Atomic Clock Ensemble in Space

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Atomic Clock Ensemble in Space (ACES) is a project led by the European Space Agency which will place ultra-stable atomic clocks on the International Space Station. Operation in the microgravity environment of the ISS will provide 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.

The payload actually 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.[1][2] 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. Further, it will allow to identify the optimal operating conditions for PHARAO and to select a compromise between frequency accuracy and stability.[3][4] The mission will also be a test-bed for the space qualification of the active hydrogen maser SHM. After 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.

The clock ensemble was planned to travel to the space station aboard a Japanese HTV by 2018. However, HTV-7[5] did not include ACES, and HTV-8[6] is scheduled to launch in 2019, The ACES module will be externally mounted to the ESA's Columbus Laboratory. [7][8] with an 18-30 month expected operations phase.[9]

See also[edit]


  1. ^ "Swiss Space Atomic Clock Technologies and Applications in Space Science" (PDF). SpectraTime. Retrieved 11 February 2017.
  2. ^ ESA. "Atomic clock ensemble in space (ACES)" (PDF). ERASMUS Centre - Directorate of Human Spaceflight and Operations. Retrieved 11 February 2017.
  3. ^ "Swiss Space Atomic Clock Technologies and Applications in Space Science" (PDF). SpectraTime. Retrieved 11 February 2017.
  4. ^ ESA (25 July 2014). "Timely Arrival of PHARAO Space Clock". ESA. Retrieved 11 February 2017.
  5. ^ "HTV-7". JAXA. Retrieved October 13, 2018.
  6. ^ "International Space Station Flight Schedule". Students for the Exploration and Development of Space. Retrieved 13 October 2018.
  7. ^ "ISS Utilization: ACES (Atomic Clock Ensemble in Space) / PHARAO". eoPortal. ESA. Retrieved 31 January 2016.
  8. ^ CNES (5 September 2016). "PHARAO". CNES. Retrieved 11 February 2017.
  9. ^ "ACES Platform". ESA. 8 September 2014. Retrieved 31 January 2016.

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