ACES

ACES [ESA]

ACES (Atomic Clock Ensemble in Space) is an European Space Agency (ESA) ISS attached payload designed to operate a highly precise time and frequency reference system.

ACES carries two ultra-stable atomic clocks: PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite) and SHM (Space Hydrogen Maser). ACES enables precision time transfer experiments between space and ground stations, allowing tests of Einstein’s theory of General Relativity and searches for potential variations of fundamental constants. Data from ACES supports applications in geodesy, fundamental physics, and the synchronization of global timekeeping systems.

PHARAO is a primary frequency standard based on cold atoms, cooled with laser techniques to near absolute zero, offering very high stability and accuracy. SHM complements PHARAO by providing excellent short-term stability using a hydrogen maser. Together, these clocks create a highly reliable time reference in orbit. The ACES payload also includes a Microwave Link (MWL) and an optical communication terminal for precise time and frequency comparisons with ground stations.

ACES was launched aboard a SpaceX Dragon CRS-32 spacecraft on 21 April 2025. The payload will be mounted on the external platform of ESA’s Columbus module.