GRGS > Research > Projects > MICROSCOPE

Bureau des longitudes Centre national d’études spatiales Institut national de l'information géographique et forestière - Laboratoire LAREG Observatoire de Paris Institut national des sciences de l'Univers Observatoire de la Côte d'Azur Service Hydrographique et Océanographique de la Marine Observatoire Midi-Pyrénées Université de la Polynésie Française Ecole Supérieure des Géomètres et Topographes - Conservatoire national des arts et métiers Ecole et Observatoire des Sciences de la Terre


The MICROSCOPE project was jointly proposed by ONERA and the OCA within the Myriade series (micro satellites). Currently, the payload is in Phase C / D while the satellite awaits further studies on the propulsion system to complete Phase B.

The MICROSCOPE space mission aims to test the universality of free fall (or the weak equivalence principle) at better than 10-15 (over 100 times better than ground experiments) using spatial accelerometers.

satellite cnes

The principle of the experiment is as follows: we seek to detect a difference in acceleration between two test masses with the same center and different compositions (sign of a violation of the equivalence between gravitational mass and inertial mass for different materials). Such a pair of concentric test masses constitutes a differential accelerometer. Each mass, ‘suspended’ electrostatically, is controlled to remain stationary relative to the rest of the satellite, and the displacement is measured by capacitive sensors. The electrostatic force exerted offsets the other accelerations (and therefore allows us to measure) of the mass relative to the satellite.

  • the non-gravitational forces exerted on the satellite but not on the masses that are inside,
  • the gravity gradient and the inertial gradient between the center of the satellite and the center of each mass,
  • different ratios of gravitational mass to inertial mass for satellite and each test mass.

The act of observing the difference in acceleration between two cylindrical test masses with the same center allows us to cancel the first two sources (common mode) and observe the difference in “free fall” of the two masses subjected to gravitational field of Earth. In addition, the electric micro-thrusters used for acceleration measured in common mode enable us to monitor effect of the movement of the satellite on the average movement of the masses; this is what is called compensated drag. This has the considerable advantage of reducing the acceleration acting on each mass individually and thus allows for greater accuracy in measuring the difference in acceleration. This drag compensation is both unique to the MICROSCOPE mission and a reason why the satellite and its payload (two differential accelerometers) should be considered as a single system.

MICROSCOPE carries two differential accelerometers: an accelerometer for the test itself with two masses of different composition (platinum and titanium) and a reference accelerometer with two masses of identical composition.