The James Webb Space Telescope

The James Webb Space Telescope (JWST) is a space observatory built for infrared astronomy. As the largest telescope ever placed in space, its highly sensitive instruments can see objects that are too old, far away, or dim for the Hubble Space Telescope. This capability allows scientists to study the earliest stars and galaxies, as well as analyze the atmospheres of exoplanets that might support life.

Although Webb’s mirror is 2.7 times wider than Hubble’s, it captures images at a similar level of detail because it works in the infrared spectrum, where wavelengths are longer than the visible light Hubble sees. To achieve good resolution at longer wavelengths, a telescope needs a larger surface to collect light—whether mirrors for infrared or antennas for radio waves.

Webb launched on December 25, 2021, aboard an Ariane 5 rocket from Kourou, French Guiana. It reached its destination in January 2022: a solar orbit near the Sun–Earth L2 Lagrange point, roughly 1.5 million kilometers (930,000 miles) from Earth. Its first public image was released on July 11, 2022.

NASA led the telescope’s design and development, partnering with the European Space Agency (ESA) and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center in Maryland managed the project, while the Space Telescope Science Institute at Johns Hopkins University in Baltimore operates it. Northrop Grumman was the primary contractor. The telescope is named after James E. Webb, NASA’s administrator from 1961 to 1968 during the Mercury, Gemini, and Apollo programs.

Webb’s primary mirror is made of 18 gold-plated beryllium hexagonal segments, forming a 6.5-meter (21-foot) diameter—compared to Hubble’s 2.4 meters (7 feet 10 inches). This gives Webb a light-collecting area of about 25 square meters (270 square feet), roughly six times Hubble’s. While Hubble observes near ultraviolet, visible, and near infrared light (0.1–2.5 μm), Webb covers longer wavelengths, from red visible light through mid-infrared (0.6–28.5 μm). To prevent its own heat from interfering with observations, the telescope must stay below 50 K (−223 °C; −370 °F). A five-layer sunshield blocks warmth from the Sun, Earth, and Moon.

Design work began in 1996 under the name Next Generation Space Telescope. Two concept studies were commissioned in 1999, aiming for a 2007 launch and a $1 billion budget. The project faced major cost overruns and delays. A significant redesign occurred in 2005, construction finished in 2016, and years of exhaustive testing followed, with a total cost of $10 billion.

Webb weighs about half as much as Hubble. Its 6.5-meter mirror has a polished area of 26.3 square meters (283 square feet), with 0.9 square meters (9.7 square feet) blocked by secondary support struts, leaving a total collecting area of 25.4 square meters (273 square feet). The gold coating boosts infrared reflectivity, and a thin glass layer protects it. Webb is mainly designed for near-infrared astronomy but can also detect orange and red visible light and mid-infrared, depending on the instrument. It can see objects up to 100 times fainter than Hubble, reaching back to a redshift of about z≈20 (roughly 180 million years after the Big Bang). By comparison, the earliest stars likely formed between z≈30 and z≈20 (100–180 million years), and the first galaxies around z≈15 (about 270 million years). Hubble cannot see beyond very early reionization at z≈11.1 (galaxy GN-z11, 400 million years).

The focus on near to mid-infrared is key: very distant objects have their visible light shifted into the infrared; infrared passes through dust clouds more easily than visible light; and cold objects like debris disks and planets emit most strongly in the infrared. These bands are hard to study from the ground because Earth’s atmosphere is opaque in many infrared regions, and common compounds like water, carbon dioxide, and methane in our air interfere. Existing space telescopes like Hubble cannot study these bands because their mirrors are too warm—Hubble’s mirror sits at about 15 °C (288 K; 59 °F), causing the telescope itself to radiate strongly in the infrared.

Webb can also observe Solar System objects at angles greater than 85° from the Sun and with apparent motion slower than 0.03 arc seconds per second. This includes Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, their moons, and comets, asteroids, and minor planets at or beyond Mars’s orbit.