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
Artificial satellites in retrograde orbit Artificial satellites in retrograde orbit Artificial satellites are rarely placed in retrograde orbit. This is partly due to the extra velocity (and fuel) required to launch into orbit against the direction of the Earth's rotation. Most commercial Earth-observing satellites use retrograde sun-synchronous orbits to ensure that observations are performed at the same local time each pass of any given location, while almost all communication satellites use prograde orbits. Israel has successfully launched seven Ofeq satellites in retrograde orbit aboard a Shavit launcher. These reconnaissance satellites complete one Earth orbit every 90 minutes and initially make about six daylight passes per