Within the last century, humans have explored challenging environments such as Antarctica, the deep sea, and outer space. Human habitation within these hostile environments is restrictive and expensive, typically limited in duration, and restricted to scientific, military, or industrial expeditions. Human presence on other celestial bodies has been the case mainly with human-made robotic spacecraft and with humans solely on the Moon, two at a time for brief intervals between 1969 and 1972. Long-term continuous human presence in space has been the case in orbit around Earth, uninterrupted since the initial crew of the International Space Station, arriving on 31 October 2000, with peaks of thirteen humans at the same time in space.