By covering this wavelength range ISO was able to explore regions of the Universe obscured in visible light. Infrared light penetrates the obscuring dust which hides much of the Universe from inspection at visible wavelengths. Light at these wavelengths originates from bodies and material which are cool and distinct from the energetic sources of visible light like the stars. But these cool sources are of fundamental importance.
A rich variety of atomic, ionic, molecular and solid-state spectral features trace the chemistry and evolution of the cold gas and dust from which the stars form, and which they in turn enrich with the heavy elements produced during their nuclear burning and terminal phases. New generations of stars and planets form from the enriched interstellar medium, revealing their presence first through the infrared emission associated with proto-stellar and proto-planetary sources. Most of the star formation which has occurred in the history of the Universe is revealed through the infrared emission of the heated dust clouds which would otherwise hide it from our view.
The ISO mission resulted from a proposal made to ESA in 1979. After a number of studies ISO was selected in 1983 as the next new start in the ESA Scientific Programme. Following a Call for Experiment and Mission Scientist Proposals, the scientific instruments were selected in mid 1985. The two spectrometers, a camera and an imaging photo-polarimeter jointly covered wavelengths from 2.5 to around 240 microns with spatial resolutions ranging from 1.5 arcseconds (at the shortest wavelengths) to 90 arcseconds (at the longer wavelengths).
The satellite design and main development phases started in 1986 and 1988, respectively, with Alcatel (Cannes, France, formerly Aerospatiale) as prime contractor. ISO was launched perfectly in November 1995 by an Ariane 44P vehicle. All went very smoothly in orbit and, at a wavelength of 12 microns, ISO was one thousand times more sensitive and had one hundred times better angular resolution than IRAS. Routine scientific operations commenced in February 1996 and continued until April 1998, with limited operations continuing through May. All data were re-processed with the end-of-mission calibration to populate the first homogeneous ISO Data Archive, which opened to the community in December 1998. By August 1999, all data had entered the public domain.
Through the ensuing 4 years of the Post-operations Phase, ESA's ISO Data Centre developed and refined the ISO Data Archive to offer the ISO data to the worldwide astronomical community. Together with the several National Data Centres in various member states and in the US worked to fill the archive with the best systematically processed and calibrated data products which could be achieved for the huge ISO database. These products allow users to select from the archive data sets of interest for deeper study with interactive analysis tools. ISO's Legacy Archive, containing this reference product set, was released at the end of February 2002.
During ISO's Active Archive Phase, which runs from January 2002 to December 2006, the ISO Data Centre continues to work with active National Data Centres in The Netherlands, Germany and the U.K. to support the community in its use of the ISO data and to leave behind a homogeneous archive as a legacy to future generations of astronomers.
Effort is concentrated into dedicated systematic reprocessing of selected observing modes with a mode-specific instrument processing pipeline or making use of interactive tools. Particular focus is also placed on the opportunity represented by the growing mass of published ISO data to gather into the archive the refined data products upon which astronomers have based their published work. In this way we will fill the archive with immediately reusable refined data products, so preparing the ISO archive for its role as part of a system of interoperable archives forming the "virtual telescopes" of the future.