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In 2016, a CNES microsatellite will test the universality of free fall for the first time in space using an experiment 100 times more precise than anything on Earth.

Back in the 17th century, Galileo conceived an experiment, without making it in practice, in which he dropped two objects of different composition and mass together from the top of the Tower of Pisa. In his theory, as the two objects hit the ground at exactly the same time, he deduced that in a vacuum all bodies fall at the same speed. This is what we call the universality of free fall or equality of gravitational and inertial mass, which Albert Einstein later stated as the equivalence principle and made it the basis of his theory of general relativity.

Although it has recently been verified with a degree of precision on the order of 10-13, this principle is nonetheless being pushed to its limits by new theories seeking to reconcile gravitation with fundamental nuclear and electromagnetic interactions, which predict that it could be violated at very weak levels. The Microscope1  satellite will probe these limits further and test the principle with a precision on the order of 10-15. In space, it is possible to study the relative motion of two bodies in almost perfect and permanent free fall on an orbiting satellite, shielded from perturbations encountered on Earth (notably seismic perturbations), over the course of several months.

To achieve this, two concentric cylindrical test masses made of different materials—one titanium and one a platinum-rhodium alloy—will be minutely controlled to maintain them motionless with respect to the satellite inside independent differential electrostatic accelerometers. If the equivalence principle is verified, the two masses will be subjected to the same control acceleration. If different accelerations have to be applied, the principle will be violated: an event that would shake the foundations of physics.

The experiment will be flown on a 300-kg microsatellite—heavier than a usual 100-150 kg microsatellite—built around CNES’s Myriade bus and equipped with cold-gas microthrusters capable of compensating for the tiniest trajectory perturbations that might otherwise skew its results. CNES is providing 90% of funding for this mission, for which it is also prime contractor in charge of satellite bus development, satellite integration and testing up to launch, and construction and operation of the mission control centre.

1  Micro-Satellite à traînée Compensée pour l'Observation du Principe d'Equivalence