Spacecraft Bus (JWST)

Spacecraft Bus is the primary support component of the James Webb Space Telescope, that hosts a multitude of computing, communication, propulsion, and structural parts, bringing the different parts of the telescope together. Along with the Sunshield, it forms the Spacecraft Element of the space telescope. The other two major elements of the JWST are the Integrated Science Instrument Module (ISIM) and the Optical Telescope Element (OTE). Region 3 of ISIM is also inside the Spacecraft Bus; region 3 includes ISIM Command and Data Handling subsystem and the MIRI cryocooler.

The structure of the Spacecraft Bus must support the 6.5 ton space telescope, while it (just the structure, not the whole bus) itself weighs 350 kg (about 772 lb, 55.1 stones). It is made primarily of graphite composite material. It was assembled in the U.S. state of California by 2015, and then it had to be integrated with the rest of the space telescope leading up to its planned 2018 launch. The bus can provide pointing of one arcsecond and isolates vibration down to two milliarcseconds (arcseconds are a unit of angle equal to 1/3600 of a degree, see Minute of arc). The fine pointing is done by the JWST fine guidance mirror, not by physically moving the whole mirror or bus.

The spacecraft bus is on the sun-facing "warm" side and operates at a temperature of about 300 k (degrees kelvin: 80 degrees Fahrenheit, 26.85 degrees celsius). Everything on the sun-facing side must be able to handle the thermal conditions of JWST's halo orbit, which has one side of continuous sunlight and the other shaded by the spacecraft sunshield.

Another important aspect of the Spacecraft Bus is the central computing, memory storage, and communications equipment. The processor and software direct data to and from the instruments, to the solid-state memory core, and to the radio system which can send data back to Earth and receive commands. The computer also controls the pointing and moment of the spacecraft, taking in sensor data from the gyroscopes and star tracker, and sending the necessary commands to the reaction wheels or thrusters.

The Bus is a carbon fibre box that houses a large number of major systems that keep the telescope functioning, such as the solar panels and computers. It also physically houses the Miri cooler and some major ISIM electronics.

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