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ESA-GEOS 2 was the first spacecraft dedicated completely to scientific measurements in an equatorial geostationary orbit. The spacecraft served as a core or reference spacecraft for the International Magnetospheric Study (IMS) and carried out correlative measurements with extensive ground-based networks in Scandinavia. The payload consisted of instruments to measure (1) dc and ac electric and magnetic fields; (2) gradient of the magnetic field; (3) thermal and suprathermal plasma parallel and perpendicular to the magnetic field; (4) energy spectra, angular distribution, and composition of positive ions; and (5) angular distribution and energy spectra of energetic electrons and protons. In the NSSDC experiment descriptions which follow, ESA Exp. S-300 is described as five separate experiments: 78-071A-05, -06, -07, -10, and -11. The spacecraft was cylindrical with a height of 1.321 m. The total mass, excluding propellants, was 273.6 kg. There were four telescopic axial booms 2.5 m in length for the wire mesh spheres of an ac electric field experiment, two 20-m cable booms for magnetic and electric field sensors and for an excitation antenna for plasma resonances, and two locking radial booms 3 m in length for a variety of instruments. There were six hydrazine thrusters; two to tilt and precess the spacecraft, two to modify the orbit so the longitude of the apogee could be changed, and two for spin up and spin down. The spin rate was nominally 10 rpm. Data were telemetered in real time at 137.2 MHz (186 and 744 bps) and at 2299.5 MHz (11.91 or 95.25 kbs). Attitude measurements were obtained by a sun sensor, a dual infrared earth sensor, and accelerometers. Power was supplied by 7200 solar cells mounted on the spacecraft surface. To prevent spacecraft differential charging, 96% of the surface was electrically conductive. Because of the importance of the magnetic field measurements, the spacecraft residual field at the magnetometer was only 0.3 nT. Except for minor modifications to certain experiments, this spacecraft and its instruments were identical to ESA-GEOS 1 (77-029A). More detailed information can be found in ESA Bulletin, n. 9, May 1977. Because one solar panel developed a short circuit soon after launch, a number of the experiments were able to obtain useful data for only one-half of the spin period.

Alternate Names

  • GEOS 2
  • 10981

Facts in Brief

Launch Date: 1978-07-14
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Mass: 571.7 kg

Funding Agency

  • European Space Agency (International)


  • Space Physics

Additional Information

Questions or comments about this spacecraft can be directed to: Dr. H. Kent Hills.



NameRoleOriginal AffiliationE-mail
Mr. John Donald KraftGeneral ContactNASA Goddard Space Flight
Mr. Derek E. MullingerProject ManagerESA-European Space Research and Technology Centre 
Dr. Karl KnottProject ScientistESA-European Space Research and Technology Centre 
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