Forget your quartz watches: this is a clock that’s expected to lose no more than one second every 300 million years. PHARAO is the French acronym for Projet d'Horloge A Refroidissement d'Atomes en Orbite (orbital cold-atom clock project), so nothing to do with Ancient Egypt. It’s now in space as part of the European ACES (Atomic Clock Ensemble in Space) payload.
ACES was sent aloft on 21 April 2025 and successfully attached the same day outside the European Columbus module on the International Space Station (ISS). PHARAO, set to be powered up this Monday, is thus the first caesium cold-atom clock ever to orbit Earth, operating as part of a time-comparison network.
To reach its targeted accuracy of 10-16, PHARAO employs advanced physics principles such as laser manipulation and cooling of atoms, a discovery that won Claude Cohen-Tannoudji from the Kastler-Brossel research laboratory the Nobel Prize for Physics in 1997.
Testing Einstein’s theories of relativity
The goal of this fundamental physics experiment is to test Albert Einstein’s theories of relativity, which turned physics on its head at the start of the 20th century (see box). His theories showed the importance of the frame of reference in which time is measured and how gravity influences such measurements.
For example, time advances more quickly on the ISS than here on Earth, since the station is further from the planet’s centre (theory of general relativity), but it also advances more slowly due to the station’s orbital velocity (theory of special relativity).
PHARAO will enable space-time to be studied by comparing data from different ground clocks developed by research laboratories around the world. The ultimate aim is to constrain the domains of validity of the different theories of relativity. PHARAO could thus help scientists to look for possible variations in time and space of the fundamental constants of physics.
CNES’s role in the PHARAO mission
CNES is the PHARAO prime contractor and was responsible for integrating and testing the flight model at its Toulouse Space Centre.
We’re also contributing to the ACES timekeeping experiment through our CADMOS centre for the development of microgravity applications and space operations, which conceives, develops and monitors microgravity science and technology experiments. Once ACES is attached to the ISS, CADMOS engineers will operate it from Toulouse.
