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How does the James Webb Telescope work? What will it see?

Updated: Jan 27


The great Carl Sagan once said, “The cosmos is within us. We are made of star-stuff. We are a way for the universe to know itself.”


The atoms in our bodies were forged deep within the bellies of red giant stars eons ago. And if we peer deep enough into the night sky, we can see stars just like them taking shape at the edge of the observable universe. The light from those stars is more than 13 billion years old and thanks to the expansion of the universe, it has stretched out into super-long infrared wavelengths—basically heat. To “see” that heat, you need a really big, and really complex telescope. In space.


Enter the James Webb Space Telescope—or JWST if you’re an astronomy buff—the most advanced space telescope ever created and one of the most complex and clever pieces of engineering in the history of humanity.


You’ve probably seen headlines or tweets or FaceBook posts about the Webb telescope, which was launched in December and is now happily zooming through space to its final destination about a million miles (1.5 million kilometers) from earth. You’ve probably seen astronomers and scientists and scifi fans frothing at the mouth, wild eyed and generally going bonkers over it. That’s because Webb will be able to see farther than any other telescope before it, unveiling some deep secrets of the universe. The telescope has been in the works since 1996 and was originally due to launch way back in 2007. Needless to say, there were some setbacks—this is one of the most complex machines we’ve ever built. So let’s talk about why Webb has taken so long to build, and why it’s so damn cool.


The James Webb Space Telescope was named after James E. Webb, NASA’s chief administrator from 1961 to 1968. He played a huge role in the Apollo moon missions and the earlier Mercury and Gemini orbital missions. Webb, the telescope, was originally designed to succeed Hubble, which is great at seeing visible light and ultraviolet, but not so great at seeing infrared. And as we mentioned, if you want to peer deep into the history of the universe, you need a telescope that can see infrared light. But there are some big challenges to making a good infrared space telescope. First, it needs to be really really big. The red-shifted light from the early universe is very faint and you need huge mirrors to capture it. Second, it needs to be cold. Heat from our own sun can easily interfere with sensitive infrared sensors. Third, it needs to be far away from earth to avoid any possible interference. All of these requirements make building and launching such a telescope extremely difficult.


A big telescope needs to somehow fit within the relatively small cargo bay of a rocket, which means it should fold up like an origami swan. Likewise, any heat shielding should also fold up to fit inside the nose cone of the rocket. The telescope also needs to carry its own fuel to make the trip from low earth orbit to its destination. And it all needs to be relatively light because there’s a limit to how much stuff you can launch into space. Last, and maybe most importantly, the thing needs to unfold by itself without any help. So that’s why it took the world’s leading scientists and engineers almost 30 years to build.


So here are the specs. The Webb telescope uses a 6.5m (21.3 foot) mirror made up of 18 gold-plated hexagonal sections. The base of each hexagon is made of beryllium, a lightweight metal that doesn’t really change shape when it heats up or cools down. It’s also non-magnetic, which is important in a finely-tuned instrument like Webb. The mirrors themselves are gold because gold is really good at reflecting infrared light. The 100 nanometer gold layer is also covered with a thin layer of glass to keep it safe. The mirror folds up for transport, and unfolds after Webb breaks free of its rocket’s nose cone. Each one of the 18 hexagonal mirror sections can be finely adjusted to dial in Webb’s focus. This is crucial—if you remember when the Hubble telescope got into orbit scientists found out its mirrors had been ground incorrectly. It needed glasses. Luckily, astronauts were able to give it a pair of snazzy spectacles and the telescope has been sending us pretty pictures ever since. Webb will be about a million miles away where no astronaut can reach it, so it needs to adjust its own prescription.


Webb uses a five-layer sunshield to protect its instruments from the harsh rays of the sun. Left unshielded, Webb’s surface would warm to about 230 degrees F (110 C), which is far too hot for detecting faint infrared light from distant, long-ago stars. The sunshield does what it says on the tin: keeps the sun from warming Webb up. The kite-shaped shield measures 21.197 m x 14.162 m (69.5 ft x 46.5 ft) and is made out of a material called Kapton, which was developed by DuPont in the late 1960s and has been used to insulate electronics ever since.


Webb’s Kapton shields are coated in reflective aluminum and silicon. The shield material is absurdly thin—the first just .04mm and the other four just .024mm. But altogether, the shields keep Webb cool so it can carry out its mission. During transport they’re neatly folded up and they unfurl on the way to Webb’s final destination. They are super complex and getting them to unfold reliably was extremely difficult. But engineers managed to lick the problem and got them to unfurl here on earth multiple times without any issues before stuffing Webb into a rocket.


Webb’s mirrors and sun shields are miraculous, but there’s more magic, or, erm, science in the telescope’s super-advanced electronics. The first—on my list anway—is the Near Infrared Spectrograph or NIRSpec. It disperses the light reflected by the mirrors into a spectrum. That spectrum can be analyzed so we can learn things about the object the light came from. Things like temperature, mass, and chemical composition. The NIRSpec is especially important in the search for extraterrestrial life. It could tell us a lot about distant planets, like their temperature and if there’s oxygen in their atmospheres. And it can tell us a lot about the composition of those long-gone stars from the early universe.


But probably the coolest thing about NIRSpec, besides its cryogenically cooled components, is the fact that it can track 100 objects at once. It can pull off this spectacular feat of multitasking thanks to a similarly spectacular feat of micro engineering. The NIRSpec sensor is covered by 62,000 “microshutters,” a new technology developed just for Webb. Each shutter measures just 100 by 200 micros, which is about the size of a few human hairs. A tiny magnetic arm sweeps across the microshutter array, opening and closing them to let in or block light. They let NIRSpec peer at particular objects in the night sky without getting interference from other nearby objects. And thanks to a bunch of cutting-edge processing power, NIRSpec can peer at 100 objects at the same time, which means MOAR SCIENCE. Webb will be able to detect and analyze a ridiculous amount of stars and planets.


Then there are Webb’s infrared detectors, which are arguably the heart of the telescope. Like everything else in Webb, they’re brand new and state of the art. There are two different types of detectors, each made to detect a different range of infrared wavelengths. And they’re made from different stuff—mercury-cadmium-telluride for "near-infrared" and arsenic doped silicon for the "mid-infrared” spectrum. The detectors are tailored for each range of infrared, giving them stellar (pun intended) fidelity.


Webb’s near-infrared detector is happy to operate at a chilly -236C (-393F), which can easily be reached in the vacuum of space with passive, radiative cooling. But the mid-infrared detector isn’t happy unless the temperature is below -266C (-447F). To make it happy, Webb carries a cryocooler, basically a super-advanced refrigerator that uses helium as a working fluid. The cryocooler uses a few high-efficiency horizontally-opposed two-cylinder pumps to move the helium around. It cools the mid-infrared detector and Webb’s mirrors.


That’s a brief overview of some of the amazing systems aboard Webb. If you want to learn more, check out https://webb.nasa.gov, the site has a wealth of detailed information about all the amazing technology developed specifically for the telescope.


Webb’s tech is awe-inspiring, but its mission is straight-up jaw dropping. To detect those faint infrared rays, Webb needs to be far, far away from earth and all its drama. The telescope has to survive the cramped rocket ride to orbit, where it has to unfold like a butterfly coming out of its chrysalis, then it has to take a million-mile trip to its final destination at the L2 Lagrange point.

A Lagrange point is essentially a stable spot in or around Earth’s orbit, a place where an object can park without either drifting out into the solar system or plunging to a fiery death in the sun. The L2 Lagrange point puts Webb on the far side of earth, facing away from the sun and out into space. It’s a good parking spot, but it’s not completely stable. There are still some gravitational forces to tug on Webb, so the telescope needs to adjust its position with thrusters. The telescope carries enough compressed propellant to keep it stable for about 10 years. After that it’ll need to be refueled, but NASA doesn’t have any planned missions to go all the way out there to gas it up. Thankfully, Webb had a stroke of luck getting into orbit and didn’t need to use as much of its propellant for course correction as NASA anticipated, so it could have about 15 years’ worth of fuel.


So far Webb has been flawless. It has unfolded and unfurled all its trick mirrors and shields and is happily zooming toward its parking spot, where it can start focusing its mirrors and running systems tests. We, or astronomers, I should say, will start receiving data from Webb later in the year.

What will Webb see? Well, it’s going to peer into deep time to see light from the very early universe. It’ll give scientists a much better idea of what the universe was like in the early days after the Big Bang. Webb will also be able to take a look at early solar systems forming, which could give us insight into how our own solar system formed. Finally, Webb will be able to look directly at exoplanets and analyze their atmospheres. Webb could show us proof of life on a planet in another solar system. That’s a pretty big deal.


I’ll keep my eyes on Webb, as will every other science-obsessed person on the planet. Hopefully one of my podcasts later this year will just be me totally freaking out about another pale blue dot somewhere out there in the galaxy.


That’s it for this one. Thanks for listening. Through the Unknown can be found on Apple Podcasts, Google Podcasts, Podbean, or wherever you get your podcasts. A full transcript of this episode can be found at throughtheunknown.net. There’s also a YouTube channel! Check it out! Like, subscribe, comment, all that good stuff.











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