ACES/PHARAO space mission

On Monday, April 21st, the ACES/PHARAO (Atomic Clock Ensemble in Space) space mission was launched from Cape Canaveral to the International Space Station (ISS). After docking at the ISS, the robotic arm moved the ACES/PHARAO payload to its final location, outside the Columbus module. In-flight validation tests have now begun and will go on for several weeks.

PHARAO will be the first primary frequency standard in space. It exploits laser atom cooling techniques which, combined with the zero-g enviroment, will enable very low atomic velocities and, consequently, ultimate performance in terms of frequency accuracy; the objective is to reach 1.1×10^-16 in relative value.

The main purpose of the ACES mission is to carry out experiments in fundamental physics, and in particular to test Albert Einstein’s theory of general relativity. Some physicists believe that a ‘small’ deviation from general relativity is necessary to ensure its compatibility with quantum physics. Because this effect is so small, a space instrument with unprecedented accuracy is needed to measure it. With the ACES ensemble: PHARAO, hydrogen maser and time transfer modules, on board the ISS, and clocks on the ground, scattered around the globe and equipped with time transfer stations, this measurement becomes possible.

Ph.Laurent (PHARAO Scientific coordinator)