The parts of ACES

The Atomic Clock Ensemble in Space (ACES) is a European facility that will test fundamental physics from the outside of ESA’s Columbus module on the International Space Station. By creating a “network of clocks”, ACES will link its own highly precise timepieces with the most accurate clocks on Earth and compare them to measure the flow of time. 

ACES includes two cutting-edge clocks: PHARAO, a caesium atomic clock developed by the French space agency CNES and the Space Hydrogen Maser, a clock which uses hydrogen atoms, produced by Safran Time Technologies. The excellent stability of SHM over a one hour period combined with the long-term stability and accuracy of PHARAO provide timekeeping for ACES with a precision of one second over 300 million years.

Today, ACES is fully assembled at Airbus in Friedrichshafen, Germany, and is undergoing rigorous testing until the end of the year, after which it will be ready for its launch on a SpaceX Falcon 9 rocket, which is expected to take place in the first half of 2025. 

Once in space, a robotic arm will position ACES onto the Columbus module, where it will remain for 30 months to collect data. ACES aims to record continuous data over at least ten sessions of 25 days each. The experiment will be operated from Europe, through CADMOS in Toulouse, France, and the Columbus Control Centre near Munich, Germany. 

The ACES clock signal is transmitted to a network of ground clocks by two time and frequency links: the microwave link (MWL) operating at microwave frequencies and the European Laser Timing (ELT) optical link. MWL ground terminals in Europe, the UK, the US and Japan will communicate with ACES to exchange time information whilst compensating for the effects of the atmosphere and the harsh conditions of space.