The generally admitted theory to describe gravitation is the General Theory of Relativity. It is based on the Principle of Equivalence. Einstein promoted this principle, considered as empirical since Galileo and Newton, as the fundamental postulate of his theory.
According to this principle, the physical laws in a reference frame 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 gravitation, the first of the known fundamental interactions, resists to the attempts of unifying it with other fundamental interactions–electromagnetic, weak nuclear and strong nuclear interactions. They are described according to a model of quantum field theory, the Standard Model of particle physics, whereas gravitation is described by a classical theory–General Relativity–which connects the geometry of space-time to the density of matter-energy it contains. The most recent theories of unification, such as string theory, thus seek to find a coherent description of gravitation and other interactions. In all cases, these theories predict the existence of a new interaction which depends on body composition. Whatever its origin, a possible new force could, if superimposed on gravitation, be highlighted by a violation of the Equivalence Principle. Testing the Equivalence Principle, and in particular the universality of free fall, implies to seek the existence and 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. a 100 times better than the accuracy of the present ground experiments performed either with a torsion pendulum or by Laser shootings to the Moon.
The results of this experiment will enlighten gravitation theories. If the EP is not violated, it will pave the way toward the direct observation of gravitational waves as predicted by General Relativity. If the EP is violated, the results will provide information on new theories which did not already benefit from so high accuracy experiments.
The Microscope mission exploits the Earth as the gravitational source for this fundamental physics space 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 from the very soft environment provided on board a drag free satellite (the non gravitational force applied on the satellite is compensated by the actuation of electrical thrusters), and from 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 integrate the 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 cold gas microthrusters. This extremely precisetechnology 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.
The satellite is on a Sun synchronous circular orbit at 700 km altitude with an ascending node at 6 or 18 hours.
In theory, the experiment of the Equivalence Principle test–which is the mission's main objective–only requires one week of continuous measurements. But the preparation of this experiment needs very long calibration periods (several months). Moreover, this experiment will be conducted several times to exclude parasit effects.
Presentation of the mission
© CNES/GEKO/ Prodigima films, 2015
(MP4 format, ~141 Mb)
During the measurements phases, the satellite will be oriented in two main modes:
- an inertial mode (fixed relative to the Sun and stars),
- a spinning mode: in slow rotation around the orbit axis to increase the gravitational signal frequency (3 or 4 times).
During calibration phases, the satellite will be moved with programmed angular and linear trajectories enabling to accurately identify the parameters of the accelerometers.
The chosen orbit enables to ensure continuous 9-month periods to the satellite without passage in the Earth's shadow. All measurement and calibration phases will therefore take place during a two-year mission.