ACES PHARAO clock

PHARAO (Projet d'Horloge Atomique à Refroidissement d'Atomes en Orbite) is the caesium atomic clock, one of two timepieces in the Atomic Clock Ensemble in Space (ACES), a European facility that will test fundamental physics from the outside of ESA’s Columbus module on the International Space Station. By creating a “network of clocks”, ACES will link its own highly precise timepieces with the most accurate clocks on Earth and compare them to measure the flow of time. 

PHARAO, developed by the French space agency CNES, uses lasers to cool caesium atoms to the absolute zero of temperature around -273 degrees Celsius; this allows extremely precise measurements of time and frequency. On Earth, a caesium fountain clock is about two or three metres high to allow enough room for the caesium atoms to be launched upwards and interact with the microwave fields of the clock, before falling due to gravity. In free fall conditions on the International Space Station, the caesium atoms can be launched more slowly over a short distance and still have plenty of time for interaction; this allows a drastic reduction of PHARAO's size while retaining a high stability.

ACES includes two cutting-edge clocks: PHARAO and SHM (Space Hydrogen Maser). The excellent stability of SHM over a one hour period combined with the long-term stability and accuracy of PHARAO provide timekeeping for ACES with a precision of one second over 300 million years.