The GALILEO Mission

Image 1: Galileo’s arrival on Jupiter (drawing)

EPI - Energetic Particles Investigation - on the Instrument Capsule PROBE of the GALILEO Mission

Goals of the EPI-Experiments

  • First-time in-situ measurements of the energy and direction distribution of electrons, protons, helium and heavier nuclei on the lower edge of Jupiter’s radiation belt between five Jupiter radii and the beginnings of the atmosphere.
  • Comparison with the synchrotron radiation which is sent out from high energy electrons from this region of space and regularly monitored from DSN.
  • Correlation of radio intensities in Jupiter’s atmosphere, like those that are measured from the Lightning and Radio Detector (LRD) on board the same capsule and those that influence the dispersion of high energy particles in the radiation belt.

Technical Data of the EPI Instruments

  • Mass: 1 kg
  • Power Input: 1 W
  • Maximum Counting Rate: 300000 1/s

A complete description of the instruments can be found in Space Science Reviews 60/1-4, S. 79-90, 1992.


  • 1978: Planning begin
  • 1983: Completion of the instrument space package
  • 18. 10. 1989: Launch
  • 12. 07. 1995: Separation of the instrument capsule PROBE from Orbiter
  • 07. 12. 1995: capsule entrance into Jupiter’s atmosphere

Results of EPI Experiments on Galileo

The intended measurements of the energy and direction dispersion of charged particles were successfully carried out during the decent of the GALILEO probe to Jupiter’s atmosphere. They began at a distance of five Jupiter radii (the radius of Jupiter is 71714 km) close to the orbit of the moon Io and ended with the entrance into the atmosphere at a height of 1.25 Jupiter radii. The region below 1.8 Jupiter radii had never before been reached from a space probe. The decent ended near the equator at 4.94 degrees west and 6.5 degrees north.

Image 2: EPI measurements

As the picture shows, Kiel’s instruments were activated for two minutes at five, four and three Jupiter radii. It remained constantly in use at two Jupiter radii until the entrance into the atmosphere.

The initial analysis of the data is as follows. The instrument discovered a new zone of the inner radiation belt under 2.5 Jupiter radii. The particle density reduces closer to Jupiter because of absorption into the material of the dust ring. A sharp lower border of the radiation belt is found at 1.25 Jupiter radii. This is shown in the picture below. Curve E1-P1 depicts the high energy electrons, P1 protons, HE helium nuclei and HV heavy nuclei

Image 3: Measured particle populations

The project was supported by the Federal Ministry of Education and Research (Bundesministerium für Forschung und Technologie, BMFT), the German Agency for Space Affairs Inc. (Deutsche Agentur für Weltraumangelegenheiten GmbH, DARA) and the German Aerospace Center (Deutsche Zentrum für Luft- und Raumfahrtforschung, DLR