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When you look at NASA's James Webb Space Telescope you can't help but

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see the huge gold mirrors. 

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But that gold is really only on the surface the rest of the mirrors

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mirrors are made out of a metal called beryllium.

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Why does Webb have beryllium Mirrors. Elements of Webb

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Let's head to the shop that worked on these mirrors to get a better idea about why

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why this metal is so remarkable. 

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music

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I’ll just follow the noises.

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Engineers chose beryllium for 3 major reasons.

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It is super lightweight. 

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It is the second lightest of all the metals and only two thirds the weight

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aluminum. It's super strong. 

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Beryllium is 6 times stiffer than steel. 

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Each of these cantilevered weights are the same

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except for the type of metal. They are the same length, the same weight 

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and size. Look at what happens when they are lifted 

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and released at the same time. The beryllium wiggles with less intensity

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and stops moving first. That is because it is far 

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stiffer than the other two. So it can also hold its shape

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over really broad range of temperatures, and that's especially important in the colds ones.

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I measured these identical aluminum and beryllium mirrors,

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and left them in some dry ice for a while. Science.

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I admit, that my measuring technique could used some work

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But the aluminum shrunk more than the beryllium. 

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Materials tend to change in shape, volume, and density

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in response to temperature. When something is heated up,

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the molecules begin to vibrate more and separate, a little. The amount the material 

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changes is predictable and is based on its

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coefficient of thermal expansion. So beryllium it stops shrinking and changing 

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its size when it cools below 100 Kelvin.

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So the mirrors we see are a little too large on Earth

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and will be the exact size they need to be once they are out in space.

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music

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Beryllium is obviously an incredible material. 

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But everything does have its nuances.

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So, why isn’t beryllium used more often? 

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Machining beryllium requires extreme care. If you think about it, 

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beryllium is actually a powder 

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that is pressed together and requires 

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very very light cuts. Any small mistake it would crack, 

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and there would probably be a piece that would fall off. 

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And you've lost a part that can be worth anywhere between 

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between $1,800.00 to $2,000.00 a pound. 

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Under the wrong circumstances, beryllium may be fragile, and it's fairly expensive. 

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But it holds its shape well, is very stiff and most

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most importantly it's light weight. Rockets need a lot of fuel 

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for each pound of payload, so any weight saving measures

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are really really worth it. 

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music