Logic might suggest that when you are flying close to the Sun, a white coating that reflects the Sun’s rays would make most sense. It is, after all, why roofs in desert countries get painted white, to keep homes cool. But there is a downside: a white surface does not give out as much heat as a black surface.

Although many of the inner layers of the heatshield are reflective, the engineers developing the spacecraft calculated that a black outer layer would be the best way of releasing heat to space. Having worked that out, next came the challenge of finding a suitable coating – a paint that stayed the same colour and did not melt, peel or degrade.

The technology they settled on is nothing new. In fact humans have been using burnt and powdered animal bone since the dawn of time for painting pictures on the walls of caves. Even the basic principle is still the same. “Animal bones are incinerated in a furnace and ground down to get this very dark powder, like charcoal,” says Draper. But despite its low-tech origins, it turns out to be one of the most robust coatings around.

Exhaustive tests

However, the next stage is much more sophisticated and was originally developed by Irish company Enbio to provide a safe coating for medical implants. “They use a special nozzle to blast the titanium surface with an abrasive material to strip off the oxide layer,” Draper explains, “and then replace this with the charcoal ink.”

So does the bone-coated titanium work? While the boxy Solar Orbiter spacecraft itself takes shape in one of the Airbus cleanrooms – a white sports-hall sized area packed with space hardware – Draper’s team has been subjecting the heat shield material to a whole range of tests.

“We heat it up to high temperatures, then drop it in liquid nitrogen to see if that has an effect,” says Draper. “We also do an adhesion test where we try to pull the coating off and, as for the doors in the heatshield, we have opened and closed them more than 20,000 times.”

So far all this heating, cooling and scraping has been successful, proving that even in the space age, some Stone Age tricks can still be useful.

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* An earlier version of this story had an incorrect distance from the Sun for the orbiter's mission.