On April 21, 2025, ESA’s Atomic Clock Ensemble in Space (ACES) began its journey to the International Space Station on a SpaceX Falcon 9 rocket launched from NASA’s Kennedy Space Center in Florida.
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.
Later this week, the Station’s Canadian robotic arm will install ACES on the exterior of ESA’s Columbus module. From its vantage point 400 kilometers above Earth, ACES will link its ultra-precise clocks with the best timekeepers on the ground, enabling groundbreaking tests of fundamental physics, including Einstein’s theory of general relativity. Over its 30-month mission, it will carry out extended measurement sessions to investigate the very nature of time and enhance global time synchronisation.
Image Credit: ESA-S. Corvaja
