Measuring time with unparalleled precision

Timekeeping has occupied human minds for as long as we know. It is based on observing a regular and repetitive phenomenon to characterize equal intervals of time.

Pharao is an atomic clock scheduled to fly on the ISS by 2013 as part of the European ACES* project.

It will measure time with unprecedented precision and stability, losing just one second every 300 million years. In comparison, even the most precise terrestrial clocks lose one second every 50 million years.

To achieve that kind of precision, Pharao will use cold caesium atoms—a world first. It measures the transitions of these atoms when they interact in resonance with a microwave source.

Operating in microgravity, Pharao will cool caesium atoms with a laser. Atomic clocks work by controlling the speed of atoms, and time can be measured more precisely if the atoms are slowed.

Their speed is also influenced by gravity. Pharao will be free from this constraint in orbit, where atoms move at a snail’s pace.

A clock so precise will find many applications in fundamental physics. For example, it will be able to verify certain principles of the general theory of relativity more precisely than ever before.

Pharao is the core element of Europe’s ACES atomic clocks mission.
Developed by French research laboratories** with CNES as prime contractor, it will be attached for 18 months to the European Columbus module on the ISS.

Pharao est l’élément central de la mission européenne ACES constitué de plusieurs horloges atomiques.
Développée par les laboratoires scientifiques français** sous maîtrise d’œuvre du CNES, elle sera installée pendant 18 mois à l’extérieur du module européen Columbus de l’ISS.

* ACES : Atomic Clock Ensemble in Space
** SYRTE laboratory (SYstèmes de Références Temps-Espace) at the Observatoire de Paris and Kastler-Brossel laboratory (LKB) at the Ecole Normale Supérieure

 _{Last updated : August 2009}