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Sun block for space astrometry

The Gaia European observatory

In May 2013, a Soyuz launcher will loft the Gaia European observatory on its way to Lagrangian point L2, 1.5 million km from Earth going away from the sun. For Gaia to carry out its five-year mission to map more than a billion stars in our galaxy it needs to guarantee extreme thermal stability for its two telescopes and their support structure. It was therefore fitted with a solar shield which has been deployed for the first time, on the ground, in Toulouse, France.

The operation took place in one of the cleanrooms at the Toulouse site of Airbus Defence and Space, prime contractor for the mission on behalf of the European Space Agency (ESA).

“Gaia’s solar shield will be deployed during the first acquisition phases following launch, after separation from the upper stage and initial attitude stabilisation,” explains Christian Lebranchu, the mission’s Mechanical, Propulsion and Launcher Interface Manager. “The aim is to achieve a stable and secure configuration as rapidly as possible, with power supply from the solar panels integrated into the shield. The observatory will be placed on station at L2 by means of 10N boosters which will not induce any load factors liable to damage the shield.”

The main problem is linked to the geometry of the system, designed to fit inside the four-metre fairing of the Soyuz launcher. In order to keep the observatory in shade once stabilised by slow rotation (four revolutions per day), with an angle stabilised at 45° relative to the direction of the sun, the shield has a diameter of 10 m and comprises two multi-layer insulation (MLI) thermal protection blankets similar to those in which satellite structures are normally wrapped. “This has never been done before on this scale,” Christian points out. “Gaia’s 12-sided base means that we have to unfurl and deploy 48 three-metre long triangles, using a mechanism with 12 degrees of freedom which has to be perfectly synchronised to avoid damaging the MLIs.”

This is no simple matter. A first reduced-scale qualification model underwent testing at ESA’s ESTEC technical centre in Noordwijk (Netherlands) in late 2009 and although thermal qualification was obtained with no problem, mechanical qualification was not so straightforward. “We had to strengthen the motor drive system, replacing the thermal regulators with stepper motors, to dampen the deployment of the springs, and also review MLI stowage,” says Christian. This new configuration was validated during testing at Intespace in Toulouse.

A weighty problem

Despite the size and the light weight of the Gaia shield, opening it up in the atmosphere posed no braking or aerodynamic damping problems. The motorised deployment took about 10 minutes. “We just turned off the air conditioning in the building,” confides Christian.

However, compensating for gravity was no mean feat. Usually, for simple deployments such as rigid solar panels, the rotation axis is placed vertically and booms with counterweights or even helium balloons are used to cancel out gravitational pull exerted on each element. The shield’s geometry makes this approach impossible and a system of booms equipped with pulleys had to be designed to vary the level of compensation force required as the angle between each element and the vertical grew. A remarkable exercise in applied trigonometry!

“Fortunately, SENER, our Spanish subcontractor for the shield, had already developed this method.”

The test establishes a benchmark for another identical test which will be carried out after the mechanical test campaign to be run on the satellite at Intespace in the next few weeks. Because of the size of the assembly, it is not possible to undertake full-scale deployment in thermal vacuum conditions.

At the same time, assembly work is continuing on the scientific payload and above all the optical instruments. Optical, mechanical and thermal qualification is scheduled for the second half of 2012, paving the way for the six months of assembly, integration and testing prior to the launch from French Guiana.

Airbus Defence and Spaces other technological contribution to Gaia

To perform its mission, Gaia will be measuring the relative positions of stars. Each star will be measured more than 70 times and its absolute position will be calculated by means of successive iterations. For these measurements to have any meaning, the Gaia instrumentation, consisting primarily of two telescopes pointing at an angle of 106.5° to each other, requires particularly rigorous levels of geometrical stability. In order to achieve this without excessive weight penalty, Airbus Defence and Space opted for its silicon carbide technology for the optical instruments, not only for the mirrors, but also for their support structure. This technology has already been used to build the Herschel observatory telescope, with its 3m diameter primary mirror, offering a surface twice as large as that of the primary mirror on Hubble, making it the largest telescope ever placed in orbit to date.

GaiaSpace Telescope