Atomic Clock Ensemble in Space Set to Transform Timekeeping and Understanding of the Universe

The launch of the Atomic Clock Ensemble in Space (ACES) on April 21, 2025, aboard a SpaceX Falcon 9 rocket, signifies a remarkable leap in our quest for precision in timekeeping and understanding the fundamental nature of time itself. Installed on the International Space Station (ISS) just days after its launch, ACES is poised to revolutionize scientific exploration and redefine our conception of the second.

Developed by the European Space Agency (ESA) in collaboration with industry partners including Airbus, ACES houses the most advanced atomic timekeepers ever sent into space. The centerpiece of this high-tech facility is the PHARAO clock, a caesium-based fountain clock engineered by the French space agency CNES, complemented by the Space Hydrogen Maser, designed by Safran Timing Technologies in Switzerland. Together, these clocks form an unparalleled timekeeping system that will enable scientists to delve deeper into the mysteries of time and relativity.

The intricate design of ACES allows for an innovative operation that utilizes cutting-edge microwave and laser link systems. This setup not only transmits time from orbit with extraordinary accuracy but also facilitates the creation of a “network of clocks.” By comparing ACES with the most precise timekeeping devices on Earth, researchers will have a powerful tool to not only test the tenets of general relativity but also explore potential revisions in the definition of a second, potentially paving the way for next-generation optical clock standards.

As the installation process concluded successfully, the focus shifted to the operational phase, with the first system activation scheduled for April 28. This crucial step will establish vital communication links between ACES and ground control, allowing telemetry and telecommanding processes to begin. The team anticipates a thorough commissioning phase spanning six months, where engineers will calibrate instruments, test time transfer links, and refine the clocks’ performance. Throughout this phase, ACES will connect with various ground-based clocks multiple times daily, yielding unprecedented accuracy in time comparison.

One of the most remarkable aspects of the ACES mission is its ability to achieve a precision of one part in 10^-17—an astounding level of accuracy that translates to approximately 10 quintillionths of a second—within just days of operation. This exceeds the precision of contemporary systems, like GPS, by one to two orders of magnitude. Such precision is not merely theoretical; it has practical implications for technology reliant on accurate timing, including communications systems and navigation technologies.

By observing how time behaves differently in varying gravitational fields and at different velocities, ACES seeks to answer vital questions about the universe, from the fundamental laws of physics to the very fabric of spacetime. For instance, by comparing distant clocks across continents, ACES will provide insights into how time flows in a way that has never been directly observed before. This will mark a significant leap in our understanding of time dilation—a phenomenon predicted by Einstein’s theory of relativity where time passes at different rates depending on speed and gravity.

Once the commissioning phase concludes, ACES will enter its two-year science phase, engaging in ten planned sessions of extensive data collection over 25 days each. The results from these sessions will not only be pivotal for scientific advancement but will also be shared with the global scientific community, opening avenues for collaboration and enhancing our collective knowledge of time.

As Simon Weinberg, ACES Project Scientist at ESA, aptly remarked: “I’m so excited and proud that our incredibly complex and hugely important project is now in space after many years of hard work. That is a major achievement for ESA and the science community, and I look forward to seeing the results of the hard work done by Airbus, Timetech, CNES, Safran Time Technologies and ESA, together with partners all over Europe.”

The implications of ACES extend far beyond mere timekeeping; they challenge our understanding of the universe on a fundamental level. As we eagerly await the insights that ACES will unveil, it stands as a testament to human ingenuity and our relentless pursuit of knowledge in the cosmos.