NASA’s James Webb Space Telescope is slated to launch on October 31, 2021. But let’s talk about Webb’s gold mirror!
The Webb Telescope aims to look back through time to when galaxies were young. And this is when Webb’s gold mirror comes into play!
The James Webb Space Telescope will be able to image the Universe as deeply and powerfully as possible. NASA wanted to use the telescope to observe galaxies that are very distant, at over 13 billion light-years away from us. And to see such far-off and faint objects, Webb needed a mirror. A large mirror.
A telescope’s sensitivity is directly related to the size of its mirror, which determines how much light the telescope can collect from the objects it observes.
However, after years of delays, Webb’s mirror is finally ready.
The mirror is so big that NASA cannot fit it inside of a rocket while fully extended. The mirror needs to fold up in order to go to space. So its ability to fold up and then unfurl, ready to get to work, is crucial.
Last year, the Webb telescope fully deployed its primary mirror into the same configuration it will have when in space.
But the Northrop Grumman engineers behind the James Webb Space Telescope had to put in a lot of effort to construct such a structure. They had to design a mirror with a diameter of 21.5 feet (6.5 meters) that can survive a rocket launch into space, orbit the Earth at a radius of about one million miles for 5-10 years, and hold its shape at temperatures near -220°Celsius.
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This mirror is not a single piece, but an array of 18 segments made of beryllium — a rare metal that is both strong and light. Engineers coated them with a microscopically thin layer of pure gold for maximum reflectivity.
Each of these hexagonal-shaped mirrors spans a diameter of 4.3 feet (1.32 meters) and weighs about 44 lbs (20 kg). Small motors control each segment, and it can move up and down, left to right, and back to front. This is necessary for unfolding and focusing the mirror. The curvature of each segment is also adjustable.
The hexagonal shape allows for a roughly circular, segmented mirror with a high filling factor, meaning the segments fit together without gaps, and six-fold symmetry.
When stitched together in a honeycomb pattern, these segments form an effective surface that will be 6.5 meters (21.5 feet) in diameter. This mirror has seven times the light-gathering power of Hubble.
The reason why scientists designed a roughly circular overall mirror shape is that that focuses the light into the most compact region on the detectors. An oval mirror, for example, would give images that are elongated in one direction. A square mirror would send a lot of the light out of the central region.
While the Hubble Space Telescope is sensitive to visible light, like the human eye, Webb will be most sensitive to infrared light.
All warm objects give off infrared light (in the form of heat). That means that the mirror itself needs to be super cold, to avoid emitting any heat that could interfere with its own observations.
To protect itself from the warmth of the Sun, the mirror will sit on a 70-ft sunshield — as long as a tennis court — made of special heat-resistant material.
Webb will launch on an Ariane 5 rocket into space to the L2 Lagrange point some 1.5 million kilometers more distant than Earth from the Sun.
In 2020, the Webb telescope successfully deployed its giant primary mirror for the first time. During the test, Webb’s mirror was hooked up to specialized gravity-offsetting equipment that simulated the zero-gravity environment in space. So, not only did the mirror deploy as designed, it did so in a space-like environment, proving it’s ready to go. Engineers and technicians will deploy Webb’s primary mirror only one more time before it’s shipped off to its launch site.
Webb’s planning began as early as 1989, the year before NASA launched the Hubble telescope into orbit around the Earth. Development started in 1996. NASA pushed an initial launch date of 2007 to 2011, then to 2014, then to 2018, and now, over 30 years later, the telescope is expected to launch in 2021.
Many think that this space telescope, whose lifetime cost now nears $9 billion, will never launch. But this is the first time that the target launch year now matches the current year, and all signs point to an on-time launch on or around October 31, 2021.
Webb will shed light on the mysteries of our solar system, it will look beyond to distant planets orbiting other stars, and probe the mysterious structures and origins of our universe and our place in it.
The James Webb Space Telescope is an international program led by NASA with its partners, the European Space Agency and the Canadian Space Agency.