James Webb Space Telescope
The James Webb Space Telescope set off for orbit with 344 potential single-point failures. Each one was a task with no alternative and no way to recover, so every one had to work. An independent review board counted them in 2018. The observatory is a joint project of NASA, the European Space Agency and the Canadian Space Agency. Its job is to study objects too old, too distant or too faint for the Hubble Space Telescope. Several questions run through this story. Why must a telescope built to catch ancient light stay so cold? How does a mirror made in pieces act as a single instrument? And how did a project run so far over its first budget that members of Congress moved to cancel it?
Hubble's mirror sits at about 15 degrees Celsius, which is warm enough that the telescope itself glows in infrared light. That glow keeps Hubble from studying the faint heat signatures Webb was built to find. Earth's surface is no better. Its atmosphere is opaque in many infrared bands, and even in the clear ones, water, carbon dioxide and methane in the air interfere with the compounds astronomers want to measure. The atmosphere also radiates in the infrared, often drowning out the object being observed.
Cosmic expansion supplies the other reason. The farther away an object is, the older it looks, because its light has taken longer to arrive. As that light crosses an expanding universe, its wavelength stretches, a shift known as redshift. Visible light from the earliest objects therefore arrives as infrared. Webb can reach back to a redshift of about 20, roughly 180 million years after the Big Bang. Hubble cannot see past about redshift 11.1, where the galaxy GN-z11 sits, 400 million years after the Big Bang. Webb can also detect objects up to 100 times fainter than Hubble can.
Molecular clouds, where stars are born, hide behind dust that scatters visible light. Infrared passes through that dust more freely. It exposes the disks around young stars that give rise to planets, and the cores of active galaxies. Objects cooler than stars, such as brown dwarfs, comets, planets and Kuiper belt objects, radiate mostly in the infrared.
Two earlier missions shaped the choice of wavelengths. The Spitzer Space Telescope showed how useful the mid-infrared is for studying dust disks around stars. The Wilkinson Microwave Anisotropy Probe showed that the universe was lit up at redshift 17. Both launched in the early 2000s, in time to influence Webb's design. The result is an instrument sensitive from 0.6 to 28.5 micrometres, spanning long-wavelength red light through the mid-infrared.
Four goals guided the design. The first is to find light from the first stars and galaxies that formed after the Big Bang. The second is to study how galaxies form and evolve. The third is to understand how stars and planets form. The fourth is to study planetary systems and the origins of life. For that last goal, Webb can tell whether an exoplanet's atmosphere holds methane, which astronomers could then weigh as a possible biosignature.
Eighteen hexagonal segments of gold-plated beryllium together form Webb's mirror, which measures 6.5 metres across. Hubble's mirror measures 2.4 metres. The polished area is 26.3 square metres, and after the shadows of the support struts are subtracted, the collecting area is 25.4 square metres. Hubble's is 4.0 square metres, so Webb gathers more than six times as much light.
A wider mirror does not simply make sharper pictures here. Webb's mirror is 2.7 times the diameter of Hubble's, yet the two produce images of comparable resolution. The reason is wavelength. The longer the wavelength a telescope observes, the larger the collecting surface it needs to reach the same resolution.
A single mirror this large would not have fitted on any existing launch vehicle. Engineers therefore built it from segments that unfolded after launch, a technique pioneered by Guido Horn d'Arturo. NASA administrator Dan Goldin, who coined the phrase "faster, better, cheaper", pushed astronomy to break the single-mirror barrier. That meant moving from eliminating moving parts to learning to live with them.
Beryllium was chosen as the mirror material because it is exceptionally stiff, has a low density of 1.845 grams per cubic centimetre, and holds a stable shape at cryogenic temperatures. The goal was to cut the mirror's mass by a factor of 10. The gold coating provides infrared reflectivity, and a thin layer of glass over it protects the surface.
A total of 132 small actuation motors position and adjust the segments, in steps as fine as 10 nanometres. Once the segments are set, they need only occasional updates every few days. Ground-based giants such as the Keck telescopes must instead adjust their segments continually to fight gravity and wind.
The optical layout is a three-mirror anastigmat. Curved secondary and tertiary mirrors deliver images free from optical aberrations across a wide field, and the secondary mirror is 0.74 metres across. A fine steering mirror adjusts many times per second to steady the image. The hexagonal segments leave a signature on every point of light in Webb's pictures: six diffraction spikes, plus two fainter ones.
Webb's sunshield has a sun protection factor of 1,000,000, compared with a range of 8 to 50 for suntan lotion. It has five layers, each about 0.1 millimetres thick. Each layer is Kapton E film coated with aluminum on both sides, and the two outermost layers carry an extra coating of doped silicon on the Sun-facing sides to reflect heat back into space. The purpose is to keep the telescope below 50 kelvin, because otherwise its own infrared radiation would overwhelm its instruments.
The shield was designed to fold twelve times to fit inside the payload fairing of the Ariane 5 rocket, which is 4.57 metres wide and 16.19 metres long. Its planned deployed size was 14.162 by 21.197 metres. Accidental tears in the delicate film during deployment testing in 2018 caused further delays. Because the shield always faces away from the telescope's view, Webb sees 40 percent of the sky from any one position. Over six months it can see all of it.
About 1.5 million kilometres beyond Earth's orbit, near the second Lagrange point of the Sun-Earth system, the arrangement works. Webb is roughly four times farther away than the Moon. There, a spacecraft can orbit the Sun in the same time Earth does, so the Sun, Earth and Moon stay on the same side of the observatory. The sunshield blocks all three at once. Staying close to Earth also allows much higher data rates for a given antenna size.
Webb does not orbit the Lagrange point so much as drift around it. Its halo orbit takes about half a year, with a radius that varies between about 250,000 and 832,000 kilometres. That path keeps the telescope out of the shadows of both Earth and Moon, which would otherwise cause small temperature swings. A stable temperature on the dark side matters for keeping the mirror segments precisely aligned.
The point is only an equilibrium, so Webb needs regular station-keeping. Its thrusters sit on the Sun-facing side. Each manoeuvre is therefore planned to slightly undershoot, since pushing too far could not be undone. Randy Kimble, the integration and test project scientist, compared the task to Sisyphus "rolling this rock up the gentle slope near the top of the hill". He added that "we never want it to roll over the crest and get away from him".
Four science instruments and a guide camera sit inside the Integrated Science Instrument Module, a frame of bonded graphite-epoxy composite that supplies power, computing, cooling and structural stability. The Near Infrared Camera, NIRCam, covers 0.6 to 5 micrometres with 10 sensors of 4 megapixels each. A team led by the University of Arizona built it, with Marcia J. Rieke as principal investigator. NIRCam also serves as the wavefront sensor that aligns the mirror segments.
The Near Infrared Spectrograph, NIRSpec, was built by the European Space Agency at ESTEC in Noordwijk, in the Netherlands, with Pierre Ferruit as project scientist. It offers a low-resolution prism mode and two higher-resolution modes. A microshutter mechanism lets it observe hundreds of individual objects at the same moment. Its wheel mechanisms descend from those on the Infrared Space Observatory.
The Mid-Infrared Instrument, MIRI, covers 5 to 27 micrometres and combines a camera with an imaging spectrometer. George Rieke of the University of Arizona and Gillian Wright of the UK Astronomy Technology Centre in Edinburgh lead it. MIRI must stay below 6 kelvin, so a helium gas mechanical cooler on the warm side of the shield serves it. Both NIRCam and MIRI carry coronagraphs that block starlight, allowing a look at faint planets and disks close to bright stars.
The Canadian Space Agency supplied the Fine Guidance Sensor and the Near Infrared Imager and Slitless Spectrograph. John Hutchings led the guidance sensor work, and René Doyon of the Université de Montréal is principal investigator for the imager, which works from 0.8 to 5 micrometres. Though often named together, they do different jobs. One is a scientific instrument, and the other steadies the line of sight of the whole observatory.
The spacecraft bus weighs only 350 kilograms yet supports the 6,200 kilogram telescope. It sits on the warm side of the sunshield at about 300 kelvin and can point the telescope to one arcsecond. Hydrazine fuel, 159 litres at launch, feeds two pairs of rocket engines for course corrections and station-keeping, with eight smaller thrusters controlling pointing.
A modified version of JavaScript, called Nombas ScriptEase 5.00e, runs Webb's operations. Its script interpreter runs under flight software written in C++. A custom chip called SIDECAR ASIC digitizes the instrument signals close to the detectors, performing the work of a 9.1 kilogram instrument box in a 3 centimetre package that draws only 11 milliwatts. Webb is designed to send 458 gigabits of data a day, and a 68 gigabyte solid-state drive is expected to shrink to 60 gigabytes of usable space by the end of a ten-year mission.
No crew can visit Webb the way shuttle astronauts visited Hubble. NASA's Thomas Zurbuchen said an uncrewed repair mission was beyond available technology when Webb was designed. Still, NASA later said it made limited accommodations for servicing, including cross-shaped guidance markers on the surface, refillable fuel tanks, removable heat protectors and accessible attachment points.
In 1993, Space Shuttle mission STS-61 replaced Hubble's camera and installed a retrofit to compensate for the spherical aberration in its primary mirror. Emboldened by Hubble's eventual success, a committee called HST and Beyond formed in 1994. Its 1996 report proposed a larger and much colder infrared telescope that could reach back to the birth of the first galaxies. The project, then called the Next Generation Space Telescope, received the highest ranking in the 2000 Decadal Survey.
In 2002, NASA administrator Sean O'Keefe renamed it for James E. Webb, who lived from 1906 to 1992. Webb led NASA from 1961 to 1968 and established scientific research as a core activity of the agency. In 2015, concerns arose about his possible role in the lavender scare, the persecution of gay people in federal employment. NASA's 2022 report examined more than 50,000 documents and found "no available evidence directly links Webb to any actions or follow-up related to the firing of individuals for their sexual orientation".
Two concept studies in 1999 assumed a launch in 2007 and a budget of about one billion dollars. In 2003, TRW won a prime contract worth US$824.8 million, and Northrop Grumman acquired TRW that same year in a hostile bid. Cost growth revealed in spring 2005 forced a re-plan that added a 22-month delay and put the life-cycle cost at US$4.5 billion. By 2008, when the project was confirmed for construction, more than US$1 billion had been spent.
In 2010, Senator Barbara Mikulski of Maryland called for an external review. The panel found the earliest possible launch was late 2015, at an extra cost of US$1.5 billion. Then, on the 6th of July 2011, a House appropriations subcommittee moved to cancel the project. About US$3 billion had been spent and 75 percent of the hardware was in production. The committee called the project "billions of dollars over budget and plagued by poor management". A 2010 article in Nature had already described Webb as "the telescope that ate astronomy".
The American Astronomical Society and Senator Mikulski defended the telescope, and editorials supporting it appeared in the international press. In November 2011, Congress reversed course and capped additional funding at US$8 billion. Hubble had gone through something similar. It was estimated at US$300 million when it began in 1972 and cost about four times that by its 1990 launch.
Gregory L. Robinson became director of the Webb program in 2018. He raised the share of measures completed on time from 50 percent to 95 percent. Zurbuchen called him "the most effective leader of a mission I have ever seen in the history of NASA". Meanwhile, launch dates slid to May 2020 and then to March 2021, and the cost cap rose by US$800 million in 2019. In August 2019, the mechanical integration of the telescope was finished, a task originally scheduled for 2007. After the COVID-19 pandemic paused launch windows in 2020, the final total came to just under US$10 billion.
A total of 258 companies, agencies and institutions took part before launch. They included 142 from the United States, 104 from 12 European countries and 12 from Canada. The European Space Agency's share grew from an agreed €300 million in 2004 to about €700 million, and the Canadian Space Agency's to about CA$200 million.
A ship carrying the telescope left California on the 26th of September 2021, passed through the Panama Canal, and reached French Guiana on the 12th of October. On the 25th of December 2021 at 12:20 UTC, an Ariane 5 rocket lifted off from the Guiana Space Centre as flight VA256. The telescope separated from the upper stage 27 minutes 7 seconds later. Space.com called the launch "flawless".
The rocket gave Webb slightly less speed than it needed to reach its destination. That was deliberate. The observatory could recover from going too slowly but not from going too fast, because the sunshield must stay between the telescope and the Sun and the craft could not turn around to brake. The precision of the launch and the first midcourse correction saved enough fuel that Webb may keep its orbit for around 20 years instead of 10.
The first of three course corrections began about 12 hours after launch, with the main engines firing for 65 minutes. On the 28th of December, controllers began deploying the sunshield. On the 31st of December, the two telescoping mid booms extended, the left in 3 hours 19 minutes and the right in 3 hours 42 minutes. On the 5th of January 2022, the secondary mirror locked into place to a tolerance of about one and a half millimetres. The mirror wings, each holding three segments, unfolded on the 7th and the 8th of January.
Mirror alignment began on the 12th of January. Each of the 132 motors had to travel 1.2 million increments, or 12.5 millimetres, so moving the segments out of their launch positions took about 10 days. The final target was 50 nanometres. NASA offered an analogy: "If the Webb primary mirror were the size of the United States, each mirror segment would be the size of Texas." The team would need to line up their heights "to an accuracy of about 1.5 inches".
The team rehearsed the seven-phase process on a model at one-sixth scale. Once the mirrors had cooled to 120 kelvin, NIRCam aimed at HD 84406, a star in Ursa Major, and took 1,560 images. At first, the view held 18 separate blurry images of the same star, one per segment. The first photons were reported on the 3rd of February 2022. Phase 1 finished on the 18th of February, and phases 2 and 3 followed a week later. Until all seven phases were done, the segments still acted as 18 smaller telescopes.
A micrometeoroid hit segment C3 between the 23rd and the 25th of May 2022. It was the fifth strike since launch and the largest, from a particle about the size of a dust mote. Engineers compensated with a mirror actuator. Micrometeoroids strike Webb once or twice a month, and the mission now avoids pointing the mirror into designated avoidance zones along its orbit.
On the 11th of July 2022, President Joe Biden revealed Webb's First Deep Field, a view of the galaxy cluster SMACS J0723.3-7327. The cluster's gravity distorts and magnifies the galaxies behind it. Among them were three ancient galaxies seen as they were 13.1 billion years ago. Full-color images and spectroscopic data followed on the 12th of July, marking the start of general science operations.
The release also included the "Cosmic Cliffs" of the star-forming region NGC 3324, about 8,500 light-years away. It showed evidence of water in the atmosphere of the giant gas planet WASP-96b, 1,120 light-years from Earth. The Southern Ring Nebula, gas and dust expelled by a dying star 2,500 light-years away, appeared as well. So did Stephan's Quintet, five galaxies with colliding gas and dust creating new stars. On the 14th of July, NASA added images of Jupiter. A preprint from NASA, ESA and CSA scientists concluded that Webb's performance was better than expected. It tracked moving objects at up to 67 milliarcseconds per second, more than twice the required speed.
Within two weeks, preprints described very luminous galaxies dating to 235 to 280 million years after the Big Bang. The candidate CEERS-93316 was estimated at redshift 16.7, but a later spectroscopic measurement revised it to about 4.91. In September 2022, primordial black holes were proposed to explain such surprisingly large early galaxies. In May 2024, Webb identified JADES-GS-z14-0, seen just 290 million years after the Big Bang at redshift 14.32. It was more luminous and massive than expected, which challenged models of early galaxy formation.
In August 2025, Juodžbalis and colleagues measured a black hole of about 50 million solar masses in the strongly lensed galaxy UHZ1. It outweighed the stars of its host. Other teams reported an active black hole in the galaxy GHZ2 at redshift 12.34, less than 400 million years after the Big Bang, the most distant black hole identified to date. In May 2026, a study in Science reported a repeating daily cloud cycle on the hot Jupiter WASP-94A b, with magnesium silicate clouds forming on the morning side and evaporating on the evening side.
Demand for the telescope is enormous. For Cycle 1, 1,173 proposals requested 24,500 hours when only 6,000 were available, and 266 programs were approved. In Cycle 4, 2,377 proposals asked for 78,000 hours, nine times the time on offer.
Updated
Common questions
What is the James Webb Space Telescope?
The James Webb Space Telescope is a NASA, European Space Agency and Canadian Space Agency space telescope designed for infrared astronomy. It is the largest telescope in space and can study objects too old, distant or faint for the Hubble Space Telescope.
When was the James Webb Space Telescope launched?
The James Webb Space Telescope launched on the 25th of December 2021 at 12:20 UTC on an Ariane 5 rocket from the Guiana Space Centre in French Guiana. It reached its orbit near the Sun-Earth L2 Lagrange point on the 24th of January 2022.
How big is the James Webb Space Telescope mirror compared with Hubble?
The James Webb Space Telescope has a 6.5 metre mirror made of 18 gold-plated beryllium hexagonal segments, against 2.4 metres for Hubble. Its collecting area of 25.4 square metres is more than six times Hubble's 4.0 square metres.
Why does the James Webb Space Telescope need a sunshield?
The James Webb Space Telescope must stay below 50 kelvin so that its own infrared radiation does not overwhelm its instruments. Its five-layer sunshield, with a sun protection factor of 1,000,000, blocks heat and light from the Sun, Earth and Moon.
How much did the James Webb Space Telescope cost?
The James Webb Space Telescope cost just under US$10 billion in total, and NASA's lifetime cost is expected to be US$9.7 billion. Early plans in the late 1990s aimed for about US$1 billion, and Congress capped additional funding at US$8 billion in November 2011 after a cancellation proposal.
Where does the James Webb Space Telescope orbit?
The James Webb Space Telescope orbits the Sun near the Sun-Earth L2 Lagrange point, about 1.5 million kilometres beyond Earth's orbit. It follows a halo orbit with a radius between about 250,000 and 832,000 kilometres that takes about half a year.
Who is the James Webb Space Telescope named after?
The James Webb Space Telescope is named after James E. Webb, who led NASA from 1961 to 1968 during the Mercury, Gemini and Apollo programs. NASA administrator Sean O'Keefe chose the name in 2002.
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- 185James Webb Space Telescope tweaks observing plans to avoid micrometeoroids16 November 2022
- 187How to track James Webb Space Telescope, mission timeline31 December 2021
- 189NewsLive Updates: Webb Telescope Launches on Long-Awaited Journey25 December 2021
- 190NewsJames Webb Space Telescope Launches on Journey to See the Dawn of Starlight – Astronomers were jubilant as the spacecraft made it off the launchpad, following decades of delays and cost overruns. The Webb is set to offer a new keyhole into the earliest moments of our universe.Dennis Overbye et al. — 25 December 2021
- 196All hail the Ariane 5 rocket, which doubled the Webb telescope's lifetimeEric Berger — Ars Technica — 10 January 2022
- 197James Webb telescope completes epic deployment sequenceJonathan Amos — BBC News — 9 January 2022
- 198'It's truly Christmas': James Webb Space Telescope's yuletide launch has NASA overjoyedPultarova — 25 December 2021
- 199Press releaseAriane 5 goes down in history with successful launch of Webb25 December 2021
- 200James Webb Space Telescope Deployment Sequence (Nominal)12 November 2021
- 201James Webb Space Telescope en route to L2; deployment sequence underwayHaygen Warren — 27 December 2021
- 202NewsNASA thrilled: Webb Space Telescope deploys sun shield, evades many potential 'single-point failures'Joel Achenbach — 4 January 2022
- 203Gimbaled Antenna AssemblyNASA
- 204The First Mid-Course Correction BurnKaren Fox — 25 December 2021
- 205Webb's Second Mid-Course Correction BurnKaren Fox — 27 December 2021
- 206Webb's Aft Momentum Flap DeployedAlise Fisher — 30 December 2021
- 207With Webb's Mid-Booms Extended, Sunshield Takes ShapePatrick Lynch — 31 December 2021
- 208First of Two Sunshield Mid-Booms DeploysPatrick Lynch — 31 December 2021
- 209Secondary Mirror Deployment ConfirmedAlise Fisher — 5 January 2022
- 210First of Two Primary Mirror Wings UnfoldsAlise Fisher — 7 January 2022
- 211Primary Mirror Wings Deployed, All Major Deployments CompleteAlise Fisher — 8 January 2022
- 212Remarkably, NASA has completed deployment of the Webb space telescopeEric Berger — 8 January 2022
- 213Space telescope's 'golden eye' opens, last major hurdle8 January 2022
- 214Webb's Journey to L2 Is Nearly CompleteAlise Fisher — 21 January 2022
- 217James Webb Space Telescope runs into technical issue2022-09-21
- 218Webb Mirror Segment Deployments Complete – James Webb Space Telescope19 January 2022
- 220Mirror, Mirror...On Its Way! – James Webb Space Telescope13 January 2022
- 223HD 84406
- 225NewsThe James Webb Space Telescope Just Detected Its First Signal – We're Watching The Future Unfold In Real TimeAbby Lee Hood — 6 February 2022
- 228Webb Team Brings 18 Dots of Starlight Into Hexagonal Formation18 February 2022
- 231Calls for Proposals & PolicySpace Telescope Science Institute
- 233Selections Made for the JWST Director's Discretionary Early Release Science ProgramSpace Telescope Science Institute
- 234Director's Discretionary Early Release Science ProgramsSpace Telescope Science Institute
- 235JournalDiscovery and properties of ultra-high redshift galaxies (9 < z < 12) in the JWST ERO SMACS 0723 FieldAdams — January 2023
- 236JournalFirst Batch of z ≈ 11–20 Candidate Objects Revealed by the James Webb Space Telescope Early Release Observations on SMACS 0723-73Haojing Yan — January 2023
- 250JWST Cycle 1 General Observer Submission StatisticsSpace Telescope Science Institute
- 252STScI Announces the JWST Cycle 2 General Observer ProgramSpace Telescope Science Institute
- 254Nearly three years since launch, Webb is a hit among astronomersStephen Clark — 6 November 2024
- 255At Last! NASA's Webb Space Telescope Is Now Fully Ready for ScienceThaddeus Cesari et al. — 2022-07-11
- 256NASA may unlock future James Webb Space Telescope dataBartels — 2022-12-12
- 258NewsHubble vs. James Webb telescope images: See the differenceMeredith Deliso et al. — 14 July 2022
- 261JournalThe First Billion Years, According to JWSTAngela Adamo et al. — 2025
- 262NewsNASA's Webb Delivers Deepest Infrared Image of Universe YetRob Garner — 11 July 2022
- 264NASA names first five targets for Webb imagesJohn Timmer — 2022-07-08
- 265First Images from the James Webb Space Telescope2022-07-08
- 266NewsGawking in Awe at the Universe, TogetherShannon Stirone — 12 July 2022
- 267NewsWebb Telescope Reveals a New Vision of an Ancient UniverseDennis Overbye et al. — 12 July 2022
- 268NewsThe James Webb telescope found 'Green Pea' galaxies in the early universeLisa Grossman — 12 January 2023
- 270JournalThe Science Performance of JWST as Characterized in CommissioningJane Rigby et al. — 2023
- 271JournalA hot terrestrial planet orbiting the bright M dwarf L 168-9 unveiled by TESSN. Astudillo-Defru et al. — 2020-04-01
- 272NewsHere's the Largest Image JWST Has Taken So FarNancy Atkinson — 17 August 2022
- 273NewsWebb's largest image of galaxies yetKelly Kizer Whitt — 18 August 2022
- 275JournalPlanck 2018 results. VI. Cosmological parametersPlanck Collaboration — 2020
- 276JournalConfirmation and refutation of very luminous galaxies in the early UniversePablo Arrabal Haro et al. — 2023-10-26
- 277JournalAccelerating Early Massive Galaxy Formation with Primordial Black HolesBoyuan Liu et al. — 2022-09-27
- 278JournalRapidly growing primordial black holes as seeds of the massive high-redshift JWST GalaxiesGuan-Wen Yuan et al. — 2024
- 279An inflation model for massive primordial black holes to interpret the JWST observationsBing-Yu Su et al. — 2023
- 281JournalExtreme properties of a compact and massive accreting black hole host in the first 500 MyrRoberta Tripodi — November 19, 2025
- 282JournalA direct black-hole mass measurement in a little red dot at high redshiftIgnas Juodžbalis — 2026
- 283Significant Evidence of an AGN Contribution in GHZ2 at z = 12.34Oscar A. Chavez Ortiz — November 4, 2025
- 284JournalCloudy mornings and clear evenings on a gas giant exoplanetSagnick Mukherjee et al. — 2026-05-21
- 285Hot Jupiter exoplanet has cloudy mornings and clear eveningsPaul Scott Anderson — 2026-05-26
- 286JournalNo thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curvesMichaël Gillon et al. — May 2026
- 288JournalThe James Webb Space TelescopeGardner — November 2006
- 289JournalScientific discovery with the James Webb Space TelescopeKalirai — April 2018