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MICRO-Satellite à traînée Compensée pour l'Observation du Principe d'Equivalence
SCIENTIFIC OBJECTIVES 
Equivalence Principle Test
Realisation of an attitude control and drag free system

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THE MICROSCOPE SCIENTIFIC OBJECTIVES
 
 
The generally admitted theory to describe the gravitation is the General theory of relativity. It is founded on the Principle of Equivalence. Einstein indeed promoted this principle, considered as empirical since Galileo and Newton, as the fundamental postulate of its theory.
According to this principle, the physical laws in a reference frame falling in free fall in a gravitational field are equivalent to the physical laws in an inertial reference frame. An elementary consequence of the principle can be stated as follows: the trajectory of a body falling in free fall (i.e. a body which is subjected to no interaction of the electromagnetic type for example) depends neither on its internal structure nor on its composition.
The principal reason to test this principle comes from the fact that the gravitation, first of the known fundamental interactions, resists to the attempts of unifying it with the other fundamental interactions (electromagnetic, weak nuclear and strong nuclear interactions). Those are described according to a quantum model of theory of the fields, the Standard Model of the physics of the particles, whereas the gravitation is described by a classical theory (in opposition to quantum), the General Relativity, which connects the geometry of the space-time to the density of matter-energy that it contains. The most recent theories of unification, such as the theory of strings, thus seek to find a coherent description of the gravitation and other interactions. In all cases, these theories predict the existence of a new interaction depending on the composition of the bodies. Whatever its origin, a possible new force could, if superimposed on the gravitation, being highlighted by a violation of the Equivalence Principle. To test the Equivalence Principle, and in particular the universality of free fall, it is thus a way to seek the existence and the characteristics of this new interaction.
 
The mission's main objective is to test the Equivalence Principle (EP) with an accuracy of 10-15, i.e. more than a 100 times better than the accuracy of the present ground experiments performed either with a torsion pendulum or Moon Laser ranging.

The results of this experiment will enlight the gravitation theories. If the EP is not violated, it will pave the way toward the direct observation of gravitational waves as predicted by the general relativity. If the EP is violated, the results will inform new theories which did not already benefit from so high accuracy experiments.

MICROSCOPE mission exploits the Earth as the gravitational source of the space fundamental physics experiment. The orbital motions of two test-masses falling in the Earth gravity field and composed of two different materials are controlled to remain perfectly identical, thus guaranteeing that both test-masses are submitted exactly to the same gravitational field.
 
This space experiment will benefit of the very soft environment provided on board a drag free satellite (the non gravitational force applied on the satellite being compensated by the actuation of electrical thrusters), and of the accurate knowledge of the gravity gradient.

The possibility of very long periods of observation of the free fall mass motions in very steady conditions leads to integration of measurements over days, providing the rejection of stochastic disturbances.

The rotation of the observational frame with respect to the gravity field helps also in the discrimination of the eventual EP violation signal. Moreover, several rotation frequencies and phases can be used.
 
The satellite drag compensation involves Field Emission Electric Propulsion (FEEP) which have never been tested in space. This technology paves the way for other scientific missions needing to compensate the drag and more generally all the non gravitational forces. It is also promising for the preparation of future missions involving several satellites in formation, as their relative trajectory needs to be controlled with a great accuracy.
 

Latest Update 23/05/2006