Spacelab was a reusable laboratory developed by the European Space Agency. It was used on multiple space flights that were flown by the Space Shuttle. The laboratory comprised multiple components, including a pressurized module, an unpressurized carrier, and other related hardware housed in the Shuttle's cargo bay. The components were arranged in various configurations to meet the needs of each spaceflight. Spacelab components flew on a total of about 32 Shuttle missions, depending on how such hardware and missions are tabulated. Spacelab allowed scientists to perform experiments in microgravity in geocentric orbit. There was a variety of Spacelab-associated hardware, so a distinction can be made between the major Spacelab program missions with European scientists running missions in the Spacelab habitable module, missions running other Spacelab hardware experiments, and other Space Transportation System (STS) missions that used some component of Spacelab hardware. There is some variation in counts of Spacelab missions, in part because there were different types of Spacelab missions with a large range in the amount of Spacelab hardware flown and the nature of each mission. There were at least 22 major Spacelab missions between 1983 and 1998, and Spacelab hardware was used on a number other missions, with some of the Spacelab pallets being flown as late as 2008.