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After the JWST's successful launch on Christmas Day, much of the world breathed a sigh of relief. However, for the team in charge of deploying the largest and most expensive space telescope ever there's still a long way to go, and many things that could prevent mission success. The passing of each potential obstacle is a victory. Currently the JWST is half way through the deployment of its sunshields, a complex and vital process that will take at least five days.
The size of the JWST's mirror (seven times the size of the Hubble's) makes it so powerful, but is not the main reason its construction and launch have been such a fraught process. The JWST will see almost exclusively in the infrared part of the spectrum, filling a gap in our knowledge of the universe, since exploration has inevitably been concentrated in light visible to our own eyes. However, to see into the infrared requires protection from the Sun's heat, making large and hard-to-deploy sunshields essential.
On Tuesday NASA announced the lowering of the Forward Sunshield, followed that day by the Aft structure. The shields contain five membranes, along with cables to support them and release mechanisms. Since the JWST's orbit — around the Sun, roughly 1.5 million kilometers (1 million miles) away from Earth — effectively prevents astronauts tugging on stuck cables or hitting things that don't work properly, there is little margin for error. Forward deployment took four hours. Failure could have destroyed the mission, so relief is strong.
Nevertheless, lowering the sunshields is not the end of the process. The next step, now completed, was Deployable Tower Assembly. This keeps the telescope and the rest of the spacecraft apart, so the telescope can be maintained at the 40 K (-233ºC or -388ºF) it needs to operate.
Still to come is the sunshield cover release, mid-boom extension, and establishing the appropriate tension in the sunshields' layers.
As NASA notes, each stage takes hours because so many steps are involved. “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,” the agency's Webb blog explains. “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.”
Sunshield deployment is the most complex, but not the final, stage before operations can begin. Once the sunshields create a safely cool environment the mirrors themselves must be deployed, starting with the secondary and followed by the primary's component wings. Once the telescope is fully unfolded it needs a fuel burn to boost it to L2, the Lagrange Point opposite the Sun from Earth.
In between the passage of theses stages, NASA had good news of a different sort – if deployment is successful the JWST should operate for longer than originally planned. The launch and first course corrections went so smoothly less propellant has been used than previously anticipated. This leaves more for the orbital corrections and orientation adjustments necessary throughout the telescope's operating life. Instead of the baseline five years considered the minimum to justify the mission, there is enough propellant for more than 10 years.
In other words, twice the science.
