`Galileo Galilei' (GG) is a proposal for a small, low-orbit satellite devoted to testing the equivalence principle (EP) of Galileo, Newton and Einstein. The GG report on the phase A study recently carried out with funding from ASI (Agenzia Spaziale Italiana) concluded that GG can test the equivalence principle to 1 part in 1017 at room temperature. The main novelty is to modulate the expected differential signal of an EP violation at the spin rate of the spacecraft (2 Hz). Compared with other experiments, the modulation frequency is increased by more than a factor of 104, thus reducing 1/f (low-frequency) electronic and mechanical noise. The challenge for an EP test in space is to improve over the sensitivity of ground-based experiments (about 1 part in 1012) by many orders of magnitude, so as to deeply probe a so far totally unexplored field; doing that with more than one pair of bodies is an unnecessary complication. For this reason GG is now proposed with a single pair of test masses. At present the best and most reliable laboratory-controlled tests of the equivalence principle have been achieved by the `Eöt-Wash' group with small test cylinders arranged on a torsion balance placed on a turntable which provides the modulation of the signal (a 1-2 h rotation period). The torsion balance is not a suitable instrument in space. We have designed and built the GGG (`GG on the Ground') prototype. It is made of coaxial test cylinders weakly coupled (via mechanical suspensions) and quickly rotating (6 Hz achieved so far); in addition, it is well suited to be flown in space - where the driving signal is about three orders of magnitude stronger and the absence of weight is very helpful - inside the coaxial, co-rotating GG cylindrical spacecraft. The GGG apparatus is now operational. Preliminary measurement data indicate that weakly coupled, fast-spinning macroscopic rotors can be a suitable instrument to detect small differential effects. Rotation (up to 6 Hz so far) is stabilized by a small passive oil damper. A finer active damper, using small capacitance sensors and actuators as in the design of the space experiment, is in preparation. The current sensitivity of the GGG system is of 10-9 m s-2/√Hz at about 300 s, which can be improved because horizontal seismic noise is rejected very well; perturbing effects of terrain tilts (due to microseismicity and tides) will be reduced by adding a passive cardanic suspension. As for the capacitance read-out, the current sensitivity (5 pm displacements in 1 s integration time at room temperature) is adequate to make GGG competitive with the torsion balance. Because of the stronger signal and weaker coupling of the test rotors in space, this sensitivity is also adequate for GG to reach its target accuracy (10-17). Information, references, research papers and photographs of the apparatus are available on the Web (http://tycho.dm.unipi.it/nobili).

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An experiment to test the equivalence of inertial to gravitational (passive) mass in space offers two main advantages: a signal about a factor of a thousand larger than on Earth and the possibility of exploiting the absence of weight. `Galileo Galilei' (GG) is a small satellite mission currently under study in Italy with the financial support of ASI (Agenzia Spaziale Italiana). The mission concerns a small, low Earth satellite ( total mass, altitude) with two objectives. One is scientific, in the field of fundamental physics, and the other technological within the framework of spacecraft propulsion and drag compensation. The scientific goal is to test the equivalence principle to one part in , four orders of magnitude better than the best ground results. The technological goal is a full, comprehensive test of FEEP (field emission electric propulsion) thrusters for accurate drag compensation, a technology developed in Europe by the ESA (European Space Agency) which is likely to become an essential component of all space experiments which require measurement of small forces. The GG experiment is based on novel concepts and does not require low temperatures.

"Galileo Galilei" (GG) is a small satellite designed to fly in low Earth orbit with the goal of testing the Equivalence Principle-which is at the basis of the General Theory of Relativity-to 1 part in 10 17 . If successful, it would improve current laboratory results by 4 orders of magnitude. A confirmation would strongly constrain theories; proof of violation is believed to lead to a scientific revolution. The experiment design allows it to be carried out at ambient temperature inside a small 1-axis stabilized satellite (250 kg total mass). GG is under investigation at Phase A-2 level by ASI (Agenzia Spaziale Italiana) at Thales Alenia Space in Torino, while a laboratory prototype (known as GGG) is operational at INFN laboratories in Pisa, supported by INFN (Istituto Nazionale di fisica Nucleare) and ASI. A final study report will be published in 2009.

Experiments to test the equivalence principle in space by testing the universality of free fall in the gravitational field of the Earth have to take into account the radiometer effect, caused by temperature differences in the residual gas inside the spacecraft as it is exposed to the infrared radiation from Earth itself. We report the results of our evaluation of this effect for the three proposed experiments currently under investigation by space agencies: SCOPE, STEP, and GG. It is found that in SCOPE, which operates at room temperature, and even in STEP, where the effect is greatly reduced by means of very low temperatures, the radiometer effect is a serious limitation to the achievable sensitivity. Instead, by axially spinning the whole spacecraft and with an appropriate choice of the sensitivity axes-as proposed in GG-the radiometer effect averages out and becomes unimportant even at room temperature.