ACES in the trunk

ESA's Atomic Clock Ensemble in Space (ACES) inside the SpaceX Falcon 9 trunk, ready for launch.

ACES carries the most accurate clocks ever flown in space – PHARAO and the Space Hydrogen Maser – designed to keep time so precisely that they would lose just one second every 300 million years. Developed by the French space agency CNES and Safran Timing Technologies in Switzerland, these European-built clocks will work with a sophisticated time transfer time using microwave and laser links to synchronise the best clocks all over Earth.

This state-of-the-art timekeeping facility was built in Europe, assembled at the Airbus facility in Friedrichshafen, Germany, and arrived at NASA's Kennedy Space Center cleanroom in March 2025, where engineers prepared it for its launch on 21 April aboard a SpaceX Falcon 9 rocket. The facility is now installed on the exterior of the Columbus laboratory on the International Space Station.