The James Webb Space Telescope’s famous golden mirror is made of beryllium, element 4. Its primary mirror is 6.5 metres across, the largest ever launched into space, and it is built from 18 hexagonal segments of beryllium, each 1.32 metres across. The gold is only a coating, a layer 100 nanometres thick, added because gold reflects infrared light so well. So the golden mirror is really a grey metal wearing a very thin coat of gold.
Why not glass?
A mirror 6.5 metres across is wider than the rocket that carried Webb, so it had to be made in segments that fold for launch. It also had to be light: according to NASA, a mirror like Hubble’s scaled up to Webb’s size would have been too heavy to launch. Two test mirrors were built and compared, one of beryllium and one of a special glass, and the experts recommended beryllium.
NASA’s engineers chose beryllium because it is very strong for its weight and good at holding its shape across a range of temperatures, including the deep cold Webb works in: its mirrors sit at around −220 °C. Beryllium has a density of only 1.85 grams per cubic centimetre, 1.85 times that of water, and it is unusually stiff; its modulus of elasticity is about 35 percent greater than that of steel.
To save more weight, most of the metal was machined away from the back of each segment, leaving a pattern of thin ribs about 1 millimetre thick. What is left still holds the segment’s shape steady, and each beryllium segment has a mass of about 20 kilograms.
The gold
If the metal does the work, why the gold? Webb is an infrared telescope. It looks for the faint infrared glow of very distant galaxies, and gold improves the mirror’s reflection of infrared light. A gold layer 100 nanometres thick, 1000 ångströms, is enough, and it is that coating that makes the honeycomb glitter in every photograph.
The emerald element
Beryllium’s name comes from the mineral beryl. In 1798 the French chemist Louis-Nicolas Vauquelin found a new “earth” while analysing emerald and beryl, which are geometrically identical crystals, and the element was later named after the mineral. Emerald and aquamarine are both forms of beryl.
The metal has a darker side: beryllium dust is toxic to breathe. In a finished mirror, far out in deep space, it is solid metal doing a very quiet job.
The Short peels the gold back from one hexagon to show the grey metal underneath. Element 4, beryllium: the strength behind the shine.
