ACES SHM clock

SHM (Space Hydrogen Maser) is 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. 

SHM, produced by Safran Time Technologies, is an active hydrogen maser, a device which uses hydrogen atoms to tell time. This clock uses hydrogen as an atomic frequency reference and its operation resembles that of passive masers used onboard the Galileo satellites, but ten times more stable.

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.