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 weather first spacecraft with the mission of analyzing the space environment. Recent spacecraft include the NASA-ESA Solar-Terrestrial Relations Observatory (STEREO) pair of spacecraft launched in 2006 into solar orbit and the Van Allen Probes, launched in 2012 into a highly elliptical Earth-orbit. The two STEREO spacecraft drift away from the Earth by about 22° per year, one leading and the other trailing the Earth in its orbit. Together they compile information about the solar surface and atmosphere in three dimensions. The Van Allen probes record detailed information about the radiation belts, geomagnetic storms and the relationship between the two. Some spacecraft