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STELLA - The centimetre over centuries

The knowledge of the physics of the Earth has improved greatly since the use of dedicated space laser targets. In 1975, the Centre National d'Etudes Spatiales (CNES) launched the first passive laser satellite, Starlette, with a Diamant rocket on a slightly eccentric orbit at 800 km altitude in a 50 degree inclined plane with respect to the equator. Many studies undertaken with the measurements on Starlette have helped considerably in the knowledge of global and large scale Earth gravity. But it is still possible to progress thanks to the increased accuracy of the laser ranging tracking system (now at the centimetre level), by better spreading out the laser tracking network or by providing satellite constellations with different inclinations. This is why the Groupe de Recherches de Geodesie Spatiale (GRGS) proposed the Stella satellite project which was accepted then developed entirely by CNES.

Like Starlette, Stella is a very dense and small sphere carrying retro reflectors on its surface. The Stella satellite is to be launched in 1993 [was launched-Ed.] from the Guyana Space Center, together with the French Earth Observation Satellite Spot 3 on top of an Ariane 40 rocket. The ejection system from the third stage of Ariane will release Stella at an altitude near 800 km in a quasi-circular orbit at an inclination of 98 degrees.

Its small size (24 cm diameter) compared to its mass (48 kg) gives it a much larger sensitivity to the gravitational attraction than to the surface forces due either to the residual atmosphere at the satellite altitude or to radiation pressure. Stella is therefore mainly dedicated to gravity research. Furthermore this passive satellite fulfils the requirements for studying slow deformations of the Earth and changes of its gravity field thanks to its long lifetime (estimated at 2,000 years) and the capability of tracking it for centuries.

Merging the observations acquired with other similar targets (Starlette, Lageos, ...) orbiting at different inclinations or altitudes will help better resolve model parameters and will allow the achievement of extended scientific objectives.

Reference