James Webb Space Telescope, the world's next great space observatory

Launch alert: JWST is scheduled to launch on Dec. 22 at 7:20 a.m. EST (12:20 UTC). Watch live on NASA TV at planetary.org/live.


  • NASA’s James Webb Space Telescope, JWST, is a multipurpose observatory expected to launch in December 2021 that will build on the legacy of the Hubble Space Telescope.
  • Like Hubble, JWST is expected to revolutionize our understanding of the cosmos. It will help us determine whether planets orbiting other stars could support life, and see galaxies that formed just after the Big Bang.
  • JWST was originally planned to launch in 2014. Behind schedule and over budget, the telescope's long road to the launch pad is nearly complete.

Why We Need JWST

Science is all about standing on the shoulders of giants, and that’s certainly the case for JWST. The observatory will build upon three decades of discoveries by the Hubble Space Telescope, which launched in 1990 and has revolutionized our understanding of the cosmos.

Earth’s atmosphere distorts our view of distant celestial objects and blocks certain wavelengths of light, including slices of infrared that human eyes cannot see. Infrared-capable space telescopes can determine the atmospheric composition of planets orbiting other stars, look through clouds of dust and gas to see newborn stars, and even peer back through time to see galaxies that formed right after the Big Bang!

JWST will be able to see a much larger portion of the infrared spectrum than Hubble, and collect six times more light. It will complement and extend Hubble’s observations to become the world’s premiere space observatory for the next decade.

So far, we know of more than 4,000 exoplanets — planets orbiting stars. Do any of these worlds host life as we know it? JWST will take us another step closer to finding out. One of the ways we study exoplanets is by watching them pass in front of their host stars, which creates a small dip in the amount of starlight we see from Earth. Not only does the amount of light change, the type changes too, due to interactions between the starlight and exoplanet’s atmosphere.

These changes show up particularly well in the infrared, which is exactly the kind of light JWST is equipped to see. What are we looking for? Life on other worlds may be more exotic than anything we’ve ever imagined, but it makes sense to start by looking for worlds like our own. An Earth-like planet would have an atmosphere made primarily of nitrogen and oxygen, with trace amounts of gases associated with life like methane and ozone. An early target for JWST will be TRAPPIST-1, a star system 40 light-years away that contains multiple planets in the habitable zone, the not-too-hot, not-too-cold region around a star where liquid water can exist on a planet’s surface.

TRAPPIST-1 System Artist Concept with Star
TRAPPIST-1 JWST will be study TRAPPIST-1, a star system that contains multiple planets in their star's habitable zone.Image: NASA / JPL-Caltech

JWST is an international observatory. In addition to providing the Ariane 5 rocket, the European Space Agency contributed to 2 of JWST’s four science instruments. The Canadian Space Agency provided one instrument as well.

How much does JWST cost?

JWST is expected to cost NASA $9.7 billion over 24 years. Of that amount, $8.8 billion was spent on spacecraft development between 2003 and 2021; $861 million is planned to support five years of operations. Adjusted for inflation to 2020 dollars, the lifetime cost to NASA will be approximately $10.8 billion.

The Planetary Society has additional context to help you understand these numbers. JWST was originally estimated to cost $4.96 billion and launch in 2014. But serious mismanagement and under-resourcing during critical early planning stages caused the ambitious spacecraft to fall behind schedule. After NASA restructured the project to launch in 2018 the total cost increased to $8.8 billion. In the intervening years, the program struggled to address serious technical problems, further delaying the launch to 2021. This final delay added yet another billion dollars to the total cost.

Six things we like about JWST
Six things we like about JWST Among other things JWST will study exoplanets, small worlds, the outer planets, and the early universe. The telescope is an international collaboration between the U.S., Canada, and Europe that will provide stunning images to inspire the public.Image: The Planetary Society

What’s in a name?

The James Webb Space Telescope is named after James Webb, who served as NASA’s second administrator from 1961 to 1968. More than 1,200 space scientists and enthusiasts — including people who have applied for observing time on the telescope — have asked NASA to change the name. They point to archival evidence showing that while serving as undersecretary of state prior to his tenure at NASA, Webb participated in discussions regarding the “lavender scare,” an effort to purge queer people from government service. In 1963, while Webb was administrator, a NASA employee in Washington, D.C. was arrested, interrogated and fired for making a "homosexual advance." In September 2021, NASA announced that a historical review found no evidence that warranted changing the telescope's name.

How JWST Works

Before JWST can do its job, it must first get to space. The telescope will launch aboard Europe’s Ariane 5 rocket, which has a payload fairing, or nose cone, 5.4 meters across — one of the world’s biggest. That’s still not wide enough for JWST’s 6.5-meter mirror, which consists of 18 hexagonal segments arranged like a honeycomb. Three segments on each side fold back for launch so the telescope can fit.

Other parts of JWST must be packed up for launch, too, making it a contortionist spacecraft. The three struts that hold the telescope’s secondary mirror fold together. The tennis-court-size sunshade folds and rolls into a tight bundle that relies on a multi-step process for unpacking. Other deployable components include the telescope’s solar panels, an antenna, and radiators that shunt extra heat into space.

For scientists, engineers, and space fans alike, watching JWST unpack itself will be a nail-biting experience. The telescope will not be located in Earth orbit, and can’t currently be visited by astronauts if anything goes wrong. Success, however, will pave the way for similar and more complex in-space deployments, which benefits future planetary exploration missions.

JWST Deployment Sequence Before JWST can do its job, it must first get to space and execute one of the most complex deployment sequences ever attempted. Credit: NASA's Goddard Space Flight Center

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In order to see infrared light, JWST must block out heat from both the Sun and its own instruments. The first line of defense is a five-layer sunshield, which will keep the telescope chilled to -233 degrees Celsius (-388 degrees Fahrenheit). At a glance, JWST’s sunshield looks a little like the solar sail aboard The Planetary Society’s LightSail spacecraft. Although JWST’s sunshield isn’t used for propulsion, the Sun’s rays will still give it a push, so the spacecraft has a reflective trim tab that deploys at a different angle to help offset the forces of solar radiation.

But even a sunshield's not enough: JWST also needs to block out light from Earth and the Moon, which is difficult in Earth orbit. Therefore JWST will travel to a special spot called L2 1.5 million kilometers (932,000 miles) away. There, the Sun and Earth’s gravity balance out in a way that allows JWST to permanently keep the Sun, Earth, and Moon at its back while it observes the cosmos.

Once JWST is at L2 with its instruments and equipment deployed, the fun begins. Light from distant objects bounces off its hexagonal mirrors, which are coated with a thin layer of gold to help it see better in the infrared. The light travels up to a secondary mirror, which focuses it into a beam the size of a dinner plate, sending it back through a hole in the center of the primary mirror. From there, it is focused further and can be sent through 4 different science instruments.

The farther away objects are, the longer it takes their light to reach us, meaning when we look at a distant galaxy, we are actually seeing what that galaxy looked like in the past. When you look at the Moon, you are seeing it as it existed 1 second ago. The sunlight warming your face on a beautiful day is eight minutes old. The Andromeda Galaxy, which looks like a smudge of light through binoculars, is actually 2.5 million years older than it appears now!

JWST can see galaxies up to 13.4 billion years old, just 400 million years after the Big Bang. Galaxies at this distance are quickly speeding away from us due to the expansion of the universe. This makes the wavelength of their light longer due to the Doppler effect — the same phenomenon that causes an ambulance siren to sound higher-pitched as it approaches you and lower-pitched after it passes you.

Long-wavelength light from objects formed even closer to the Big Bang is shifted deep into the infrared; in some cases beyond Hubble’s capabilities. JWST may be able to see objects as far back as 100 million years after the Big Bang, which will help us better understand our cosmic origin story.

JWST sunshield
JWST sunshield JWST sits with its five-layer sunshield fully deployed during testing at Northrop Grumman in Redondo Beach, California.Image: NASA/Chris Gunn

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Acknowledgements: This page was initially written by Max King in 2020.