Category: James Webb Space Telescope

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.

“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

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.

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.

After a successful launch of NASA’s James Webb Space Telescope Dec. 25, and completion of two mid-course correction maneuvers, the Webb team has analyzed its initial trajectory and determined the observatory should have enough propellant to allow support of science operations in orbit for significantly more than a 10-year science lifetime.  (The minimum baseline for the mission is five years.)

The analysis shows that less propellant than originally planned for is needed to correct Webb’s  trajectory toward its final orbit around the second Lagrange point known as L2, a point of gravitational balance on the far side of Earth away from the Sun. Consequently, Webb will have much more than the baseline estimate of propellant – though many factors could ultimately affect Webb’s duration of operation.

Webb has rocket propellant onboard not only for midcourse correction and insertion into orbit around L2, but also for necessary functions during the life of the mission, including “station keeping” maneuvers – small thruster burns to adjust Webb’s orbit — as well as what’s known as momentum management, which maintains Webb’s orientation in space.

The extra propellant is largely due to the precision of the Arianespace Ariane 5 launch, which exceeded the requirements needed to put Webb on the right path, as well as the precision of the first mid-course correction maneuver – a relatively small, 65-minute burn after launch that added approximately 45 mph (20 meters/sec) to the observatory’s speed.  A second correction maneuver occurred on Dec. 27, adding around 6.3 mph (2.8 meters/sec) to the speed.

The accuracy of the launch trajectory had another result: the timing of the solar array deployment. That deployment was executed automatically after separation from the Ariane 5 based on a stored command to deploy either when Webb reached a certain attitude toward the Sun ideal for capturing sunlight to power the observatory – or automatically at 33 minutes after launch. Because Webb was already in the correct attitude after separation from the Ariane 5 second stage, the solar array was able to deploy about a minute and a half after separation, approximately 29 minutes after launch.

From here on, all deployments are human-controlled so deployment timing – or even their order — may change. Explore what’s planned here.

 

 

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.

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.

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.

At 7:20 pm EST – 60 hours after liftoff — Webb’s second mid-course correction burn began. It lasted 9 minutes and 27 seconds and is now complete. This burn is one of three planned course corrections to put the telescope precisely in orbit around the second Lagrange point, commonly known as L2.

While the team continues to work on unfolding Webb, we take a moment to  learn more about the launch from two European Space Agency (ESA) representatives, Daniel de Chambure, Acting Head Ariane 5 Adaptation & Future Missions for ESA and Maurice Te Plate, JWST NIRSpec Systems and AIV Engineer for ESA. They provide details about the launch trajectory, which is the first phase of Webb’s journey before additional planned course corrections:

Europe’s Ariane 5 delivered Webb, with a launch mass of about 13,700 lbs (6200 kg), into the first phase of its planned trajectory toward its final position orbiting the L2 Lagrange point. As we all held our collective breath, lift-off was on December 25, 2021 at 7:20 am EST from Europe’s Spaceport in Kourou, French Guiana, for a flight lasting about 27 minutes before spacecraft separation.

About seven seconds after start of the ignition of the main stage cryogenic engine, the two solid propellant boosters were ignited, enabling liftoff. The launcher first climbed vertically for about 13 seconds, and then rotated towards the East. The solid boosters were jettisoned 2 mins and 14 sec after liftoff.

The fairing that protected Webb from the acoustic, thermal and aerodynamic stresses during the ascent, was jettisoned 3 minutes and 19 sec after liftoff. In order to protect the delicate thermal sunshield blankets of Webb, the fairing had been modified to minimize the shock of depressurization at separation. Updated venting ports allowed the pressure inside the fairing to properly equalize prior to opening. The recorded residual pressure was successfully below the required allowed maximum.

Once the atmospheric part of the flight was completed, Ariane’s onboard computers optimized the trajectory in real time, and brought the launcher to the intermediate orbit targeted at the end of the main stage propulsion phase, at 8 minutes and 35 seconds after launch. Ten seconds later, the HM7B engine of the cryogenic upper stage, with Webb still on top, was ignited and operated for a duration of 16 minutes. After engine shut-down, the upper stage underwent a number of positioning maneuvers with its attitude control system in order to separate Webb at the required attitude.

After separation, the upper stage underwent a delicate series of contamination and collision avoidance maneuvers, making sure that its thruster plumes did not impinge on Webb and its precious optics.

Finally, an end-of-life maneuver was performed to avoid potential long term collision risks with Webb.

Webb had started its journey to explore the Universe. Webb is on its way!

-Daniel de Chambure, Acting Head Ariane 5 Adaptation & Future Missions, ESA
-Maurice Te Plate, JWST NIRSpec Systems and AIV Engineer, ESA

 

 

On Dec. 25, the Webb team successfully executed the first of three planned orbit corrections to get Webb into its halo orbit around the second Lagrange point, L2. To hear more about these important maneuvers, here is Randy Kimble, the Webb Integration, Test, and Commissioning Project Scientist, at NASA Goddard:

In sending the Webb Observatory into its orbit around the Sun-Earth L2 point, the vast majority of the energy required was provided by the Ariane 5 rocket. After release of the observatory from the rocket, several small tweaks to the trajectory are planned, to ease the observatory into its operating orbit about one month after launch.

The largest and most important mid-course correction (MCC), designated MCC-1a, has already been successfully executed as planned, beginning 12.5 hours after launch. This time was chosen because the earlier the course correction is made, the less propellant it requires. This leaves as much remaining fuel as possible for Webb’s ordinary operations over its lifetime: station-keeping (small adjustments to keep Webb in its desired orbit) and momentum unloading (to counteract the effects of solar radiation pressure on the huge sunshield).

The burn wasn’t scheduled immediately after launch to give time for the flight dynamics team to receive tracking data from three ground stations, widely separated over the surface of the Earth, thus providing high accuracy for their determination of Webb’s position and velocity, necessary to determine the precise parameters for the correction burn. Ground stations in Malindi Kenya, Canberra Australia, and Madrid Spain provided the necessary ranging data.  There was also time to do a test firing of the required thruster before executing the actual burn. We are currently doing the analysis to determine just how much more correction of Webb’s trajectory will be needed, and how much fuel will be left, but we already know that the Ariane 5’s placement of Webb was better than requirements.

One interesting aspect of the Webb launch and the Mid-Course Corrections is that we always “aim a little bit low.” The L2 point and Webb’s loose orbit around it are only semi-stable. In the radial direction (along the Sun-Earth line), there is an equilibrium point where in principle it would take no thrust to remain in position; however, that point is not stable. If Webb drifted a little bit toward Earth, it would continue (in the absence of corrective thrust) to drift ever closer; if it drifted a little bit away from Earth, it would continue to drift farther away. Webb has thrusters only on the warm, Sun-facing side of the observatory. We would not want the hot thrusters to contaminate the cold side of the observatory with unwanted heat or with rocket exhaust that could condense on the cold optics. This means the thrusters can only push Webb away from the Sun, not back toward the Sun (and Earth). We thus design the launch insertion and the MCCs to always keep us on the uphill side of the gravitational potential,  we never want to go over the crest – and drift away downhill on the other side, with no ability to come back.

Therefore, the Ariane 5 launch insertion was intentionally designed to leave some velocity in the anti-Sun direction to be provided by the payload. MCC-1a similarly was executed to take out most, but not all, of the total required correction (to be sure that this burn also would not overshoot). In the same way, MCC-1b, scheduled for 2.5 days after launch, and MCC-2, scheduled for about 29 days after launch (but neither time-critical), and the station-keeping burns throughout the mission lifetime will always thrust just enough to leave us a little bit shy of the crest. We want Sisyphus to keep rolling this rock up the gentle slope near the top of the hill – we never want it to roll over the crest and get away from him. The Webb team’s job, guided by the Flight Dynamics Facility at NASA Goddard, is to make sure it doesn’t.

-Randy Kimble, JWST Integration, Test, and Commissioning Project Scientist, NASA Goddard Space Flight Center