Webb’s Aft Momentum Flap Deployed

Shortly after 9 a.m. EST today, the Webb team completed deployment of the observatory’s aft momentum flap. In a process that took about eight minutes, engineers released the flap’s hold-down devices, and a spring brought the flap into its final position.

The aft momentum flap helps minimize the fuel engineers will need to use throughout Webb’s lifetime, by helping to maintain the observatory’s orientation in orbit. As photons of sunlight hit the large sunshield surface, they will exert pressure on the sunshield, and if not properly balanced, this solar pressure would cause rotations of the observatory that must be accommodated by its reaction wheels. The aft momentum flap will sail on the pressure of these photons, balancing the sunshield and keeping the observatory steady.

Just as a ship’s mast must be set in position and the rigging established before the ship unfurls its sails, Webb’s pallet structures, momentum flap, and mid-booms will soon all be in place for Webb’s silver sunshield to unfold. The next steps in Webb’s planned deployment timeline are outlined here.

Webb Ready for Sunshield Deployment and Cooldown

With Webb’s first major structural deployments completed and the observatory’s Deployable Tower Assembly extended, we are taking a step back to learn more about Webb’s sunshield. Observatory Project Scientist Michael McElwain, from NASA’s Goddard Space Flight Center, provided these thoughts:

“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.

“For launch, the sunshield was folded like a parachute and stowed onto the forward and aft unitized pallet structures (UPSs). Both the telescope and sunshield’s support structures are mechanically connected to each other and the spacecraft bus in order to fit within the Ariane 5’s fairing and withstand the dynamic launch environment.

Deployment Graphic

“There are 50 major deployments that transform Webb from its stowed, launch configuration into an operational observatory. The sunshield deployment sequence started with the forward, then aft, UPSs’ mechanical release from the telescope and motorized lowering into position. The telescope and science instruments, mounted on a deployable tower assembly, were then mechanically released and raised. There is a momentum flap attached to the end of the aft UPS that is released and positioned, whose function is to balance the solar pressure on the deployed sunshield. The sunshield covers are released via retraction of membrane release devices and roll out of the way, readying the system for the deployment of the sunshield layers. The telescopic mid-booms sequentially push out from the spacecraft bus perpendicular to the telescope line of sight, pulling the folded stack of sunshield layers out into the final, but still untensioned, configuration. Finally, each sunshield layer is tensioned into position, starting with the Sun-facing layer first and finishing with the telescope-facing layer. The deployed sunshield begins a rapid cooldown of the telescope and the science instruments, but on-board heaters within the science instruments will be used to control their cooldown and prevent contamination.

“While these steps have been tested on the ground and operationally rehearsed in the Mission Operations Center, these critical activities must be executed for a successful mission. Best wishes to our team, and stay cool, Webb!”

– Michael McElwain, Webb observatory project scientist, NASA’s Goddard Space Flight Center

Webb’s Deployable Tower Assembly Extends in Space

This afternoon, the Webb team successfully extended the observatory’s Deployable Tower Assembly (DTA), creating critical distance between the two halves of the spacecraft.

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.

Engineers perform the final deployment test of the James Webb Space Telescope’s Deployable Tower Assembly in June 2021 at Northrop Grumman Space Park in Redondo Beach, California. Credit: NASA’s Goddard Space Flight Center

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.

This step furthers the team’s progress in deploying Webb’s sunshield – a human-controlled, multi-day process that will continue with the release of aft momentum flap and the sunshield covers.

Webb Team Begins Process of Extending Deployable Tower Assembly

Shortly after 9:00 a.m. EST today, engineering teams began the process of extending Webb’s Deployable Tower Assembly (DTA). When deployed, the DTA will create space between the spacecraft and the telescope, to allow for better thermal isolation and provide room for the sunshield to deploy.

This deployment is expected to take six or more hours. It is a human-controlled process that provides the team with the flexibility to pause, assess the data, and make adjustments as needed.

Aft Sunshield Pallet Deployed

Webb is beginning to resemble the form it will take when it is fully deployed – now that the mission operations team has successfully deployed and latched into place the observatory’s forward and aft Unitized Pallet Structures.

The team began working through the deployment of the forward pallet this morning, concluding at approximately 1:21 p.m. EST. The team then moved on to the aft pallet deployment, completing the process at approximately 7:27 p.m. EST. While the actual motion to lower the forward pallet from its stowed to its deployed position took only 20 minutes, and the lowering of the aft pallet took only 18 minutes, the overall process took several hours for each because of the dozens of additional steps required. These include closely monitoring structural temperatures, maneuvering the observatory with respect to the sun to provide optimal temperatures, turning on heaters to warm key components, activating release mechanisms, configuring electronics and software, and ultimately latching the pallets into place.

The unfolding of the pallets marks the beginning of Webb’s major structural deployments and also the beginning of the sunshield deployment phase – which will continue through at least this Sunday, Jan. 2.

The planned timeline of these deployments is laid out here but could change as the operations team gets deeper into the schedule.

Forward Pallet Structure Lowered, Beginning Multiple-Day Sunshield Deployment

Early this afternoon the Webb mission operations team concluded the deployment of the first of two structures that hold within them Webb’s most unpredictable and in many ways complicated component: the sunshield.

The structures – called the Forward and Aft Unitized Pallet Structures – contain the five carefully folded sunshield membranes, plus the cables, pulleys, and release mechanisms that make up Webb’s sunshield. The team completed the deployment of the forward pallet at approximately 1:21 p.m. EST, after beginning the entire process about four hours earlier. The team will now move on to the aft pallet deployment.

Webb's final fold test in April 2021
Engineers at Northrop Grumman Space Park in Redondo Beach, California, oversee Webb’s final mirror fold test in April 2021. The forward pallet structure is seen here in the foreground, in its unfolded state. Credit: Northrop Grumman

The deployment of the forward pallet required several hours of the mission operations team carefully walking through dozens of steps – only one of which was the actual motor-driven deployment to move the pallet from its stowed position to its deployed state. The lowering of the forward pallet also marks the first time that structure has conducted that movement since it underwent its final unfolding and deployment test in December 2020 at Northrop Grumman Space Park in Redondo Beach, California.

The deployment of the pallet structures begins what will be at least five more days of necessary steps to deploy the sunshield – a process that will ultimately determine the mission’s ability to succeed. If the sunshield isn’t in place to keep Webb’s telescope and instruments extremely cold, Webb would be unable to observe the universe in the way it was designed.

The steps involved – outlined here – will continue after today with the extension of the Deployable Tower Assembly, followed by the release of the sunshield covers, the extension of the mid-booms, and finally the tensioning of the five Kapton layers of the sunshield itself.

As the deployment of the sunshield will be one of the most challenging spacecraft deployments NASA has ever attempted, the mission operations team built flexibility into the planned timeline, so that the schedule and even sequence of the next steps could change in the coming days.

Join us to Watch the Webb Launch Live

Webb has been installed onto the Ariane 5 rocket in preparation for launch at 7:20 a.m. Eastern time on Saturday, Dec. 25, 2021. We asked Amber Straughn, Webb deputy project scientist for communications, to tell us how people can watch the launch live online – and how to join the worldwide virtual Webb launch watch party:

As we are all eagerly anticipating launch, you might be wondering how to watch the launch and what to expect. Good news! NASA and its partners are planning a launch broadcast celebration that will be viewable in several different ways. Live countdown commentary and launch broadcast will begin at 6 a.m. EST (11:00 UTC) on Dec. 25 and air on NASA Television and the agency’s website, as well as YouTube, Twitter, Facebook, LinkedIn, Twitch, Daily Motion, Theta.TV, and NASA’s App. You can also join the Facebook event to watch the launch live and interact with others watching the launch. The launch broadcast will continue until approximately one hour past launch, to follow the first several critical milestones post-launch. The actual launch window opens at 7:20 a.m. EST (12:20 UTC) and lasts for 31 minutes – we can launch at any point during that window.

The launch broadcast will include exciting scenes from the launch site in Kourou (naturally!), along with commentary from mission experts from both NASA’s Goddard Space Flight Center and the Mission Operations Center at the Space Telescope Science Institute. In the program you can also expect to see extraordinary video sequences showing construction of the telescope, in-person presentations about key technology, and cutting-edge animations depicting how it will unfold once it gets to space. Tune in to see an exciting new way to learn about the mission science and engineering. There might even be a launch event near you.

After the broadcast is over, follow the progress of commissioning by keeping an eye on our website and social media (Twitter, Facebook, & Instagram), and of course this blog.  We will have almost-daily updates here for the first couple of weeks post-launch, and then will go back to a roughly weekly cadence for technical updates.

But you don’t have to wait until launch day to join in on the excitement. Check out the Journey to Space YouTube series, the Elements YouTube series, and if you are the creative type, submit art for our #UnfoldTheUniverse challenge. Find out even more “Need to Know” information here!

I personally cannot believe the day is almost here. I’m so very excited for launch!

—Amber Straughn, Webb deputy project scientist for communications, NASA’s Goddard Space Flight Center


By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute

Webb Placed on Top of Ariane 5

On Saturday, Dec. 11, NASA’s James Webb Space Telescope was secured on top of the Ariane 5 rocket that will launch it to space from Europe’s Spaceport in French Guiana.

Webb placed on top of Ariane 5
Credit: ESA-M.Pedoussaut

After its arrival in the final assembly building, Webb was slowly hoisted nearly 130 feet and then perfectly aligned on top of the Ariane 5, after which technicians bolted Webb’s launch vehicle adapter down to the rocket.

Webb placed on top of Ariane 5
Credit: ESA-M.Pedoussaut

This whole process was performed under strict safety and cleanliness policies, as it was one of the most delicate operations during the entire launch campaign for Webb. A custom ‘shower curtain,’ already installed between the two platforms where technicians worked to connect Webb to its launch vehicle, served as the walls of a clean room to keep the observatory dirt-free.

Webb placed on top of Ariane 5
Credit: ESA-M.Pedoussaut

The next step ahead is to encapsulate Webb inside the Ariane 5’s specially adapted fairing.

Webb will be the largest, most powerful telescope ever launched into space. As part of an international collaboration agreement, the European Space Agency (ESA) is providing the telescope’s launch services using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the procurement of the launch services by Arianespace and for the development and qualification of Ariane 5 adaptations for the Webb mission.

Webb is an international partnership between NASA, ESA, and the Canadian Space Agency.

Find out more about Webb at: https://webb.nasa.gov.


By Thaddeus Cesari, Webb science writer, NASA’s Goddard Space Flight Center, Greenbelt, Md.

Webb’s Mid-Infrared Instrument Is Ready for Launch

Two weeks until launch! Things are moving forward in Kourou, and so we check in with the two leads (one from the U.S., one from the U.K.) of the final instrument in Webb’s suite:

“Webb’s Mid-Infrared Instrument (MIRI) is special – in the wavelengths it covers, the science that enables, its technology challenges, and in the way it was built.

“With the other three instruments, Webb observes wavelengths up to 5 microns. Adding wavelengths out to 28.5 microns with MIRI really increases its range of science. This includes everything from studying protostars and their surrounding protoplanetary disks, the energy balance of exoplanets, mass loss from evolved stars, circumnuclear tori around the central black holes in active galactic nuclei, and a lot more.

“The universe is relatively unexplored at mid-infrared wavelengths. Since anything at room temperature emits mid-infrared light, infrared astronomers working with ground-based telescopes peer through the huge foreground infrared emission of the telescope and atmosphere. With perseverance, some interesting mid-infrared results have been obtained from ground-based telescopes, but the limitations are severe.

“Dramatic results in the mid-infrared have come from telescopes in the vacuum of space, where they are cooled to cryogenic temperatures to eliminate their emission and are clear of Earth’s atmosphere. This brings big technical challenges. To keep the ice off the telescope before it was launched, the first infrared telescopes in space were built into thermos flasks, or Dewars, with thick walls to hold a vacuum. This meant that these telescopes had to be small, around a tenth the diameter of Webb. Despite their small size, these telescopes were very sensitive and have surveyed the entire sky as well as conducted pioneering studies of individual sources.

“Webb is built on a scale approaching the largest telescopes on the ground, and it will be cold enough to provide the full potential for the mid-infrared. The sensitivity gains and the image clarity will both be nearly a factor of 100 better than ever before. This was so exciting that ten European countries plus the United States pooled resources to make MIRI possible. The National Space Agencies of these ten European countries committed additional funding, beyond their ESA membership, specifically to build MIRI and enable its compelling science.

“While Webb provides the kind of capabilities in the mid-infrared that have only been dreamt of since the beginnings of infrared astronomy, we could only fit a single mid-infrared instrument into Webb and the available international resources. So, we designed the MIRI optics to cover almost everything – imaging, low- and medium-resolution spectroscopy, and high-contrast coronagraphy. About a third of the space for MIRI is empty to allow for long, thermally-isolating hexapod legs. MIRI includes a helium-filled closed-cycle cryogenic refrigerator to bring it to an operating temperature 33 degrees colder than the rest of Webb, reaching less than 7 degrees above absolute zero.

“Our multinational, transatlantic team has pulled together for more than two decades, with both exciting additions and painful losses, to provide Webb and the astronomical community with a mid-infrared instrument. Finally, the moment has arrived when the scientific results will reward everyone who has contributed. The instrument is ready, our cooler is full of helium and connected up, and the team is raring to go.”

—George Rieke, professor of astronomy at the University of Arizona, and Gillian Wright, director of the UK Astronomy Centre


By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute