Written By John Bradley
The James Webb Space Telescope is actually lighter than its predecessor, the Hubble Space Telescope. This one is a real shocker to most people. Under most circumstances, if you want to build a bigger version of something, it’s going to be heavier and more massive.
Hubble was 2.4 meters in diameter, with a solid primary mirror, and a collecting area of 4.0 square metres. James Webb is 6.5 meters in diameter, made out of 18 different mirror segments, with a collecting area of 25.37 square metres. Hubble weighs 27,000lbs and James Webb a mere 14,000lbs!
James Webb’s mirrors are the lightest large telescope mirrors of all-time. By time they’re completed, however, that mass has been reduced to a mere 21 kg (44 pounds), or a 92% reduction in weight. First, the mirrors are cut into their hexagonal shape, which offers a slight reduction in mass. But then — and here’s where it gets brilliant — practically all of the mass on the “back side” of the mirror is machined away.
Although they appear gold, James Webb’s mirrors are actually made out of beryllium. Yes, there’s a gold coating applied to each of the mirrors, but it would have been catastrophic to manufacture the mirrors entirely out of gold. No, not because of the very high density, nor because of gold’s malleability, both of which are properties it definitely possesses. The big problem would be thermal expansion.
The total amount of gold in the James Webb Space Telescope’s mirrors is only 48 grams: less than 2 ounces. Each one of James Webb’s 18 mirrors needs to be outstanding at reflecting the type of light it’s designed to observe: infrared light.
The gold itself will not be directly exposed to space; it’s coated in a thin layer of amorphous silicon dioxide glass. Why wouldn’t you just expose the gold itself to the depths of space? Because it’s so soft and malleable, it’s highly susceptible to damage from even a mild or tiny impact. Whereas the beryllium is largely unaffected by micrometeoroid impacts
The “telescope side” of James Webb will passively cool itself down to no higher than ~50 K: cool enough to make nitrogen liquify. With active, cryogenic cooling, Webb will get all the way down to ~7 K. James Webb should maintain its cold temperatures for its
entire lifespan.
