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  • Article
  • Open Access

The Ion Mass Imager on the Planet-B spacecraft

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Earth, Planets and Space201450:BF03352104

  • Received: 28 July 1997
  • Accepted: 5 January 1998
  • Published:


The Ion Mass Imager (IMI) is a light-weight ion mass composition instrument for the Japanese Planet-B mission to be launched to Mars in 1998. The objective of the Planet-B mission is to study the Martian environment with emphasis on the upper atmosphere interaction with the solar wind. IMI measures positive ions with energies between 10 eV/q and 35 keV/q and with a mass resolution high enough to resolve the most important ion species (H+, He++, He+, O++, O+, O2+). The instrument has a 360° field-of-view aperture and uses the spacecraft spin to cover almost the full unit sphere to obtain three-dimensional distribution functions every 4 s (half a spacecraft spin period). Particles are energy-filtered by a spherical electrostatic analyzer, and then mass-analysed by the magnetic separation method. The ions hit a microchannel plate assembly with a position sensitive anode divided into 32 mass channels. Together with 16 angular sectors, this system “images” the direction of motion and mass of ions. A pre-acceleration voltage of 0–4000 V is used to select the mass range, e.g., modes optimized for light ions (up to O+) and heavy ions (O+ to charged dust grains). A loss-less data compression algorithm is used in the in-flight processing software to optimize the amount of data that can be returned from Mars.


  • Solar Wind
  • Dusty Plasma
  • Spacecraft Spin
  • Target Data Rate
  • Data Compression Scheme