Tommaso Ghidini, Head of ESA’s Materials Technology section
Materials make the space mission: manufacturing materials and processes must be chosen with great care.
Space is a not a single environment, but many: simply getting into orbit stresses materials to their limits. A rocket launch combines high temperatures and pressure with additional stresses due to shock, vibration, aerodynamic and acoustic forces. The earliest satellites launched by Europe simply went into Earth orbit, but then came interplanetary missions like Ulysses, Huygens and Rosetta which had to contend with the cold darkness of deep space.
Meanwhile probes to the inner Solar System were faced with the opposite problem of escalating solar flux and temperatures, starting with Venus Express, then the BepiColombo mission to Mercury (to be launched in April 2018) – which could at least shelter behind the innermost planet during part of its orbit.
But ESA’s Solar Orbiter, also launching in 2018, will have nowhere to hide, tasked with high-resolution imaging of the Sun from as near as 42 million km away – a little more than a quarter Earth’s distance from its parent star. In the process it will be operating in the continuous direct view of the Sun, enduring 13 times the intensity of terrestrial sunlight, resulting in surface satellite temperatures of 500°C or more.
As the head of ESA’s Materials Technology section – focused mainly on metallic materials and their related preparation and manufacturing processes – Solar Orbiter is a good example of the kind of challenges that come our team’s way.
The spacecraft’s main body takes cover behind a multi-layer 3.1m by 2.4 heat shield. This heat shield works to absorb sunlight, then convert it to infrared to radiate it back to space. To achieve this its surface has to maintain constant ‘thermo-optical properties’ – to keep the same colour despite years of exposure to extreme ultraviolet radiation.
At the same time, the shield surface cannot crack, shed material or release vapour, because this might contaminate Solar Orbiter’s highly-sensitive instruments. Any build-up of static from the close-up solar wind was also unacceptable. Tests in our STAR ‘Synergistic Temperature Accelerated Radiation’ Chamber showed that our current suite of materials and coatings were unable to meet the performance we needed.
It was time to look for a solution outside the space industry. As materials processes experts, we are constantly scanning the European industrial landscape, looking for emerging technologies with potential for space.
In this instance we found it with an Irish company called ENBIO and a technique called ‘Co-Blast’, developed to coat titanium-made medical implants. It works with reactive metals like titanium, aluminium and stainless steel, which possess a natural surface oxide layer. This surface is stripped away and switched for with a ‘dopant’ material, ending up bonded to the metal, rather than painted or stuck on – effectively becoming part of it. The material to be added to the outermost titanium layer of Solar Orbiter’s heat shield is called ‘SolarBlack’ – a type of black calcium phosphate processed from burnt-bone charcoal, commonly used by industry – and found also in prehistoric cave paintings.
See the full story on ESA’s site.