Today marks the 20th anniversary of the first long-term mission to the International Space Station, known as Expedition 1. Ever since then, there have always been a handful of humans living and working in orbit on the ISS — a continuous presence of people in space.
Prior to that mission, most of NASA’s human spaceflight program revolved around launching relatively quick, weeks-long trips to orbit on the agency’s Space Shuttle. But in the mid-1990s, NASA began sending its astronauts to space for much longer trips to live on Russia’s old Mir space station. Once the US, Russia, and their international partners started piecing together the International Space Station, NASA started sending some of its astronauts to stay for months at a time…