A pallet holding three of the James Webb Space Telescope’s 18 hexagonal mirror segments rotated into position Saturday, filling out the observatory’s 21.3-foot-wide primary mirror to wrap up the most complicated set of spacecraft deployments ever attempted.
Fourteen days after launch on Christmas Day and now 665,000 miles outbound from Earth, the right-side “wing” of Webb’s iconic segmented mirror swung into place at 10:28 a.m. EST, prompting cheers and applause from elated scientists and engineers at the Space Telescope Science Institute in Baltimore.
Several hours will be needed to fully lock the pallet in place, but the final deployment marked the conclusion to the riskiest, most complex set of steps ever attempted to essentially unfold the largest, most powerful telescope ever launched.
“NASA’s James Webb Space Telescope has fully unfolded!” NASA science chief Thomas Zurbuchen tweeted. “Congrats to the entire @NASAWebb team for an amazing week! Your hard work is a huge testament to what humanity can do when we come together for the betterment of all humankind! Go Webb!”
NASA’s James Webb Space Telescope has fully unfolded! Congrats to the entire @NASAWebb team for an amazing week! Your hard work is a huge testament to what humanity can do when we come together for the betterment of all humankind! Go #UnfoldtheUniverse, Webb! https://t.co/lUgLPfM4gr
— Thomas Zurbuchen (@Dr_ThomasZ) January 8, 2022
At a cost of nearly $10 billion, Webb is the most expensive science spacecraft ever built and by far the most powerful space telescope, 100 times more sensitive than the 31-year-old Hubble, the observatory it will eventually replace.
It’s also one of the most complex, with success riding on the flawless operation of 178 release mechanisms that all had to work perfectly to carry out 50 major deployments to unfold the telescope after it was packed into the nose cone of a European Space Agency-supplied Ariane 5 rocket.
Bound for an orbit around the sun a million miles from Earth, Webb already is far beyond the reach of any foreseeable astronaut repair crews. Its complex, multi-step metamorphosis, requiring all those non-redundant mechanisms to work exactly as planned, simply had to work.
And it did.
“I’m emotional about it,” Zurbuchen said. “What an amazing milestone. One hundred seventy-eight out of 178 of these actuators that had to fire the right way, I’m just so amazed and in awe of this team.”
Since its December 25 launch, Webb successfully unfolded its critical solar array, the high-gain antenna it will use to beam data back to Earth, a stabilizing “momentum flap” to counteract the effects of the solar wind and radiators to dissipate excess heat.
But radiators alone were not enough.
Webb was designed to capture light from the first stars and galaxies to form after the Big Bang, radiation that has been stretched into the infrared region of the spectrum by the expansion of space itself over the past 13.8 billion years.
To register that faint heat, Webb must be cooled to within 50 degrees of absolute zero, or down to around 370 degrees below zero Fahrenheit.
In the riskiest deployment procedure of all, two pallets unfolded and five hair-thin Kapton layers were pulled out to form a sunshade the size of a tennis court using 90 motor-driven cables running through scores of pulleys.
As the layers were slowly pulled taut, they were lifted and separated to provide a gap between each one, allowing heat to reflect out the sides. While the layer facing the sun will experience temperatures as high as 290 degrees, the telescope will be cooled to unprecedented levels to register infrared light from the first galaxies.
Midway through the sunshade’s deployment, a tower was extended, elevating the primary mirror and instruments 4 feet above the shield to provide clearance for membrane tensioning and to further distance the optics from the heat generated by the spacecraft’s electronic gear.
With sunshade deployment complete, engineers sent commands to unfold Webb’s 2.4-foot-wide secondary mirror atop an articulating tripod made up of three 25-foot-long booms. An instrument radiator then was deployed to help dissipate heat generated by Webb’s science instruments.
That set the stage for the final two deployments in the telescope’s initial commissioning.
Webb’s segmented primary mirror was too large to fit inside any existing rocket’s nose cone. So six of its 18 segments, three on each side, were folded back out of the way on hinged pallets. The left-side pallet was rotated into position Friday and the right-side pallet followed suit Saturday.
With the deployments complete, flight controllers now will begin work to precisely align all 18 segments so they act as a single 21.3-foot-wide mirror.
Each segment features six mechanical actuators allowing movement in six directions. A seventh actuator can push or pull on the center of a segment to ever so slightly distort its shape if needed.
Before alignment, the 18 segments will produce 18 separate images. Using using one of Webb’s infrared cameras, engineers will map the alignment of each segment and send commands to adjust the orientation and curvature as required to produce a single, sharply-focused image.
“You’ve got to get these 18 mirrors to act as one mirror,” NASA Project Manager Bill Ochs said before launch. “So when we get on orbit, we go through what we call a wavefront sensing process, but really it’s the focusing process. When you start out, if you’re looking at one star, you’re going to have 18 images and you need to get that down to one.
“So we use the actuators on the back of the mirror, there are motors on the back of each mirror that allow you to move the mirrors up and down, back and forth, in and out as well as change shape slightly.”
That process will take several months to complete, along with checkout and calibration of Webb’s four science instruments. All the while, the telescope will be slowly cooling down to its operational temperature.
As of Saturday morning, the cold side of the sunshade was registering minus 327 degrees but the mirror was “only” down to minus 278 degrees. The first science images are expected by late June.
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