The scientific goal of GG is to improve the current best ground tests of the Equivalence Principle (EP) by 5 orders of magnitude, searching for a new composition dependent effect to 1 part in 10¹⁷. Such an experiment would probe –flying a small satellite in low Earth orbit– a totally unexplored field of physics which is inaccessible to ground laboratories and where new findings are expected no matter whether the Equivalence Principle is confirmed or violated. GG has been studied to Phase A level by the Italian Space Agency (ASI) in 1998; the result of the study was consistent with the target proposed here. Proposals for space missions to test the Equivalence Principle go back almost to the very beginning of the space age; all major space agencies around the world have seriously considered –and still consider– EP missions for flight. The Stanford proposed STEP project has been studied twice by ESA at Phase A level; both times the studies reported a target in EP testing of 1 part in 10¹⁷.

As a small, 1-axis spin stabilized spacecraft in low Earth orbit, GG poses no problems and its subsystems can be taken from available busses. The only novelty is drag-free control with FEEP mini-thrusters (an ESA technology largely developed in Italy), which is of considerable interest for the LISA mission. While pursuing a major scientific goal GG would also fully test the FEEP for accurate drag-free control at low frequency and at room temperature. ESA itself has officially stated that the STEP drag-free control was to be seen as a preliminary test for LISA. If this is the case for STEP, which needs a different technology (He thrusters and not ion thrusters), and in cryogenic conditions, it must be the more so for GG, since it would test the technology of interest and not just the software. FEEP could also be tested with the proposed ELITE technology mission. Yet, why only a technology mission if a major scientific result can be achieved in combination with a technology test? It is a trade off game. However, unless a technology is of immediate interest to the ordinary people (which is not the case here), it is always the science achieved by a space mission to capture the imagination of the media, the public and –ultimately– of the taxpayers which provide the resources for all space activities.

It is not unusual that challenging scientific experiments need, at some point, to be rethought completely anew. This is the case with GG, which is based on new concepts. These concepts have been debated in the open literature and within space institutions for a few years by now, and proved to be sound. More importantly, the GG prototype experiment in the laboratory demonstrates that these concepts are sound. The challenge in this field is to exploit the stronger signal in space and the absence of weight to fly an experiment able to improve, by many orders of magnitude, the current sensitivity. Very accurate EP tests require (on Earth and in space) that spurious relative motions of the test bodies be greatly reduced, leaving them essentially motionless. Achieving that in space, with more than one pair of test bodies, is an unnecessary complication if the issue is to prove high sensitivity. For this reason GG is proposed with a single pair of test masses, whose composition can be carefully selected.

ESA has identified an EP mission as a high priority since 1996, and has allocated the sum of 21.7 Meuro as a contribution to a NASA-led STEP mission, should NASA decide to fly it. ESA is therefore totally dependent on NASA with respect to the implementation of this priority. As the future of STEP is uncertain (Nature 402, 7, 1999), we argue that GG is a viable back up for a mission which has been identified as being of prime importance to ESA. The argument is twofold: (i) the scientific and technological goals of GG; (ii) the cost of GG to ESA, which can be limited to the cost of the spacecraft (19 Meuro estimated by Alenia; see Letter I), thus placing GG in the category of particularly cheap missions (Sec. 2.8 of Call). Reference is to the annexed Letters IV and VI, by the president of ASI, in which he seeks collaboration between ASI and the Indian space authority for the launch of GG, expresses the willingness of ASI to provide the GG payload, offers the use of the Malindi tracking station for ground operations, envisages the possibility for GG to be injected in its orbit with the qualification launch of Vega, should Vega evolve positively. Reference is also to Letter VII, by the Principal of Pisa University, to confirm that GG science operations and archiving would be carried out entirely at the University of Pisa, at no cost to ESA.