radiators will be expected to expel excess heat.”
“We even have simulated propellant in the fuel tanks to see whether any leakages occur,” he added. “It is also important to discover how much outgassing from the spacecraft structure takes place as the temperature changes.”
The Rosetta team expects the thermal tests to be completed in the second week of March.
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