Editing EXOSAT (section)

==History of Exosat==

During the period from 1967 to 1969, the [[European Space Research Organisation]] (ESRO) studied two separate missions: a European X-ray observatory satellite, as a combined X- and gamma-ray observatory (Cos-A), and a gamma-ray observatory (Cos-B). Cos-A was dropped after the initial study, and [[Cos-B]] was proceeded with.

Later in 1969 a separate satellite (the Highly Eccentric Lunar Occultation Satellite - Helos) was proposed. The Helos mission was to determine accurately the location of bright X-ray sources using the lunar occultation technique. In 1973 the observatory part of the mission was added, and mission approval from the European Space Agency Council was given<ref name=Taylor>{{ cite journal |vauthors=Taylor BG, Andresen RD, Peacock A, Zobl R |title=The Exosat mission |journal= Space Sci. Rev. |volume=30 |issue=1–4 |date=Mar 1981 |pages=479–94 |doi=10.1007/BF01246069 |bibcode = 1981SSRv...30..479T |s2cid=120475350 }}</ref> for Helos, now renamed Exosat.

It was decided that the observatory should be made available to a wide community, rather than be restricted to instrument developers, as had been the case for all previous ESA (ESRO) scientific programmes. For the first time in an ESA project, this led to the approach of payload funding and management by the Agency. Instrument design and development became a shared responsibility between ESA and hardware groups.

In July 1981 ESA released the first Announcement of Opportunity (AO) for participation in the Exosat observation programme to the scientific community of its Member States. By 1 November 1981, the closing of the AO window, some 500 observing proposals had been received. Of these, 200 were selected for the first nine months of operation.<ref name=Hoff/>

Exosat was the first ESA spacecraft to carry on board a digital computer (OBC), with its main purpose being scientific data processing. Spacecraft monitoring and control were secondary. To provide the data handling subsystem with an exceptional flexibility of operation, the OBC and Central Terminal Unit were in-flight reprogrammable. This flexibility far exceeded any other ESA spacecraft built up to then.

It was originally planned to launch on an [[Ariane 1]] but concerns over delays to the rocket resulted in it being transferred to a [[Thor-Delta]].<ref name=Dickson>{{cite journal |last=Dickson |first=David |title=Ariane Loses One to NASA |journal=Science |volume=219 |issue=4589 |date=11 March 1983 |issn=0036-8075 |doi=10.1126/science.219.4589.1202 |page=1202}}</ref><ref name=calder65>{{cite book |last=Calder |first=Nigel |title=Giotto to the Comets |publisher=Presswork |publication-place=London |date=1992 |isbn=0-9520115-0-6 |page=65}}</ref>