The Science objectives of ACES/PHARAO are both fundamental and technical in nature. The fundamental aspects deal with the physics of the cold atom clock. For the first time cold atoms will be operated in conditions which are not accessible on Earth in order to perform fundamental physics tests (relativity, possible drift of fundamental constants with time). At the same time, a number of new technologies requested by space environment will be validated. Furthermore, the science community could take advantage worldwide of the ACES frequency stability by using ground station to download the ACES time reference. These aspects will become increasingly important with future developments of navigation and positioning systems, new "matter wave" inertial sensors and fundamental physics tests in solar orbit.
Thanks to ACES, inter-comparison of distant atomic ground clock will soon be possible and very useful even if some of them become more accurate than PHARAO in the decade to come. Any new ground clock will increase the interest of ACES/PHARAO as it could be cross-compared to any clock worldwide.
The objectives and associated performances are:
- To operate a laser cooled caesium clock in micro-gravity with a relative frequency stability of 10-13-½ where is the time measurement in seconds. Averaged over 10 days, the stability will reach 10-16. The chart below shows the expected PHARAO frequency stability on the ISS. PHARAO in space will explore the domain of long interaction times between the atoms and the probing microwave while this time is limited in the presence of gravity.
- To distribute, within the ACES ensemble, and using a specific high performance microwave link, the optimized time scale of the combined system SHM-PHARAO, to ground users. The accuracy of this time scale will be 30 picoseconds for one day. Users belong to various areas of applications including: Time and Frequency comparisons, covering a large number of laboratories contributing to the construction of TAI (Temps Atomique International), geodesy, Very Long Baseline Interferometry (VLBI), Atmospheric propagation of microwave signals, etc.
- To perform fundamental physics experiments: a new measurement of the gravitational red-shift with an accuracy of 10-6, almost a 100-fold improvement over the Gravity Probe A experiment of 1976; a search for a possible anisotropy of the speed of light at the relative level of 2 10-10 will be made; a search for a possible time (or space) variation of the fine structure constant, which is one of the fundamental constants of physics, will be possible through the comparison of PHARAO time with the ones of atomic clocks on the ground based on atoms other than caesium.
Once fully integrated within the ACES payload, the PHARAO clock will be placed in the US Dragon vehicle and lifted into orbit by the SpaceX Falcon-9 launcher. The Dragon module has the capability to transport pressurized payloads as well as payloads exposed to vacuum. ACES falls within the second category because it is designed to operate in space vacuum (it could easily be placed on a conventional satellite if need be). Once the vehicle is docked to the ISS, ACES will be extracted by a robotic arm and will reach a temporary location on the ISS. It will then be moved in order to be locked in its final destination outside the Columbus module, again by a robotic manipulator. The installation of PHARAO/ACES does not require extravehicular activity by astronauts. The payload is designed to operate during at least 18 months with a possible extension to three years.