Atomic Clock Ensemble in Space

The payload contains two clocks: a caesium laser cooled atomic clock (Projet d'horloge atomique par refroidissement d'atomes en orbite) developed by CNES, France for long-term stability and an active hydrogen maser (SHM) developed by Spectratime, Switzerland for short-term stability. 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. 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 years.

• After earlier plans for launch readiness in 2012, the clock ensemble was expected to travel to the space station aboard a Falcon 9 in 2021. 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 • ACES was launched to the ISS on 21 April 2025 on a SpaceX Falcon 9 rocket • 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 • The first activation of ACES, which will establish communications with ground control and stabilise thermal systems, is scheduled for 28 April • ESA expects a 30-month operations phase for ACES

Atomic Clock Ensemble in Space is associated with European Union. Recorded from 2010.