The background

At the end of the nineteen sixties satellites characteristically had masses in the range 150 to 300 kg. By the end of the nineteen nineties many, if not most, satellites are being designed to mass budgets between one and ten metric tonnes. Satellites have expanded to fill the launchers available would be one conclusion. In fact, many changes have taken place during the past thirty years and most of these have led to a growth in satellite masses. The scientific problems being solved from space have grown more and more exacting in terms of the equipment required. As more is discovered, seemingly, more is left to be discovered and ever more sensitive instruments are demanded. Greater sensitivity usually requires large collecting areas, cooled telescopes or massive detectors, all strong factors in determining the mass of the payload. In the fields of Astronomy and Earth Observation the scientific problems seem best tackled by ‘Observatory Class’ missions in which a payload of five or ten separate instruments is compiled to provide the varied individual measurements necessary to address the mission objectives. In some cases these instruments could be launched on separate platforms if their observations could be properly coordinated, in other cases the full set of measurements must be simultaneous in space and time. Launch vehicles which are principally designed to meet the growing needs of geostationary communications satellites are available with the lift capability of many tonnes and so there has been little pressure to identify missions which can achieve the highest quality science from small, and hence inexpensive, satellites.