Space stations allow the crew to work in space for long periods in conditions of zero gravity. While conditions in space capsules and the space shuttle are cramped, space stations are more suitable for longer stays in space. Rockets or the space shuttle bring supplies of air and food to the space station, and often a replacement crew. Space stations are constructed from modules that are small enough to be carried by rockets or the space shuttle, which are assembled once they are in orbit.
Skylab was the first US space station. One of its tasks was to test how humans behaved in microgravity.