How the James Webb Telescope Works

01 Origins and Purpose

The James Webb Space Telescope was conceived in 1996 as the Next Generation Space Telescope, a successor to Hubble that would focus on the infrared spectrum. The scientific motivation was clear: the universe's earliest galaxies are so distant that their light has been stretched by cosmic expansion — redshifted — from visible wavelengths into the infrared. To see the first stars and galaxies forming after the Big Bang, an infrared telescope was required, and it had to be large enough to collect photons from objects more than 13 billion light-years away. The project was renamed in 2002 after James E. Webb, NASA's administrator during the Apollo era.

What began as a relatively modest concept grew into the most complex robotic spacecraft ever built. JWST was designed from the outset as an infrared-only telescope, observing wavelengths from 600 nanometres (red light) to 28,000 nanometres (mid-infrared). This range covers the redshifted light of the earliest galaxies, the dust-penetrating wavelengths needed to observe star formation, and the molecular fingerprints of planetary atmospheres. The telescope's sensitivity targets were set so high that every aspect of the design — mirror material, operating temperature, orbit, and deployment mechanism — had to be invented rather than adapted from existing technology.

02 The Segmented Mirror

No single rocket fairing could carry a 6.5-metre mirror fully assembled, so Webb's primary mirror was built as a mosaic of eighteen hexagonal segments, each approximately 1.32 metres across. The segments are made of beryllium, a metal chosen for its stiffness and thermal stability — beryllium barely expands or contracts with temperature changes, making it ideal for a mirror that must hold its shape at cryogenic temperatures. Each segment was polished to an accuracy of about 20 nanometres — a fraction of the wavelength of visible light — and then coated with a layer of gold approximately 100 nanometres thick, chosen because gold reflects infrared light more efficiently than aluminium or silver.

After launch, the segments were folded like the leaves of a table and deployed over a period of two weeks. But deployment was only the first step: the eighteen segments then had to be aligned to act as a single optical surface, a process called wavefront sensing and control that took approximately three months. Each segment carries seven actuators — six for position control and one for adjusting curvature — allowing ground controllers to adjust each mirror's orientation to within nanometres. The alignment process produced the first sharp image in March 2022, and the fully aligned telescope achieved diffraction-limited performance at 2 micrometres, meeting its design specification.

03 The Five-Layer Sunshield

Infrared detectors are exquisitely sensitive to heat — any warm surface emits infrared radiation that would swamp the faint signals from distant galaxies. JWST's instruments must be cooled to approximately -233°C (40 Kelvin) or colder without carrying a cryogenic refrigerator. The solution is passive cooling: a sunshield the size of a tennis court, composed of five thin layers of Kapton polyimide film, each coated with aluminium and vapour-deposited gold. The shield blocks 99.98% of the Sun's heat, allowing the cold side to reach equilibrium with deep space.

The five layers are separated by gaps that allow heat to radiate outward between layers rather than conducting through. Layer one, facing the Sun, operates at roughly 85°C; each subsequent layer is cooler, until the fifth layer, nearest the instruments, reaches about -233°C. The sunshield was the single most complex deployment on the spacecraft, involving 139 actuators, eight deployment motors, and approximately 90 tensioning cables. It was unfolded over a period of eight days in January 2022, each step controlled and verified from the ground. If the sunshield had failed to deploy correctly, the telescope would have been unable to operate — there was no backup and no means of repair.

04 The Instruments

JWST carries four science instruments, each serving a different spectral range and purpose. NIRCam (Near-Infrared Camera) is the primary imaging instrument, covering 0.6 to 5 micrometres with two identical modules, each with a 5-megapixel detector. NIRCam is also responsible for the wavefront sensing that aligns the mirror segments. NIRSpec (Near-Infrared Spectrograph) is the first multi-object spectrograph in space, capable of simultaneously observing up to 200 targets using a programmable microshutter array. Its spectral resolution reaches R = 2700, allowing detection of individual emission and absorption lines from distant galaxies and exoplanet atmospheres.

MIRI (Mid-Infrared Instrument) covers 5 to 28 micrometres and is the only instrument that reaches the mid-infrared, where dust-embedded stars, planetary debris discs, and the thermal emission of cool objects are brightest. MIRI carries an active cryocooler because passive cooling alone cannot reach its required operating temperature of about 7 Kelvin. Finally, FGS/NIRISS (Fine Guidance Sensor / Near-Infrared Imager and Slitless Spectrograph) provides the pointing precision that keeps JWST stable during long exposures, while also conducting slitless spectroscopy for exoplanet transit observations. Together, these instruments give Webb a spectral reach from visible-red light through the mid-infrared, a regime no previous space telescope has fully explored.

05 The L2 Orbit and Deployment

Webb orbits the Sun at the second Lagrange point (L2), approximately 1.5 million kilometres beyond Earth on the anti-Sun side. At L2, the gravitational pull of the Sun and Earth combine such that a spacecraft can maintain a fixed position relative to both, allowing continuous observation with the Sun always behind the sunshield. JWST does not sit precisely at L2 but orbits around it in a halo orbit, a three-dimensional Lissajous pattern that keeps the telescope in stable thermal conditions. The orbit requires station-keeping thruster burns every 21 days to maintain, using a modest supply of hydrazine propellant that limits the mission to roughly 20 years of operations.

Deployment was the most critical phase of the mission. Over a period of approximately one month following launch on December 25, 2021, Webb executed 344 single-point failures — individual mechanical actions that, if any one had gone wrong, could not have been corrected from Earth. The sequence included: deploying the solar array, deploying the high-gain antenna, unfolding the forward and aft sunshield pallets, releasing and tensioning five sunshield layers, deploying the secondary mirror support, unfolding and latching the two primary mirror wings, and turning on each instrument in sequence. Every step was commanded from the ground with verification between steps. The deployment concluded successfully in January 2022, and the first science images were released on July 12, 2022.

06 First Discoveries

Webb's first science images, released in July 2022, immediately demonstrated the telescope's transformative capability. The deep-field image of galaxy cluster SMACS 0723 revealed thousands of galaxies in a fraction of the exposure time Hubble would have required, including galaxies whose light had travelled more than 13 billion years. The transmission spectrum of exoplanet WASP-96 b showed the unmistakable signature of water vapour in the planet's atmosphere, the first of many such measurements. The image of the Carina Nebula revealed previously invisible stellar nurseries, and the spectrum of the exoplanet WASP-39 b detected carbon dioxide in an exoplanet atmosphere for the first time.

As observations have accumulated through 2026, Webb has pushed further. Deep-field surveys have found galaxies at redshifts exceeding 14, corresponding to light emitted less than 300 million years after the Big Bang — earlier than most models predicted. Some of these galaxies appear more massive and structured than theorists expected, prompting active debate about whether the standard cosmological model needs revision. Webb has also produced direct chemical measurements of rocky exoplanet atmospheres, detected carbon-bearing molecules in protoplanetary discs, and captured unprecedented mid-infrared images of the Solar System's giant planets, including new views of Neptune's rings and Jupiter's aurora. Each observation demonstrates that the telescope's twenty-five years of development produced a machine that has fundamentally changed infrared astronomy.