Discussions for a successor to the Hubble Space Telescope began in 1989, even before Hubble's launch, at a workshop held at the Space Telescope Science Institute (STScI) in Baltimore, Maryland.¹² In 1996, an eighteen-member committee chaired by astronomer Alan Dressler released the HST and Beyond report, which formally recommended that NASA develop a large, infrared-optimized space telescope with a mirror aperture of more than four meters, cooled to cryogenic temperatures to observe the earliest epochs of the universe.¹³¹⁴ NASA subsequently initiated the project under the name Next Generation Space Telescope (NGST), with Nobel laureate John C. Mather of NASA's Goddard Space Flight Center serving as senior project scientist.¹⁵ The National Academy of Sciences ranked the mission as the highest priority for United States astronomy in its 2001 decadal survey.

Because a solid primary mirror larger than four meters could not fit inside the payload fairing of any existing launch vehicle and would be prohibitively heavy, engineers devised a lightweight, segmented folding mirror architecture alongside a deployable multi-layer sunshield.¹⁶¹⁷¹⁸ In 2002, NASA awarded the prime contract to build the observatory to TRW Inc., which was acquired shortly thereafter by Northrop Grumman. Overall project management and systems engineering were assigned to the Goddard Space Flight Center in Greenbelt, Maryland, while mission and science operations were assigned to STScI. Under formal international agreements, ESA agreed to provide the Near-Infrared Spectrograph (NIRSpec), the optical bench assembly for the Mid-Infrared Instrument (MIRI), the Ariane 5 launch vehicle, and operational personnel in exchange for a minimum 15 percent share of observing time for European astronomers. CSA agreed to supply the Fine Guidance Sensor and Near-Infrared Imager and Slitless Spectrograph (FGS/NIRISS) in exchange for a guaranteed share of observing time for Canadian researchers.

Initial estimates in the late 1990s projected a launch by 2007 to 2010 at a cost of $1 billion to $3.5 billion, which was revised to $4.96 billion with a 2014 launch date when the mission passed its preliminary design review in 2008. However, the novelty of its technologies led to persistent cost growth and schedule delays. In 2010, an Independent Comprehensive Review Panel chaired by John Casani concluded that the project's budget and timeline were unrealistic due to inadequate financial reserves and management flaws. Following the report, NASA restructured the program management and revised the total cost estimate to $8.8 billion, prompting the United States Congress to impose a strict $8 billion legislative cap on pre-launch development costs in 2011.

During the 2010s, mirror segments and instruments were manufactured, tested at cryogenic temperatures in historic Vacuum Chamber A at the Johnson Space Center in Houston, Texas, and integrated at Northrop Grumman's facilities in Redondo Beach, California. In 2018, an independent review board reported that human errors and technical issues during spacecraft testing—including the use of an improper solvent to clean propulsion valves and the loosening of dozen of fasteners during a sunshield acoustic vibration test—required further delays. In early 2019, Congress raised the development cost cap by $800 million, bringing NASA's total lifecycle expenditure to roughly $9.7 billion ($8.8 billion for development and $861 million for five years of operations), alongside roughly $1 billion combined from ESA and CSA.

In September 2002, NASA Administrator Sean O'Keefe renamed the Next Generation Space Telescope in honor of James E. Webb, who served as the agency's second administrator from 1961 to 1968. While Webb was widely recognized for leading NASA through the Mercury, Gemini, and Apollo programs while maintaining support for unmanned space science, the choice departed from NASA's customary practice of naming major observatories after prominent scientists through a committee review process.

Beginning in 2015 and intensifying in March 2021 with an opinion article published in Scientific American by astronomers Chanda Prescod-Weinstein, Sarah Tuttle, Lucianne Walkowicz, and Brian Nord, critics called on NASA to rename the observatory. A petition signed by more than 1,700 astronomers and students argued that Webb, during his tenure as Under Secretary of State from 1949 to 1952 and later as NASA Administrator, had been complicit in the "Lavender Scare"—the systematic federal purge of gay and lesbian government employees.¹⁹²⁰ Critics cited archival records of a 1950 meeting in which Webb discussed Senate investigations into homosexuality with President Harry S. Truman and Senator Clyde R. Hoey, as well as the 1963 arrest, interrogation by NASA security, and dismissal of NASA budget analyst Clifford L. Norton on suspicion of homosexual conduct.²¹

In response, NASA instructed its chief historian, Brian Odom, to conduct an archival investigation. In September 2021, NASA announced it would retain the name, prompting Walkowicz to resign from NASA's Astrophysics Advisory Committee in protest over the lack of a public report. In November 2022, after reviewing more than 50,000 pages of archival documents from federal collections and the Truman Presidential Library, NASA released an 89-page historical report. The report concluded that Webb's 1950 actions were aimed at limiting congressional access to State Department personnel records, that there was no evidence linking Webb to actions or follow-ups pursuing the firing of employees for their sexual orientation, and that no evidence indicated Webb had personal knowledge of Norton's 1963 dismissal.

The James Webb Space Telescope has a launch mass of approximately 6,200 to 6,500 kilograms—roughly half the mass of the Hubble Space Telescope—and consists of three primary subsystems: the Spacecraft Element (comprising the spacecraft bus and sunshield), the Optical Telescope Element (OTE), and the Integrated Science Instrument Module (ISIM).

The James Webb Space Telescope launched at 12:20 UTC on December 25, 2021, aboard an Ariane 5 ECA rocket (Flight VA256) from the Guiana Space Centre in Kourou, French Guiana. Separating from the upper stage 27 minutes after liftoff, the spacecraft deployed its solar array ahead of schedule at 29 minutes post-launch, indicating that the rocket had placed the observatory directly into its desired orientation and transfer trajectory.²⁹³⁰ The accuracy of the launch and the subsequent mid-course correction burns required significantly less hydrazine and dinitrogen tetroxide propellant than budgeted, roughly doubling the telescope's estimated propellant-limited operational lifespan from its 10-year goal to more than 20 years.³¹

Over the following two weeks, mission controllers at STScI guided the spacecraft through a sequence of complex mechanical deployments Involving 178 release mechanisms and 344 potential single-point failures. Between December 28, 2021, and January 4, 2022, engineers deployed the forward and aft sunshield pallets, extended the telescope's deployable tower assembly to separate the warm and cold halves of the spacecraft, released the sunshield protective covers, extended the mid-booms, and successfully tensioned all five Kapton layers. On January 5, the secondary mirror tripod unfolded and latched into place, followed by the deployment of the port and starboard primary mirror wings on January 7 and 8, completing the structural transformation of the observatory. On January 24, 2022, a final thruster burn inserted JWST into its operational halo orbit around $L_2$.

A five-month optical and instrument commissioning phase followed as the telescope passively cooled and MIRI's cryocooler brought its detectors below 7 K. Using light from an isolated target star, engineers executed a seven-stage mirror alignment process with NIRCam, moving the 18 individual mirror segments from millimeter-scale launch offsets into a unified optical surface phased to within 50 nanometers—exceeding diffraction-limited design requirements across all instruments. Between May 22 and 24, 2022, mirror segment C3 was struck by a micrometeoroid larger than pre-launch dust models had anticipated, increasing the segment's local wavefront error from 56 to 258 nanometers root-mean-square (rms). By adjusting the segment's actuators, engineers reduced the local error to 178 nanometers rms, leaving the overall telescope wavefront error at roughly 59 nanometers rms—still well outperformed compared to its 150-nanometer mission requirement. To reduce future high-energy impacts, mission planners subsequently instituted a "micrometeoroid avoidance zone," restricting the time the primary mirror spends pointing directly into the orbital ram direction.³²

Commissioning concluded on July 11, 2022, when United States President Joe Biden unveiled the observatory's first full-color scientific image, Webb's First Deep Field, a 12.5-hour exposure of the galaxy cluster SMACS 0723 showing gravitationally lensed galaxies from the early universe.³³ On July 12, NASA, ESA, and CSA released the remaining Early Release Observations: an atmospheric transmission spectrum of the hot gas-giant exoplanet WASP-96b showing distinct water vapor signatures, high-resolution images of the planetary nebula NGC 3132 (the Southern Ring Nebula) and the interacting galaxy group Stephan's Quintet, and a detailed infrared panorama of star-forming cliffs in the Carina Nebula (NGC 3324).³⁴

Since the start of Cycle 1 science operations in July 2022, JWST has altered models across observational cosmology, galactic evolution, stellar physics, and planetary science. Because cosmic expansion stretches ultraviolet and visible light emitted by the earliest stars and galaxies into longer infrared wavelengths—a phenomenon known as cosmological redshift ($z$)—JWST has extended the observational frontier into the first few hundred million years after the Big Bang.³⁵³⁶ Through programs such as the JWST Advanced Deep Extragalactic Survey (JADES) and the Mirage or Miracle (MoM) survey, the telescope has repeatedly broken the record for the most distant spectroscopically confirmed galaxy, identifying JADES-GS-z14-0 at redshift $z = 14.18$ (seen roughly 300 million years after the Big Bang) in 2024 and MoM-z14 at redshift $z = 14.44$ (roughly 280 million years after the Big Bang) in 2025.³⁷³⁸

These high-redshift surveys revealed an unexpected abundance of luminous, massive, and chemically enriched galaxies—containing heavy elements such as carbon, nitrogen, and oxygen—existing within 300 to 500 million years of the Big Bang.³⁹ Dubbed the "impossible early galaxies" problem in scientific literature, the discovery of galaxies with stellar masses and structures larger than predicted at those epochs by the standard Lambda Cold Dark Matter ($\Lambda$CDM) cosmological model has prompted astrophysicists to investigate revised models of early star-formation efficiency, bursty star formation, and primordial black hole growth. Simultaneously, JWST has uncovered a population of compact, high-redshift sources termed "Little Red Dots" ($z \approx 4\text{--}9$), which appear to host rapidly growing supermassive black holes or dense stellar cores in the infant universe, and has mapped dark matter distributions via strong gravitational lensing in ancient structures such as the galaxy cluster XLSSC 122, located 10.4 billion light-years away.⁴⁰⁴¹

In planetary and exoplanetary science, JWST has conducted transit and emission spectroscopy across dozens of extrasolar worlds. In 2022, observations of the gas giant WASP-39b yielded the first detection of sulfur dioxide ($SO_2$) in an exoplanet atmosphere, providing direct evidence of active photochemistry driven by host star radiation, alongside measurements of water, carbon monoxide, carbon dioxide, sodium, and potassium. Observations of the sub-Neptune K2-18b, orbiting within its star's habitable zone 120 light-years away, confirmed abundant methane and carbon dioxide with a paucity of ammonia—consistent with a hydrogen-rich atmosphere overlying a global water ocean ("Hycean" world)—and yielded tentative three-sigma evidence of dimethyl sulfide, a compound produced on Earth primarily by marine life. In the TRAPPIST-1 system of seven Earth-sized planets orbiting an ultracool red dwarf, MIRI thermal emission measurements demonstrated that the innermost rocky planets, TRAPPIST-1b and TRAPPIST-1c, lack thick carbon dioxide atmospheres, spurring a 500-hour Director's Discretionary Time program beginning in 2025 to test whether rocky planets around M-dwarf stars can retain atmospheres. Closer to Earth, JWST has mapped multi-layered weather patterns and auroral processes on brown dwarfs such as SIMP 0136, imaged dusty protoplanetary disks and stellar nurseries including the Pillars of Creation in the Eagle Nebula, and observed plumes, rings, and atmospheric dynamics across Jupiter, Saturn, Uranus, Neptune, and Kuiper Belt objects.⁴²

As of 2026, the James Webb Space Telescope remains in full scientific operation at $L_2$ and is executing its fifth annual observing cycle (Cycle 5), which began in July 2026. Demand for observing time among the international astronomical community has increased in every cycle since launch, reaching a record 2,900 submitted proposals for Cycle 5—an oversubscription rate under which only roughly 8 percent of requested observing hours can be accommodated within the approximately 8,000 hours available annually for scientific programs. Following minor operational anomalies earlier in the mission that were resolved through software and operational updates—including brief friction in a MIRI medium-resolution spectrometer grating wheel in 2022 and an intermittent electrical short in NIRISS—all four science instruments continue to operate nominal observation modes, with sufficient onboard propellant to sustain station-keeping and momentum management into the 2040s.