CRS-32

One second of error across 300 million years. That is the target precision of the Atomic Clock Ensemble in Space, a pair of clocks riding inside Dragon's unpressurized trunk when CRS-32 launched on 21 April 2025. ACES combines PHARAO — a cold-atom cesium clock developed by CNES — with an active hydrogen maser built by Safran in Switzerland. The two instruments cross-check one another: PHARAO delivers long-term accuracy; the maser provides short-term stability.

On 25 April, ACES was installed on the Earth-facing exterior of ESA's Columbus module using Canadarm2. The clocks, now orbiting 400 km above the surface, are positioned to measure the tiny shift in the rate of time predicted by Einstein's general relativity — time passing faster in weaker gravitational fields. Comparing the station's clock rate against ground-based references allows physicists to verify the theory at a precision no previous space-based experiment had reached.