The Hubble telescope was built by the United States space agency NASA with contributions from the European Space Agency. The Space Telescope Science Institute (STScI) selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster (1986). It was finally launched by Space Shuttle Discovery in 1990, but its main mirror had been ground incorrectly and created a spherical aberration, compromising the telescope's capabilities. The optics were corrected to their intended quality by a servicing mission in 1993.