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The James Webb Space Telescope: Origins, design and mission objectives

An illustration of the James Webb Space Telescope
An artist’s impression of how the James Webb Space Telescope will look after deployment. (Image credit: ESA)

The James Webb Space Telescope (JWST) has a lot to live up to as the successor of the Hubble Space Telescope. In the three decades since it was launched in 1990, Hubble has revealed the wonders of the universe in unprecedented detail. It's been used to study cutting-edge topics like dark energy and exoplanets that were scarcely dreamed of when it began operation. Plus, it has captured the public's imagination to the extent that it is now a household name. 

The James Webb Space Telescope, known as Webb (like "Hubble"), will be operated primarily by NASA, which is providing the bulk of the funding, with the European Space Agency (ESA) as a junior partner. The telescope is named after one of NASA's early administrators, James E. Webb, who oversaw the creation of the Apollo program in the 1960s, according to NASA.

It was way back in 2002, almost 20 years ago, that Webb's name was first applied to what had previously been referred to as the "Next Generation Space Telescope." This was originally planned to cost half a billion dollars and be ready for launch in 2007, the Atlantic reported.

However, these estimates turned out to be hopelessly over-optimistic, given the enormously complex and innovative design of the spacecraft. By the time it's finally launched, hopefully before the end of 2021, it will have cost almost ten billion dollars, according to Space.com.

James Webb, after whom the telescope is named, was NASA administrator in the 1960s.  (Image credit: NASA)

Nevertheless, the scientists involved in the project believe the results will more than compensate for the time and money invested in it. NASA is keen to emphasise that Webb isn't simply a bigger and more powerful replacement for Hubble. While it is both those things — with more than two and a half times the diameter and a hundred times the sensitivity — at its heart the JWST is a different type of instrument altogether. 

Related: How are asteroids and space debris detected before they hit Earth?

Ordinary optical telescopes see in the same part of the spectrum as our own eyes, covering a range of wavelengths between roughly 380 and 740 nanometres (nm), as Live Science has previously reported. Hubble spanned all of this, plus a little way into the ultraviolet at shorter wavelengths and infrared at longer ones. 

But the JWST will primarily be an infrared telescope, optimized for 600 to 28,000 nm, according to NASA’s JWST website. So it won’t be able to see green or blue light, just orange and red – plus a wide range of longer wavelengths beyond that.

For many astronomical objects – including star-forming regions, exoplanets and the most distant galaxies — these very long wavelengths are more useful to astronomers than the visible spectrum. But infrared poses problems for Earth-based observers, because much of it is blocked by our planet's atmosphere, according to the University of St Andrews

On top of that, the Earth produces its own infrared emissions via heat radiation, which tend to swamp the fainter astronomical sources. So the best place for an infrared telescope is out in space, as far as possible from the Earth and all its unwanted sources of heat.

Related: How many satellites orbit Earth?

Following in the footsteps of ESA's Herschel infrared observatory, the Webb telescope will be located 1.5 million kilometers from Earth at the so-called L2 point, according to NASA’s JWST website. 

This will give it a much clearer view of the universe than the one Hubble has in low-Earth orbit, but it does have a downside. Unlike its predecessor, it won't be a relatively simple matter to send astronauts up to repair it if it breaks down. Everything has to work perfectly the first attempt, which is one of the reasons it's taken NASA the best part of two decades to get it ready for launch.

How does the James Webb Space Telescope work?

Externally, the JWST looks very different from Hubble. The latter, just like a traditional telescope, is enclosed in a cylindrical tube that shields the optics from stray light. Depending on its position in its orbit, Hubble can be exposed to a lot of this: blazing sunshine from one direction, reflections from the Earth's surface in another, and maybe even the moon

But Webb is more fortunate. Seen from the L2 point all these bright sources are in more or less the same direction, so all the telescope needs is a single large sunshield. The bare optics, in the form of primary and secondary mirrors, then sit on top of this. The result, at first glance, looks more like a radio telescope than an optical one.

Functionally, however, both Webb and Hubble are constructed on the same principles. They’re both built around a large primary mirror, which has the crucial job of capturing as much light as possible from objects that may be on the very edge of the observable universe. In essence, the bigger this mirror is, the better. 

In Hubble's case it's 8 feet (2.4 meters) in diameter, and made from a single circular piece of glass. If this was scaled up to the size needed for the JWST — around 21.3 feet (6.5 meters) across — then not only would it be extremely difficult to fabricate, but the result would be too large and heavy to launch into space, according to NASA.

The huge primary mirror of the JWST during ground testing by NASA engineers. (Image credit: NASA)

Instead, Webb's mirror is constructed from 18 hexagonal segments, which can fold up for launch and then deploy into an operational configuration once in space. Although NASA considered making the segments from glass, like Hubble's mirror, in the end they used beryllium: a very strong, lightweight metal commonly employed in high-speed aircraft and space vehicles. 

This needs to be shaped and polished to extremely high accuracy in order to produce images with the necessary clarity; NASA estimates the polishing error to be less than a millionth of an inch. After achieving the desired shape, the mirror segments were then coated with a thin layer of pure gold, to maximize reflectivity at infrared wavelengths.

When all the segments are put together, they achieve the desired 6.5 metre diameter for the main mirror. That’s around 2.7 times as big as Hubble, but the actual performance improvement is much greater than this. 

That’s because the light-collecting power of a mirror is proportional to its area rather than its diameter. Allowing for the hexagonal shape of the segments and the hole in the centre, the effective area of Webb's mirror is 269 square feet (25 square meters), compared with 43 square feet (four square meters) for Hubble. That equates to a performance improvement of better than a factor of six.

Related: Cosmology: Uncovering the story of the Universe

JWST’s sunshield

Located at the L2 point, the JWST will sit in constant bright sunshine. This is healthy for the equipment in the spacecraft bus, but bad news for the optical instruments and science module. Because they observe via infrared, these need to be kept as cold as possible in order to function correctly. 

So the two halves of the spacecraft will be separated by a huge, kite-shaped, five-layer sunshield roughly the size of a tennis court. While the sunlit side may reach temperatures of 212 degrees Fahrenheit (100 degrees Celsius), the cold side will be as low as minus 394 F (minus 237 C) according to NASA’s JWST website.

The Sunshield on NASA's James Webb Space Telescope.  (Image credit: NASA/Chris Gunn)

Why do JWT'S optical instruments observe in infrared?

We normally think of astronomy in terms of visible light, because that's what our eyes and traditional telescopes see. But astronomical objects produce emissions across the whole of the electromagnetic spectrum, from very long wavelength radio waves to very short wavelength X-rays and gamma rays. Our eyes evolved to see the wavelengths they do because that's where the Sun emits most of its energy, but cooler objects, such as planets and newly formed stars, tend to radiate at longer wavelengths than this, according to research published in the journal Eye

This is one reason why infrared telescopes such as Webb (and its predecessor, NASA's Spitzer space telescope, which operated between 2003 and 2020) are so important. A second reason is that while the dust in galaxies absorbs visible light, it's virtually transparent to infrared waves. This means even sun-like stars can be easier to see in the infrared if there's a lot of intervening dust, according to NASA.

What are the mission objectives of JWST?

Objective 1: The early universe

Webb is sometimes described as a "time machine," which in a sense it is. Because light from distant objects travels at finite speed, we see them as they used to be in the past. Hubble has shown us galaxies as they were many billions of years ago, but the JWST will be even more sensitive. NASA hopes it will see all the way back to when the first galaxies formed, around 13.6 billion years ago. And Webb has another advantage over visible-band telescopes like Hubble. 

Because the universe is expanding, light from distant objects is stretched out, increasing its wavelength. This means light emitted in the visible waveband actually reaches us in the infrared, the band that the JWST is optimized for. One of its first tasks will be a survey, called COSMOS-Webb, of the most distant galaxies in a specific patch of sky, to explore conditions at the dawn of the universe.

The COSMOS-Webb survey will explore an area equivalent to three full moons. (Image credit: NASA)

Objective 2: Galaxies over time

Thanks to Hubble's spectacular imagery, most people know what galaxies look like: huge collections of stars, often arranged in elegantly symmetric spiral patterns. But these tend to be relatively nearby galaxies, and hence mature ones. The tantalising glimpses Hubble has provided of very early galaxies suggests they are considerably smaller and scrappier-looking. 

As yet, no one knows how these proto-galaxies formed, or how they subsequently clumped together to produce the larger, regular-looking galaxies we see today, according to the California Institute of Technology. It's hoped that Webb will be able to answer questions like these with its ultra-deep view of the early universe.

Another well-established feature of galaxies is the presence of supermassive black holes in the centres of most of them. In the early universe, these black holes often powered enormously bright galactic nuclei called quasars, and Webb is scheduled to study six of the most distant and luminous examples of these.

An artist's impression of a quasar

A NASA artist’s rendering of a powerful quasar of the type Webb will study.  (Image credit: NASA)

Objective 3: Lifecycle of stars

The galaxies that fill the universe originated very early on, and they've steadily evolved ever since then. But that's not true of the stars inside them, which go through life cycles more akin to living creatures. They're born, develop, age and die, and the remnants of old stars contribute to the raw material needed to make new stars. Much of this process is well understood, but there's still a mystery surrounding the actual birth of stars, and the planetary discs that may form around them. 

That's because these are initially enveloped inside a cocoon of dust, which ordinary telescopes using visible light can’t penetrate. But all this dust will be virtually transparent at the infrared wavelengths used by Webb, so NASA hopes it will finally reveal the ultimate secrets of star formation. In turn, this may teach us something about the origins of our own Sun and Solar System.

Objective 4: Other worlds

One of the most exciting areas of contemporary astronomy is the search for exoplanets orbiting other stars, particularly Earth-like planets that may have the chemical ingredients and conditions necessary for life to evolve. The JWST will contribute to this search in several ways, using infrared imaging and spectroscopy to study the chemical and physical properties of planetary systems. 

Its ability to peer through dust and snap super-high resolution images should provide us with a direct view of planetary systems — such as that of the newly formed star Beta Pictoris — in their very earliest stages, according to NASA's JWST website. Webb will also analyse the chemical composition of exoplanet atmospheres, looking in particular for tell-tale signatures of the building blocks of life. This again is something an infrared telescope is ideally suited for, because the molecules making up planetary atmospheres tend to be most active at these wavelengths.

Hubble’s view of Beta Pictoris

Hubble’s view of the planetary disc around Beta Pictoris, which the JWST will study in greater depth. (Image credit: NASA)

When will JWST launch?

A key feature of Webb's design is that it has a "cold side" and a "hot side." The cold side is the one that does the observing, while the hot side carries the spacecraft’s solar panels and an antenna for two-way communication with Earth. But this arrangement only works if the Sun and Earth are always in the same direction from the spacecraft’s point of view. 

This wouldn’t be the case if it was simply placed in Earth orbit like Hubble, nor would it be true if the spacecraft orbited the Sun at a slightly different distance from the Earth’s orbit. But it turns out there is one special distance at which an object can orbit the Sun and always see the Sun and Earth in the same direction. This is the so-called L2 point — and it's where the Webb telescope will operate.

L2 is one of several locations called Lagrange points, after Joseph-Louis Lagrange who studied them in the 18th century. At these locations the gravity of two massive bodies (in this case the Sun and Earth) conspire to keep a third, smaller body (such as an asteroid or spacecraft) in a fixed position relative to the first two. The Lagrange points aren't stationary, but they revolve around the Sun at exactly the same rate as the Earth, so the distance from us always stays the same. In the case of L2, it's around 1.5 million kilometres away: around four times as far away as the Moon.

Diagram of JWST launch

A diagram of how the JWST will be stored and launched aboard the Ariane 5 rocket.  (Image credit: ESA)

To get the telescope all the way to L2 requires a powerful launch vehicle, which will be the European Space Agency's Ariane 5 rocket. In just 26 minutes following lift-off from French Guiana, this will carry Webb free of Earth’s atmosphere and put it on course for L2. The spacecraft will then separate from the rocket and cruise for around a month before finally arriving at its destination, according to NASA's JWST website.

Q&A with an Astrophysicist

We asked NASA's Dr Mike McElwain about his hopes for the new telescope.

Headshot of Mike McElwain

Michael McElwain is JWST Observatory Project Scientist at NASA’s Goddard Space Flight Centre.  (Image credit: NASA/Jolearra Tshiteya)

What sort of science will the telescope do in its first year?

In the first year, Webb's observing program will run the cosmic gamut from the first light in the early universe to exoplanet atmospheres. Webb will observe the most interesting objects in the universe with a combination of improved resolution, sensitivity and wavelength coverage. This will enable new and enhanced characterisation of the famous objects in the sky. If you can name it, Webb is likely to observe it, though maybe not all in the first year.

What are the most exciting discoveries the JWST might make?

When you have an observatory as transformational as Webb, the most exciting discoveries are likely to be the ones that we don’t even anticipate! Webb’s infrared eyes on the universe will enable us to see space where we were previously blind. Its unprecedented infrared sensitivity will help astronomers compare the earliest galaxies to today's grand spirals and ellipticals, helping us to understand how galaxies assemble over billions of years. It will be able to see right through and into massive clouds of dust that are opaque to visible-light observatories like Hubble, where stars and planetary systems are being born. Webb will tell us more about the atmospheres of extrasolar planets, and perhaps even find the building blocks of life elsewhere in the universe.

Do you think Webb will become a household name like Hubble?

I fully expect it will, and that people around the globe will be discussing Webb imagery while sitting around the dinner table. Similar to Hubble, Webb will produce spectacular images of the cosmos that will captivate the imagination. We expect Webb imagery to go viral on the internet, show up in calendars, and occupy space on household coffee tables.

Additional resources

Andrew May holds a PhD in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer where he has written for Fortrean Times, How It Works, All About Space, Popular Science, among others. He has also written a selection of books including Cosmic Impact and Astrobiology: The Search for Life Elsewhere in the Universe, published by Icon Books.