JWST Spacecraft Bus
The JWST spacecraft bus provides the telescope with electrical power, attitude control, thermal control, command and data handling, communications services, and propulsion.
Its solar array provides the electrical power needed for the mission, with a capacity of up to 3 kilowatts.
The attitude control subsystem (ACS) provides attitude determination and control for all mission phases and modes of the observatory. The ACS interfaces with the Fine Guidance Sensor (FGS), located in the Integrated Science Instrument Module (ISIM), and with the telescope's fine steering mirror (FSM) for fine pointing control during observations.
JWST's propulsion subsystem provides the means to correct the telescope orbit, control the observatory attitude in certain modes, and unload reaction wheel momentum. Thrusters are used for orbit maintenance, momentum unloads, and some attitude control functions. The observatory carries enough propellant for more than 20 years of science operations.
The spacecraft Command & Data Handling (C&DH) subsystem supports command processing for the spacecraft bus, command routing to the ISIM and Optical Telescope Element (OTE), as well as telemetry recording and routing to the communications subsystem.
The spacecraft's solid state recorder (SSR) provides at least 65 Gbytes† of storage for science data. The ISIM Command and Data Handling (ICDH) computer creates science data files as the detectors are read out, and transfers these files to the SSR where they are staged until they can be transmitted to the ground.
† This article uses the S.I. definition of gigabyte: 1 Gbyte = 109 bytes.