How Webb's Origami Mirror Is Supposed to Work

The James Webb Space Telescope is sitting in French Guiana right now, going through final checks before it rides an Arianespace Ariane 5 rocket into space from the Guiana Space Centre in Kourou. A December liftoff has been the plan, and every time I think about the actual hardware involved, I get a little nervous on NASA’s behalf.

Here’s the thing that makes Webb different from basically every telescope that’s come before it: the mirror is too big to launch in one piece. Hubble’s primary mirror was 2.4 meters across and fit inside its rocket fairing just fine. Webb’s primary mirror is 6.5 meters across. There is no fairing on Earth wide enough to carry that up fully assembled. So the engineers did something that sounds almost too clever for its own good — they built the mirror out of 18 separate hexagonal segments, each coated in a thin layer of gold, and designed the whole assembly to fold up like a piece of furniture from a box, then unfold once it’s in space.

Gold is the interesting detail here. It’s not decorative. Gold reflects infrared light extremely well, and infrared is the part of the spectrum Webb is built to observe — it’s how you see the light from the earliest, most distant galaxies, whose visible light has been stretched into infrared wavelengths by the expansion of the universe over billions of years. Beryllium was picked for the segment substrate underneath the gold because it’s light, stiff, and holds its shape across a huge range of temperatures, which matters when one side of the telescope will be facing the sun and the other side will be facing the cold of deep space.

The part that should make anyone nervous is the unfolding itself. Once Webb separates from the rocket, it has to unfold its sunshield and then swing those mirror segments into position, one by one, using tiny actuators behind each segment that can nudge it into place with incredible precision — we’re talking alignment tolerances measured in fractions of the wavelength of light. There’s no repair mission possible out where Webb is headed. Hubble got serviced by astronauts multiple times because it orbits close to Earth. Webb is going to sit roughly a million miles out, at the L2 Lagrange point, where a shuttle crew simply can’t reach it. Whatever ships is what we get, and if a segment doesn’t lock into place correctly, there’s no going up to fix it with a wrench.

I don’t say this to be doom and gloom about it — the engineering behind this thing has been in development for over a decade, and folding mechanisms have been tested and retested on the ground more times than I can count. But it’s worth sitting with just how audacious the design really is. We’re about to fold up a giant gold mirror, cram it into a rocket, and trust it to unfold itself correctly a million miles from home. If it works, we get a telescope that can peer further back in cosmic time than anything before it. If any part of that unfolding sequence goes wrong, well, it’s a very expensive origami crane sitting silently in space.

Launch prep continues in Kourou, and I’ll be watching closely once a firm date is locked in.