Your Guide to NASA's James Webb Space Telescope

The world's next great space observatory

At a Glance

  • NASA’s James Webb Space Telescope, also known as Webb or JWST, is a multipurpose observatory launching in 2021 that will build on the legacy of the Hubble Space Telescope.
  • Like Hubble, Webb 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.
  • Webb has had a long, tumultuous road to the launch pad. Follow its progress as it crosses the finish line by signing up for The Downlink, our weekly newsletter.

Why do we need the James Webb Space Telescope?

Science is all about standing on the shoulders of giants, and that’s certainly the case for NASA’s James Webb Space Telescope. Webb will build upon 3 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!

Webb will be able to see a much larger portion of the infrared spectrum than Hubble, and collect 6 times more light. It will complement and extend Hubble’s observations to become the world’s premiere space observatory for the next decade.

James Webb Space Telescope Mirror
James Webb Space Telescope Mirror The James Webb Space Telescope's primary mirror is seen here during spacecraft assembly and testing. NASA

Space Missions

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So far, we know of more than 4,000 exoplanets—planets orbiting stars. Do any of these worlds host life as we know it? Webb 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 Webb 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.

Will the James Webb Space Telescope observe TRAPPIST-1?

Yes! An early target for Webb 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.

The Planetary Post - James Webb Space Telescope Bill Nye and Robert Picardo get a special tour of NASA’s Goddard Space Flight Center to see the amazing new James Webb Space Telescope.

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How the James Webb Space Telescope works

Before Webb 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 Webb’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 Webb must be packed up for launch, too, making it a contortionist spacecraft. The 3 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.

James Webb Space Telescope Deployment Sequence Before Webb 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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For scientists, engineers, and space fans alike, watching Webb 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.

An international observatory

In addition to providing the Ariane 5 rocket, the European Space Agency contributed to 2 of Webb’s 4 science instruments. The Canadian Space Agency provided 1 instrument as well, making Webb an international observatory.

Shields up for James Webb Space Telescope
Shields up for James Webb Space Telescope NASA’s James Webb Space Telescope sits with its five-layer sunshield fully deployed during testing at Northrop Grumman in Redondo Beach, California. NASA/Chris Gunn

The LightSail connection

At a glance, Webb’s sunshield looks a little like the solar sail aboard The Planetary Society’s LightSail spacecraft. Although Webb’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.

LightSail 2 solar sail

Size: Boxing ring
Deployment mechanism: Metallic booms that unwind from a central spindle
Sail material: Mylar

Webb sunshield

Size: Tennis court
Deployment mechanism: Unfolding pallets and telescoping booms
Sail material: Kapton

In order to see infrared light, Webb 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). But even that’s not enough: Webb also needs to block out light from Earth and the Moon, which is difficult in Earth orbit. Therefore Webb 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.

Webb's Special Orbit
Webb's Special Orbit The James Webb Space Telescope will orbit a special spot called L2 1.5 million kilometers (932,000 miles) away from Earth. NASA / Space Telescope Science Institute

Once Webb 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.

James Webb Space Telescope Light Path This animation shows the path light follows to get to the James Webb Space Telescope's science instruments. Credit: NASA, ESA, and G. Bacon (STScI)

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How Webb sees back in time

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 8 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!

Hubble 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. Webb 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.

How you can support the James Webb Space Telescope

Webb has had a long, tumultuous road to the launch pad, suffering delays and budget overruns not uncommon for ambitious, one-of-a-kind projects that involve so many new technologies. Webb’s problems, however, have been particularly severe: when the telescope was named in 2002, it was scheduled to launch in 2010, while its price tag has skyrocketed from $3.8 billion to $8.8 billion. The situation was so dire that Congress threatened to cancel the project in 2011.

Better days are hopefully ahead. The telescope is now going through final tests and has a launch date of 31 October 2021. Sign up for our weekly newsletter, The Downlink, to follow Webb as it launches, performs its nerve-racking deployment, and begins sending home beautiful pictures of the cosmos.

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Acknowledgements: This page was initially written by Max King and is regularly updated by Planetary Society staff writers.