The James Webb Space Telescope’s five-layer sunshield, seen here during ground testing at Northrop Grumman’s factory in Redondo Beach, California. Credit: NASA/Chris Gunn
Mission controllers started the delicate work of tightening the five razor-thin layers of the James Webb Space Telescope’s sunshade Monday. Managers confirmed that the observatory costing nearly $10 billion is complete. “hunky-dory” after pausing deployments over the weekend to adjust the observatory’s power levels and ensure motors needed for the tensioning are in tip-top shape.
Ground teams at Baltimore’s Space Telescope Science Institute hoped to complete tensioning Monday’s largest and hottest layer in the tennis court-sized sunshield. The remaining four will be pulled taut with a complex system of electric motors, pulleys, and cables Tuesday and Wednesday, assuming everything goes according to plan, said Bill Ochs, NASA’s Webb project manager.
But managers have flexibility in the schedule, and there’s no rush to push Webb through its remaining deployments, a sequence of events required to transform Webb from its launch configuration into a telescope tuned to register the faint infrared light from the first galaxies in the universe.
“We do not want to burn out our team along the way,”Ochs spoke Monday during a conference call with reporters.
The first week of Webb’s mission went like clockwork, but engineers changed the schedule over the weekend to make adjustments to the spacecraft’s power-generating solar array and cool higher-than-expected temperatures on the motors required to pull the sunshade tight.
Officials waited to tension the sunshield to give ground teams enough time. “rebalance” Webb’s 20-foot-long (6-meter) solar array to improve its power generating capacity. The change involved adjusting the solar array, made of five individual panels, off its factory settings, according to Amy Lo, a Webb systems engineer at Northrop Grumman, NASA’s prime contractor for the mission.
Webb was able generate enough electricity with the solar array. However, engineers prefer to have as much power as possible as the power-hungry observatory continues to require more deployments and commissioning.
Lo said engineers knew they would reset the array’s operating parameters after monitoring its performance in space. The power throughout from the solar array can depend on a range of factors, including the temperature of the system’s solar cells and regulators feeding power to Webb’s batteries. Lo said that a solar panel that is hotter than the others is slightly less efficient in generating power.
“The factory preset makes all of the panels and all of the regulators operate at the same duty cycle (or operating regime), so it’s not very optimized,”Monday was Lo’s turn
“As we got on orbit and understood what temperatures the arrays were operating at, and tracked the load we expected to see, we then went ahead and made the decision that we did want to rebalance the array and set a duty cycle so that each panel of the array … could be optimized to work at their best duty cycle possible.”
The reset Sunday changed the system’s voltage from 58.6 volts to between 65 and 69 volts,Lo said.
“So the change wasn’t much, but it’s what we needed,” Lo said. “Everything is hunky-dory and doing well now. The observatory was never in danger.”
Artist’s concept of the James Webb Space Telescope, as it appeared after opening the mission’s five-layer sunshield. Credit: NASA
The other technical issue Webb’s ground team addressed over the weekend was with the sunshade tensioning motors.
Motors are essential components that drive sunshade layers to fully tensioned states. Although their temperatures were slightly higher than expected over the weekend they were still within acceptable operating limits.
“As we looked at the temperature of the motors, it did not have as much margin as we preferred,”Monday was Lo’s turn “One way of cooling the motor is to re-point the observatory, so there’s less incident sun on these motors, so they can have time too cool off. So that was executed last night.”
The Webb telescope was launched Dec. 25 aboard an European Ariane 5 rocket. It has performed well as it unfolded the solar array, high-gain communication antenna, and two large pallets that held the sunshade layer for the journey to space.
Webb extended a tower assembly last week to provide distance between the telescope’s mirrors and instruments, which must be chilled to cryogenic temperatures, and the much hotter spacecraft, with the solar array that needs to be constantly pointed at the sun.
The sunshield is designed for a 600-degree thermal gradient between hot spacecrafts and cold telescopes. It is made from kapton, a material that is as thin as a human hair and treated with shiny aluminium. It has five layers. To increase reflectivity, the two outermost layers are coated with a purple-hued silicone coating.
Ground teams linked commands to Webb to remove the sunshield membranes from the spacecraft Thursday. Friday’s mid-booms extended two sides of the spacecraft, revealing the five sunshade layers.
“Mid-boom deployment was huge,”Ochs stated. “That was really a huge achievement for us. The tensioning of the layers is the next big step that we go through.”
Before Webb’s launch, most NASA managers and astronomers gave 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, in a press conference before launch. “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.
Menzel said that predicting the behavior and properties of the sunshield layers was like predicting what a string will do if you push it on a 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.”
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 stop tears or holes and metallic ribbons that give the kapton some structural support.
“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 the five ultra thin membranes. They will be spaced a few inches apart at the center and a couple of feet at the edges. The tapered spacing allows heat to reflect between the layers and radiate back into the universe.
Six motors will pull on a quarter mile of cable on the left, right, and front of the sunshield. The cables are used to tighten up the five layers of kapton. Webb can be asked to halt any motors that get too hot.
“JWST deployment is designed to stop any any point,”Lo said. “We can stop at any time and do a little bit of reverse, and then keep going. That is a basic fundamental design driver that we have in our requirements.”
Guides will help you navigate the movement. Friday’s deployment of the mid-booms worked exactly as planned. 107 critical release mechanisms activated to allow the solar blanket unfurl.
“The observatory is designed very carefully to be snag-free, and we have tested this multiple times on the ground,”Lo said. “Given that our telemetry matches the ground testing, we are expecting to have smooth sailing.”
Ochs stated that once all five sunshade layer are fully tensioned the mission will be complete. “retired somewhere between 70 and 75%”One-point failures were identified in 344 cases before launch. Webb must be able to operate its mission without interruptions if there are 344 possible failure points.
Webb’s instruments and telescope need to be super-cold to make them sensitive to infrared light. Astronomers also want to observe the cosmos using infrared wavelengths. This allows them to see the oldest galaxies whose light waves were stretched by the expansion.
Infrared Astronomy also shows star-forming regions that are obscured by clouds made of gas and dust. This makes it difficult for telescopes to see in visible wavelengths. This is the type of light that can be seen by the human eye.
Mark McCaughrean, an Astronomer at the European Space Agency tweeted last week that infrared Astronomy can be done with a warm telescope “is like trying to observe in the visible in broad daylight with a telescope made of light bulbs. Possible, but you won’t see faint things very well.”
Once the sunshield is deployed and tensioned, ground teams will turn their attention to unfolding Webb’s huge primary mirror to its full 21.3-foot-wide (6.5-meter) diameter, the biggest telescope ever sent into space. A secondary mirror, mounted on a tripod-like boom apparatus, also needs to move into place to reflect light into Webb’s instrument module, and ultimately onto detector arrays to make images and spectral measurements.
Ochs indicated that those events will start at the end of the week, a few more days later than originally planned following the weekend’s pause in deployments.
Webb is cruising towards its operations post in an orbit resembling a halo around the L2 Lagrange points, a gravitational balance spot nearly a million miles (1 million kilometers) from Earth. Webb’s arrival in that orbit is expected around Jan. 23, followed by five more months of instrument activations, optical focusing, and other calibration work before the science mission begins.
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