Editing Hubble Space Telescope (section)

=== Spacecraft systems ===
The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to withstand frequent passages from direct sunlight into the darkness of Earth's [[shadow]], which would cause major changes in temperature, while being stable enough to allow extremely accurate pointing of the telescope. A shroud of [[multi-layer insulation]] keeps the temperature within the telescope stable and surrounds a light aluminum shell in which the telescope and instruments sit. Within the shell, a [[carbon fiber reinforced polymer|graphite-epoxy]] frame keeps the working parts of the telescope firmly aligned.<ref>{{cite web|url=http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01/hubble_space_telescope_systems.htm|title=Hubble Space Telescope Systems|publisher=Goddard Space Flight Center|access-date=April 26, 2008|url-status=dead|archive-url=https://web.archive.org/web/20030317035553/http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01/hubble_space_telescope_systems.htm|archive-date=March 17, 2003}} {{PD-notice}}</ref> Because graphite composites are [[hygroscopic]], there was a risk that water vapor absorbed by the truss while in Lockheed's clean room would later be expressed in the vacuum of space; resulting in the telescope's instruments being covered by ice. To reduce that risk, a nitrogen gas purge was performed before launching the telescope into space.<ref>Ghitelman, David (1987) ''The Space Telescope,'' New York: Michael Friedman Publishing, p. 50.</ref>

As well as electrical power systems, the '''Pointing Control System''' controls HST orientation using five types of sensors (magnetic sensors, optical sensors, and six gyroscopes) and two types of [[actuator]]s ([[reaction wheel]]s and [[magnetic torquer]]s).<ref name=PCS/>

While construction of the spacecraft in which the telescope and instruments would be housed proceeded somewhat more smoothly than the construction of the OTA, Lockheed experienced some budget and schedule slippage, and by the summer 1985, construction of the spacecraft was 30% over budget and three months behind schedule. An MSFC report said Lockheed tended to rely on NASA directions rather than take their own initiative in the construction.{{sfn|Dunar|Waring|1999|p=508}}