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How the James Webb Space Telescope Will Work


The Nickel Tour of the James Webb Space Telescope
NASA engineer Ernie Wright studies Webb's first six flight-ready primary mirror segments as they're prepped to begin final cryogenic testing. The primary mirror will have 18 segments in all.
NASA engineer Ernie Wright studies Webb's first six flight-ready primary mirror segments as they're prepped to begin final cryogenic testing. The primary mirror will have 18 segments in all.

Before you sign on the dotted line, we know you'll want to kick the tires and give the vehicle a quick walk-around. Take your time -- this baby's one of a kind.

Webb looks a bit like a diamond-shaped raft sporting a thick, curved mast and sail -- if the sail were built by giant, beryllium-chewing honeybees. Pointed bottom-toward-the-sun, the "raft" portion consists of five gap-separated layers of Kapton-based heat shield. Each separated by a vacuum-filled gap for effective cooling, they together protect the main reflector and instruments.

Kapton is a very thin (think human hair!) polymer film made by DuPont capable of keeping stable mechanical properties under extremes of heat and vibration, and it's already done time in space. If you were so inclined, you could boil water on one side of the shield and liquefy nitrogen on the other. Oh, and it folds up rather nicely, too, which it'll need to do for launch.

The ship's "keel" consists of a Unitized Pallet Structure, which stores the sunshield during liftoff, and solar cells for power. In the center lies the spacecraft bus, which packs all of the support functions that keep Webb running, including electrical power, attitude control, communications, command and data handling, and thermal control. A high-gain antenna adorns the bus's exterior, as do a set of star trackers that work with the Fine Guidance Sensor (see next section) to keep everything pointed in the right direction. Finally, at one end of the heat shield, and perpendicular to it, lies a momentum trim tab that offsets the pressure that photons exert on the ship, much like a trim flap does on a sailing ship.

On the spaceward side of the shield lies the "sail," Webb's giant mirror, part of an optics suite and instrumentation package. Its 18 hexagonal beryllium sections unfold after launch, then coordinate to act like one whopping primary mirror that stretches 21.3 feet (6.5 meters) across.

Opposite this mirror, held in place by three supports, stands the secondary mirror, which focuses light from the primary mirror onto the aft-optics subsystem, a wedge-shaped box jutting from the main mirror's center. This structure deflects stray light and directs the light from the secondary mirror to the instruments housed within the backplate "mast," which does double duty by also maintaining the segmented main mirror's structure.

Once the ship completes its six-month commissioning period after launch, it will last 5-10 years and, we hope, longer, depending on fuel consumption, but it will orbit too far out for servicing. Actually, Hubble and the International Space Station are the exceptions in that regard, but like Hubble and other general observatories, its missions will then derive from competing, peer-reviewed and ranked proposals, submitted by scientists around the globe. The results will find their way into published studies and data available on the Internet.

Let's take a closer look at the instruments that will make all of those studies possible.