The Gaia mission, part of ESA’s Cosmic Vision programme, will create an extraordinarily precise three-dimensional map of more than one billion stars in our Milky Way by around 2022. The Gaia spacecraft, launched on 19 December 2013, has been making scientific observations of the universe since July 2014 and has so far already delivered spectacular data. TNO developed a diagnostic system on the basis of silicon carbide to monitor the behaviour of Gaia’s telescopes, which behave differently in space than predicted by models, and thus the TNO monitoring system plays a crucial role in the mission’s success.
The Gaia spacecraft succeeds Hipparcos, which mapped over 100,000 stars since 1989. Gaia will lead to an unprecedented number of new discoveries with about 100 new asteroids a day in our solar system, 10 new stars with planets, 50 new exploding stars in other galaxies, and 300 new distant quasars. Exo-planets will be detected by measuring the tiny movement of the star, caused by the small gravitation pull of the planet. It is also estimated that Gaia will discover about 15 000 new exo-planets during its five-year operational lifetime.
The Gaia spacecraft measures the angles between stars using two telescopes set at a fixed angle of 106.5°, named the Basic Angle. The astrometric measurements will be accurate to 24 microarcsec (at 15 magnitude), comparable to measuring the diameter of a human hair at a distance of 1000 kilometres. This requires ultra high stability, which can only be achieved by using Silicon Carbide for the optical bench and telescopes. In addition, Gaia is equipped with a metrology system to monitor the angle between the two telescopes (the Basic Angle Monitoring Opto-Mechnical Assembly – BAM OMA, developed by TNO). This system consists of two laser interferometers. Two pairs of parallel laser bundles are sent to the two telescopes, which create two interference patterns on a detector. If the basic angle varies, the interference patterns will shift. The Basic Angle variation is measured within a precision of 0.5 microarcsec, during an observation period of 5 minutes, which equals an Optical Path Difference (OPD) as small as 1.5 picometers.
The BAM OMA consists of two optical benches, a number of flat mirrors, beam splitters, fibre collimators and periscopes. In order to fulfil the stability requirements for such accurate OPD measurements, the entire BAM OMA is constructed from Silicon Carbide. BAM OMA's components were developed in close collaboration with Eindhoven University of Technology and with the support of the Netherlands Space Office (NSO).
Silicon Carbide (SiC) has a number of advantages:
- High specific stiffness
- High thermal conductivity
- Low Coefficient of Thermal Expansion
- Good isotropy
The sintered SiC components, designed by TNO, were manufactured by Airbus DS/Mersen-Boostec. SiC is brittle and very hard. Therefore machining of SiC components after sintering, shall be minimised as far as possible. This requires a different approach in design, compared to working with conventional materials such as Aluminium. The BAM OMA shall have an extremely small Wave Front Error (WFE), less than 25 nm RMS over the entire optical path. TNO has developed processes for in house polishing of SiC mirrors to very low surface errors.