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
Halo orbit each Lagrange point. Because halo orbits tend to be unstable, stationkeeping is required to keep a satellite on the orbit. Most satellites in halo orbit serve scientific purposes, for example as space telescopes. Robert W. Farquhar first used the name "halo" for these orbits in his 1968 Ph.D. thesis. Farquhar advocated using spacecraft in a halo orbit on the far side of the Moon (Earth–Moon ) as a communications relay station for an Apollo mission to the far side of the Moon. A spacecraft in such a halo orbit would be in continuous view of both the Earth and the