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EUVS - Extreme Ultraviolet Spectrometer

MISSION OBJECTIVES

• Study the composition and structure of the upper Jovian atmosphere.

• Determine the loss rates of volatile gases from the Galilean satellites.

• Examine the physical processes occurring in the Io plasma torus.

• Do follow up studies to the Voyager UVS discoveries.

SUMMARY

Volcanic eruptions on Io are believed to be the source of the Io torus. Temperatures of the sulfur and oxygen ions in this plasma torus can be more than 10 times the temperatures at the surface of the Sun. These ultraviolet observations will help provide a picture of Io's evolution and its relationship with Jupiter's magnetic field.

The Cassegrain telescope of the UVS has a 250-millimeter aperture and collects light from the observation target. Both the UVS and EUV instruments use a ruled grating to disperse this light for spectral analysis. This light then passes through an exit slit into photomultiplier tubes that produce pulses or "sprays" of electrons. These electron pulses are counted, and these count numbers are the data that are sent to Earth.

The UVS is mounted on the scan platform and can be pointed to an object in inertial space. The EUV is mounted on the spun section of the spacecraft. As Galileo spins, the EUV observes a narrow ribbon of space perpendicular to the spin axis.

The two instruments combined weigh about 9.7 kilograms and use 5.9 watts of power.

EUVS DESCRIPTION

• The EUV is an objective grating spectrometer with a mechanical collimator.

• There is a concave dispersion grating that focuses extreme ultraviolet via a single reflection onto a detector consisting of a photocathode, microchannel plate electron multiplier, and an anode array.

• The EUV is a modified Voyager spare Ultraviolet Spectrometer with an electrical interface to adapt it to the Galileo command and data bus.

DESIGN DETAILS

• The EUV has a 20 cm focal length and an aperture of 4.0 x 6.0 cm.

• The EUV covers 54 to 128 nm in 128 contiguous intervals of 0.59 nm.

• The EUV field of view is 0.17 Deg. FWHM in the dispersion direction and about 0.87 Deg. in the cross-dispersion direction.

• The wavelength resolution of the EUV is 3.5 nm for extended sources and 1.5 nm for point sources.

INSTRUMENT PARAMETERS

• Instrument Mass: 12.23 Kg
• Power Consumption:
  • Instrument - 6.65 W
• Microprocessor: 1802 type
• ROM/RAM: 0 b/1.3 Kb
• Size :
  • EUV Channel - 44.3 x 16.9 x 14.9 cm
  • Logic Module - 27.9 x 17.8 x 6.4 cm
• Thermal Range:
  • Operating (GLL3-210)
    • EUV Logic Module: 5 to 50 Deg. C
    • EUV Sensor: -20 to 30 Deg. C
  • Non-operating (GLL 3-210)
    • EUV Logic Module: 5 to 50 Deg. C
    • EUV Sensor: -20 to 35 Deg. C
• Instrument Modes:
  • Encounter - Off
  • Cruise - POR
  • Cruise - On