The Shuttle Pallet Satellite (SPAS-II), seen from Shuttle Discovery during Mission STS-39 of 28 April to 6 May 1991. SPAS-II is here being used to carry the Infrared Backgound Signature Survey (IBSS) payload for the US Department of Defense. IBSS is part of the Strategic Defense Initiative (SDI or 'Star Wars') programme, and is intended to provide a thorough database of the Earth's background infrared reflectance. This will enable future detectors to discriminate between the background and other IR sources, such as a missile being launched. SPAS is adaptable to a variety of experiments, and may be launched and retrieved within the duration of one Shuttle mission.