NASA holds up final Webb sunshade deploy to double check readiness

A frame from a NASA animation showing the James Webb Space Telescope with its five-layer sunshade, secondary and primary mirrors deployed. Image: NASA

Since launch 25 December, the James Webb Space Telescope has sailed through its initial activation, carrying out two precision thruster firings to fine tune its trajectory, deploying its solar array, unlimbering the high-gain antenna it will use to send science data back to Earth and unfolding two pallets holding the observatory’s five-layer sunshade.

The sunshade’s actual deployment began on New Year’s Eve when two telescoping booms at right angles to the pallets began extending, one at a time, slowly pulling out both sides of the shield and unfolding the membranes in the process.

Flight controllers initially planned to begin the process of tensioning the layers, pulling all five taut with motor-driven cables running through scores of pulleys, on New Year’s Day. But managers opted to hold up the two-day procedure to give engineers time off after a hectic week.

Tensioning was held up another day to give the team a chance to assess Webb’s actual performance in the space environment, to characterise the behaviour of its power distribution system and to give the sunshade tensioning motors time to reach optimum operating temperatures.

“We’ve spent 20 years on the ground with Webb, designing, developing and testing,” said Mike Menzel, Webb’s lead systems engineer. “We’ve had a week to see how the observatory actually behaves in space. It’s not uncommon to learn certain characteristics of your spacecraft once you’re in flight. That’s what we’re doing right now.

“So far, the major deployments we’ve executed have gone about as smoothly as we could have hoped for. But we want to take our time and understand everything we can about the observatory before moving forward.”

The sunshade tensioning procedure is critical to Webb’s ability to register infrared light from the first stars and galaxies to light up in the wake of the Big Bang. To detect that faint radiation, Webb must be cooled to within 50 degrees of absolute zero, using the tennis court-size sunshade to block out the heat of the Sun.

On New Year’s Eve, Webb’s sunshade was pulled out by two telescoping booms at right angles to the primary mirror. To work properly, the shield’s five layers must now be tightened and separated, producing gaps between each membrane to help dissipate heat. Image: NASA

But simply pulling the layers out into the now-iconic kite-like shape is not enough. All five membranes must be pulled taut and separated, producing gaps between each layer to allow heat to escape out the sides. The sun-facing side of the shield will reach a blistering 110 C (230 F), but temperatures will drop to around minus 223C (-370 F) on the side facing deep space.

Given the fragility of the Kapton membranes and multiple seams – and given the critical nature of the sunshade – flight controllers opted to carry out additional checks before starting the tensioning process.

“Nothing we can learn from simulations on the ground is as good as analysing the observatory when it’s up and running,” Bill Ochs, the Webb project manager, said in a NASA blog post. “Now is the time … to learn everything we can about its baseline operations. Then we will take the next steps.”

If all goes well, the sunshade will be fully deployed later this week, clearing the way for erection of Webb’s still-stowed secondary mirror and the unfolding of six segments needed to complete its 6.5-metre (21-foot 3-inch) primary mirror. Once that work is complete, engineers will begin aligning each segment to achieve a razor-sharp focus, a process expected to take several weeks.