When Solar Orbiter launches on its journey to the Sun, there’s one key piece of engineering making this ESA-NASA mission possible: the heat shield.
Seeking a view of the Sun’s north and south poles, Solar Orbiter will journey out of the ecliptic plane – the belt of space, roughly in line with the Sun’s equator, through which the planets orbit. Slinging repeatedly past Venus in order to draw near the Sun and climb higher above the ecliptic, the spacecraft bounds from the Sun and back toward the orbit of Earth throughout its mission.
“Although Solar Orbiter goes quite close to the Sun, it also goes quite far away,” said Anne Pacros, the payload manager at the European Space Agency’s, or ESA’s, European Space Research and Technology Centre in the Netherlands. “We have to survive both high heat and extreme cold.” In the dark of space, Solar Orbiter faces temperatures of minus 300 degrees Fahrenheit. At closest approach, 26 million miles from the Sun, it will encounter intense heat and radiation.
But Solar Orbiter’s 324-pound heat shield reflects and guides heat away from the spacecraft and can withstand up to 970 F.
The heat shield is built like a 10-foot-by-8-foot sandwich. The front layer – wafer-thin sheets of titanium foil – strongly reflects heat. A honeycomb-patterned aluminum base, covered in more foil insulation, forms the inner slice closest to the spacecraft and provides support.
Star-shaped titanium brackets keep the layers in place, like a toothpick tasked to hold bread together, but notably, this sandwich is missing its filling. The nearly 10-inch gap in the shield funnels heat out to space. A smaller, second gap lies between the inner slice and the spacecraft. Overall, the shield is 15 inches thick. It also has several eyes: peepholes for five of the spacecraft’s remote-sensing instruments to peer through.
Solar Orbiter’s heat shield is coated with a thin, black layer of calcium phosphate, a charcoal-like powder much like pigments used in cave paintings thousands of years ago.
“It’s funny that something as technologically advanced as this is actually very old,” Pacros said. But the coating resists degradation under the blow of intense solar ultraviolet radiation. Although the black powder does absorb some heat, it’s excellent at shedding that heat back out to space.
Solar Orbiter also has to contend with its own heat. Its instruments work up a sweat while they’re running; panels of radiators on the side of the spacecraft eject heat and ensure the instruments don’t get too hot.
Tight control of the spacecraft’s position and tilt is key to protecting the instruments. Once the spacecraft is past the 88-million-mile mark in its solar flybys – that’s 95% of the distance between the Sun and Earth – the heat shield must be pointed straight at the Sun. That means Solar Orbiter will crab walk through space, keeping the spacecraft and instruments tucked in the heat shield’s shadow.
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