"ACES Payload: Recent Developments and Design Status ahead of Launch"

The Atomic Clock Ensemble in Space (ACES) payload is a European Space Agency (ESA) mission in cooperation with the French Space Agency (CNES) which is flying on the ISS in 2025. Accommodated on the nadir pointing Columbus External Payload Facility, ACES will represent a new scientific experiment in fundamental physics based on a high accuracy comparison of atomic clocks in space and on ground. ACES will accommodate two state-of-art atomic clocks operated together in the microgravity environment for the first time ever: PHARAO (developed by CNES), a long-term and stable frequency standard based on laser cooled Cs atoms, and SHM (Space Hydrogen Maser), an ultra-stable short-term frequency generator. Both on-board clocks will provide a time signal with an unprecedented level of accuracy in space. The ambitious project began over three decades ago, a testament to the highly complex nature of the technology in ACES. The paper aims to review the progress of the system in general from the last 10 years until the launch campaign. During this time frame the flight models of ACES clocks and main subsystems underwent an extensive testing campaign to verify performance and survivability requirements in an integrated configuration under flight-like environmental conditions. Moreover, the Thermal Control Subsystem (TCS) of ACES has been upgraded to cope with the new transport scenario baseline with SpaceX Dragon 2 vehicle. In this context, the description of the thermal design upgrade process is presented. The latest results of ACES thermal vacuum test campaign at iABG facilities in Munich, Germany, together with its model correlation activities are covered in this paper. Finally, the status of the two atomic clocks PHARAO and SHM is presented in detail.