GEOS 3: Report on the Phase-A study

A European mission into the distant geomagnetic tail of the Earth is outlined. The mission can be realized comparatively quickly and inexpensively because it is based on existing hardware and designs from the GEOS program. Three major exceptions are the propulsion system, which is taken from the SYMPHONIE program, part of the attitude-measuring subsystem, and the telecommunications subsystem. Although the experiment sensors will have to be modified or even completely redesigned to operate in the far tail, it will be possible to maintain the existing spacecraft interfaces for power, telemetry, telecommand, booms, etc. The GEOS spacecraft design has sufficient flexibility to allow an open announcement of opportunity. Like its prodecessors, GEOS-3 will be placed initially into a geostationary transfer orbit; its mass and dimensions being compatible with an Ariane double launch. The SYMPHONIE motor will then be used to inject the satellite into its final deep-tail orbit. The apogee of this orbit, which can be as much as 1.5 million km from Earth, will be kept in the tail by means of regular lunar flyby or by placing the spacecraft into an orbit about the antisunward Lagrangian point L2.