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
Bigelow Commercial Space Station was formed in 1999. By 2004, plans made public included assembly of multiple modules "into a manned space facility in low Earth orbit for both and funded research and for space tourism." Two more formal concepts have since been made public. By 2005, Bigelow space station plans had been further conceptualized into "Commercial Space Station Skywalker", or CSS Skywalker. In mid-2010, Bigelow announced their "Next-Generation Commercial Space Station"—later named "Space Complex Alpha". The initial dates for the Alpha complex were not achieved. In January 2013, the Alpha complex was specified to be an in-space assemblage of only two B330 modules,