Protecting the James Webb Space Telescope

Aluminium is an essential building block of any space-bound device.
5 December, 2016
Satellites use aluminium to weather harsh space conditions, and NASA's new telescope is no different.
The world's largest telescope took 20 years to build, but NASA has finished the project and plans to launch the James Webb Space Telescope (JWST) in October 2018.

The JWST is considered the successor to the Hubble telescope, which is likely to remain in concurrent service – so it's not technically a replacement yet. But it has finally come together, and NASA scientists at the Goddard Space Flight Center in the United States were excited to share the milestone with the public.
We're about to prove that it works. We've done two decades of innovation and hard work, and this is the result — we're opening up a whole new territory of astronomy.
John Mather, astrophysicist, senior project scientist of the JWST
Image: NASA
The conference was held overlooking 18 large mirrors designed to collect infrared light, all sheltered behind a sun shield the size of a tennis court. The $8.8 billion telescope will collect seven times more light than the Hubble, which was launched in 1990 with state-of-the-art equipment for its time. More than a quarter century later, the JWST will be able to see more and farther in order to capture the events of the last 13 billion years. Its surface is relatively flawless, and it can detect a bee-sized object on the moon.

In order to do that, the JWST scientists relied on aluminium to help solve a primary problem. Infrared light – essentially heat energy – is what JWST science relies on, and in order to accurately interpret data, the telescope needs to be kept super-cold. The mirrors and other scientific instruments have to be kept at -223° Celsius, which means the sunshield has to protect them from all external heat sources—especially the sun. In fact, the units themselves have to be kept cold so they don't generate heat themselves.
The solution relies on five thin layers of Kapton E with aluminium and doped silicon applied as coatings, so that they reflect the sun's heat back into space. The Kapton is a commercially available polyimide film that is made by DuPont, while the other coatings are applied based on a JWST formulation.

The layers are remarkably thin, with the first one being just two-thousandths of an inch thick and the other four half that. The coating on the sun-facing side of the hottest two layers is only 50 nanometers thick, and uses doped silicon; doping means that a small amount of conductive material was added to the silicon coating to make it electrically conductive.
Image: NASA
The high-reflectivity aluminium layer is about 100 nanometers thick and is applied to all the other surfaces, so that any remaining energy is "bounced" back out of the gaps between the layers. These membranes, with layers no thicker than a human hair, were the last part of the project to be completed.

Now, the NASA scientists will complete the rigorous testing that is part of the project's final stage. Unlike the Hubble, the JWST will be too far away to fix from Earth, so these performance tests are make or break.

The telescope will be blasted with noise levels that simulate the launch conditions for the Ariane 5 rocket that will send the James Webb on its mission. It will undergo cryogenic testing, to ensure that the aluminium and silicon layers work and that it can survive the extreme temperatures in space, and then go through final phases to make sure there are no optical defects like those that plagued Hubble.
Banner image: NASA