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The James Webb Space Telescope

 
NASA Webb Telescope on Twitter: "You’re hot and you’re cold… 🎵 ..." / Twitter
You’re hot and you’re cold… 🎵

#NASAWebb is split into a “hot side” and “cold side” by its sunshield. The sunshield will always be facing the Sun to block out heat and light, as Webb's mirrors need to stay extremely cold to observe faint heat signals in the universe!

How large is the temperature difference between the two sides? On the hot side, parts of Webb will reach temperatures as high as 85 degrees Celsius, or 185 degrees Fahrenheit! Meanwhile, on the cold side Webb will be about -233 degrees Celsius, or -388 degrees Fahrenheit.

NOW LIVE: You can track the temperatures of Webb’s hot and cold sides on our website at Where Is Webb? NASA/Webb! Note that temperatures will continue to change as Webb unfolds and then cools down to operating temperatures over the next months.
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Where Is Webb? NASA/Webb -- with "?units=metric" appended to the base URL.
The locations:
a) Sunshield UPS Average Temperature (hot side: Sunshield Structure)
b) Spacecraft Equipment Panel Average Temperature (hot side: Spacecraft Bus)
c) Primary Mirror Average Temperature (cold side: Mirrors)
d) Instrument Radiator Temperature (cold side: ISIM)
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PlaceCKFR
Hot-side expected85358185645
Cold-side target-23340-38872
a - pallets-726620480
b - body928349509
c - mirror-57216-70390
d - instruments-153120-244216

The JWST is currently at 0.58 gigameters (M km), 1.5 times the distance to the Moon, 40% of the distance to its destination.

R = Rankine, Fahrenheit adjusted to make absolute zero = 0, like Kelvin from Celsius. Absolute zero is -273.15 C = -459.67 F.
 
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Webb’s Deployable Tower Assembly Extends in Space – James Webb Space Telescope
The DTA extended about 48 inches (1.22 meters), putting room between the upper section of the observatory, which houses the mirrors and scientific instruments, and the spacecraft bus, which holds the electronics and propulsion systems. This creates enough distance to allow the sensitive mirrors and instruments to cool down to the necessary temperatures to detect infrared light. This gap will also provide room for the sunshield membranes to fully unfold.

The deployment took more than six and a half hours, as engineers activated release devices and configured heaters, software, and electronics, before commanding the DTA itself to extend. The movement of the DTA, which looks like a large, black pipe, is driven by a motor. The team began the deployment at approximately 9:45 a.m. EST and completed it at approximately 4:24 p.m. EST.
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NASA Webb Telescope on Twitter: "Moving on up! ⬆️
Our team has just confirmed that we have successfully extended our Deployable Tower Assembly (DTA) upwards, making space for our sunshield deployments in the next few days -- another step completed as we #UnfoldTheUniverse: (links)" / Twitter
 
About 12 hours after launch:
Phil Plait on Twitter: "Amazing that we can see @NASAWebb / @ESA_Webb on its journey to the L2 point 1.5 million km away! Look for the two faint dots moving to the upper left near the center. One's JWST, the other is the upper stage Ariane rocket booster. (streaks are likely satellites)" / Twitter
noting
Ruari Mackenzie on Twitter: "My attempt at observing JWST earlier tonight from far north in Scotland. The two slowly moving dots are the telescope and upper stage, moving past Eridanus. (vid link)" / Twitter

The sunshield's size: 22*10 m (72*33 ft).

At its current distance of 627,000 km, that's an angular size of 3.5*10-8 radians or 7.2 milliarcseconds. At the JWST's final location, those numbers are 1.5*10-8 rad or 3.0 mas.

That means that it will be *very* hard to resolve the JWST from the ground. But if one can see it, one can use its brightness to estimate the cross section that it makes relative to the Sun.

 James Webb Space Telescope sunshield
 Spacecraft bus (James Webb Space Telescope)
 Timeline of the James Webb Space Telescope
 
Webb Ready for Sunshield Deployment and Cooldown – James Webb Space Telescope
From Observatory Project Scientist Michael McElwain of NASA’s Goddard Space Flight Center:
The Webb telescope and science instruments are ready to enter the shade, never again to see direct sunlight. One of Webb’s unique design features is using passive cooling by a five-layer sunshield to reach the telescope’s operational temperatures of 45 Kelvin (-380 degrees Fahrenheit). The enormous sunshield is about 70 by 47 feet (21 by 14 meters) when deployed, or approximately the size of a tennis court. The sunshield geometry and size were determined such that the telescope can point within a field of regard that covers 40% of the sky at any time and can observe anywhere in the sky over six months. This innovative architecture enables Webb’s sensitivity to be limited by the natural sky background (mostly zodiacal light) rather than being compromised by thermal glow of the observatory itself, for all wavelengths shorter than 15 microns, for the duration of the mission.
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In addition to its passive cooling with its sunshield, the telescope also has active cooling: a closed-cycle refrigerator for its coldest parts: Cryocooler Webb/NASA I say closed-cycle to distinguish it from open-cycle ones that work by evaporating liquid helium into space.

Webb’s Aft Momentum Flap Deployed – James Webb Space Telescope
Deployment took about 8 minutes, by releasing its hold-down devices with a spring doing the rest. As with the solar array, the JWST has only one of these.

Webb Team Releases Sunshield Covers – James Webb Space Telescope

NASA Webb Telescope on Twitter: "We just successfully deployed our aft (back) momentum flap, which helps balance pressure from solar radiation on Webb's sunshield, much like a trim tab helps stabilize a boat or plane! ⛵ ✈️

Sail on, Webb! (link) #UnfoldTheUniverse (pic link)" / Twitter

NASA Webb Telescope on Twitter: "🔆 Photons of sunlight hitting the sunshield surface will exert pressure on the sunshield. The aft momentum flap uses the pressure of these photons to balance the sunshield and keep Webb steady." / Twitter

regs on Twitter: "@NASAWebb @SpaceHub_SL BAH. I can't believe the first images arent going to be here for months. Such hype and were still unfolding it? I really wanna know what this thing can do!" / Twitter

NASA Webb Telescope on Twitter: "@regularregs @SpaceHub_SL The long commissioning period isn't because of the unfolding but because the telescope has to cool down to operating temperatures before the mirror alignment can be fine-tuned, and the instruments calibrated. The deployments take ~2 weeks, and another 2 to get into orbit." / Twitter
 
ESA on Twitter: "🎥 Great footage from @RealtraSpace camera of NASA/ESA/CSA James #Webb Space Telescope as it separates from its @Arianespace @ariane5 upper stage and unfolds its solar panel, 25 December 2021 👉 (link)
#WebbFliesAriane #VA256 #JWST @ESA_Webb @NASAWebb @csa_asc (vid link)" / Twitter
noting
ESA - Webb separation from Ariane 5

ESA on Twitter: "Very special thanks to @CHatherley79, one of our @spacerockslive friends, for permission to use her amazing music! 👍" / Twitter

ESA on Twitter: "Also available on our #ESA @YouTube channel 😮👇
(link)" / Twitter

The part of our planet that you see in that video is the Gulf of Aden between East Africa and the Arabian Peninsula. The spacecraft is moving eastward relative to the camera, which is on the Ariane upper stage, so what you see is upside down relative to the usual map view.


NASA Webb Telescope on Twitter: "📺 Tune into NASA TV in the coming days for LIVE coverage of #NASAWebb's final deployments as we #UnfoldTheUniverse. (link)

Post final deployments, there will be a media briefing. See this release for more detail: (links)" / Twitter

NASA Webb Telescope on Twitter: "Find it hard to get out from under the covers in the morning? Imagine doing it in space!

Our sunshield covers protected the sunshield while it was folded for launch. Today those covers were removed to prep for unfolding the sunshield! (link) #UnfoldTheUniverse (vid link)" / Twitter


Here is a detailed schedule: James Webb Space Telescope - Launch Timeline

Along with Deployment Explorer Webb/NASA which gets updated for whichever deployment stage has been completed.
 
lpetrich said:
ESA on Twitter: "🎥 Great footage from @RealtraSpace camera of NASA/ESA/CSA James #Webb Space Telescope as it separates from its @Arianespace @ariane5 upper stage and unfolds its solar panel, 25 December 2021 👉 (link)
#WebbFliesAriane #VA256 #JWST @ESA_Webb @NASAWebb @csa_asc (vid link)" / Twitter
noting
ESA - Webb separation from Ariane 5

ESA on Twitter: "Very special thanks to @CHatherley79, one of our @spacerockslive friends, for permission to use her amazing music! 👍" / Twitter

ESA on Twitter: "Also available on our #ESA @YouTube channel 😮👇
(link)" / Twitter

The part of our planet that you see in that video is the Gulf of Aden between East Africa and the Arabian Peninsula. The spacecraft is moving eastward relative to the camera, which is on the Ariane upper stage, so what you see is upside down relative to the usual map view.


NASA Webb Telescope on Twitter: "📺 Tune into NASA TV in the coming days for LIVE coverage of #NASAWebb's final deployments as we #UnfoldTheUniverse. (link)
Post final deployments, there will be a media briefing. See this release for more detail: (links)" / Twitter

NASA Webb Telescope on Twitter: "Find it hard to get out from under the covers in the morning? Imagine doing it in space!
Our sunshield covers protected the sunshield while it was folded for launch. Today those covers were removed to prep for unfolding the sunshield! (link) #UnfoldTheUniverse (vid link)" / Twitter


Here is a detailed schedule: James Webb Space Telescope - Launch Timeline

Along with Deployment Explorer Webb/NASA which gets updated for whichever deployment stage has been completed.
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Very excited about the James Webb telescope!
 
 
Speaking of the mouse on the Moon. It is certainly capable of that because it is cold enough to be sensitive to 1 photon per second rates.

And another interesting thing is that it has basically ordinary refrigerator on board, unlike previous telescopes which used liquid gases as coolant which were limited resource. The only thing which limits the telescope is fuel for thrusters, but it seems NASA thinks about refueling it. So it can last much longer than 10 years.

And I agree, future telescopes should be assembled in space (remotely)
It should be cheaper that this insane engineering.
 
When I saw this tweet, I got worried.
NASA Webb Telescope on Twitter: ""Liftoff from a tropical rainforest to the edge of time itself..."

Get out your tissues and watch Webb placed into its @ariane5 rocket fairing, then moved to its launch pad. Witness again our historic launch and the start of Webb's epic million-mile (1.5 million km) journey. (link)" / Twitter
The video showed the Ariane 5 rocket in its assembly building, the JWST being lowered into place on top of it, the fairing also lowered into place, then the rocket going out on its launch pad to its launch site, then the rocket's launch, then the JWST being released from the rocket's upper stage.

However nice that video was, it seemed to be filler, and I wondered what had happened to deployment of the mid-booms. I was concerned about the disasters that Skylab and Galileo suffered.

 Skylab - Skylab: First U.S. Space Station | Space
It was built from the third stage of a Saturn V rocket and launched atop the first two stages. About a minute into the launch, a micrometeoroid shield got ripped off of it by the air rushing by, and that ripped off one of the solar arrays and damaged the other.

The first astronauts to visit the station put into place a replacement sunshield and they fully deployed the remaining solar array.

They were the first of three crews, with three astronauts each, staying in space 28, 68, and 84 days. However, a fourth mission was canceled.

 Galileo project
The Galileo spacecraft was a Jupiter orbiter and atmosphere probe that was launched in 1989, arriving at Jupiter in 1995. On the way, it made three gravity-assist flybys, one of Venus then two of the Earth, and it also flew by two asteroids, returning pictures of them.

After the Venus flyby, it opened its high-gain antenna. That was much like an umbrella, with 18 graphite-epoxy ribs and a wire-mesh fabric for reflecting wadio waves. The spacecraft was launched with the antenna folded up, with the ribs' ends in sockets on the antenna's central pole. But the antenna did not fully open it. Only 15 ribs moved outward, with 3 stuck in place. The spacecraft's operators were unable to unstick those 3 ribs, and they did the rest of the mission using the spacecraft's low-gain antenna.

Galileo did not return very many pictures, though it did return some of Jupiter's four big moons, and its operators felt satisfied that it had done much of what it was sent to do.

Looking at James Webb Space Telescope (JWST) - YouTube I find that it has a series: "Elements of Webb". In order, gold, beryllium, Kapton, carbon, silicon

 Kapton is not a chemical element, but a kind of plastic that is stable over a very large temperature range: 4 to 673 K (-269 to 400 C).

Back to JWST deployment, the deployment of the mid booms was delayed for checking out the opening of the sunshield covers. But one of them is now deployed, as I write this.
 
First of Two Sunshield Mid-Booms Deploys – James Webb Space Telescope

These arms have 5 telescoping segments, and deploying this one took around 3h 19m.

With Webb’s Mid-Booms Extended, Sunshield Takes Shape – James Webb Space Telescope

Deploying this one took around 3h 13m, around the same time as the first one.

Both arms are now fully deployed, and the JWST is now ready for the next step: tensioning the five layers of the sunshield. This will be done upward, from the side facing the Sun to the side facing the telescope proper. It is scheduled for two days, but since the JWST's operators want to be cautious, they may take longer. They have already demonstrated that earlier today.

NASA Webb Telescope on Twitter: "@WorldSpaceflig1 The sunshield covers had been rolled back to the extent necessary yesterday. Part of the mid-boom deployment involved rolling them the rest of the way back. This final preparation to begin extending the mid-boom was what the team was analyzing before beginning the deployment." / Twitter

NASA Webb Telescope on Twitter: "We successfully deployed ..." / Twitter
We successfully deployed #NASAWebb’s port sunshield mid-boom, which pulls out our 5 sunshield layers. While scheduled for earlier today, our team paused to confirm the sunshield cover had fully rolled up: (link) #UnfoldTheUniverse

Thread ⬇️ (pic link)

When switches did not trigger to indicate the sunshield covers were rolled up, team members used temperature data & gyroscope sensors to confirm that they had. This analysis took extra time, but allowed the team to move forward. #UnfoldTheUniverse

The deployment of the 5 telescoping segments of the mid-boom began around 1:30pm ET and reached full deployment at 4:49pm.

Webb's deployment steps are all human-controlled, so the schedule can change. The team plans to deploy the starboard mid-boom tonight. #UnfoldTheUniverse
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NASA Webb Telescope on Twitter: "Shine bright like a diamond 💎 ..." / Twitter
Shine bright like a diamond 💎

With the successful deployment of our right sunshield mid-boom, or “arm,” Webb’s sunshield has now taken on its diamond shape in space. Next up: tensioning the 5 sunshield layers! (link) #UnfoldTheUniverse (pic link)

Webb's sunshield was secured for flight with 107 individual pins, called 'non explosive actuators' which were designed to keep all 5 layers of the sunshield tidy for launch. With the completion of this deployment, all of them have now been successfully released.
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NASA on Twitter: "Start spreading the news! Before the @NASAWebb space telescope works to #UnfoldTheUniverse, it’s making a cameo in Times Square.

Wishing you and yours a #HappyNewYear full of light & discovery. (link)

📸: @MorganStanley (pic link)" / Twitter

I've decided to estimate how observable the JWST is by calculating its horizontal cross section.

 Ariane 5 -- A5 Tech Info

First, the part of its rocket that got into space: the upper stage of the Ariane 5 ECA rocket. It has diameter 5.4 meters and height 4.711 m. I don't know if that height figure includes the engine. I will leave aside that issue.

Its horizontal cross section is 23 square meters and its vertical cross section is 25 m^2.

The Ariane 5 has several payload fairings that can be used with it, and they have a usable diameter of 4.57 meters. Since the JWST had a close fit with the fairing, and since it had a square horizontal cross section when it was released, I estimate that square's side length by dividing by sqrt(2): 3.23 m

 James Webb Space Telescope -- Key Facts - Webb/NASA -- The James Webb Space Telescope — Technical Details at Northrop Grumman

From the NASA specs, the dimensions of the sunshield are 21.197 m * 14.162 m.

Here are my cross-section estimates:
  • Initial: 10 m^2
  • One pallet deployed: 39 m^2
  • Both pallets deployed: 68 m^2
  • One mid-boom deployed: 126 m^2
  • Both mid-booms deployed: 184 m^2
The JWST's cross section has gone up by a factor of 18, or luminosity magnitude being less by 3.1.
 
Deployment Timeline Adjusted as Team Focuses on Observatory Operations – James Webb Space Telescope
Specifically, the team is analyzing how the power subsystem is operating now that several of the major deployments have been completed. Simultaneously, the deployments team is working to make sure motors that are key to the tensioning process are at the optimal temperatures prior to beginning that operation.

Using an approach to keep mission operations focused on as few activities as necessary at a time, mission managers have chosen to wait to resume sunshield deployment steps after better understanding the details of how Webb is functioning in its new environment.
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Seems like they are being very careful.

NASA Webb Telescope on Twitter: "To ensure that #NASAWebb ..." / Twitter
To ensure that #NASAWebb is in prime condition for its next major step, our team has decided to focus today on learning more about how Webb behaves in space. Sunshield tensioning has been moved to no earlier than tomorrow, Jan. 3. (link) #UnfoldTheUniverse (pic link)

“So far, the major deployments we’ve executed have gone about as smoothly as we could have hoped for. But we want to take our time and understand everything we can about the observatory before moving forward.” - #NASAWebb lead systems engineer Mike Menzel #UnfoldTheUniverse

“Nothing we can learn from simulations on the ground is as good as analyzing the observatory when it’s up and running. Now is the time to take the opportunity to learn everything we can about its baseline operations.” - Bill Ochs, #NASAWebb project manager
#UnfoldTheUniverse
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NASA on Twitter: "NOW: @NASAWebb experts summarize the space telescope’s first week of deployments and the next steps as we #UnfoldTheUniverse: (links)" / Twitter
NASA had a press conference:
Media Briefing: Webb Telescope Week One Deployments Update - YouTube

They found that some of the motors and pulleys had become a little bit too hot, so they tilted the spacecraft to cool them down.

They also said that once the sunshield is deployed, then the deployment process will be past 70 - 75% of the failure points of the process.
 
The Sunshield Webb/NASA
The sunshield separates the observatory into a warm, sun-facing side (thermal models show the max temperature of the outermost layer is 383K or approximately 230 degrees F), and a cold side (with the coldest layer having a modeled minimum temp of 36K or around -394 degrees F). The five-layer sunshield keeps sunlight from interfering with the sensitive telescope instruments. The telescope operates under 50K (~-370F) Photo: Northrop Grumman

...
The sunshield will allow the telescope to cool down to a temperature below 50 Kelvin (-370°F, or -223°C) by passively radiating its heat into space. The near-infrared instruments (NIRCam, NIRSpec, FGS/NIRISS) will work at about 39 K (-389°F, -234°C) through a passive cooling system. The mid-infrared instrument (MIRI) will work at a temperature of 7 K (-447°F, -266°C), using a helium refrigerator, or cryocooler system.

In addition to providing a cold environment, the sunshield provides a thermally stable environment. This is essential to maintaining proper alignment of the primary mirror segments as the telescope changes its orientation to the Sun.
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Why five layers?

The layers glow in infrared light, and that gets reflected between them until it can escape into space. The five layers are for extra effect, with their four gaps.

The layers are made of aluminum-coated Kapton, a plastic. The sunward sides of the two hottest layers have doped silicon for additional reflectivity.
Each layer of the sunshield is incredibly thin. Layer 1 will face the sun and is only 0.05 millimeters (0.002 inches) thick, while the other four layers are 0.025 mm (0.001 inches). The thickness of the aluminum and silicon coatings are even smaller. The silicon coating is ~50 nanometers (nm) (1.9 microinches) thick, while the aluminum coating is ~100 nm (3.93 microinches) thick.

The layers are slightly different sizes and different shapes. Layer 5 (just under the primary mirror) is smallest and Layer 1 is largest. Layer 1 is relatively flat and layer 5 is more curved. The layers are closer together at center and further apart at the edges to direct heat from center to the outside of the layers.

Thermal models show that the max temperature of Layer 1 is 383K. The max temperature of Layer 5 is 221K, and its minimum temperature is a chilly 36K.
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The layers have protection against rips spreading through them.
The membrane material is tough, but if it gets a small tear or hole, the hole could become much larger. So, there is a special process called a Thermal Spot Bond (TSB) - areas where each layer are melted together. In addition, reinforcing strips of membrane material are Thermal Spot Bonded to the parent membrane about every 6 feet or so, forming a grid pattern of "rip-stops."

"This has been shown through testing to arrest a tear and keep it from extending outside of a given grid area," said Cooper. So, if a hole occurs in a layer of the sunshield from a meteoroid, or small meteor, the size of the damage is limited. These are not intended to stop a meteoroid, but rather to contain the area of damage.
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Sunshield Coatings Webb/NASA

"Fun fact: the pink hue of the lower side of the sunshield is actually due to the coating."

NASA is going ahead with the final step in the sunshield deployment: tensioning its layers so that they will be completely stretched out.

Webb Team Moving Forward With Sunshield Tensioning – James Webb Space Telescope

First Layer of Webb’s Sunshield Tightened – James Webb Space Telescope
The layer that is directly illuminated by the Sun.

Second and Third Layers of Sunshield Fully Tightened – James Webb Space Telescope

Timeline (times are EST):
  • First one: started 10 AM, finished 3:48 PM
  • Second one: started 4:09 PM, took 74 min: finished 5:23 PM
  • Third one: started 5:48 PM, took 71 min: finished 6:59 PM
The process took 5 1/2 hours, suggesting that they began the first layer around 1:30 PM. I'm not sure why there is a discrepancy between the stated and calculated start times.

They plan to work on the fourth and fifth layers the next day.

NASA Webb Telescope on Twitter: "Today we start tensioning ..." / Twitter
Today we start tensioning the most Sun-facing layer of #NASAWebb's 5-layer, tennis court-sized sunshield. Read in depth how our sunshield works ➡️ (link)

More about tensioning activities: (link) #UnfoldTheUniverse (pic link)

Read more about today's tensioning activities on our latest blog, now out: (link)

“Onions have layers. Ogres have layers.” And so does our sunshield!

Layer 1 of our 5-layer sunshield is now pulled taut, getting us even closer on our journey to #UnfoldTheUniverse. We have begun tensioning Layer 2: (links)

The layers start coming and they don’t stop coming…

Layers 2 and 3 are now complete! Tensioning for the final two layers of Webb’s 5-layer sunshield is planned for tomorrow. More: (link) #UnfoldTheUniverse (pic link)
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Tensioning Begins for Two Layers of Webb’s Sunshield – James Webb Space Telescope
The final two ones.

Webb Team Tensions Fifth Layer, Sunshield Fully Deployed – James Webb Space Telescope
Done on 11:59 AM EST

JWST detailed deployment schedule - updated to the fifth layer being done.

NASA Webb Telescope on Twitter: "Can you feel the tension? ..." / Twitter
Can you feel the tension? Webb is feeling it...in a good way! We tensioned 3 of 5 sunshield layers yesterday. Today we start with Layer 4: (link)

Follow along LIVE for the final layer starting ~9:30 am ET (14:30 UTC): (link) #UnfoldTheUniverse (pic link)

We just tensioned #NASAWebb’s sunshield some more, that was Layer 4. Now everybody look alive, it’s time to tension Layer 5! And better yet — keep watching along live: (link) #UnfoldTheUniverse (pic link)

If you’re tuned in to our live broadcast, now’s the perfect time to go grab a drink of water! Coverage resumes at 11:30 am ET (16:30 UTC) for the tensioning of Webb’s fifth and final sunshield layer 🤩

More on our sunshield: (link) #UnfoldTheUniverse (pic link)
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So they started the fourth layer at around 9:30 AM and finished at 10:23 AM, thus taking 53 minutes.

NASA on Twitter: "It's time: @NASAWebb is GO to begin tensioning its fifth and final sunshield layer, a major milestone for the telescope. Watch the live feed from mission control: (link)

#UnfoldTheUniverse (pic link)" / Twitter
I watched it, and it was mainly two people discussing the deployment while also playing the deployment team's announcements. I would have wanted top priority for the deployment team. But they did mention an interesting tidbit. The primary mirrors are gold-plated beryllium, and they were deliberately made a bit off-shape at room temperature so they will get to their proper shapes when they cool down to their operating temperature.

NASA Webb Telescope on Twitter: "This is it: we’ve just wrapped up one of the most challenging steps of our journey to #UnfoldTheUniverse.

With all five layers of sunshield tensioning complete, about 75% of our 344 single-point failures have been retired! (pic link)" / Twitter
That means 86 single failure points remaining.

NASA on Twitter: "Shields up! @NASAWebb has completed the tensioning of its tennis-court-size sunshield.

This five-layered shade will protect the telescope from the heat of the Sun, Earth and Moon, helping it #UnfoldTheUniverse in infrared light. Details & next steps: (links)" / Twitter

Sunshield Successfully Deploys on NASA’s Next Flagship Telescope | NASA
The five-layered sunshield will protect the telescope from the light and heat of the Sun, Earth, and Moon. Each plastic sheet is about as thin as a human hair and coated with reflective metal, providing protection on the order of more than SPF 1 million. Together, the five layers reduce exposure from the Sun from over 200 kilowatts of solar energy to a fraction of a watt.

This protection is crucial to keep Webb’s scientific instruments at temperatures of 40 kelvins, or under minus 380 degrees Fahrenheit – cold enough to see the faint infrared light that Webb seeks to observe.

“Unfolding Webb’s sunshield in space is an incredible milestone, crucial to the success of the mission,” said Gregory L. Robinson, Webb’s program director at NASA Headquarters. “Thousands of parts had to work with precision for this marvel of engineering to fully unfurl. The team has accomplished an audacious feat with the complexity of this deployment – one of the boldest undertakings yet for Webb.”

...
The unfolding and tensioning of the sunshield involved 139 of Webb’s 178 release mechanisms, 70 hinge assemblies, eight deployment motors, roughly 400 pulleys, and 90 individual cables totaling roughly one quarter of a mile in length. The team also paused deployment operations for a day to work on optimizing Webb’s power systems and tensioning motors, to ensure Webb was in prime condition before beginning the major work of sunshield tensioning.
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That leaves 39 remaining release mechanisms, 22% of the total.
 
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