Space provides unique opportunities to test gravitation. By using an interplanetary spacecraft as a test mass, it is possible to test General Relativity at the Solar System distance scale. This requires to compute accurately the trajectory of the spacecraft, a process which relies on radio tracking and is limited by the uncertainty on the spacecraft non-gravitational acceleration. The Gravity Advanced Package (GAP) is designed to measure the non-gravitational acceleration without bias. It is composed of an electrostatic accelerometer supplemented by a rotating stage. This article presents the instrument and its performances, and describes the method to make unbiased measurements. Finally, it addresses briefly the improvement brought by the instrument as far as orbit reconstruction is concerned.
With the ever-increasing precision of measurements, space has become a privileged place to test the two fundamental theories which emerged during the 20 th century: General Relativity and Quantum Theory. In addition to providing a very clean environment, it opened new ways of testing these theories: as an example of interest for this article, precise navigation of interplanetary spacecrafts allows probing the scale dependence of gravitation at the Solar System distance scale (Jaekel & Reynaud 2006).
Even if most experimental tests support General Relativity (Will 2006), there are still open windows for deviations. Indeed, the fact that these two fundamental theories are difficult to reconcile suggest that General Relativity may not be the final description of gravitation. The reason is that gravitation is the only interaction not having a quantum description. The validity of the Newton potential has been extensively tested for distances between the millimeter and the characteristic size of planetary orbits (Fischbach & Talmadge 1999). But there remain open windows outside this distance range for violations of the inverse square law (Adelberger et al. 2003, Fig. 4): below the millimeter or for distances of the order or larger than the Solar System characteristic size.
Long range tests are performed using the motion of planets and interplanetary probes. Monitoring of the Moon and Mars delivers high precision tests of the validity of General Relativity at these distances (e.g. Kolosnitsyn & Melnikov 2004;Williams et al. 1996). However, the navigation data of the Pioneer probes show a discrepancy with respect to the predictions of General Relativity (Anderson et al. 1998(Anderson et al. , 2002a;;Lévy et al. 2009). This discrepancy can be described as an anomalous acceleration directed toward the Sun with a roughly constant amplitude of approximately 8×10 -10 m.s -2 . The origin of this anomaly is yet unexplained despite a huge effort of the scientific community (Turyshev & Toth 2010, and references therein): it may be an experimental artifact as well as a hint of considerable importance for fundamental physics (Brownstein & Moffat 2006;Jaekel & Reynaud 2005). At larger scales, the rotation curves of galaxies and the relation between redshift and luminosities of supernovae are accounted for by introducing respectively “dark matter” and “dark energy”, which represent 25 % and 70 % of the energy content of the Universe (Frieman et al. 2008). Since these dark components have been introduced on the basis of gravitational observations solely, the hypothesis that General Relativity is not a correct description of gravitation at these scales needs to be considered (Carroll et al. 2004).
It is therefore essential to test gravitation at all distance scales. To this extend, several mission concepts have been proposed to improve the experiment made by the Pioneer probes (Anderson et al. 2002b;Bertolami & Paramos 2007;Johann et al. 2008;Christophe et al. 2009Christophe et al. , 2011;;Wolf et al. 2009). In many proposals, the addition of an accelerometer being able to measure without bias the non-gravitational acceleration of the spacecraft is central. ESA included this idea in the roadmap for fundamental physics in space (ESA 2010) and recommended the development of accelerometer compatible with spacecraft tracking at the 10 pm.s -2 level. This article presents such an instrument, called the Gravity Advanced Package (Lenoir et al. 2011b). First, a description of the instrument and its performances is given. Then, the method used to make absolute measurement is described (Lenoir et al. 2011a). Finally, the expected improvements of the orbit reconstruction process using the instrument are briefly discussed.
The Gravity Advanced Package is an important technological upgrade for future fundamental physics missions in space. It is composed of two subsystems: MicroSTAR is a three-axis electrostatic accelerometer (Josselin et al. 1999) based on Onera’s experience (Touboul et al. 1999;Touboul & Rodrigues 2001), and the Bias Rejection System is a rotating stage with piezo-electric actuator used to rotate MicroSTAR around its x axis. The accelerometer aims at measuring the non-gravitational acceleration of the spacecraft but other quantities are also measured. In fact, MicroSTAR measures the components of the vector a on its three orthogonal measurement axis called x, y and z :
where m S and m A are the masses of the satellite and the proof mass respectively, F NG ext→S is the non-gravitational force acting on the spacecraft, Ω is the rotation vector of the instrument with respect to a Galilean reference frame, l is the vector between the center of mass of the satellite and the instrument (lever arm), F G S→A is the gravity of the spacecraft, and the last term in parenthesis is the gravity gradient expressed in term of acceleration, F G ext→S and F G ext→A being the gravitational forces acting on the satellite and the proof mass
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