The James Webb Space Telescope Mission

The JWST mission consists of an observatory, a ground system provided by Space Telescope Science Institute and launch services provided by Arianespace under the direction of ESA. The observatory includes all the on-orbit hardware; the prime contractor for the observatory was Northrop Grumman Aerospace Systems (NGAS). The observatory consists of an optical telescope element provided by Ball Aerospace, a spacecraft and sunshield provided by NGAS and an Integrated Science Instrument Module (ISIM) constructed by Goddard Space Flight Center. The ISIM houses the four science instruments and provides them with thermal, electrical, structural, and data handling support.

The Near-Infrared Camera [NIRCam; 109, this issue], was built by Lockheed Martin under the direction of Principal Investigator Marcia Rieke of the University of Arizona. All of the near-infrared instruments include detectors from Teledyne Imaging Systems. The Near-Infrared Spectrograph [NIRSpec; 15, this issue] was built by EADS Astrium under the direction of ESA, and includes a microshutter assembly (MSA) and detector system built by Goddard Space Flight Center. The Near-Infrared Imager and Slitless Spectrograph [NIRISS 29, this issue], was built by Honeywell under the direction of CSA. The Fine Guidance System (FGS) is included with the NIRISS and shares the same optical bench. The Mid-Infrared Instrument [MIRI; 143, this issue] consists of an optical bench assembly built by a consortium of European countries organized by ESA, a cryocooler built by Northrop Grumman under the direction of Jet Propulsion Laboratories and a detector system by JPL and Raytheon Intelligence & Space.

The mission design and its performance is summarized here. Details provided about the telescope are given by, (this issue), construction, integration, and test of the observatory by, (this issue), the on-orbit performance measured during commissioning by, (this issue), and the on-orbit backgrounds measured during commissioning by, (this issue).

IV.1 Launch, Orbit, Deployments, and Commissioning

The JWST observatory was launched from Centre Spatial Guyanais at 12:20 UTC on 2021 December 25 by an Ariane 5 ECA+ rocket. The launch mass was 6161.4kg. The launch provided a near-perfect trajectory. Three mid-course correction (MCC) burns placed the observatory in an L2 halo orbit, approximately 1.5×10⁶km from Earth. The successful launch and MCC burns used less on-board fuel than allocated; the remaining propellant will enable a fuel-limited lifetime of more than 20 years.

Launch was followed by more than 50 major deployments, which were completed successfully enroute to the final orbit. The major deployed systems included the solar panel, the high-gain antenna, the deployed tower assembly, the sunshield, the secondary mirror, an instrument radiator and the primary mirror wings. The sunshield deployment included 140 membrane release mechanisms, 70 hinge assemblies, 8 deployment motors, 400 pullies, and 90 cables totaling more than 400m. Following the major deployments, which were completed in the first 14 days, the primary mirror segments and secondary mirror were moved off their launch locks.

At the completion of the deployments, the telescope was pointed at HD 84406 to begin the telescope alignment and phasing process. The major steps included (1) Segment Image Identification, (2) Segment Alignment, (3) Image Stacking, (4) Coarse Phasing, (5) Fine Phasing, (6) Telescope Alignment Over Instrument Fields of View, and (7) Iterate Alignment for Final Correction. Following telescope alignment, commissioning of the instruments included activating all instrument systems and commissioning the 17 science instrument modes. The final commissioning tasks included taking the Early Release Observations (ERO). Commissioning was completed by 2022 July 11 and 12 with the release of the EROs.

IV.2 Optical Telescope Element

The optical telescope element (OTE) consists of a primary mirror made up of 18 hexagonal beryllium primary mirror segment assemblies (PMSAs), a 0.8m convex secondary mirror, and an aft optical assembly subsystem containing a tertiary mirror and a fine steering mirror. All the mirrors are coated with gold. The PMSAs and secondary mirror were aligned and phased on-orbit using actuators that had a total of 132 degrees of freedom. The total collecting area of the primary mirror is 25.4 m², as measured on-orbit using the NIRCam pupil imaging lens. The telescope was designed to be diffraction-limited at 2 μm wavelength, defined as having a Strehl ratio >0.8 at the end of a 5-year post-commissioning lifetime, equivalent to 150nm, root-mean squared (RMS) wavefront error (WFE). The WFE at the end of commissioning was ∼80 nm RMS, equivalent to a diffraction limit at 1.1 μm. The expected degradation of the wavefront error due to micrometeoroid impacts and other effects over the operational lifetime of the mission will be closely monitored [86, 112, this issue].

IV.3 Spacecraft

The spacecraft provides power, pointing, orbit maintenance, data storage and communications for the observatory. At the end of commissioning, the solar array provided an average of 1.5 kW of power with the ability to provide 3 kW when needed. The pointing system uses star trackers, inertial reference units (IRUs) containing gyros, reaction wheels, and a fine steering mirror (FSM) to point the telescope. There are three star trackers, one of which provides redundancy. There are two IRUs, one of which is redundant; each IRU includes four gyros, one of which provides redundancy. There are six reaction wheels, of which two provide redundancy. The star trackers, gyros and reaction wheels maintain the attitude and coarse pointing of the observatory. During fine guiding, after guide star acquisition, the FGS provides a 16 Hz positional update to the fine steering mirror to adjust the pointing of the telescope. At the end of commissioning, the pointing system delivers pointing stability of ∼1mas (1σ per axis), greatly exceeding the pre-launch estimates of ∼6mas. Orbit maintenance currently requires firing the on-board thrusters about once every 6 weeks. The thruster firings are also used to maintain angular momentum by de-spinning the reaction wheels. The frequency of orbit maintenance and angular momentum management is determined through ranging measurements and reaction wheel telemetry.

The on-board solid-state recorder holds 471 Gbits of science and engineering data. The data are downlinked via Ka band through the Deep Space Network (DSN) on a nominal schedule of two contacts per day totaling up to 12 hours. The actual downlink schedule varies from day to day with which antenna is in range and DSN scheduling with respect to other mission needs. Commands and observation plans are uplinked to the observatory through the DSN using S band.

IV.4 Sunshield and Cooling

The sunshield consists of 5 layers of Kapton, about 14m×22m in size, that separate the ∼300K spacecraft from the telescope, and attenuate the ∼ 200 kW of incident Solar radiation to mW levels. The sunshield size and geometry provides an instantaneous field of regard of 40% of the sky in an annulus that sweeps around the full sky once per year; each point on the sky is visible at least once in each six-month period. The telescope can point 5deg towards the Sun and 45deg away from the Sun, and can spin around the Sun-anti-Sun axis.

Immediately after launch, the observatory began passively cooling, a process that was completed within 120 days. The cooldown was controlled using heaters to ensure that the instruments would not be contaminated by condensation of outgassing water and other volatiles. The MIRI cryocooler (the only active cooling in the observatory) was turned on after the deployments and reached its final temperature on day 104. The final temperature reached by the secondary mirror is 29.2K. The primary mirror segments range from 34.7K to 54.5K, with the mirror segments closest to the core region near the sunshield at the bottom of the telescope warmer than the mirror segments at the top and wings. With these temperatures, JWST broad-band observations are background limited by zodiacal light out to about 12.5μm wavelength, and limited by thermal self-emission at longer wavelengths [113, this issue]. The near-infrared instrument detector plane temperatures are actively maintained using heaters at 38.5K for NIRCam and NIRISS, and 42.8K for NIRSpec. The MIRI optical assembly is kept at 6K by the cryocooler, while the MIRI shield around the instrument is about 20K.