And that will take a little longer: the Webb team has decided to postpone the tensioning of the solar shield.

Last weekend, the mighty James Webb telescope successfully deployed its solar shield. A great relief. The kite-shaped, silver shiny sun shield is crucial for the functioning of the telescope. So nothing could go wrong during the rollout. Incidentally, we are not quite there yet. Because the sun shield may have been unfolded, but it still needs to be pulled tight. And the team takes just a little more time for that step.

Rolling out the sun shield

The roll-out of the solar shield was still quite an undertaking. In total, no fewer than 107 release mechanisms were activated – not one of which was allowed to fail – to unfold the sun shield. Everything went according to plan, which means that the telescope now has a diameter of more than fourteen meters. The two arms in the center of the boom are also locked in their final position. And that means it’s time for the final step: tightening the sun shield.

Pull tight

The team will separate and then individually stretch each of the five solar shield layers so that they are stretched into their final, taut shape. The Webb team starts with the bottom tier; the largest and the flattest closest to the sun and will experience the highest temperatures. In this way they work incrementally towards the fifth and smallest layer, which is closest to the primary mirror.

The James Webb telescope will primarily observe infrared light from faint and very distant objects. And in order to detect those weak signals, the telescope itself has to be extremely cold. That is why a 5-layer sun shield, the size of a tennis court, has been manufactured. This shield will protect the telescope from external light and heat sources (such as the sun, earth and moon) and from the heat radiated by the observatory itself. Why the ingenious space telescope has a 5-layer sun shield instead of one thick one? That’s because each successive layer of the solar shield is cooler than the layer below it. The opening between the layers also provides an extra insulating effect. In short, one large thick solar shield conducts much more heat than a solar shield consisting of five layers separated by vacuum.

Tightening each layer still takes some work. The team has to activate several engines on board Webb in order to roll out a total of 90 cables: a complex process that will take at least two days.


However, the team has now decided to take a little more time for this next step. They will first focus on optimizing Webb’s energy systems and study how the observatory is currently behaving in space. This will ensure that the telescope is in top condition when the solar shield tensioning is started. “Nothing we can learn from Earth simulations is better than analyzing the observatory when it’s in use,” said Webb project manager Bill Ochs. “Now is the time to take the opportunity and learn all about the basic activities. Then we take the next steps.”


Among other things, the team is analyzing how the flow subsystem is working now that several major implementations have been completed. At the same time, the team ensures that the motors are at the right temperature before starting the tensioning process. It means that the team wants to gain a little more insight into the functioning of the telescope to ensure that the unfolding of all elements that make up the telescope continues to be successful.

“We spent 20 years on the ground with Webb,” said NASA’s Mike Menzel. “And now we’ve had a week to see how the observatory behaves in space. It’s not uncommon to only really get to know certain features of your spacecraft when the mission is underway. That’s what we do now. So far, the major deployments we’ve done have run smoothly. But we want to take our time and understand everything about the observatory before moving forward.”