Atomic clocks are an outstanding tool for the experimental verification of general relativity and more generally for fundamental astronomy (VLBI, pulsar timing, navigation, etc). Recent years have seen a rapid improvement in the performance of such clocks, promising new improved tests of relativity, in particular onboard terrestrial and interplanetary space missions. We present the scientific motivations of such tests taking the ACES Salomon et al. and SAGAS Wolf et al. (2009) projects as particular examples.

The discovery of Dark Energy and the fact that only about 5% of the mass of the universe can be explained on the basis of the current laws of physics have led to a serious impasse. Based on past history, physics might indeed be on the verge of major discoveries; but the challenge is enormous. The way to tackle it is twofold. On one side, scientists try to perform large scale direct observations and measurements – mostly from space. On the other, they multiply their efforts to put to the most stringent tests ever the physical theories underlying the current view of the physical world, from the very small to the very large. On the extremely small scale very exciting results are expected from one of the most impressive experiments in the history of mankind: the Large Hadron Collider. On the very large scale, the universe is dominated by gravity and the present impasse undoubtedly calls for more powerful tests of General Relativity – the best theory of gravity to date. Experiments testing the Weak Equivalence Principle, on which General Relativity ultimately lies, have the strongest probing power of them all; a breakthrough in sensitivity is possible with the “Galileo Galilei” (GG) satellite experiment to fly in low Earth orbit.

Two pairs of solid test-masses have been considered to perform in space the test of the universality of free fall with an accuracy of at least 10−15. These cylindrical masses are precisely at the heart of the MICROSCOPE mission instrument comprising two differential electrostatic accelerometers. These masses shall exhibit material quality, shapes, positions and alignments in regard to stringent experimental requirements. Indeed the space experiment is based on the control of the two masses submitted to the same gravity acceleration along the same orbit at 810 km altitude with an accuracy of 10−11 m. Thus effects of Earth and satellite gravity gradients shall be contained as well as any other disturbances of the mass motions induced by their magnetic susceptibility or electrical dissymmetries, by outgassing of the materials or radiation emissivity. Furthermore, the electrostatic levitation of the two masses depends dramatically on the mass shapes and electrical properties in particular for the definition of the sensitive axes orientation. All these aspects will be presented from the mass characteristics to the space MICROSCOPE experiment performance.

In the last decade, a variety of high-precision experiments have searched for miniscule violations of Lorentz symmetry. These searches are largely motivated by the possibility of uncovering experimental signatures from a fundamental unified theory. Experimental results are reported in the framework called the Standard-Model Extension (SME), which describes general Lorentz violation for each particle species in terms of its coefficients for Lorentz violation. Recently, the role of gravitational experiments in probing the SME has been explored in the literature. In this talk, I will summarize theoretical and experimental aspects of these works. I will also discuss recent lunar laser ranging and atom interferometer experiments, which place stringent constraints on gravity coefficients for Lorentz violation.

Improved accuracy in measurement of the gravitational time delay of electromagnetic waves passing by the sun may be achieved with two drag-free spacecraft, one with a stable clock and laser transmitter and one with a high-stability transponder. We consider one spacecraft near the Earth-Sun L1 point with an advanced optical clock, and the transponder on a second satellite, which has a 2 year period orbit and eccentricity e = 0.37. Superior conjunctions will occur at aphelion 1, 3, and 5 years after launch of the second spacecraft. The measurements can be made using carrier phase comparisons on the laser beam that would be sent to the distant spacecraft and then transponded back. Recent development of clocks based on optical transitions in cooled and trapped ions or atoms indicate that a noise spectral amplitude of about 5 × 10−15/ at frequencies down to at least 1 microhertz can be achieved in space-borne clocks. An attractive candidate is a clock based on a single laser-cooled Yb+ trapped ion. Both spacecraft can be drag-free at a level of 1×10−13m/s2/ at frequencies down to at least 1 microhertz. The corresponding requirement for the LISA gravitational wave mission is 3 × 10−15m/s2/ at frequencies down to 10−4 Hz, and Gravitational Reference Sensors have been developed to meet this goal. They will be tested in the LISA Pathfinder mission, planned by ESA for flight in 2011. The requirements to extend the performance to longer times are mainly thermal. The achievable accuracy for determining the PPN parameter γ is about 1 × 10−8.

The French space mission MICROSCOPE aims at testing the weak Equivalence Principle (EP) with an accuracy of 10−15. The payload, which is developed and built by the French institute ONERA consists of two high-precision capacitive differential accelerometers. The detection of the test mass movement and their control is done via a complex electrode system. The German department ZARM is member of the MICROSCOPE performance team. In addition to drop tower tests, mission simulations and the preparation of the mission data evaluation are realized in close cooperation with the French partners CNES, ONERA and OCA. Therefore a comprehensive simulation of the real system including the science signal and all error sources is built for the development and testing of data reduction and data analysis algorithms to extract the EP violation signal. In this context the focus lays on the correct modeling of the environmental disturbances. Currently new effort to study the influence of the solar radiation and the Earth albedo to the MICROSCOPE mission scenario is underway.

MICROSCOPE is a ESA/CNES space mission for testing the validity of the weak equivalence principle. The mission's goal is to determine the Eötvös parameter η with an accuracy of 10−15. The French space agency CNES is responsible for designing the satellite which is developed and produced within the Myriade series. The satellite's payload T–SAGE (Twin Space Accelerometer for Gravitation Experimentation) consists of two high–precision capacitive differential accelerometers and is developed and built by the French institute ONERA.

As a member of the MICROSCOPE performance team, the German department ZARM performs free fall tests of the MICROSCOPE differential accelerometers at the Bremen drop tower. The project's concepts and current results of the free fall tests are shortly presented.

The exact modeling of external and internal perturbations acting on spacecraft becomes increasingly important as the scientific requirements become more demanding. Disturbance models included in orbit determination and propagation tools need to be improved to account for the needed accuracy. At ZARM (Center of Applied Space Technology and Microgravity) algorithms for the simulation and analysis of thermal perturbations have been developed. The applied methods are based on the inclusion of the actual spacecraft geometry by means of Finite Element (FE) models in the calculation of the disturbance forces. Thus the modeling accuracy is increased considerably and also housekeeping and sensor data can be included in the calculations. Preliminary results for a test case geometry of the Pioneer 10/11 mission are presented and discussed with respect to the Pioneer anomaly. It is shown that thermal effects cannot be neglected for the magnitude scale of the observed anomalous effect.