The first version of this mission was approved by the European Space Agency (ESA) Council in 1973 but did not infact start its Phase B study until 1977. The early baseline design had been constrained by the requirement to make the spacecraft compatible in size and weight with the performance of a NASA Delta rocket, since this was seen as a necessary back-up to Ariane, then at an early stage of development. The mission payload then evolved with time due to the changing role dictated by the technical successes and observations made by the series of well-known X-ray satellites. The final spacecraft has emerged to have a unique capability since all the other X-ray satellites except the small Hakucho have now expired.