Maleki L., Prestage J. (2001) SpaceTime Mission: Clock Test of Relativityat Four Solar Radii. In: Lämmerzahl C., Everitt C.W.F., Hehl F.W. (eds) Gyros, Clocks, Interferometers...: Testing Relativistic Graviy in Space. Lecture Notes in Physics, vol 562. Springer, Berlin, Heidelberg
SpaceTime is a mission concept developed to test the Equivalence Principle. The mission is based on a clock experiment that will search for a violation of the Equivalence Principle through the observation of a variation of the fine structure constant, α. A spatio-temporal variation of α is expected in some string theories aimed at unifying gravity with other forces in nature. SpaceTime uses a special tri- clock instrument on a spacecraft which approaches the sun to within four solar radii. The instrument consists of three trapped ion clocks based on mercury, cadmium, and ytterbium ions, in the same environment. This configuration allows for a differential measurement of the frequency of the clocks and the cancellation of perturbations common to the three. The observation of any frequency drift between each of the clocks, as the tri-clock instrument approaches the sun, signals the existence of a scalar partner to the tensor gravity. Some relevant details of the mission design are discussed in the paper.