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Prehistoric cave pigment to shield ESA’s solar probe

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Posted February 13, 2014

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.

ESA’s next generation Sun explorer, Solar Orbiter will be launched in 2017. It will investigate the connections and the coupling between the Sun and the heliosphere, a huge bubble in space created by the solar wind. The solar wind can cause auroras and disrupt satellite-based communication. Copyright ESA/AOES

ESA’s next generation Sun explorer, Solar Orbiter will be launched in 2017. It will investigate the connections and the coupling between the Sun and the heliosphere, a huge bubble in space created by the solar wind. The solar wind can cause auroras and disrupt satellite-based communication. Copyright ESA/AOES

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.

The 30 000-year-old Chauvet Cave paintings in southern France employ burnt bone from fires as the source of their black pigment. Copyright Wikimedia

The 30 000-year-old Chauvet Cave paintings in southern France employ burnt bone from fires as the source of their black pigment. Copyright Wikimedia

“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.

Front view of the Solar Orbiter spacecraft, highlighting its protective heatshield. The shield has feed-throughs with doors for the mission's remote-sensing instruments to observe the Sun. Copyright: ESA

Front view of the Solar Orbiter spacecraft, highlighting its protective heatshield. The shield has feed-throughs with doors for the mission’s remote-sensing instruments to observe the Sun. Copyright: ESA

“We soon identified a problem with the heatshield requirements,” said Andrew Norman, a materials technology specialist.

“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.

The Enbio team wield a blow torch to demonstrate the heat absorption of Solar Black CoBlast-treated titanium foil. A similar coating will form the outermost layer of the protective heatshield for ESA's 2017 Solar Orbiter mission. Pictured at Enbio’s manufacturing facility at the NovaUCD centre on the University College Dublin campus are (l-r) Nigel Cobbe, vice-president, business development, Enbio; John O’Donoghue, CEO, Enbio; and Dr James Carton, industrial surfaces programme manager, Enbio. Copyright Enbio/NovaUCD

The Enbio team wield a blow torch to demonstrate the heat absorption of Solar Black CoBlast-treated titanium foil. A similar coating will form the outermost layer of the protective heatshield for ESA’s 2017 Solar Orbiter mission. Pictured at Enbio’s manufacturing facility at the NovaUCD centre on the University College Dublin campus are (l-r) Nigel Cobbe, vice-president, business development, Enbio; John O’Donoghue, CEO, Enbio; and Dr James Carton, industrial surfaces programme manager, Enbio. Copyright Enbio/NovaUCD

“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.

Medical solution

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.

Solar Black dopant powder used in the Co-Blast process to coat reactive metals with specially-tailored surface characteristics, being used for Solar Orbiter's heatshield. The particle size is between three and 24 micrometres (a micrometre is a millionth of a millimetre). Copyright EnBio

Solar Black dopant powder used in the Co-Blast process to coat reactive metals with specially-tailored surface characteristics, being used for Solar Orbiter’s heatshield. The particle size is between three and 24 micrometres (a micrometre is a millionth of a millimetre). Copyright 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.”

ESA's Synergistic Temperature Accelerated Radiation (STAR) facility has been used to test the candidate Solar Black titanium coating for Solar Orbiter's protective heatshield. STAR allows test items to be exposed to intense ultraviolet radiation in combination with electron and proton particle radiation, across an extreme range of temperatures (-150°C to +350°C), while in ultra-high vacuum. Based at the Materials and Electrical Components Laboratory in ESA's techncial centre ESTEC in Noordwijk, the Netherlands, it is often used for accelerated lifetime testing. Copyright: ESA

ESA’s Synergistic Temperature Accelerated Radiation (STAR) facility has been used to test the candidate Solar Black titanium coating for Solar Orbiter’s protective heatshield. STAR allows test items to be exposed to intense ultraviolet radiation in combination with electron and proton particle radiation, across an extreme range of temperatures (-150°C to +350°C), while in ultra-high vacuum. Based at the Materials and Electrical Components Laboratory in ESA’s techncial centre ESTEC in Noordwijk, the Netherlands, it is often used for accelerated lifetime testing. Copyright: ESA

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.

Burnt ‘char bone’ is widely used in everyday life, employed for everything from fertiliser and metal alloy production to purifying white sugar and filtering heavy metals out of water.

Its robustness is demonstrated by the immaculate appearance of the 30 000-year-old Chauvet Cave paintings in southern France – burnt bone from fires being the source of the very first black pigment.

The CoBlasted Solar Black titanium has passed a gamut of testing in ESA’s Materials and Electrical Components Laboratories in the ESTEC technical centre in Noordwijk, the Netherlands – everything from accelerated lifetime exposure to sunlight and ultraviolet radiation to the deceptively simple-sounding ‘tape pull’ – applying then pulling off adhesive tape to see if anything comes with it.

Testing time

The treated titanium is now baselined for the heatshield, being developed by Thales Alenia Space in Italy. A prototype ‘structural and thermal model’ is due to be tested inside ESTEC’s Large Space Simulator, a giant vacuum chamber featuring a simulated Sun.

While Solar Black is being used for Solar Orbiter's heatshield, another EnBio product, ‘Solar White’ is being tested for other elements of Solar Orbiter exposed to the Sun, including its main antenna, instrument booms and solar array attachments. The Solar Black is used as a ‘tie layer’ foundation on metal, allowing another layer to be placed on top – in this case a carefully tailored conductive white ceramic. Copyright EnBio

While Solar Black is being used for Solar Orbiter’s heatshield, another EnBio product, ‘Solar White’ is being tested for other elements of Solar Orbiter exposed to the Sun, including its main antenna, instrument booms and solar array attachments. The Solar Black is used as a ‘tie layer’ foundation on metal, allowing another layer to be placed on top – in this case a carefully tailored conductive white ceramic. Copyright EnBio

Another Enbio product, ‘Solar White’ is also being tested for other elements of Solar Orbiter exposed to the Sun, including its main antenna, instrument booms and solar array attachments.

“We’ve found that the Solar Black works well as a ‘tie layer’ foundation on metal, allowing us to place another layer on top – in this case a carefully tailored conductive white ceramic,” added Mr O’Donoghue.

“The rigorous ESA evaluation we’ve gone through has increased our knowledge and opened up new product possibilities, including a range of different tints.”

ESA is also studying wider uses for the coating process, including boosting the long-term surface robustness of telecom satellites, which typically operate for 15 years or more.

CoBlast and Enbio

CoBlast is a unique low-temperature and environmentally friendly process, chemical free and able to coat a range of different metal shapes to a thickness of a few thousandths of a millimetre.

Enbio is based at the University College Dublin campus in Ireland, allowing it to make use of the university’s test facilities, while ESA has also installed specialised equipment for dedicated space testing.

“It’s very much a symbiotic relationship, which has turned out well,” concluded Mr O’Donoghue. “Our work with Solar Orbiter has had a catalytic effect, opening doors for our company.”

Source: ESA

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