The French space agency CNES has announced that PHARAO, a cold-atom clock built in France, completed its in-orbit acceptance testing at the end of June 2026. It is the main instrument of the European Space Agency’s ACES mission, mounted outside the Columbus module of the International Space Station.
According to CNES, it is the most precise clock ever sent into space: its accuracy reaches the 16th decimal place and it could detect the change in the flow of time caused by a difference of just one metre in altitude.
The clock was launched in April 2025 and switched on in orbit on 30 April that year. About six months of tuning and eight months of comparisons with clocks on the ground followed. The mission’s second clock, a maser, only produced a signal after about ten months of attempts and now supports the measurements.
All operations are run from Toulouse. A final review will mark the transition of the ACES mission to full operations, although some adjustments remain.
