WASHINGTON — While the impending launch of the James Webb Space Telescope has astronomers excited and nervous, liftoff only marks the beginning of the riskiest part of the mission.
NASA and other mission partners said Dec. 22 they remained on schedule for a Dec. 25 launch of JWST on an Ariane 5 from French Guiana after weather conditions forced a slip of a day on December 21. The rocket is scheduled to emerge from its assembly building at the launch pad on December 23.
Liftoff is scheduled between 7:20 a.m. and 7:52 a.m. Eastern Time on December 25. If all goes as planned, the JWST will separate from the rocket’s upper stage 27 minutes after liftoff.
Although the launch is usually the riskiest part of a spacecraft mission, it pales in comparison to what JWST will experience. As the spacecraft flies on a rocket that has been in service for more than two decades, with its last catastrophic launch failure in 2002, JWST will have to perform a series of one-of-a-kind intricate deployments and maneuvers once it gets going. will separate from the upper floor.
The first deployment takes place 33 minutes after takeoff, when he releases his solar panel. This will be followed by its first mid-course correction burn, called MCC-1a, which will take place 12.5 hours after liftoff.
Those two events are the most urgent for the spacecraft, Greg Robinson, JWST program manager at NASA Headquarters, said in a Dec. 21 call with reporters. “Everything else is flexible, but at the moment we are definitely planning on a nominal schedule,” he said.
This nominal delay foresees the beginning of the deployment of its sun visor three days after the launch. Two paddles on either side of the spaceship will swing down with the sun visor folded inside. Two days later, the covers protecting the sun visor come free, followed the next day by the deployment of two booms to extend the sun visor in all its dimensions. The visor’s five foil-coated Kapton layers are deployed over the next two days, completing deployment of the visor eight days after launch.
Deployment of the telescope mirror begins 10 days after liftoff when a tripod holding the small secondary mirror moves into place. Two wings, each containing three of the 18 primary mirror segments, then lock into place, completing the telescope’s deployment 13 days after launch.
Months of work lie ahead after these deployments to align the telescope mirrors and commission the instruments as they cool to their operating temperatures, a process that will not be completed until six months after launch. A maneuver 29 days after launch will place JWST in its final halo orbit around the Earth-sun Lagrange point L-2, 1.5 million kilometers from Earth.
These initial deployments, however, are among the most critical and risky. During a briefing in November, Mike Menzel, principal mission systems engineer JWST at NASA’s Goddard Space Flight Center, said there were 344 point failures in the spacecraft, 80% of which are associated with mechanisms. of deployment. “When you have a release mechanism, it’s hard to put complete redundancy into it,” he said.
The sun visor, for example, includes 140 release mechanisms, 70 hinge assemblies, eight deployment motors, about 400 pulleys and 90 cables with a total length of 400 meters, said Krystal Puga, gear systems engineer JWST spacecraft at Northrop Grumman, during this November briefing. .
“I think of the sunshade deployment as similar to a Rube Goldberg machine, in that it uses a series of reactions that work in succession, triggering one event after another until the whole sunshade is fully deployed. deployed,” she said, but added that she was confident. it would work fine.
“We’ve done multiple deployment tests over several years on both small and large models,” she said. “It gives us confidence that Webb will deploy successfully.”
“The things we’ve focused on for the rollout are definitely the sun visor,” Menzel said. “The sun visor is one of those things that’s almost inherently indeterministic.”
If something goes wrong during the deployment process, there are various contingency plans to attempt to resolve the issue. Alphonso Stewart, JWST deployment systems manager at Goddard, said the simplest fixes are to re-send deployment commands and check telemetry for any erroneous signals.
More complex steps, he said, involve a “shimmy” where the spacecraft is rocked back and forth or a “whirlwind” to spin the spacecraft on an axis to release a stuck item. Another stage, dubbed “fire and ice,” would reorient the spacecraft to put sunlight on specific areas to heat the components. “This particular exercise, this contingency, is probably the last step,” he said. “We have quite a few alternative contingencies in the system.”
If a deployment issue cannot be resolved, it does not necessarily mean the end of the mission. “I don’t think I would, if half weren’t deployed we wouldn’t have a problem. We definitely would,” Menzel said of the sun visor. “If parts of it didn’t deploy exactly the way we wanted, a lot of that would depend on where the misalignment was.”
The telescope has “cryogenic margins” that could absorb some sunshade deployment issues. “We might have unknown thermal loads, for example, one that might arise from the [sunshield] layers touching,” he said, especially if that contract was localized to an area of the sun visor.
Bill Ochs, JWST project manager at Goddard, said the procedures are ready for a number of potential issues during deployment. “We don’t talk about what we will do if we fail. We talk about how we fix the issues we see in orbit and how we progress from there,” he said.
Although the sun visor and telescope deployments are among the riskiest phases of flight, he said he would not relax once they were completed. “As project manager, I won’t breathe a sigh of relief until we declare that we are operational 180 days after launch.”