General Relativity and all metric theories of gravity rely on the principle of
equivalence between inertial and gravitational mass. Its fundamental character and far
reaching implications make it necessary for it to be tested as accurately as possible by
testing the Universality of Free Fall. If the test bodies orbit around the Earth
the signal is about 3 orders of magnitude bigger than at its surface, and this is what
makes space experiments to test the equivalence principle so attractive despite their
inevitable difficulties. We have presented here a nondrag-free version of the Galileo
Galilei (GG) mission proposal, arguing that it could detect any deviation from the Universality
of Free Fall - hence from the equivalence between inertial and gravitational mass - to
the level of 1 part in 
, four
orders of magnitude better than the most recent ground tests (Adelberger et al., 1990; Su et al., 1994). The main features of this concept
are to be nondrag-free and non cryogenic, to make the test bodies spin at a relatively
high frequency chosen by the experimentalist (e.g. 
) and to exploit the zero-g space environment in order to
naturally obtain self-centering of the test bodies and a very low level of vibrational
noise. The spacecraft is small, compact and essentially passive so as to minimize
disturbances on the test masses; no active control, neither of the orbit nor of the
attitude is needed. The signal from a violation of equivalence would be modulated at the
spin frequency of the test bodies (and the spacecraft) while the common rotation of the
entire apparatus makes many internal perturbing effects DC. The read out system is
capacitive. In the perturbation analysis we have tried to keep all directly competing
effects well below the target signal unless otherwise distinguishable. Thermal
perturbations can be sufficiently reduced by passive insulation thanks to fast spin and
vacuum. It is concluded that a violation of equivalence to the level of 1 part in can be detected with an integration time of
a few hours. A partial compensation of air drag would help reduce the intensity of
inertial forces on test bodies and - correspondingly - the required level of common mode
rejection. However, drag-free control is advantageous only if the thrusters are
proportional rather than impulsive and the propellant to be carried on board does not
itself give rise to perturbations. An interesting possibility would be to use FEEP (Field
Emission Electric Propulsion) thrusters, which need a negligible amount of Cesium and are
meant to be highly proportional. A drag-free GG mission with FEEP thrusters (Nobili et al., 1995; GALILEO GALILEI, 1996) can indeed aim at a 1
order of magnitude better sensitivity, i.e. at an EP test to 1 part in 
. As for the possibility to run the
experiment at low temperature, the advantages must be weighed against the disturbances due
to the cryogenic system itself. Taking also into account that a room temperature
capacitive read out is adequate to the task, we have preferred to consider a non cryogenic
experiment. The choice for a space mission completely devoted to one single scientific
objective (indeed the entire spacecraft, its orbit and attitude control are driven by this
objective) was done on purpose, to reduce the complexity, cost and realization time were
any space agency interested in the experiment. To this end it is worth stressing that the
major components of the space experiment can be tested in the ground laboratory, and even
though the entire apparatus is designed for zero g a modified, less accurate (due
to the weaker signal), 1-g version of it is possible and is underway.
Copyright © 1998 Elsevier Science B.V., Amsterdam. All Rights Reserved.
(Anna Nobili- nobili@dm.unipi.it)
