Like a flower unfurling, the largest and most complex space telescope ever built has fully deployed its tennis court-sized sunshield, guaranteeing it the cool zone needed to operate at infrared wavelengths. The process took a day longer than originally planned, but after a 14 year-delay getting off the ground that looks minor, and the operating team can now turn to the next task of mirror deployment. After that, all that will stand between the mighty telescope and first observations will be a boost into its final orbit...oh, and months of performance optimization. Twenty-five years and countless setbacks since the JWST was first approved, confidence is growing.

With a mirror seven times the size of Hubble's, the JWST pushed the maximum size that could be carried aboard an Ariane 5 rocket. Future telescopes may be clipped together from components carried up separately, but the JWST's planned orbit at the Lagrange 2 point of the Earth-Sun system meant that wasn't really viable with current technology.

Consequently, the JWST needed to be packed up tight, with its 22-meter (70-foot) shields and mirrors opening out once safely above the Earth's atmosphere, including a distance between the shields and main telescope to prevent heat transfer.

Prior to launch, the multi-stage sunshield deployment process was identified as a particularly risky part of the process. When JWST is in position, much further out than Hubble, humans won't be able to fix it, so everything has to work perfectly first time. Needing to be sure the sunshield would open was one of the reasons the launch was delayed so many times. Deployment took an entire week. If this sounds like overkill, 400 pulleys, 139 release mechanisms, 90 cables, 70 hinges, and eight deployment motors all had to operate, with little to no margin for error. Conducted within the 12 days of Christmas, it's a bigger task than wrangling all those lords-a-leaping and partridges in pear trees.

The process of fully deploying the sunshield was delayed for a day to make sure motors that would put the right tension in the cables were at the ideal temperatures.

Now, however, all that is behind us, with NASA announcing successful completion.

Although the mirrors also need to be unfolded, starting with the secondary, NASA’s Associate Administrator for the Science Mission Directorate Dr Thomas Zurbuchen described the sunshield as the JWST's “most challenging deployment” and called its success: “An incredible testament to the human ingenuity and engineering skill that will enable Webb to accomplish its science goals.”

Even once the JWST is safely inserted in its orbit – planned for 18 days from now – observations cannot start immediately. Instead, the telescope will need to keep cooling down as the shields block new heat from the Sun and it radiates what it acquired before they were up. Then the optics need to be aligned (owners of backyard telescopes know the pain) and instruments calibrated. This could all take another five months before proper observations begin, so it's just as well the telescope's expected operational life has been doubled to 10 years.