Atomic Clock Ensemble in Space

The Atomic Clock Ensemble in Space (ACES) is a project led by the European Space Agency (ESA), placing ultra-stable atomic clocks on the International Space Station (ISS). It was launched^[1] and installed on the Columbus External Payload Facility in April 2025.^[2] Operation in the microgravity environment of the ISS provides a stable and accurate time base for different areas of research, including general relativity and string theory tests, time and frequency metrology, and very long baseline interferometry.

Launch of the Falcon 9 rocket with the CRS-32 Cargo Dragon capsule carying ACES to the International Space Station

Instruments

The payload contains two clocks: a caesium laser cooled atomic clock (PHARAO) developed by CNES, France for long-term stability and an active hydrogen maser (SHM) developed by Spectratime, Switzerland for short-term stability.^[3][4] The onboard frequency comparison between PHARAO and SHM will be a key element for the evaluation of the accuracy and the short/medium-term stability of the PHARAO clock, it will allow the identification of the optimal operating conditions for PHARAO and to select a compromise between frequency accuracy and stability.^[5][6] The mission will also be a test-bed for the space qualification of the active hydrogen maser SHM. Afterwards, optimisation performances in the 2 × 10⁻¹⁶ range for both frequency instability and inaccuracy are intended, this corresponds to a time error of about 1 second over 300 million (300 × 10⁶) years.

Timeline

• After earlier plans for launch readiness in 2012,^[7] the clock ensemble was expected to travel to the space station aboard a Falcon 9 in 2021.^[8] Major delays due to difficulties in the development and testing of the active hydrogen maser and the time transfer microwave system extended the launch to 2025^[9]
• ACES was launched to the ISS on 21 April 2025 on a SpaceX Falcon 9 rocket^[1]
• On April 25, 2025, ACES was installed on the Earth-facing side of ESA’s Columbus laboratory module using the Canadian robotic arm. A six-month commissioning phase will follow^[2][10]
• The first activation of ACES, which will establish communications with ground control and stabilise thermal systems, is scheduled for 28 April^[2]
• ESA expects a 30-month operations phase for ACES^[11]

See also

• Scientific research on the ISS
• Columbus External Payload Facility
• European contribution to the International Space Station
• List of European Space Agency programmes and missions