ACES lifted off on 21 April 2025 aboard a SpaceX Falcon 9 from NASA’s Kennedy Space Center in Florida, United States, as part of the 32nd SpaceX commercial resupply services mission to the International Space Station. On 25 April, the Station’s Canadian robotic arm installed the payload on the Earth-facing side of ESA’s Columbus laboratory, where it is set to operate for 30 months.

Developed by ESA in collaboration with European industry led by Airbus, ACES carries the most accurate atomic timepieces ever launched into space: PHARAO, a caesium-based fountain clock developed by the French space agency CNES, and the Space Hydrogen Maser built by Safran Timing Technologies in Switzerland. These clocks will work in tandem with a cutting-edge microwave and laser link system to deliver time from orbit with unprecedented precision and allow ACES to establish a “network of clocks”, comparing the most accurate clocks on Earth and in space to explore the nature of time, test general relativity and help pave the way for a redefinition of the second based on next-generation optical clocks.