How do you dress on a hot day? Knowing that Solar Orbiter will venture closer to the Sun than any previous ESA mission, it might seem odd that a spacecraft needing to endure an unprecedented level of sunlight ends up almost entirely black in colour. The reason why comes down to practicalities of thermal design, a coating treatment devised for the very different environment of the human body and a pigment whose history stretches all the way back to the Stone Age.
Solar Orbiter will come as close as 0.28 times Earth’s distance from the Sun, encountering sunlight 13 times stronger than in Earth orbit.
Accordingly the spacecraft was designed to operate behind an advanced multi-layer heat shield. The challenge its thermal design team faced was deciding on the precise ‘thermo-optic’ properties of the shield’s front face: how much solar energy would be reflected back directly, how much would be absorbed, and how much would be re-radiated in the form of infrared radiation, or heat?
This choice dictates the maximum temperature the surface could reach, influencing in turn the extent of the intervening layers, which work to dissipate this unwanted heat to insulate the main body of the spacecraft.
Just 0.05 mm thick – about the width of a human hair – this front surface is made from strong but light titanium alloy. The plan was to add a surface coating in turn to optimise the metal’s reflectivity, absorption and emissivity, but this proved harder than it sounds.
“The mission had three main requirements for the heat shield covering,” explains ESA materials engineer Andrew Norman. “Firstly it had to keep the same colour during years of exposure to intense solar flux, including high ultraviolet radiation – the mission team would have loved a white coating, to reflect back more solar energy, but testing showed it would darken unacceptably over time. So we settled on black, to keep its thermo-optical properties stable over the mission lifetime.”
More on ESA website.