Instrument Microprocessor and Electronics
The UVS uses an RCA 1802 CMOS microprocessor for command parsing, spacecraft time recognition and synchronization, and instrument control. In addition, the UVS design incorporates additional electronics called the Cold Start Logic (CSL) that places it into a cyclical F-G scan mode until microprocessor control is initiated by spacecraft command. The instrument receives commands and spacecraft timing information via the Bus Adaptor and associated Direct Memory Access (DMA) logic. The Bus Adaptor serves as the bi-directional interface between the Galileo spacecraft and the UVS> It's circuitry serves to isolate the UVS electrically from the spacecraft and to allow for 8-bit information flow to and from the UVS.
The scientific objectives of the Galileo Ultraviolet Spectrometer (UVS) investigation include the following:
THE INTERPLANETARY MEDIUM: By carrying out a systematic program of H and He measurements over the course of the mission, UVS will improve our knowledge of the interstellar wind (ISW) and of the processes that affect its passage through the solar system..
VENUS: The geometry of the Galileo flyby permitted pole-to-pole and dawn-to-dusk measurements by UVS of the abundance of SO2 in the cloud-top region, and of the abundances of H, O, C, and CO in the thermosphere.
EARTH AND MOON: The post-encounter passage near the subsolar point at long range allows the near-simultaneous measurement of pole-to-pole and dawn-to-dusk variations in the UV airglow and in reflected sunlight, allowing investigation of the global O/N2 ratio and the distribution of ozone. It is also of interest to establish the Earth's UV albedo in the Schumann-Runge band region near and below 200 nm. A search for a tenuous lunar atmosphere using the resonance emissions of H, O, and OH will address the question of the rate of bombardment of the Moon by small bodies, and of the fate of solar wind protons that strike the surface. The flybys also allow the Earth-Moon system to be mapped, and these data contain an image from each encounter of the hydrogen geocorona from a unique sunward vantage point.
ASTEROIDS: The UVS measured the albedo of the asteroids Gaspra and Ida during flyby. Spatial resolution on the bodies surfaces will not be possible, but their scattering properties as a function of phase angle will be measured, and the presence of absorption features at wavelengths longer than 200 nm will be determined. At these and shorter wavelengths the asteroid's albedo may be directly compared to that of the Moon measured during the two Earth encounters.
JOVIAN CLOUDS AND HAZES: The Galileo orbiting mission offers the opportunity to observe Jupiter's clouds and hazes repeatedly over a wide range of phase angle and wavelength. Since its ability to examine small scattering angles is restricted by solar protection considerations, the contribution of UVS will be to determine the imaginary parts of the aerosols' refractive indices by obtaining the single-scattering albedo from photometric measurements. It will sample the lower end of the aerosol size distribution due to its sensitivity down to 200 nm. The distribution of aerosols with altitude will be measured in the stratosphere by measuring limb radiance profiles and in the troposphere by making nadir-to-limb scans. Temporal variability in the properties of clouds and hazes will be investigated at timescales ranging from days to the duration of the mission.
COMPOSITION AND CHEMISTRY OF THE JOVIAN STRATOSPHERE: UVS will use reflectance spectroscopy during disc and limb scans to compile and inventory numerous hydrocarbons (such as methane, acetylene, and ethane) as a function of location and altitude. UVS limb scans will yield stratospheric temperatures through the scale height of the signal from Rayleigh-scattered sunlight.
JOVIAN THERMOSPHERE: The thermosphere of Jupiter is characterized by unexpectedly high temperatures (of order 1100 K in the upper thermosphere) and by unexpectedly bright UV emissions from molecular hydrogen. Lyman-alpha emission from H shows an equatorial bulge that sometimes extends across the morning terminator. None of these phenomena have been totally explained. A careful study of spectral, horizontal, vertical, and diurnal and other time variations is an important objective for the Galileo UVS and EUV experiments, with the goal of gaining insight into these phenomena.
JOVIAN AURORA: Galileo's mostly equatorial orbits mean that the aurora will be observed near the northern or southern limbs, allowing excellent longitudinal resolution at the cost of lesser latitude resolution. The spectral effects of atmospheric absorption will be enhanced. Jupiter's rapid rotation will facilitate the determination of longitudinal dependencies of the emissions on each orbit. The possibility of correlations between the aurora and conditions in the Io torus will be explored. Galileo will be able to compare day and night-side auroral emissions.
JOVIAN SATELLITES: While close-range observations of Io and Europa by the UVS will be prevented by the radiation environment, the outer two Galilean satellites will be visited a few times in close encounters: Ganymede in orbits G1, G2, G7; Callisto in orbits C3, C10; and Europa in orbits E4, E6, E11. The Galileo UVS will measure and map the UV albedos of areas of these moons. The measurements will be compared with those of the Moon and of the asteroids Gaspra and Ida. The rich variety of surface terrain and materials will greatly expand our knowledge of the UV scattering properties of satellite surfaces. The UVS will also look for evidence of tenuous and possibly sporadic atmospheres that might be produced by sublimation, sputtering by co-rotating plasma, or even eruptive events.
IO TORUS: In conjunction with the EUV instrument, the UVS will measure the abundance and distribution of the neutral and ionized species existing in the Io torus. The surface and atmospheric composition of Io and the nature and efficiency of escape and ionization processes, as well as the complex interaction of the ionized material with the magnetic and gravitational fields of Jupiter and with the rest of the magnetosphere, will be investigated. The data are expected to reveal many dynamical aspects of the torus in addition to its composition.
JOVIAN MAGNETOSPHERE: There are many processes in the exosphere of Jupiter, on the constantly irradiated satellites, in the Io torus, and in the magnetosphere in general, that might provide sources of neutral atoms in the magnetosphere, including H and even OH in addition to oxygen and sulfur. The UVS will search for such material at times when the radiation noise in the instrument is at a minimum.
JOINT INVESTIGATIONS: Collaborative studies are planned with the fields and particles investigators, with the goal to improve our understanding of the transportation of sulfur and oxygen ions from the Io plasma torus to their ultimate precipitation in the Jupiter auroral region. Joint investigations with the Photopolarimeter Radiometer (PPR) experiment will help define the particulate properties of the Jupiter atmosphere, providing constraints on cloud particle size, shape, and composition. Complementary UVS and PPR observations will also provide information about the spatial extent and altitude distributions of these clouds. Properties of the satellite surfaces will be measured in cooperation with the Near Infrared Mapping Spectrometer (NIMS), the Solid State Imaging (SSI) instrument, and the PPR. Scattering properties as well as ultraviolet absorbers, e.g., sulfur dioxide, will be measured to add leverage to our understanding of the Galilean satellites.
Shoemaker-Levy 9 Galileo UVS obtained a unique 292nm data set during the S-L 9 fragment G impact showing a brief "flash" characterized by a brightness temperature near 8000K.
from Space Science Reviews, Galileo Ultraviolet Spectrometer Experiment, Vol 60: 503-530, 1992.