The James Webb Space Telescope has only gone and deployed its primary mirror

And you thought unfolding the table for Christmas dinner was tricky


The gold-coated primary mirror of the James Webb Space Telescope (JWST) was fully deployed this weekend, marking the end of the epic major deployments of the spacecraft, but only the beginning of months of alignment and calibration.

The deployment of the sunshield required to keep things cool was successfully completed last week as the spacecraft hurtled toward its final destination, an orbit around the second Lagrange point (1.5 million km, or nearly 1 million miles from Earth.) Following the deployment of the secondary mirror, the final step was the unfolding of the JWST's primary mirror.

The segmented primary mirror is 6.5 metres across and consists of 18 gold-coated mirror segments. Folded in order to fit in the fairing of the Ariane 5 rocket that launched it, the first panel was deployed on 7 January and the second locked into place on 8 January. The completed mirror is the largest ever launched into space.

It is quite the engineering feat. Although it was exhaustively tested prior to launch, any issues the team might have faced during the deployment in space could have resulted in an abrupt curtailment of the mission. Unlike the Hubble Space Telescope during the Shuttle days, there is currently no way for astronauts to reach the JWST. However, things have gone well.

As NASA administrator Bill Nelson put it: "Each feat already achieved and future accomplishment is a testament to the thousands of innovators who poured their life's passion into this mission."

There is still months of work to do before the European Space Agency (ESA), NASA and Canadian Space Agency collaboration can begin observations. Another burn is coming up and those 18 primary mirror segments must be moved to align the optics. The science instruments also need to be calibrated.

As the agencies ticked off each JWST milestone, the deployments were represented during briefings by computer generated graphics, prompting the question of: "Why no cameras to watch the deployment of the multi-billion dollar observatory?"

Ostensibly a reasonable question, and one that had come up and been rejected during the design of the observatory. Aside from the issue of lighting (the sun-facing side would suffer from extreme glare and contrast issues) and temperature (a camera on the cold side would have to work at cryogenic temperatures) there was the question of benefit of the extra complexity. Certainly when compared to the telemetry that would flow from the various mechanical, thermal and electrical sensors built into the spacecraft.

The movement of the individual mirror segments out their launch configuration is expected to be completed approximately 27 days following launch. A day and a half later, the JWST will complete its insertion into a halo orbit around L2. It will take a good few weeks for the instruments to cool in the shade of the sunshield and the remaining five months will be all about calibration. ®

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