A moment of tension as the James Webb Space Telescope stretches sunshield on way to L2 destination

Engineers on Earth have five and a half months more of this


Updated The James Webb Space Telescope has continued to notch up the milestones on its journey to its L2 destination with the tensioning of its sunshield.

After a successful launch atop an Ariane 5 on 25 December, the observatory has begun unfurling as it hurtles towards its final location. After antennas and sunshield pallets popped out of the spacecraft, the sunshield itself, consisting of five folded membranes, is in the process of being tensioned.

The first layer of the observatory's kite-shaped sunshield, and the one that is both the largest and closest to the Sun, was tightened to its final, completely taut position on 3 January. The second layer took 74 minutes and the third 71 minutes. By the completion of the third layer, NASA reported that the process (up to that point) had taken five and a half hours.

The fourth layer was successfully tensioned earlier today. The fifth is expected to be completed by the end of the day should things continue to go to plan.

The sunshield is 21.197m x 14.162m (69.5ft x 46.5ft).

"The membrane tensioning phase of sunshield deployment is especially challenging because there are complex interactions between the structures, the tensioning mechanisms, the cables and the membranes," said James Cooper, NASA's Webb sunshield manager, based at Goddard Space Flight Center.

"This was the hardest part to test on the ground, so it feels awesome to have everything go so well today."

The sunshield is vital because the telescope must be protected from the Sun's radiation. Tensioning and keeping the layers apart is also essential since if they touch, they conduct heat. "Thus the cold side can't, well, get very cold," remarked Mark McCaughrean, senior advisor for Science & Exploration at ESA.

Unlike other infrared observatories, which have required active cooling via cryogenics, the James Webb Space Telescope uses passive cooling to get temperatures down to -233°C for three of its instruments – the Near Infrared Camera (NIRCam), the Near Infrared Spectrograph (NIRSpec), and the Near Infrared Imager and Slitless Spectrograph (NIRISS). NIRISS includes the Fine Guidance Sensor (FGS) used to point the observatory precisely.

A fourth instrument, the Mid-Infrared Instrument (MIRI), uses a dedicated cryo-cooler to bring the temperature down even further, to 7 Kelvin (or just under -266.1°C).

The tensioning of the sunshield, while a critical step, does not mean that scientists and engineers on Earth will able to retreat from the edge of their seats. There remains the deployment of the secondary mirror and the unfolding of the iconic primary mirror among other steps ahead of the L2 insertion burn some 29 days after launch.

And then there is the commissioning of the payload.

"There's another five & a half months of this to go," McCaughrean tweeted.

Five and a half months that, just a few short years ago, seemed impossibly distant. ®

Updated to add on 5 January 2022 at 1630 UTC:

Youtube Video

JWST has just this second deployed its secondary mirror. Now it's an actual telescope!


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