Paint it black: behind the James Webb Space Telescope's signature color

NASA's James Webb Space Telescope has to stay super cool to observe the cosmos. How does it beat the heat? Black paint. 

As the agency explained in its new YouTube series "Elements of Webb," the James Webb Space Telescope's radiator is painted black to absorb heat. Just like how black asphalt gets hot in the summertime, objects that are black are generally hotter as they absorb all wavelengths of light and convert it into heat. (Comparatively, white objects reflect light and do not absorb heat.)

Webb engineers use this principle to keep the telescope cool. 

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 NASA's James Webb Space Telescope mission

This NASA image shows the deployment of the James Webb Space Telescope's secondary mirror out in front of its main mirror assembly. (Image credit: NASA)

Webb essentially has two sides, a hot side and a cool side, which are separated by the spacecraft's sunshield. The cool side is where its highly sensitive scientific instruments reside, and sunshield blocks any heat from the sun from reaching those instruments.

The cool side "even has a radiator to keep it extra cool," NASA multimedia specialist Sophia Roberts said in the video. That radiator and everything except Webb's bright gold mirrors are black on that side, she explained.

Now, this black paint is important not only because black objects absorb heat well, but also because they emit heat well, Roberts noted, adding that objects painted black are highly emissive (a measurement of how well something radiates heat).

To ensure that the black paint used on Webb did the best job possible at soaking up and radiating heat away from the telescope's instruments, NASA used a special ultra-black paint made by Ball Aerospace called Ball Infrared Black (BIRB).

Webb's cold side is about minus 388 degrees Fahrenheit (minus 233 degrees Celsius). So, why do Webb's instruments need to be so cold?

Webb observes primarily infrared light, which we humans perceive as heat. If the telescope is too warm, that heat could interfere with observations. Since the scope's instruments naturally create heat as they work, Roberts noted, Webb has to be proactive about cooling off.

This is why, in addition to shielding the instruments from solar rays with the sunshield, Webb also has a radiator — painted with BIRB — that helps to keep things as chilly as possible.

Webb arrived at its final home in space on Jan. 24 after a month of traveling nearly 1 million miles (1.5 million kilometers) through space. By observing in infrared from this distant vantage point, scientists hope that Webb will be able to peer back at some of the earliest moments in the universe's history, detecting the heat from the earliest stars and galaxies to ever exist. 

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Chelsea Gohd
Senior Writer

Chelsea “Foxanne” Gohd joined in 2018 and is now a Senior Writer, writing about everything from climate change to planetary science and human spaceflight in both articles and on-camera in videos. With a degree in Public Health and biological sciences, Chelsea has written and worked for institutions including the American Museum of Natural History, Scientific American, Discover Magazine Blog, Astronomy Magazine and Live Science. When not writing, editing or filming something space-y, Chelsea "Foxanne" Gohd is writing music and performing as Foxanne, even launching a song to space in 2021 with Inspiration4. You can follow her on Twitter @chelsea_gohd and @foxannemusic.