VSOP (VLBI Space Observatory Programme) was a space VLBI mission. Data collected using a 8-m radio dish on board the Japanese satellite HALCA (Highly Advanced Laboratory for Communications and Astronomy) were correlated with data collected from ground-based radio arrays such as the VLBA, EVN and LBA to form the world's largest-ever telescope. The maximum baseline was over twice the diameter of Earth, resulting in unprecedented sub-milliarcsecond resolution at the operating frequencies of 1.6 and 5 GHz, suitable for detailed studies of high brightness temperature sources such as the cores and jets in radio galaxies and AGN. VSOP was led by the Japanese Institute of Space and Aeronautical Science (ISAS) of the Japanese Aerospace Exploration Agency (JAXA), with international collaborators. HALCA was launched into Earth orbit in February 1997 and conducted its last observation in October 2003. The next-generation space VLBI mission, VSOP-2, is expected to be launched in 2012.