U.S. space exploration history on U.S. stamps also the only space station NASA launched alone. The 100-ton space station was in Earth's orbit from 1973 to 1979 and it was visited by crews three times in 1973 and 1974. Circling 50 degrees north and south of the equator at an altitude of , Skylab had an orbital period of 93 minutes. There were a plethora of UV astronomy experiments done during the Skylab lifetime, as well as detailed X-ray studies of the Sun. The station was active until July 11, 1979, when it fell out of orbit. The 10-cent Skylab commemorative stamp first day of release took
Space logistics significant focus of space logistics is on "upmass", or payload mass carried up to orbit from Earth, space station operations also have significant downmass requirements. Returning cargo from low-Earth orbit to Earth is known as transporting "downmass", the total logistics payload mass that is returned from space to the surface of the Earth for subsequent use or analysis. Downmass logistics are important aspects of research and manufacturing work that occurs in orbital space facilities. For the International Space Station, there have been periods of time when downmass capability was severely restricted. For example, for approximately ten months from the time