The Primary Atomic Reference Clock in Space (PARCS) will compare the frequency of an ultracold cesium atomic clock against a hydrogen microwave laser, or maser. Launch: 2004 or 2005.
The European Space Agency's Atomic Clock Ensemble in Space (ACES) will also consist of a cesium clock and a hydrogen maser. Launch: 2004 or 2005.
The Superconducting Microwave Oscillator (SUMO) will keep time with a microwave-filled superconductor cavity. It will make measurements on its own and in conjunction with The Rubidium Atomic Clock Experiment (RACE). Launch: 2007.
RACE will carry a laser-cooled rubidium clock, which should be more stable and accurate than the cesium variety. Launch: early 2008.
The proposed OPTIS satellite would contain both an atomic clock and a pair of optical cavities, arranged to perform the classic Michelson-Morley and Kennedy-Thorndike experiments. (OPTIS originally stood for improved Optical Test of Isotropy in Space, but the mission's objectives have grown beyond that.)
The proposed Space Time Mission (STM) would slingshot a satellite containing three atomic clocks around Jupiter for a close view of the sun. The high speeds involved might offer more sensitive tests of relativity.