A European spacecraft scheduled for launch towards the sun in 2017 will be shielded from extreme temperatures by a type of paint once used in prehistoric cave art.
Carrying instruments to perform high-resolution imaging of the sun, the European Space Agency’s Solar Orbiter will travel as close as 42 million kilometers, or a little more than a quarter of the distance to Earth, from the sun—the closest a spacecraft has ever been to the star.
According to ESA officials, burnt bone charcoal, a type of pigment once used by early humans to create art on the walls of caves in France, will be applied to the spacecraft’s titanium heatshield to help protect it from extreme conditions. Operating in direct view of the Sun, the mission will be exposed to 13 times the intensity of terrestrial sunlight and temperatures as high as 520°C.
“The main body of the spacecraft takes cover behind a multi-layered 3.1 m by 2.4 m heatshield,” Pierre Olivier, Solar Orbiter’s safety engineer, said of the spacecraft’s design. “And Solar Orbiter’s instruments will operate at the far end of ‘feed-through’ lines that run through the shield, some under protective covers of beryllium or glass.”
The decision to include the burnt bone charcoal in the design was made after ESA scientists realized that conventional manufacturing methods and materials would not be able to meet requirements for the heatshield.
“To go on absorbing sunlight, then convert it into infrared to radiate back out to space, [the heatshield’s] surface material needs to maintain constant ‘thermo-optical properties’ – keep the same color despite years of exposure to extreme ultraviolet radiation,” Andrew Norman, a materials technology specialist, explained.
“At the same time, the shield cannot shed material or outgas vapor, because of the risk of contaminating Solar Orbiter’s highly sensitive instruments, [and] it has to avoid any build-up of static charge in the solar wind because that might threaten a disruptive or even destructive discharge.”
To solve this problem, ESA scientists turned to a technique originally developed to coat titanium medical implants.
“The process works for reactive metals like titanium, aluminium and stainless steel, which possess a surface oxide layer,” John O’Donoghue, managing director of Enbio, the developer of the CoBlast technique, said.
“We spray the metal surface with abrasive material to grit-blast this layer off, but—as the CoBlast name suggests—we also include a second ‘dopant’ material possessing whatever characteristics are needed. This simultaneously takes the place of the oxide layer being stripped out.”
“The big advantage is that the new layer ends up bonded, rather than only painted or stuck on. It effectively becomes part of the metal – when you handle metal you never worry about its surface coming off in your hands.”
According to ESA officials, the material Enbio will apply to the outermost titanium sheet of Solar Orbiter’s multi-layered heatshield is known as “Solar Black,” a type of black calcium phosphate processed from burnt bone charcoal.
