JWST’s sunshield deployment during a ground test at Northrop Grumman. Credit: NASA

The James Webb Space Telescope opened covers that protected the mission’s folded sunshield Thursday, and deployed a momentum flap to help the observatory balance against the unending light pressure from the sun.

The steps pave the way for a critical three days of work to open the sunshield and tension all five of its ultra-thin layers, made of kapton with aluminum and silicon coatings.

The sunshield covers released Thursday after ground teams at the Space Telescope Science Institute uplinked commands to Webb, on its way to an operational orbit around the L2 Lagrange point nearly a million miles (1.5 million kilometers) from Earth.

“Webb’s engineers have released and rolled up the sunshield covers that protected the thin layers of Webb’s sunshield during launch,” NASA said in an update Thursday. “After the team electrically activated release devices to release the covers, they executed commands to roll the covers up into a holding position, exposing Webb’s sunshield membranes to space for the first time.”

The five membranes were folded and stowed for launch to fit inside the nearly 18-foot (5.4-meter) diameter of the payload fairing on the European Ariane 5 rocket, which hurled Webb into space on Christmas Day.

Mission controllers confirmed the covers were released at 12:27 p.m. EST (1727 GMT) Thursday.

Earlier in the day, Webb deployed a momentum tab on the back side of the observatory. The eight-minute process involved releasing the flap’s hold-down devices, then a spring moved the flag into its final position, according to NASA.

The flap will help keep Webb stable against the bombardment of solar photons, or light energy, from the sun throughout the observatory’s astronomy mission. The mission’s giant sunshield, once deployed, will catch the solar photons like a kite moves with the wind, but with more subtle effects.

Without the aft momentum flag, the influence of the sun would require Webb’s six reaction wheels to counteract the movement to keep the telescope properly pointed. In turn, Webb would need to fire its thrusters and consume fuel more often to offload momentum from the reaction wheels.

The deployments Thursday followed the unfolding of the sunshield pallets containing Webb’s thermal barrier Tuesday. On Wednesday, Webb extended a telescoping tower holding the mission’s primary mirror segments and science instruments, creating some distance between the hardware, which must be cooled to cryogenic conditions, and the relatively warm spacecraft, with its solar array pointed at the sun.

That clears the way for critical work, set to begin Friday, to open the sunshield to its full dimension, roughly the size of a tennis court.

Made of five fragile kapton membranes, each as thin as a human hair, the sunshield will keep Webb’s mirrors, instruments, and detectors in constant shadow, allowing their operating temperature to plummet to near minus 400 degrees Fahrenheit. Such cold conditions are required to allow Webb to see the faint infrared light from the first galaxies in the universe more than 13.5 billion light years away.

Most NASA managers and astronomers waiting to use the nearly $10 billion Webb telescope give the same answer about the most stressful moment of the mission: Sunshield deployment.

“The sunshield is one of these things that is almost inherently indeterministic,” said Mike Menzel, Webb’s mission systems engineer at NASA’s Goddard Space Flight Center in Maryland. “NASA is used to deploying rigid beams on hinges, because they’re deterministic, you can determine how they move.”

“Given that there are 40 different major deployments, and hundreds of pulleys and wires, the whole thing makes me nervous and will until its fully deployed,” said John Grunsfeld, an astrophysicist, former astronaut, and head of NASA’s science mission directorate from 2012 until 2016, a key period in Webb’s development.

But it’s the sunshield that got the biggest share of Menzel’s attention during the design and testing of Webb.

Menzel compares predicting the behavior of the sunshield layers to guessing what a string will do when you push it on a table top.

“So it is with the membranes of the sunshield,” he said. “So we can’t really predict their shape, but we can constrain it. “We can try to prevent it from going in places that we don’t want it to go, places where it could snag or tear, or maybe impede the deployment of other members.”

Two booms will extend from each side of Webb as soon as Friday. With the assistance of deployment motors, the structural support booms will pull the five sunshade membranes out into their distinctive diamond shape.

It all happens slowly, with sensors across the observatory tracking how the sunshield opens. Ground controllers can pause in between steps to ensure everything is working as designed.

Each layer of the sunshield is slightly different in size and shape, created using thermally bonded sections of kapton with around 10,000 seams, according to Krystal Puga, Webb’s lead spacecraft systems engineer at Northrop Grumman.

There are reinforcement strips, or rip stops, to contain any tears or holes, and metallic ribbons giving the kapton some structural support.

The sunshield membranes are coated with aluminum, and two of the outermost layers are treated with silicon, giving the skin-like material a purple hue.

Webb has 344 devices that must work exactly as intended. Of those, 107 are membrane release devices, non-explosive actuators that pin the sunshield in place for launch.

In total, the mission’s deployment sequence relies on 140 release mechanisms, 70 hinge assemblies, eight deployment motors, 400 pulleys, and 90 cables running a quarter-mile in length. There are also an array of bearings, springs, and gears to transform Webb from its launch to operational configuration.

With the sunshield in its diamond shape, covering an area the size of a tennis court, Webb controllers will send commands for the observatory to tension each of the five layers over two days — currently planned on Saturday and Sunday.

“Once we get the sunshield out, that’s great, but then we have to sort of tighten it up,” said Keith Parrish, NASA’s commissioning manager for Webb, in an interview before launch. “All five layers have different points around them where they’re connected up, and then we’ll pull on cables in each one of those corners to actually tighten up the sunshield.”

“The very last step is super important,” Puga said. “We need to tension all of the membranes using a series of pulleys and cables to create the separation between each of the five layers.”

The tensioning will separate each of the five ultra-thin kapton membranes, spacing them a few inches at the center and a few feet at the outermost edges. The tapered spacing helps allow heat from the sun to reflect between the layers, and eventually radiate back into space.

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– Spaceflight Now