Satellites Need Sunscreen, Too
(Bloomberg Businessweek) -- The European Space Agency had a problem: A satellite it was building to study solar flares and wind risked burning up as it reached its final orbit, nearer to the sun than Mercury’s closest approach. At 26 million miles from our neighborhood star, the ESA’s Solar Orbiter, scheduled to launch in February, will face temperatures above 500C (932F). In such intense heat, traditional coatings made from organic compounds break down and form a vapor that can condense on cooler parts of the spacecraft, blurring the lenses of optical instruments and fouling other sensors. The ESA considered using a heat shield of woven carbon-fiber fabric but discovered that the intense vibrations during launch would shake off carbon particles, reducing their effectiveness.
The solution turned out to be one of humanity’s earliest inventions: black pigment made from the burned bones of prey, similar to the stuff used to decorate the walls of prehistoric caves. “This is really a back-to-the-future story,” says John O’Donoghue, co-founder and chief executive officer of Enbio Ltd., the Irish company behind the coating planned for the Solar Orbiter. “The oldest-known art in the world was painted with charred bone.”
Founded in 2006, Enbio developed a process called “co-blasting” for medical implants such as artificial hips. A high-powered airjet sprays titanium implants with a mixture of tiny particles of coating and gritty aluminum oxide, which removes the oxidized layer that forms on the surface of most metals and makes them difficult to paint. Before oxidation can recur (in just 1/300th of a second), Enbio’s powder bonds with the metal.
The ESA thought the process might work for its heat shield, but the coating needed to be dark to absorb radiation. In its implants, Enbio had used light-colored synthetic bone, which helps the new joints bond with a patient’s body. For the ESA, the company’s scientists suggested charred animal bone. The substance “is highly stable for going to space because there’s nothing combustible left in it,” O’Donoghue says. It’s also easy to obtain: Slaughterhouses sell bone meal that’s burned for use as fertilizer, paint dye, and water filters.
The dark coating, which Enbio calls SolarBlack, seemed so promising that the ESA asked whether the company could use a similar process to make a white veneer for aluminum parts on the satellite—for instance, the antennae and a long arm designed to hold instruments—that must reflect sunlight rather than absorb it. Although Enbio’s implant coatings were light-colored, space scientists know that white tends to discolor when exposed to ultraviolet radiation. And the company had already discovered that a lighter material applied with its technology “doesn’t stay white. It turns gunmetal gray,” O’Donoghue says. “At first, we said no.”
Then Barry Twomey, an Enbio researcher, proposed using SolarBlack as a primer on the aluminum parts before applying the white coating. Twomey figured that since the chemistry of the two materials is more closely matched, they could achieve a stronger bond. It took five years to perfect the process, but today Enbio makes what it calls SolarWhite by applying a white ceramic slurry that’s cured and baked onto pieces of the satellite. “It was massively satisfying,” Twomey says. “It was the first time I’d brought something from an original idea all the way through to seeing it applied.”
SolarBlack has since been adopted by several European satellite makers, and Enbio is working with the ESA to promote the use of SolarWhite on commercial orbiters. Meanwhile, space companies are considering other uses of Enbio’s co-blasting process, including coating threaded titanium bolts with Teflon to prevent them from spontaneously welding to surrounding metal.
The ESA helped save Enbio itself from flaming out: When the agency came calling, the company had few takers for its medical devices and had spent most of the €2 million-plus in funding it had received over the years. It had recently returned to University College Dublin, where many of its techniques were developed, so it could use the school’s electron microscopes and X-ray equipment—the corporate equivalent of moving back in with your parents. Because of the ESA work, the company last year won a €1.5 million ($1.8 million) grant from the European Union. It employs 18 people in Dublin and at a plant dedicated to its space business in the town of Clonmel, a two-hour drive south of the Irish capital. O’Donoghue expects to have a staff of 28 by the end of the year and to secure further funding within three years. That’s an eternity in startup time, but by then the Solar Orbiter will be approaching the sun to start its expected 10-year mission, and SolarBlack’s true test will begin.
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