ACES begins precision time mission aboard the International Space Station
Launched on April 21, 2025, aboard a SpaceX Falcon 9 from NASA's Kennedy Space Center, ACES traveled to the ISS as part of the 32nd SpaceX commercial resupply mission. Just four days later, the Station's Canadian-built robotic arm successfully mounted the advanced timekeeping payload onto the Earth-facing side of ESA's Columbus laboratory, where it is scheduled to operate for the next 30 months.
Developed through a partnership between ESA and European industry led by Airbus, ACES hosts two of the most sophisticated atomic clocks ever deployed in space. These include PHARAO, a caesium fountain clock crafted by the French space agency CNES, and the Space Hydrogen Maser, engineered by Safran Timing Technologies in Switzerland. Integrated with a highly advanced microwave and laser link system, ACES will synchronize orbital and terrestrial clocks with unprecedented precision. This capability will enable critical experiments testing Einstein's theory of general relativity and support efforts to redefine the second using future optical clocks.
With installation complete, ACES is set for its initial power-up on April 28. During this first activation, the payload will establish data links with ground controllers, enabling two-way communication for telemetry and telecommand functions. Concurrently, engineers will stabilize thermal systems to prepare the clocks for operation.
A six-month commissioning phase will follow, during which ACES' performance will be thoroughly tested. Engineers and scientists will fine-tune the time transfer links and calibrate the instruments. As the ISS circles Earth, ACES will connect several times daily with ground-based atomic clocks. When multiple terrestrial stations are within its view, such as two in Europe, the system can achieve a synchronization accuracy of one part in 10^17 within a few days-significantly surpassing the precision of current GPS-based timing systems. Clock comparisons across continents, a milestone never before achieved at this accuracy, are expected to take about one week.
Upon completing the commissioning phase, optimal operational parameters for PHARAO will be finalized. ACES will then transition into its primary science mission, conducting ten 25-day sessions of data collection. Once the science team validates the findings, the results will be made available to researchers worldwide, opening new pathways for breakthroughs in fundamental physics and the measurement of time itself.
Related Links
Atomic Clock Ensemble in Space (ACES)
Understanding Time and Space
