Starting in 1979, scientists will be offered a new opportunity for space research when NASA begins halfyearly launchings of the Long Duration Exposure Facility (LDEF). The instrument is a large passive unmanned structure on which over 70 separate self-contained experiment packages will be mounted. It will be carried aloft by the space shuttle transportation system, left to orbit the earth for a number of months, and then retrieved and brought back to earth, and the experiments will be returned to their owners for analysis. LDEF offers prolonged exposure to the conditions at its 556-km orbital altitude: weightlessness, extreme vacuum, and high particle and radiation fluxes. A unique feature is the return of materials and instruments at the end of the mission.