The James Webb Space Telescope

At a glance: the James Webb Space Telescope is a space-based infrared telescope launched on 25 December 2021 from the Guiana Space Centre near Kourou, French Guiana, on an Ariane 5 rocket. It is the successor to the Hubble Space Telescope and to the Spitzer Space Telescope, and is operated by a consortium of NASA, the European Space Agency, and the Canadian Space Agency. The telescope’s primary mirror is 6.5 metres in diameter, segmented into 18 hexagonal beryllium elements that unfolded in space after launch. Its instruments observe primarily in the infrared, from 0.6 to 28 micrometres, and are designed to image the first galaxies that formed after the Big Bang, to characterise the atmospheres of exoplanets, and to study the formation of stars and planetary systems. The telescope is positioned at the second Sun-Earth Lagrange point, about 1.5 million kilometres from Earth. Its first full year of science operations has produced results that justify, in the assessment of most working astronomers, the two and a half decades of development and the roughly ten billion dollars of total mission cost.

Background: why a successor to Hubble was wanted

The case for a successor to the Hubble Space Telescope was being made, in various forms, from the mid-1990s. Hubble, launched in 1990 and serviced five times by Space Shuttle astronauts between 1993 and 2009, had transformed observational astronomy across a wide range of fields. Its angular resolution in visible light, sharpened by the corrective mirrors installed in 1993 and 1997, allowed astronomers to resolve structures in distant galaxies that had previously been inaccessible; its ultraviolet and near-infrared instruments opened windows into the chemistry of the interstellar medium and the physics of accretion onto supermassive black holes. By the early 2000s, however, it was becoming clear that the questions that Hubble could answer were approaching their natural limits, and that the next generation of questions would require a substantially larger telescope observing primarily in the infrared.

The infrared is important for several reasons. Light from the most distant galaxies, which has been redshifted by the expansion of the universe from the ultraviolet into the infrared, is best observed at infrared wavelengths; the warm dust that obscures star-forming regions is transparent in the infrared; and the cool objects such as brown dwarfs and exoplanets radiate predominantly in the infrared. The Hubble telescope was capable of observing in the near-infrared, but its primary mirror at 2.4 metres was too small to collect sufficient photons from the most distant galaxies, and its instruments were not designed to optimise the infrared signal-to-noise ratio. The next major observatory would need a primary mirror of several metres and a set of instruments optimised for infrared observation.

The project that became the James Webb Space Telescope was formally proposed in the mid-1990s under the name Next Generation Space Telescope. The design that emerged over the following two decades specified a primary mirror of 6.5 metres, segmented because no existing launch vehicle could carry a monolithic mirror of that size, made of beryllium because beryllium is both stiff and light and because it holds its figure at the cryogenic temperatures required for infrared observation. The mirror was to be cooled by a five-layer sunshield, the size of a tennis court when deployed, that would keep the optical bench at a temperature of about 50 Kelvin.

Mirror: how the 6.5 metre segmented optic was built and tested

The primary mirror of the James Webb Space Telescope is composed of 18 hexagonal segments arranged in a honeycomb pattern. Each segment is 1.32 metres across the flats and weighs about 20 kilograms; the total mass of the mirror assembly is about 705 kilograms. The segments are made of beryllium, chosen for its combination of stiffness, low density, and dimensional stability at cryogenic temperatures. Each segment was coated with a thin layer of gold, chosen for its high reflectivity in the infrared; the gold layer is only about 100 nanometres thick, but it gives the mirror a reflectivity of more than 98 per cent across the wavelengths at which the telescope operates.

The mirror was manufactured by Ball Aerospace and Technologies Corporation, in Boulder, Colorado, and the segments were polished by L3Harris Technologies, in Rochester, New York. Each segment was polished to a figure accurate to about 20 nanometres across the optical surface, a precision substantially finer than the wavelength of visible light. The polished segments were then cryogenically tested at the Marshall Space Flight Center in Huntsville, Alabama, to confirm that the figure remained stable at the operating temperature of about 50 Kelvin. The testing took several years and revealed, in 2011, a small budget shortfall in the optical budget that led to a substantial delay in the launch and a substantial increase in the mission cost.

After launch, the segments were unfolded from their stowed configuration, in which the mirror was folded into three sections for launch, and aligned using a set of actuators on the back of each segment. The actuators, of which there are 132 in total, allow each segment to be moved in tip, tilt, and piston with sub-nanometre precision. The alignment process, which took several months after launch, produced a wavefront error at the operating wavelength of about 50 nanometres, well within the budget required for diffraction-limited performance at 2 micrometres.

Sunshield: the tennis-court-sized thermal blanket

The sunshield is the second of the two engineering innovations that distinguish the James Webb Space Telescope from its predecessors. The sunshield consists of five layers of Kapton, a polyimide film manufactured by DuPont, each layer about as thick as a human hair and coated with aluminium and doped silicon to give the layers their specific thermal and optical properties. The sunshield is shaped like a kite, about 21 metres by 14 metres when fully deployed, and separates the warm side of the spacecraft, facing the Sun and the Earth, from the cold side, facing deep space. The warm side of the spacecraft is at a temperature of about 85 Celsius; the cold side is at a temperature of about minus 233 Celsius, a difference of more than 300 degrees maintained across the five layers.

The sunshield was designed to fold up like an origami into a compact package about the size of a coffee table for launch and to unfold in a precisely choreographed sequence of motor-driven cable releases. The unfolding took place in the first week after launch and was the most anxious phase of the deployment. There were 344 single-point failure modes associated with the deployment, any one of which would have prevented the telescope from operating as designed; the deployment was, in the words of one NASA engineer, “the most complex sequence ever attempted in space”. The deployment proceeded without incident.

Position: why the second Lagrange point

The James Webb Space Telescope is positioned at the second Sun-Earth Lagrange point, designated L2, about 1.5 million kilometres from Earth in the anti-Sun direction. The L2 point is one of five points in the Sun-Earth system at which a small object, subject to the gravitational pull of the Sun and the Earth, can orbit the Sun with the same period as the Earth and so remain in a fixed position relative to the Sun and the Earth. The L2 point has been used for several previous infrared observatories, including the Wilkinson Microwave Anisotropy Probe, the Herschel Space Observatory, and the Planck spacecraft.

The L2 point offers several advantages for infrared observation. The point is far enough from the Earth that the Earth’s thermal radiation, which peaks in the infrared, does not contaminate the observations. The point is far enough from the Sun, with the sunshield deployed, that the satellite can be cooled to the operating temperature of the infrared detectors. The point is gravitationally stable, so that the satellite can remain in a small orbit around the point with only modest station-keeping. The principal disadvantage of the L2 point is that it is too far from the Earth to be serviced by the Space Shuttle or by any other crewed vehicle; the James Webb Space Telescope was designed, accordingly, to be serviceable only by remote intervention.

Instruments: NIRCam, NIRSpec, MIRI, and FGS-NIRISS

The James Webb Space Telescope carries four principal science instruments. The Near Infrared Camera, NIRCam, built by the University of Arizona, is the principal imaging instrument for wavelengths from 0.6 to 5 micrometres. NIRCam is equipped with a set of filters and a coronagraph for blocking the light of bright stars; the coronagraph allows NIRCam to image faint objects such as exoplanets and circumstellar disks that would otherwise be lost in the glare of the parent star. NIRCam was the first of the four instruments to return science-grade images, and its first deep field, released on 11 July 2022, was the deepest infrared image of the universe yet obtained.

The Near Infrared Spectrograph, NIRSpec, built by Airbus Defence and Space for the European Space Agency, is the principal spectroscopic instrument for the near infrared. NIRSpec is equipped with a micro-shutter array that allows it to take spectra of up to 100 objects simultaneously; the micro-shutter array is the first such device to fly, and it allows NIRSpec to survey the spectra of galaxies in deep fields at a rate that would have been impossible with previous spectrographs. The Mid Infrared Instrument, MIRI, built by a European consortium led by the Max Planck Institute for Astronomy in Heidelberg, is the principal instrument for wavelengths from 5 to 28 micrometres. MIRI is equipped with a camera, a spectrograph, and a coronagraph, and is the only instrument on the telescope that operates at the longer infrared wavelengths; MIRI must be cooled to a temperature of about 7 Kelvin, colder than the rest of the telescope, by a dedicated cryocooler.

The Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph, FGS-NIRISS, built by the Canadian Space Agency, serves a dual function. The FGS portion of the instrument provides the fine pointing of the telescope, allowing the spacecraft to hold a target to within a few milliarcseconds; the NIRISS portion of the instrument provides a set of imaging and spectroscopic modes optimised for the study of exoplanets. The NIRISS instrument is, in particular, equipped with a single-object spectrograph that uses a technique called aperture mask interferometry to achieve very high contrast imaging of faint companions to bright stars.

First results: the deep fields

The first major science result from the James Webb Space Telescope was the release, on 11 July 2022, of the first deep field. The deep field is a long-exposure image of a small patch of sky near the handle of the Big Dipper, in the direction of the galaxy cluster SMACS 0723, obtained over a period of about 12.5 hours. The image shows, in a region of sky about the size of a grain of sand held at arm’s length, thousands of galaxies, the faintest of which are at a redshift of about 8.5, corresponding to a time about 600 million years after the Big Bang. The image was the deepest infrared image of the universe at the time of its release and has been used, in the months since, as a reference for a substantial fraction of the first year of JWST science.

The deep field has also produced, in subsequent observations, images of galaxies that appear to be at redshifts greater than 12, corresponding to a time less than 350 million years after the Big Bang. The existence of such galaxies, if confirmed, would imply that the first generation of galaxies formed substantially earlier, and that the first generation of stars was substantially more luminous in the ultraviolet, than had been predicted by the standard models of early galaxy formation. The first papers to report these candidates were published in late 2022 and early 2023; the candidates are now the subject of intensive follow-up observation, both with JWST and with ground-based facilities such as the Atacama Large Millimeter Array and the Keck Observatory.

Exoplanet atmospheres: the spectroscopic characterisation of distant worlds

The second major science result from the James Webb Space Telescope is the spectroscopic characterisation of exoplanet atmospheres. Before JWST, the characterisation of exoplanet atmospheres was confined to a small number of bright, close-in planets observed with the Hubble Space Telescope and with the Spitzer Space Telescope; the spectra that could be obtained were of low signal-to-noise ratio and could be interpreted only with considerable uncertainty. JWST has changed the situation in two ways. The larger collecting area of the JWST primary mirror allows spectra of much higher signal-to-noise ratio to be obtained, and the broader wavelength coverage, particularly into the mid-infrared, allows a wider range of molecular species to be detected.

The first major JWST result on exoplanet atmospheres was the detection, in August 2022, of carbon dioxide in the atmosphere of WASP-39b, a hot Saturn-mass planet orbiting a star about 700 light-years from Earth. The detection, made with NIRSpec, was the first unambiguous detection of carbon dioxide in an exoplanet atmosphere and demonstrated that the precision of JWST spectroscopy was sufficient to detect molecules at the abundance levels predicted by models. Subsequent observations of WASP-39b detected water, carbon monoxide, sodium, and sulphur dioxide; the detection of sulphur dioxide, in particular, was the first detection of a photochemical product in an exoplanet atmosphere.

The JWST exoplanet programme has also produced the first detection of carbon-bearing molecules in the atmosphere of a rocky planet. In a series of observations released in 2023, JWST detected carbon dioxide and carbon monoxide, but not methane, in the atmosphere of TRAPPIST-1c, a rocky planet in the habitable zone of the ultracool dwarf star TRAPPIST-1. The detection demonstrated that rocky planets around ultracool dwarfs can have substantial atmospheres; the absence of methane, however, suggests that the atmosphere of TRAPPIST-1c is not, in the strict sense, habitable. The result is the first direct constraint on the atmospheres of rocky planets in the habitable zones of nearby stars and is widely regarded as a milestone in the search for habitable exoplanets.

Star formation and protoplanetary disks

The third major area in which JWST has produced substantial early results is the study of star formation and protoplanetary disks. The telescope’s infrared capability is ideally suited to the study of cool, dusty objects such as protostars and protoplanetary disks, which radiate predominantly at infrared wavelengths. The first JWST images of the Pillars of Creation, the iconic Eagle Nebula star-forming region previously imaged by Hubble, were released in October 2022; the JWST images show, in much greater detail than the Hubble images, the young stars and protoplanetary disks embedded in the pillars, and have been widely regarded as the most spectacular astronomical images obtained to date.

The JWST study of protoplanetary disks has produced, in particular, the first direct detection of water in the inner regions of a disk. In a series of observations published in 2023, JWST detected water vapour in the disk of PDS 70, a young star with two giant planets in the process of forming. The detection demonstrated that the conditions in the inner regions of protoplanetary disks are consistent with the presence of liquid water, and therefore with the conditions that may be necessary for the emergence of life. The detection is widely regarded as a milestone in the study of the conditions under which planetary systems form.

Mission cost and operational life

The James Webb Space Telescope is one of the most expensive scientific instruments ever built. The total cost of the mission, including design, development, launch, and ten years of operations, is estimated by NASA at about ten billion US dollars, of which about eight billion was spent on development and about two billion on operations. The development cost was substantially higher than the original estimate of about five billion, and the launch was delayed by several years from the original target of 2007. The overruns and delays were the subject of substantial criticism from the United States Congress and from the wider scientific community, and they have influenced the design of subsequent large space observatories.

The mission is designed to operate for a minimum of ten years, with the expectation that the operational life may be as long as twenty years, depending on the consumption of propellant for station-keeping at the L2 point. The propellant budget at launch was sufficient for twenty years of station-keeping; the budget was augmented by the precision of the Ariane 5 launch, which placed the telescope into its operational orbit more accurately than had been planned and so conserved propellant. The first year of operations has proceeded without major anomaly, and the consumption of propellant has been substantially lower than had been planned. The operational life of the telescope is now expected to be sufficient to support a programme of observations through the late 2030s.

JWST and the future of astronomy

The James Webb Space Telescope has, in the assessment of most working astronomers, transformed infrared observation in a manner comparable to the transformation that Hubble produced in visible-light observation in the 1990s. The first year of operations has produced results in cosmology, in exoplanet science, in star formation, and in galactic evolution that were not possible with previous instruments and that would have required decades of observation with existing facilities. The pace of publication in the first year of operations has been substantially higher than that of any previous space observatory; the rate of high-impact publications has been substantially higher than that of any previous space observatory. The telescope has, in effect, become the principal platform for infrared astronomy in the second quarter of the twenty-first century.

The JWST has also reshaped the broader scientific landscape by altering the relative weight of different approaches to astronomy. The next-generation ground-based observatories, including the Extremely Large Telescope under construction in Chile and the Thirty Meter Telescope under construction in Hawaii, will be complementary to JWST rather than competitive; the ground-based telescopes will provide the high-angular-resolution imaging and the high-resolution spectroscopy that JWST cannot provide, while JWST will provide the infrared sensitivity and the wavelength coverage that ground-based telescopes cannot match. The combination of JWST and the next-generation ground-based observatories will, in the assessment of most working astronomers, define the observational astronomy of the 2030s.

Why JWST still matters

The James Webb Space Telescope matters because it has opened a new window on the universe. The window was opened by the combination of a large segmented mirror, a precision sunshield, a set of infrared-optimised instruments, and a position at the L2 point that allows the telescope to operate at the cryogenic temperatures required for infrared observation. The window has revealed the first galaxies that formed after the Big Bang; the first chemical compositions of exoplanet atmospheres; the first structures in protoplanetary disks that may be the cradles of planetary systems. The window is, by any measure, the most capable infrared observatory ever built, and the results that it has produced in its first year of operation justify, in the assessment of most working astronomers, the two and a half decades of development and the roughly ten billion dollars of total mission cost.

For the broader public, JWST matters because it has made the universe visible in a way that no previous observatory has. The first deep field, the Pillars of Creation, the images of galaxies near the edge of the observable universe, the spectra of exoplanet atmospheres — all of these have been widely circulated, widely discussed, widely admired. The telescope has, in effect, become a cultural artefact as well as a scientific instrument; the images that it produces are recognised and remembered in a way that no previous astronomical images have been. The telescope is, in this sense, the first scientific instrument of the twenty-first century that has entered the broader cultural imagination.

For the future of astronomy, JWST matters because it has established a model for what a large infrared space observatory can do. The model will inform the design of subsequent observatories, including the Nancy Grace Roman Space Telescope, scheduled for launch in the late 2020s, and the proposed Habitable Worlds Observatory, scheduled for launch in the 2040s. The model is, in effect, a demonstration that the technological and managerial challenges of building and operating a large infrared space observatory can be met, and that the scientific return on the investment can be substantial. The James Webb Space Telescope is, in this sense, the foundation of the next generation of astronomical observation.

Operational challenges: how a deep-space observatory is run

The James Webb Space Telescope is operated by the Space Telescope Science Institute, in Baltimore, the same institute that operates Hubble. The institute receives the observing proposals from the worldwide astronomical community, evaluates them through a peer-review process, schedules the accepted observations on the telescope, and processes the data into a form that is useful for the observers. The principal difference between the Hubble operations model and the JWST operations model is the absence of crewed servicing. The Hubble telescope was serviced five times by Space Shuttle astronauts between 1993 and 2009; the JWST cannot be serviced by crewed spacecraft and was designed, accordingly, to operate without intervention.

The observatory is operated by a small team of engineers and scientists at the Space Telescope Science Institute, supported by a larger team at the Goddard Space Flight Center and at the prime contractors. The team has developed, over the first year of operations, a substantial body of operational expertise; the team has also developed, in collaboration with the instrument teams, a set of observing modes and strategies that allow the instruments to be used to their full capability. The operational model has been successful; the telescope has, in its first year, completed substantially more science observations than was originally planned, and the rate of observing is now close to the maximum that the telescope can support.

The principal operational challenge in the first year has been the management of micrometeoroid impacts on the primary mirror. The mirror is exposed to the flux of small particles that travel through the inner solar system, and a small number of these particles have struck the mirror in the first year. The impacts have caused measurable, but not catastrophic, degradation in the figure of one of the mirror segments; the impact of 18 May 2023, in particular, produced a measurable wavefront error in segment C3. The team has developed a procedure for correcting the wavefront error by adjusting the alignment of the affected segment; the procedure has been demonstrated to be effective, and the telescope continues to operate at its full diffraction limit.

The infrared universe: what JWST sees that others cannot

The James Webb Space Telescope observes principally in the infrared because the infrared is the wavelength range at which the most informative observations of distant, cool, and obscured objects can be made. The infrared has three principal advantages for astronomy. First, the infrared penetrates dust clouds that are opaque at visible wavelengths; the dust clouds in which stars form and in which the most active regions of galaxies are embedded are transparent in the infrared, and the infrared therefore allows astronomers to see into the regions that are otherwise hidden. Second, the infrared is the wavelength range at which the most distant galaxies emit; the expansion of the universe has redshifted the ultraviolet light from the first galaxies into the infrared, and the infrared is therefore the wavelength range at which the most distant galaxies must be observed. Third, the infrared is the wavelength range at which cool objects such as brown dwarfs, exoplanets, and protoplanetary disks emit most strongly; the infrared is therefore the wavelength range at which these objects are best studied.

The principal disadvantage of infrared observation is the requirement of low temperature. The telescope itself, and the detectors that it carries, must be cooled to temperatures of a few tens of Kelvin to avoid contamination of the infrared signal by thermal radiation from the telescope itself. The cooling is achieved, in JWST, by the sunshield and by the dedicated cryocooler that cools the MIRI instrument to about 7 Kelvin. The cryocooler is one of the more challenging components of the mission; it uses a closed-cycle Joule-Thomson cooler with a neon-hydrogen mixture as the working fluid, and it is the only actively cooled component of the observatory.

Why it took so long

The James Webb Space Telescope took substantially longer to develop, and cost substantially more than originally planned, than any previous NASA science mission. The project was formally initiated in 1996, with an estimated cost of about five hundred million dollars and an estimated launch date of 2007. By the time of the launch, in December 2021, the estimated cost had grown to about ten billion dollars and the launch had been delayed by fourteen years. The overruns and delays were caused by a combination of factors that have been analysed in detail by the National Academy of Sciences and by various investigative bodies.

The principal causes of the delay were the technical challenges associated with the mirror and the sunshield. The mirror required the development of new polishing techniques and the development of new test facilities; the polishing of the segments alone took several years longer than planned, and the test facilities required the construction of a new cryogenic test chamber at the Marshall Space Flight Center. The sunshield required the development of new deployment mechanisms and the development of new materials; the unfolding sequence in particular required extensive ground testing and several redesigns before the final configuration was judged ready for flight. The combination of these technical challenges, the layered management structures that NASA put in place to oversee the development, and the political pressures from Congress that accompanied the cost overruns, all contributed to the delay.

The overruns and delays have been the subject of substantial criticism. The criticism has focused on three principal points. First, the project was under-managed in its early years; the management structures put in place by NASA were inadequate to the scale of the technical challenge. Second, the project was under-resourced in its early years; the annual budget allocated by Congress was substantially less than the budget that the project office estimated was required to complete development on schedule. Third, the project was over-optimistic in its estimates; the cost and schedule estimates presented to the development team and to Congress were substantially less than the estimates that the project office had prepared for internal use. The combination of these factors produced the cost and schedule outcome that has been widely criticised.

The lessons learned from the development of JWST have informed the design of subsequent large space observatories. The Roman Space Telescope, scheduled for launch in the late 2020s, has been managed according to a more conservative cost-and-schedule model; the proposed Habitable Worlds Observatory, scheduled for launch in the 2040s, is being designed according to a model that emphasises the manageability of the technical challenges. The model is, in effect, a response to the experience of JWST: large ambitious projects require very conservative cost and schedule planning, and the management structures put in place must be adequate to the scale of the technical challenge.

The science returns of the first year

The first year of operations of the James Webb Space Telescope has produced a body of science results that exceeds, in volume and in significance, the first-year results of any previous space observatory. The results have been published in a substantial number of peer-reviewed papers; the rate of publication has been substantially higher than the rate of publication from Hubble, Spitzer, or any previous infrared observatory. The results have also been widely discussed in the popular press and on social media; the images and spectra produced by the telescope have entered the broader cultural imagination in a way that no previous astronomical images have.

The first year of science operations has been allocated to a set of Guaranteed Time Observations, in which the instrument teams and a small number of competitively selected observers have been allocated a substantial fraction of the available observing time. The remaining time has been allocated through a competitive proposal process that has been open to the worldwide astronomical community. The competition for time on the telescope has been intense; the oversubscription factor for the first cycle of General Observer programmes was about four to one, meaning that the demand for time on the telescope was about four times the time available. The competition has produced, in turn, a body of science proposals of substantially higher quality than the proposals submitted to any previous space observatory.

The first year has also demonstrated that the telescope is capable of operating at the diffraction limit across the full range of wavelengths at which it was designed to operate. The angular resolution of the telescope, measured at 2 micrometres, is about 0.07 arcseconds; the angular resolution at 10 micrometres is about 0.35 arcseconds; both figures are in good agreement with the predictions of the optical model. The sensitivity of the telescope, measured by the time required to obtain a signal-to-noise ratio of unity on a point source of a given brightness, is within about a factor of two of the pre-launch predictions. The principal limitation on the sensitivity has been the thermal background from the zodiacal light and from the small amounts of residual warm dust and gas in the solar system; the thermal background has been slightly higher than predicted.

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