Abstract
Atomic Clock Ensemble in Space (ACES) is a mission in fundamental physics which will operate a new generation of atomic clocks in the microgravity environment of the International Space Station. Designed to be installed at the external payload facility of the Columbus module, ACES will accommodate the cold-atom clock PHARAO and the active hydrogen maser SHM. The on-board time scale will reach fractional frequency instability and inaccuracy of few parts in 1016. The ACES clock signal will be distributed on Earth by a link in the microwave domain and used for the comparison of atomic frequency standards, both space-to-ground and ground-to-ground. Based on these comparisons, ACES will perform accurate tests of Einstein's theory of general relativity and develop applications in time and frequency metrology, global positioning and navigation, geodesy, and gravimetry. After a general overview of the mission concept and its scientific objectives, the present status of the ACES mission is discussed.
