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
Assembly of the International Space Station combinations. The last two Shuttle flights added one of the two final elements of ISS, followed by a Proton launch to deliver the European Robotic Arm in 2019. Other assembly flights consisted of modules lifted by the Russian Proton rocket or in the case of Pirs and Poisk by a Soyuz-U rocket. Some of the larger modules include: The space station is located in orbit around the Earth at an altitude of approximately , a type of orbit usually termed low Earth orbit (the actual height varies over time by several kilometers due to atmospheric drag and reboosts). It orbits