The mirror, a perfect hexagon of gunmetal gray, stands vertically on a low platform. It is about two inches thick and more than four feet wide, a precisely carved slab of beryllium that gleams in the low light of this optics laboratory near San Francisco Bay. My guide, chief engineer Jay Daniel, watches my footing as I step gingerly in front of the mirror to see my reflection. “It’s like your bathroom mirror,” Daniel says, chuckling.
The other side of this looking glass, though, is nothing like a household vanity. The slab of metal is mostly hollow, drilled out by machinists to leave an intricate triangular scaffold of narrow ribs. It is beautiful in its geometric precision, and I resist the urge to touch one of the knifelike edges. The polished front layer that remains, Daniel says, is a mere 2.5 millimeters thick. From its starting weight of 250 kilograms, the entire mirror now weighs just 21 kilos. That is light enough for a rocket to hoist 18 of them deep into space, where the curved mirrors will join as one to form the heart of the most audacious space observatory ever launched.