Prehistoric Cave Pigment to Shield ESA’s Solar Probe

A pigment once daubed onto prehistoric cave paintings is set to protect ESA’s Solar Orbiter mission from the Sun’s close-up glare.
Burnt bone charcoal will be applied to the spacecraft’s titanium heatshield using a novel technique.

Solar Orbiter, due for launch in 2017, will carry a portfolio of instruments to perform high-resolution imaging of our parent star from as close as 42 million km – a little more than a quarter of the distance to Earth.

Operating in direct view of the Sun, the mission must endure 13 times the intensity of terrestrial sunlight and temperatures rising 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,” explained Pierre Olivier, Solar Orbiter’s safety engineer.

“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.”

Back in 2010, during the ‘Phase-A’ planning stage, ESA’s materials specialists began checking that the mission was indeed achievable with current manufacturing methods and materials.
“We soon identified a problem with the heatshield requirements,” said Andrew Norman, a materials technology specialist.

They found it in the shape of Irish company Enbio and its CoBlast 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,” commented John O’Donoghue, Managing Director of Enbio.

“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.”

The material Enbio will apply to the outermost titanium sheet of Solar Orbiter’s multi-layered heatshield is called ‘Solar Black’ – a type of black calcium phosphate processed from burnt bone charcoal.

“To go on absorbing sunlight, then convert it into infrared to radiate back out to space, its surface material needs to maintain constant ‘thermo-optical properties’ – keep the same colour despite years of exposure to extreme ultraviolet radiation.

“At the same time, the shield cannot shed material or outgas vapour, 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.”

The initial choice – carbon-fibre fabric – was ruled out. Instead the team began looking for the answer outside the space business.