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During the 1970s, ESRO and ESA elaborated projects for quite complex missions to determine two ill-defined parameters required by General Relativity and other theories of gravitation, and to measure the oblateness of the Sun. The first mission would be a reflight of the Helios Sun probe carrying a sensitive accelerometer, laser transponder and other high sensitivity apparatus, while the second was a small perihelion solar probe that also spurred similar studies in the United States. Important contributions to both of these projects were made by the famed Italian space scientist Giuseppe “Bepi” Colombo.
Et à propos du CACTUS:
ONERA had been studying for some time a high accuracy accelerometer for use in space for the French space agency CNES (Centre National d'Etudes Spatiales, national space studies center), which it called the CACTUS experiment (Capteur Accélérométrique Capacitif Triaxiale Ultra-Sensible, ultra-sensitive capacitance three-axis accelerometric sensor), able to measure accelerations of the order of 10-8 m/s2. A prototype of CACTUS was successfully flown on a Vesta sounding rocket in November 1969 and was being adapted for flight on a satellite. CACTUS consisted of a small proof mass of 550 g enclosed in a very tightly-fitting cavity (leaving a gap just 0.06 mm wide around the 4 cm platinum sphere). Capacitors would sense the displacement of the proof mass with respect to the walls of the cavity and, in a drag-free system, would command thrusters to fire to counteract it, bringing the sphere back at the center of the cavity.
The idea behind a drag-free system was that as the proof mass would respond to gravitational forces, the spacecraft shell around it would respond to surface forces (radiation pressure etc.), and to a variety of other perturbations. Therefore, to ensure that the spacecraft would be following a truly gravitational-only trajectory, its displacement with respect to the test mass would have to be measured and corrected at all times. When perturbations would cause the shell to move relative to the proof mass, an error signal would be generated that activated thrusters and moved the shell to follow the proof mass. Non gravitational forces acting on the spacecraft would include solar radiation pressure (by far the largest), the tiny but measurable pressure of the solar wind, magnetic and electric fields, radiative emissions of the spacecraft etc. A variety of other perturbations of similar entity to the effect being measured could still arise within the spacecraft itself, including gravitational, magnetic, electrostatic and electric effects, radiation pressure on the sphere due to the different temperature of the walls of the cavity, and non perfect vacuum in it. Note in particular that the proof mass would still get electric charges during the flight by interacting with cosmic rays and solar and galactic high energy particles, and ways to de-electrify it would have to be provided.
For SOREL, ONERA at first studied an adaptation of the CACTUS but soon recognized that a CACTUS-like accelerometer alone would reach at best 10-10 m/s2, which was not yet acceptable, 10-12 m/s2 being the objective. A new system was then designed, completely abandoning the CACTUS architecture. It was considered that the largest perturbations on the test mass would be caused by the unbalance of masses in the probe, and therefore a much larger empty cavity, up to 1 m in diameter was suggested, to remove as much as possible perturbing masses from the proof mass. Just to fix ideas, consider that a 15 gram mass located 10 cm away from the proof mass would exert a gravitational attraction on it of the order of 10-10 m/s2. The use of a large cavity spherical accelerometer would have its merits but would also have its shortcomings. In order to measure the gravitational motion of the spacecraft, it would have to be placed as close as possible to the center of mass of the probe, taking into account the inhomogeneities in the spacecraft body itself. Moreover, the shape and mass distribution of the probe would not be constant, due to the heating it would receive from the Sun and the different thermal expansion coefficients of its various components. To even out and average these effects, the spacecraft would have to be spin stabilized.
In the end, ONERA adopted the large cavity accelerometer, using an optical detector, called the Optical Probe Centering System (OPCS) to replace the CACTUS capacity pick-up, which would be impractical for use over such a large gap. The OPCS was envisaged to consist of six light sources and as many sensors, plus all the associated electronics. The six light beams would bound the 9.6 cm gold-platinum alloy proof mass on its position at the center of the cavity and would pick up any of its displacements.
ESRO was then informed that the French CNES was planning a satellite test flight of CACTUS in 1973, using the Diamant-B rocket and that a CACTUS-controlled drag-free geodesy satellite could follow. The Mission Definition Group encouraged ESRO to propose a reduced-size optical system for flight on the French satellite. In fact, ESRO and CNES soon reached an agreement to study the adaptation of the SOREL accelerometer to the D5B satellite. This satellite had the scientific mission of measuring the gravitational potential of the Earth and its variations (with tides, for example). For this, the small spacecraft would need to measure accelerations acting on it with an accuracy of the order of 10-10 m/s2 (i.e. some 100 times that of SOREL). As a result, ONERA was contracted by CNES to study an adaptation of the optical accelerometer to the satellite, with reduced performance and size to enable it to fly on the Diamant-B.
The proposal was not eventually implemented and D5B (renamed Castor) flew with the planned CACTUS accelerometer. Its first launch, in 1973, failed because the launcher fairing did not jettison, but it was successfully launched on 15 May 1975. It was an extremely successful mission, and the model of high atmosphere densities obtained by the CACTUS is still in use today.