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James Webb Space Telescope
JWST spacecraft model 3.png
Rendering of the James Webb Space Telescope fully deployed.
Names Next Generation Space Telescope (NGST; 1996–2002)
Mission type Astronomy
Operator STScI (NASA) / ESA / ASC
Mission duration
  • 1 year, 11 months, 6 days (elapsed)
  • 5½ years (primary mission)
  • 10 years (planned)
  • 20 years (expected life)
Spacecraft properties
  • Northrop Grumman
  • Ball Aerospace
  • L3Harris
Launch mass 6,161.4 kg (13,584 lb)
Dimensions 20.197 m × 14.162 m (66.26 ft × 46.46 ft), sunshield
Power 2 kW
Start of mission
Launch date 25 December 2021 (2021-12-25), 12:20 UTC
Rocket Ariane 5 ECA (VA256)
Launch site Centre Spatial Guyanais, ELA-3
Contractor Arianespace
Entered service 12 July 2022
Orbital parameters
Reference system Sun–Earth L2 orbit
Regime Halo orbit
Periapsis 250,000 km (160,000 mi)
Apoapsis 832,000 km (517,000 mi)
Period 6 months
Main telescope
Type Korsch telescope
Diameter 6.5 m (21 ft)
Focal length 131.4 m (431 ft)
Focal ratio f/20.2
Collecting area 25.4 m2 (273 sq ft)
Wavelengths 0.6–28.3 μm (orange to mid-infrared)
JWST Launch Logo.png
James Webb Space Telescope mission logo

James Webb Space Telescope (JWST) is a telescope that was launched on 25 December 2021. It is a replacement for the Hubble Space Telescope which was launched in 1990.

The telescope is named after James E. Webb, who was a director at NASA and created the Apollo program that put astronauts on the moon.

It has a main mirror that is 6.5 metres (21 feet) wide. This is 6 times larger in area than Hubble. It is so large that it is made in 18 pieces that fold together during the launch, so that it can fit into a rocket. It is mainly an infrared telescope but also works in the red part of the visible light (the pictures will be coded with false color so we can see them). It is plated with gold because gold reflects infrared very well. It is able to see things that the Hubble Space Telescope cannot. Infrared vision can be used to see heat radiation (like some kinds of night vision goggles), so the telescope itself must be kept as cool as possible. It is protected by a large sunshield the size of a tennis court to keep it cool and dark.

NASA released the first image from JWST on 11 July 2022, the oldest and highest resolution image of the Universe.


The James Webb Space Telescope has a mass that is about half of Hubble Space Telescope's mass. The JWST has a 6.5-meter (21 ft)-diameter gold-coated beryllium primary mirror made up of 18 separate hexagonal mirrors. The mirror has a polished area of 26.3 m2 (283 sq ft), of which 0.9 m2 (9.7 sq ft) is obscured by the secondary support struts, giving a total collecting area of 25.4 m2 (273 sq ft). This is over six times larger than the collecting area of Hubble's 2.4-meter (7.9 ft) diameter mirror, which has a collecting area of 4.0 m2 (43 sq ft). The mirror has a gold coating to provide infrared reflectivity and this is covered by a thin layer of glass for durability.

JWST is designed primarily for near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument. It can detect objects up to 100 times fainter than Hubble can, and objects much earlier in the history of the universe, back to redshift z≈20 (about 180 million years cosmic time after the Big Bang). For comparison, the earliest stars are thought to have formed between z≈30 and z≈20 (100–180 million years cosmic time), and the first galaxies may have formed around redshift z≈15 (about 270 million years cosmic time). Hubble is unable to see further back than very early reionization at about z≈11.1 (galaxy GN-z11, 400 million years cosmic time).

The design emphasizes the near to mid-infrared for several reasons:

  • high-redshift (very early and distant) objects have their visible emissions shifted into the infrared, and therefore their light can only be observed today via infrared astronomy;
  • infrared light passes more easily through dust clouds than visible light
  • colder objects such as debris disks and planets emit most strongly in the infrared;
  • these infrared bands are difficult to study from the ground or by existing space telescopes such as Hubble.

Ground-based telescopes must look through Earth's atmosphere, which is opaque in many infrared bands (see figure of atmospheric absorption). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis. Existing space telescopes such as Hubble cannot study these bands since their mirrors are insufficiently cool (the Hubble mirror is maintained at about 15 °C [288 K; 59 °F]) thus the telescope itself radiates strongly in the infrared bands.

JWST can also observe nearby objects, including objects in the Solar System, having an apparent angular rate of motion of 0.030 arc seconds per second or less. This includes all planets and satellites, comets, and asteroids beyond Earth's orbit, and "virtually all" known Kuiper Belt Objects. In addition, it can observe opportunistic and unplanned targets within 48 hours of a decision to do so, such as supernovae and gamma ray bursts.


The James Webb Space Telescope in the Cleanroom at the Launch Site (51604442070)
The Webb telescope days before its launch in December 2021

The JWST is in orbit far from Earth, to avoid heat radiating from the Earth and moon. This special orbit is beyond the moon, at the second Lagrange point (L2) of the Sun-Earth system, a place of stable gravity. This orbit is 1,500,000 kilometres (930,000 miles) from Earth, about four times farther away from us than the moon. This keeps it in the Earth's shadow most of the time; it does not actually go around the Earth, but goes around the sun at the same speed as the Earth.

Mission goals

The James Webb Space Telescope has four key goals:

These goals can be accomplished more effectively by observation in near-infrared light rather than light in the visible part of the spectrum. For this reason, JWST's instruments will not measure visible or ultraviolet light like the Hubble Telescope, but will have a much greater capacity to perform infrared astronomy. JWST will be sensitive to a range of wavelengths from 0.6 to 28 μm (corresponding respectively to orange light and deep infrared radiation at about 100 K or −173 °C).

JWST may be used to gather information on the dimming light of star KIC 8462852, which was discovered in 2015, and has some abnormal light-curve properties.

Additionally, it will be able to tell if an exoplanet has methane in its atmosphere, allowing astronomers to determine whether or not the methane is a biosignature.

Ground support and operations

JWST orbit (not to scale)
James Webb Telescope DC
Life-sized model of JWST shown at the 2007 AAS meeting in Seattle, Washington. It stands two stories high and weighs several tons. Credit: Rob Gutro, NASA/GSFC.

The Space Telescope Science Institute (STScI), in Baltimore, Maryland, on the Homewood Campus of Johns Hopkins University, was selected as the Science and Operations Center (S&OC) for JWST with an initial budget of US$162.2 million intended to support operations through the first year after launch. In this capacity, STScI will be responsible for the scientific operation of the telescope and delivery of data products to the astronomical community. Data will be transmitted from JWST to the ground via the NASA Deep Space Network, processed and calibrated at STScI, and then distributed online to astronomers worldwide. Similar to how Hubble is operated, anyone, anywhere in the world, will be allowed to submit proposals for observations. Each year several committees of astronomers will peer review the submitted proposals to select the projects to observe in the coming year. The authors of the chosen proposals will typically have one year of private access to the new observations, after which the data will become publicly available for download by anyone from the online archive at STScI.

The bandwidth and digital throughput of the satellite is designed to operate at 458 gigabits of data per day for the length of the mission (equivalent to a sustained rate of 5.42 megabits per second [Mbps]). Most of the data processing on the telescope is done by conventional single-board computers. The digitization of the analog data from the instruments is performed by the custom SIDECAR ASIC (System for Image Digitization, Enhancement, Control And Retrieval Application Specific Integrated Circuit). NASA stated that the SIDECAR ASIC will include all the functions of a 9.1 kg (20 lb) instrument box in a 3 cm (1.2 in) package and consume only 11 milliwatts of power. Since this conversion must be done close to the detectors, on the cold side of the telescope, the low power dissipation is crucial for maintaining the low temperature required for optimal operation of JWST.

Comparison with other telescopes

The desire for a large infrared space telescope traces back decades. In the United States, the Space Infrared Telescope Facility (SIRTF, later called the Spitzer Space Telescope) was planned while the Space Shuttle was in development, and the potential for infrared astronomy was acknowledged at that time. Unlike ground telescopes, space observatories were free from atmospheric absorption of infrared light. Space observatories opened up a whole "new sky" for astronomers.

The tenuous atmosphere above the 400 km nominal flight altitude has no measurable absorption so that detectors operating at all wavelengths from 5 μm to 1000 μm can achieve high radiometric sensitivity.

However, infrared telescopes have a disadvantage: they need to stay extremely cold, and the longer the wavelength of infrared, the colder they need to be. If not, the background heat of the device itself overwhelms the detectors, making it effectively blind. This can be overcome by careful spacecraft design, in particular by placing the telescope in a dewar with an extremely cold substance, such as liquid helium. The coolant will slowly vaporize, limiting the lifetime of the instrument from as short as a few months to a few years at most.

In some cases, it is possible to maintain a temperature low enough through the design of the spacecraft to enable near-infrared observations without a supply of coolant, such as the extended missions of Spitzer Space Telescope and Wide-field Infrared Survey Explorer, which operated at reduced capacity after coolant depletion. Another example is Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument, which started out using a block of nitrogen ice that depleted after a couple of years, but was then replaced during the STS-109 servicing mission with a cryocooler that worked continuously. The James Webb Space Telescope is designed to cool itself without a dewar, using a combination of sunshields and radiators, with the mid-infrared instrument using an additional cryocooler.

Selected space telescopes and instruments
Name Launch Year Wavelength
Spacelab Infrared Telescope (IRT) 1985 1.7–118 0.15 Helium
Infrared Space Observatory (ISO) 1995 2.5–240 0.60 Helium
Hubble Space Telescope Imaging Spectrograph (STIS) 1997 0.115–1.03 2.4 Passive
Hubble Near Infrared Camera and Multi-Object Spectrometer (NICMOS) 1997 0.8–2.4 2.4 Nitrogen, later cryocooler
Spitzer Space Telescope 2003 3–180 0.85 Helium
Hubble Wide Field Camera 3 (WFC3) 2009 0.2–1.7 2.4 Passive, and thermo-electric
Herschel Space Observatory 2009 55–672 3.5 Helium
James Webb Space Telescope 2021 0.6–28.5 6.5 Passive, and cryocooler (MIRI)

JWST's delays and cost increases can be compared to those of its predecessor, the Hubble Space Telescope. When Hubble formally started in 1972, it had an estimated development cost of US$300 million (or about US$1 billion in 2006 constant dollars), but by the time it was sent into orbit in 1990, the cost was about four times that. In addition, new instruments and servicing missions increased the cost to at least US$9 billion by 2006.

Scientific results

The release of the first full-color images and spectroscopic data was on 12 July 2022, which also marked the official beginning of Webb's general science operations; President Joe Biden revealed the first image, Webb's First Deep Field, on 11 July 2022. NASA announced the list of observations targeted for release:

  • Carina Nebula – young, star-forming region called NGC 3324 displaying "Cosmic Cliffs" about 8500 light-years from Earth.
  • WASP-96b – including an analysis of atmosphere with evidence of water around a giant gas planet orbiting a distant star 1120 light-years from Earth.
  • Southern Ring Nebula – clouds of gas and dust expelled by a dying star 2500 light-years from Earth.
  • Stephan's Quintet – a visual display of five galaxies with colliding gas and dust clouds creating new stars; four central galaxies are 290 million light-years from Earth.
  • SMACS J0723.3-7327 – a gravitationally lensed view called Webb's First Deep Field 4.6 billion light-years from Earth, with distant galaxies as far away as 13.1 billion light-years.

A paper about the science performance from commissioning, released by NASA, ESA and CSA scientists, describes that "almost across the board, the science performance of JWST is better than expected". The paper describes a series of observations during the commissioning, when the instruments captured spectra of transiting exoplanets with a precision better than 1000 ppm per data point and tracked moving objects with speeds up to 67 milliarcseconds/second, more than twice as fast as the requirement. It also obtained the spectra of hundred of stars simultaneously in a dense field towards the galactic center. Other targets described in the paper:

  • Moving targets: Jupiter (including rings and the moons Europa, Thebe and Metis), asteroids 2516 Roman, 118 Peitho, 6481 Tenzing, 1773 Rumpelstilz, 216 Kleopatra, 2035 Stearns, 4015 Wilson-Harrington and 2004 JX20
  • NIRCam grism time-series, NIRISS SOSS and NIRSpec BOTS mode: the Jupiter-sized planet HAT-P-14b
  • NIRISS aperture masking interferometry (AMI): A clear detection of the very low-mass companion star AB Doradus C, which had a separation of only 0.3 arcseconds to the primary. This observation was the first demonstration of AMI in space.
  • MIRI low-resolution spectroscopy (LRS): a hot super-earth planet L168-9b (TOI-134) around a bright M-dwarf star

Images for kids

See also

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