True Story, Space 1999 was origianlly meant to be the second season of U.F.O.
In 1972, Sir Lew Grade, head of ITC Entertainment, proposed financing a second series of the Century 21 production UFO to show-runners Gerry and Sylvia Anderson. Grade had one stipulation: the new series would be set primarily on the Moon within the environs of an expanded SHADO Moonbase; the ratings indicated the Moon-centric episodes had proved more popular with the viewers. The Andersons and their team would quickly revamp the production, flashing ahead nearly twenty years for UFO: 1999 with Commander Ed Straker and the forces of SHADO fighting their alien foes from a large new Moonbase facility.
However, toward the end of its run, UFO experienced a drop in ratings in both America and the UK; nervous ITC executives in both countries began to question the financial viability of the new series, and support for the project collapsed. In the meantime, Production Designer Keith Wilson and the art department had made considerable progress in envisioning the look and design of the new series. Their work was then shelved for the foreseeable future.[6]
UFO - Space 1999