Near-Infrared Mapping Spectrometer experiment on Galileo

Abstract

The Galileo Near-Infrared Mapping Spectrometer (NIMS) is a combination of imaging and spectroscopic methods. Simultaneous use of these two methods yields a powerful combination, far greater than when used individually. For geological studies of surfaces, it can be used to map morphological features, while simultaneously determining their composition and mineralogy, providing data to investigate the evolution of surface geology. For atmospheres, many of the most interesting phenomena are transitory, with unpredictable locations. With concurrent mapping and spectroscopy, such features can be found and spectroscopically analyzed. In addition, the spatial/compositional aspects of known features can be fully investigated. The NIMS experiment will investigate Jupiter and the Galilean satellites during the two year orbital operation period, commencing December 1995. Prior to that, Galileo will have flown past Venus, the Earth/Moon system (twice), and two asteroids; obtaining scientific measurements for all of these objects.

The NIMS instrument covers the spectral range 0.7 to 5.2 μ, which includes the reflected-sunlight and thermal-radiation regimes for many solar system objects. This spectral region contains diagnostic spectral signatures, arising from molecular vibrational transitions (and some electronic transitions) of both solid and gaseous species. Imaging is performed by a combination of one-dimensional instrument spatial scanning, coupled with orthogonal spacecraft scan-platform motion, yielding two-dimensional images for each of the NIMS wavelengths.

The instrument consists of a telescope, with one dimension of spatial scanning, and a diffraction grating spectrometer. Both are passively cooled to low temperatures in order to reduce background photon shot noise. The detectors consist of an array of indium antimonide and silicon photovoltaic diodes, contained within a focal-plane-assembly, and cooled to cryogenic temperatures using a radiative cooler. Spectral and spatial scanning is accomplished by electro-mechanical devices, with motions executed using commandable instrument modes.

Particular attention was given to the thermal and contamination aspects of the Galileo spacecraft, both of which could profoundly affect NIMS performance. Various protective measures have been implemented, including shades to protect against thruster firings as well as thermal radiation from the spacecraft.