ESA has its headquarters in Paris. The European Space Research and Technology Centre (ESTEC) is at Noordwijk in the Netherlands and is concerned with management of science and applications programs. The European Space Operations Centre (ESOC) is at Darmstadt in Germany and is concerned with satellite and spaceprobe operations and data acquisition and processing; it controls a network of ground stations for data reception from satellites and interplanetary probes. ESRIN (European Space Research Institute) at Frascati, Italy, houses a major information-retrieval service and handles data from remote-sensing satellites. The European Astronaut Centre (EAC) at Cologne, Germany, selects and trains people for missions aboard the International Space Station. ESA's launch base for its Ariane launcher is at Kourou, French Guiana. It also operates sounding-rocket launch stations in Norway and Sweden, a meteorological program office at Toulon, in France, and satellite-tracking stations in Belgium, Germany, Italy, and Spain.
The programs carried out under the general budget and the science program budget are mandatory for all member countries. Other programs are optional, and members are free to decide on their level of scientific and financial involvement. Several of ESA's missions are carried out in collaboration with other space agencies, notably NASA, and ESA has made essential contributions to such long-term projects as the Hubble Space Telescope. The Ariane series of launch vehicles, Spacelab, Giotto, SOHO, ISO, XMM-Newton, Mars Express, and the Huygens probe to Titan (see Cassini-Huygens) are among ESA's most successful ventures into space. Arianespace, the world's first commercial space transportation company and a division of ESA, now conducts more than half of all commercial satellite launches. See also Horizon 2000.