The James Webb Space Telescope Mission

We summarize the history, concept, scientific program, and technical performance of the James Webb Space Telescope [45, JWST, Webb;]. This paper points to and extracts key results from detailed papers in this issue on the four instruments: Near-Infrared Camera [NIRCam; 109, this issue], Near-Infrared Spectrograph [NIRSpec; 15, this issue], Near-Infrared Imager and Slitless Spectrograph [NIRISS 29, this issue], and Mid-Infrared Instrument [MIRI; 143, this issue]; the telescope [86, this issue]; the observatory [88, this issue]; the scientific performance [112, this issue]; and the brightness of the sky and stray light [113, this issue].

Launched 2021 December 25, the JWST (See figure) is a 6.5m diameter cold space telescope with cameras and spectrometers covering 0.6μm to 28μm wavelength. Orbiting the Sun-Earth L2 point, it extends the discoveries and technologies of the 2.4m warm Hubble Space Telescope (HST) and the 85cm cold Spitzer Space Telescope. JWST enables observations of the distant early universe, potentially reaching beyond redshift z∼15, within ≲300Myr of the Big Bang. JWST’s infrared wavelength range penetrates dust clouds, to observe obscured active galactic nuclei (AGN) and star and planet formation. It shows objects too cool to radiate visible light. It includes the fundamental vibration-rotation bands of important molecules.

JWST was conceived from the beginning as an international project, led by NASA in partnership with the European and Canadian Space Agencies (ESA and CSA). Observing time allocations are open to all astronomers worldwide. JWST was launched from French Guiana on an ESA-provided Ariane 5 rocket. JWST cost NASA $8.8B to get to launch, and will make observations for a projected fuel-limited lifetime potentially as long as 20 yrs. JWST’s observations cannot be obtained in any other way: Hubble has 1/6.25 of the collecting area, and, being a room-temperature telescope emits its own infrared beyond 1.7μm. Spitzer was cold but had less than 2% of JWST’s collecting area. For ground-based telescopes, the Earth’s warm atmosphere results in a background that is 10⁶−10⁷ times higher and also blocks large regions of IR wavelengths.

JWST includes a general-purpose instrument package in its Integrated Science Instrument Module (ISIM). NIRCam covers 0.6μm to 5μm with filters, a coronagraph, 0.032 arcsec pixels in its short wavelength channel, and a dichroic filter enabling simultaneous observation in both short and long wavelength bands. NIRSpec covers the same wavelength range with prisms and gratings, with fixed slits, an integral field unit (IFU), and a microshutter array (MSA) to select up to 100 simultaneous targets. NIRISS covers the same wavelength range with slitless spectroscopy, and provides a non-redundant mask for coronagraphy. MIRI covers 5μm to 28μm with imaging, IFU spectroscopy, and coronagraphs. The Fine Guidance Sensor (FGS) senses pointing errors and feeds the attitude control system to maintain sharp images. ESA provided the NIRSpec with the detectors and micro-shutter array provided by Goddard, CSA provided the NIRISS and FGS, and a partnership of JPL and a European consortium provided MIRI. NIRCam, and the US portions of MIRI and NIRSpec, were funded by NASA.

JWST’s first year of observations includes nearly half of a year of guaranteed time observations allocated to the teams building the instruments, 6 interdisciplinary scientists, and the telescope scientist. There is also a set of 13 competitively-selected Director’s Discretionary Early Release Science (DD-ERS) programs with data that become public immediately. The remaining observations were selected from over 1200 proposals through a dual-anonymous peer review, in which the identities of the proposers were not known to reviewers. Anyone can propose, regardless of nationality or institution. There were 286 programs selected in Cycle 1, including 263 pointed programs, 3 pure parallel programs, and 20 archival or theory programs. The Cycle 2 call received more than 1600 proposals submitted by more than 5000 scientists from around the world. NASA provides funding for data analysis to US-based investigators who are awarded time on JWST, and for archival and theoretical research related to JWST.

JWST is a technical pioneer, the first deployable segmented space telescope, with mirror segments that are aligned and focused after launch, with its remarkable cryogenic optical system providing diffraction-limited images (Strehl ratio 0.8) as short as 1.1μm. Its detectors are larger and more sensitive than previous generations. Its 6.0K cryocooler for the mid IR instrument (MIRI) has no expendables and should last the duration of the mission. It uses cryogenic ASICs to control and read the near IR detectors. Its 5-layer sunshield cools the telescope to between 35K and 55K. Based on JWST’s successes and lessons learned, NASA is positioned to proceed to the 6m near-IR, optical, and UV flagship telescope recommended by the 2020 Decadal Survey, with the ability to directly detect Earth-like planets around Sun-like stars.