ACES mission delivers record-breaking atomic clock to ISS for precision timekeeping

ACES mission delivers record-breaking atomic clock to ISS for precision timekeeping
by Erica Marchand
Paris, France (SPX) Apr 23, 2025

ESA's Atomic Clock Ensemble in Space (ACES) has begun its journey to the International Space Station (ISS), where it will revolutionize timekeeping from orbit. Equipped with two ultra-precise atomic clocks and a sophisticated time-transfer system, ACES is set to broadcast the most accurate time signals ever generated in space, while enabling key experiments in fundamental physics, including real-world tests of Einstein's general relativity.

Launched aboard a SpaceX Falcon 9 from NASA's Kennedy Space Center in Florida, ACES is part of the 32nd commercial resupply mission to the ISS. The payload underscores Europe's commitment to advancing high-precision metrology and collaborative space science.

"The launch of ACES marks a major milestone for European science and international cooperation in space. With this mission, we are placing the most precise timepiece ever sent to orbit aboard the International Space Station - opening new frontiers in fundamental physics, time transfer, and global synchronization. ACES is a shining example of what Europe can achieve when we unite cutting-edge technology, scientific ambition, and strong partnerships", said Daniel Neuenschwander, Director of Human and Robotic Exploration at ESA.

Spearheaded by ESA and built in partnership with Airbus, the ACES mission integrates the PHARAO clock, developed by CNES, and the Space Hydrogen Maser (SHM) from Safran Timing Technologies. These instruments are capable of producing a time reference so stable that it would deviate by only one second every 300 million years. Complementing this is an advanced time transfer system built by TimeTech in Germany, which uses both microwave and optical links to synchronize time between orbit and Earth with unparalleled precision.

Mission operations will be managed by CADMOS in France and the Columbus Control Centre in Germany. Once aboard the ISS, the Station's robotic arm is scheduled to install ACES on ESA's Columbus module on April 25, securing it in the Earth-facing nadir position on the External Payload Facility.

Throughout its 30-month deployment, ACES will conduct at least ten dedicated measurement campaigns, each spanning 25 days, while circling Earth 16 times per day. These efforts are expected to push the boundaries of space-based metrology and foster breakthroughs in our understanding of time and gravity.