James Webb Space Telescope

The James Webb Space Telescope — known as Webb or JWST — is the premier observatory of the next decade, serving thousands of astronomers worldwide, and NASA's infrared flagship observatory¹². It studies every phase in the history of our Universe, from the search for the first galaxies formed after the Big Bang to the evolution of galaxies, the lifecycle of stars, and other worlds including our own Solar System³. It is the next great space science observatory following Hubble, designed to answer outstanding questions about the Universe and make breakthrough discoveries in all fields of astronomy⁴. Following the technical and scientific legacy of observatories such as Hubble, Spitzer, and Herschel, JWST offers orders-of-magnitude improvements in sensitivity and spatial resolution². One of its main goals is to detect the very first star formation, thought to occur between redshift 15 and 30 — 100 to 250 million years after the Big Bang⁵.

Webb is an international partnership between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA)⁴. It launched on 25 December 2021 at 12:20 UTC aboard an Ariane 5 ECA rocket from Europe's Spaceport near Kourou, French Guiana³⁵. Rather than circling Earth just above the atmosphere like Hubble, Webb orbits the Sun about 1.5 million kilometers from Earth around the second Lagrange point, or L2⁶.

Webb carries a folding mirror: the 6.5-meter primary is so large that it could not fit in the launch shrouds of available rockets, so it unfolds in space⁵. A five-layer sunshield protects the telescope from the infrared radiation of the Sun, Earth, and Moon — like having sun protection of SPF 1 million¹. As the largest, most technically advanced telescope ever built, its infrared views let it peer back over 13.5 billion years, witnessing the first stars and galaxies forming out of the darkness of the early universe⁶.

The telescope was originally called the Next Generation Space Telescope (NGST) — "Next Generation" because it builds on and continues the science exploration started by the Hubble Space Telescope⁵. On 10 September 2002 it was named in honor of James E. Webb, NASA's second administrator⁵. The name is apt in an engineering sense as well: Webb introduces new technologies such as the lightweight, deployable primary mirror that will pave the way for future missions⁵.

James E. Webb (1906–1992) is best known for leading Apollo, the series of lunar exploration programs that landed the first humans on the Moon⁵. During his tenure as NASA's second administrator he also initiated a vigorous space science program that was responsible for more than 75 launches, including America's first interplanetary explorers⁵. The telescope's name thus honors a leader associated with both the Moon landings and a broad scientific legacy⁵.

Webb's primary mirror is 6.5 m (21.3 ft) in diameter, with a clear aperture of 25 m² and a total mirror mass of 705 kg, and the observatory's total payload mass is approximately 6,200 kg³. The mirror consists of 18 hexagonal beryllium segments, each bare segment massing 20.1 kg⁴³. Beryllium was chosen for its light weight, stiffness, and stability at very cold temperatures⁵. The segments are coated with gold — a layer 1,000 angstroms thick over 25 m², using about 48.25 grams of gold, roughly the mass of a golf ball⁵. Because the mirror is so large it could not fit in the launch shrouds of available rockets, it was designed to unfold; designing, building, and operating a mirror that unfolds is one of the major technological developments of Webb⁵.

Webb's sunshield measures 21.197 m × 14.162 m (69.5 ft × 46.5 ft) — a tennis-court-sized structure of five thin layers of Kapton E with aluminum and doped-silicon coatings that reflect the Sun's heat back into space³. The sunshade is Webb's largest structure, and it must be able to shield the deployed primary mirror and the tower that holds the secondary mirror⁵. Across its five layers the sunshield reduces the temperature between the hot and cold sides of the spacecraft by almost 600 °F, from about 185 °F (85 °C) on the hot side to about −388 °F (−233 °C) on the cold side³.

Webb carries four scientific instruments: the Near Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor/Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS)⁵. NIRSpec was provided by ESA, while MIRI was provided by a consortium of European countries together with ESA and NASA's Jet Propulsion Laboratory, and NIRISS by the Canadian Space Agency⁵. NIRCam was built by the University of Arizona working with Lockheed-Martin⁵.

The near-infrared instruments — NIRCam, NIRSpec, and FGS/NIRISS — work at about 39 K through a passive cooling system, while the mid-infrared instrument MIRI works at 7 K using a helium refrigerator, or cryocooler⁵. MIRI's detectors need to be at a temperature of less than 7 kelvin to operate properly, which is not possible by passive means alone³. Such cold operation is essential because infrared is heat radiation, and a warm telescope would swamp the faint astronomical signals Webb was built to capture⁵.

Webb observes primarily the infrared light from faint and very distant objects, a band in which it complements and extends the discoveries of Hubble with longer wavelength coverage and greatly improved sensitivity⁵. Infrared is heat radiation, so all warm things — including telescopes — emit it; to avoid swamping the very faint astronomical signals with radiation from the telescope itself, the telescope and its instruments must be very cold, and Webb's operating temperature is below 50 kelvin (−223 °C or −370 °F)⁵. Webb's infrared vision cuts through the dust and gas of massive clouds where stars and planetary systems form⁶.

Webb orbits the Sun about 1.5 million km from Earth around the second Lagrange point, where the Sun, Earth, and Moon always stay in the same part of the sky, allowing the enormous sunshield to block their light and keep the telescope cool⁶. At roughly one million miles from Earth — four times farther from Earth than the Moon — there is no current servicing capability or plan for Webb, and the telescope's design does not rely on servicing missions⁶. By contrast, because Hubble circles just above Earth's atmosphere, astronaut servicing missions were part of its long-term plan⁶.

Webb's deeper infrared vision spans 0.6 to 28.5 microns, while Hubble was optimized for shorter ultraviolet and visible wavelengths from 0.1 to 2.5 microns⁶. Webb can detect objects ten billion times fainter than the faintest stars visible without a telescope — 10 to 100 times fainter than Hubble can see⁵. Its angular resolution is somewhat better than 0.1 arc-seconds at 2 micrometers: details the size of a US penny at 40 km, or a soccer ball at 550 km⁵.

Webb launched on 25 December 2021 at 07:20 EST (12:20 UTC) on an Ariane 5 ECA rocket³. The launch took place from Arianespace's ELA-3 complex at Europe's Spaceport near Kourou, French Guiana⁵. ESA provided the launch vehicle and associated services on a no-exchange-of-funds basis; in exchange, NASA guaranteed European scientists roughly 15 percent of Webb's observing time⁵.

Webb's deployment was planned over a roughly two-week period and involved hundreds of individual steps, monitored in real time and human-controlled rather than an automatic hands-off sequence⁵. The first deployment — the solar array — came approximately 30 minutes after launch, and all major deployments were completed on 8 January 2022⁵. The trip to L2 took about a month, with Webb arriving at its orbit around the second Lagrange point on 24 January 2022⁵¹. Check-out procedures then continued until six months after launch, when routine scientific operations began⁵.

Webb is expected to be operational for at least 5 years, with a goal of 10, and carries enough propellant for more than 20 years of operation²⁵. It is a general observatory, accessible to the worldwide community through regular calls for observing proposals that are peer-reviewed and selected by an external time allocation committee². In May 2022 the telescope took a larger-than-expected micrometeoroid strike on a particularly sensitive part of the mirror and structure; the team determined it was a statistically rare event, and the telescope is still performing at a level that exceeds all mission requirements⁵.

On 12 July 2022, NASA revealed the first five full-color images and spectrographic data from Webb at a live event streamed from the agency's Goddard Space Flight Center in Greenbelt, Maryland⁷. The five targets were the galaxy cluster SMACS 0723, the exoplanet WASP-96 b, the Southern Ring Nebula, the compact galaxy group Stephan's Quintet, and the Carina Nebula⁷. The release kicked off Webb's science operations, open to astronomers around the world⁷.

Webb's First Deep Field — the galaxy cluster SMACS 0723 — delivered the deepest and sharpest infrared image of the distant universe so far, in only 12.5 hours⁷. The image is teeming with thousands of galaxies, including the faintest objects ever observed in the infrared, in a slice of sky approximately the size of a grain of sand held at arm's length⁸. Webb's detailed observation of the hot, puffy exoplanet WASP-96 b revealed the clear signature of water along with evidence of haze and clouds that previous studies had not detected — Webb's first detection of water in the atmosphere of an exoplanet⁷.

In October 2023 and January 2024, the JWST Advanced Deep Extragalactic Survey (JADES) program used Webb's NIRSpec to obtain a spectrum of a record-breaking galaxy observed only 290 million years after the Big Bang, at a redshift of about 14⁹. That galaxy, JADES-GS-z14-0, was determined to be at a redshift of 14.32, making it the current record-holder for the most distant known galaxy — seen less than 300 million years after the Big Bang and shattering the previous record of z = 13.2, held by JADES-GS-z13-0⁹. The source is over 1,600 light-years across, and its light comes mostly from young stars rather than from emission near a growing supermassive black hole, implying a mass of several hundred million times that of the Sun⁹. The presence of oxygen so early in the galaxy's life is a surprise, suggesting that multiple generations of very massive stars had already lived and died before we observed it⁹. Above all, JADES-GS-z14-0 is not like the types of galaxies that theoretical models and computer simulations predict for the very early universe⁹.

Webb also mapped the weather on the hot gas-giant exoplanet WASP-43 b, a Jupiter-sized planet roughly 280 light-years away in the constellation Sextans that orbits its star every 19.5 hours and is probably tidally locked¹⁰. The measurements show a dayside averaging nearly 1,250 °C — hot enough to forge iron — and a nightside at 600 °C, with supersonic winds shifting the hottest spot eastward of the point that receives the most stellar radiation¹⁰. They also suggest thick high clouds on the nightside, clear skies on the dayside, and equatorial winds upwards of 5,000 miles per hour mixing atmospheric gases around the planet¹⁰. Earlier, in Webb's first science release, its spectrum of WASP-96 b had revealed the clear signature of water along with evidence of haze and clouds⁷.

In the cluster NGC 346 in the Small Magellanic Cloud, Webb found planet-forming disks that are longer-lived than those seen around young stars in our Milky Way galaxy¹¹. Hubble had revealed many stars there about 20 to 30 million years old that still had planet-forming disks, contradicting the conventional belief that such disks dissipate after 2 or 3 million years¹¹. The massive star cluster NGC 346 has only about ten percent of the heavier elements present in the chemical composition of our Sun¹¹. In other young systems, Webb's MIRI revealed the richest hydrocarbon chemistry seen to date in a protoplanetary disk — 13 carbon-bearing molecules up to benzene, including the first extrasolar detection of ethane, plus the first disk detections of ethylene, propyne, and the methyl radical CH3¹². And in early-stage protostars, where planets have not yet formed, Webb discovered molecules ranging from methane to acetic acid and ethanol — key ingredients for making potentially habitable worlds; the low-mass protostar IRAS 2A may resemble the primordial stages of our own Solar System, so the chemicals identified there may have been delivered to the primitive Earth¹³.

Webb's light-echo images near the supernova remnant Cassiopeia A are allowing astronomers to map the true 3D structure of interstellar dust and gas — the interstellar medium — for the first time¹⁴. The images reveal tightly packed sheets and filaments on remarkably small scales of about 400 astronomical units, less than one-hundredth of a light-year¹⁴. Armin Rest of the Space Telescope Science Institute compared the three-epoch observation to a medical CT scan, with three slices taken at three different times that reveal the true 3D structure of the interstellar medium¹⁴. On the expansion of the universe, Webb confirmed that Hubble's measurements of the expansion rate were right all along, erasing lingering doubt about them¹⁵. The Hubble Tension — the persistent difference between the Hubble constant measured with a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang — persists, and the two telescopes' tag-team measurements strengthen the case that something other than measurement errors is influencing the expansion rate¹⁵.