Since the interaction duration in a cold atom fountain is determined by the device height, it does not seem reasonable to expect to increase this duration by one magnitude order, which would lead to an increase in the device height by two magnitude orders due to this relationship.

In order to overcome this limitation, one solution consists in operating the clock in microgravity space: the cold atoms then no longer fall relative to the clock (very strange sentence, in french as well in english), which makes it possible to reach interaction times much greater than those available on Earth. Thus, a cold atom space clock, PHARAO, was developed in France by laboratories (SYRTE, LKB), industrialists (Sodern, Thales) and the French space agency CNES.

The PHARAO clock is unique in the world has and required two decades of development. Planned to be installed in 2017 on board the International Space Station as part of the European Space Agency ESA European space mission ACES (Atomic Clock Ensemble in Space). The ACES mission will allow to operate for the first time in space a cold atom clock associated with an active hydrogen maser, with main objectives in metrology and fundamental physics, including a very precise gravitational shift measurement. This mission will also be a precursor to future missions aimed at testing in space different aspects of the equivalence principle, stated by Einstein, which is the pillar of the relativity theory.