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on the previous Galileo and Ulysses missions. With these systems, heat from the natural decay of plutonium-238 is used to generate electricity to operate Cassini’s systems. The Cassini


spacecraft is equipped with


18 instruments, 12 on the orbiter and another six on the Huygens probe, which is designed to separate from


the main spacecraft and


parachute through the atmosphere of Titan, Saturn’s largest moon. The 12 instruments on the orbiter are currently conducting in-depth studies of Saturn, its moons, rings and magnetic environment (previous page, bottom). Key to Cassini’s science mission is the Ultraviolet Imaging Spectrograph (UVIS), an instrument based on Schlumberger sensors and packaging, and designed to operate in harsh environments like those found in oil and gas logging operations (previous page, right). The UVIS is now helping scientists


determine


atmospheric chemistry, the nature of clouds and ring systems, and the atmospheric energy balance on Saturn and its moon Titan. The UVIS comprises a set of telescopes that measure ultraviolet light from the Saturn system’s atmospheres, instrument


has two spectrographs:


rings and surfaces. The the


far


ultraviolet channel (FUV), 110 to 190 nm, and the extreme ultraviolet channel (EUV), 5 6 to 118 nm. The FUV and EUV channels in the UVIS spectrometer require different detectors


V enus swingby 0 4 / 2 6 / 9 8 V enus swingby 0 6 / 2 4 / 9 9 O rbit of E arth


Deep-space maneuver 1 2 / 0 3 / 9 8


O rbit of V enus L aunch 1 0 / 1 5 / 9 7 to


optimize sensitivity to the wavelength range required by the Cassini project. In cooperation with the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Schlumberger designed the detector response to meet these requirements. The FUV detector was assembled using a cesium iodide photocathode with a magnesium fluoride window. This detector was vacuum- sealed and included an integrated pump that maintained an ultrahigh vacuum during the spacecraft assembly and launch. Once in space, the detector was equalized to the vacuum of space for the voyage to Saturn. The EUV detector utilizes a potassium bromide photocathode and has no window since transmission of all known substances is very poor in this


E arth swingby 0 8 / 1 8 / 9 9


J upiter swingby 1 2 / 3 0 / 0 0


O rbit of Saturn


Saturn Arrival 0 7 / 0 1 / 0 4


O rbit of J upiter


> Launching Cassini. A Titan IVB/ Centaur launch vehicle propelled the Cassini spacecraft and its attached Huy gens prob e into space from Cape K ennedy Air Station’ s Launch Com plex 4 0, Florida. Visib le in this view are the 20-m [ 66-ft] long, 5 -m [ 17 -ft] w ide pay load atop the vehicle holding the Cassini spacecraft. Cassini’ s planned interplanetary  ight path ( chart inset) b egan w ith launch from Earth on Octob er 15 , 19 9 7 , follow ed b y gravity assist  y b y s of Venus, Earth and J upiter. The gravity -assist  y b y s of the different planets are designed to increase the spacecraft’ s velocity relative to the Sun so it can reach Saturn. With the gravity -assist traj ectory , it took m ore than 61 arrive at Saturn. ( Im ages courtesy of NASA. )


 2 y ears for the Cassini spacecraft to


Q uality-control procedures during manufacturing allowed only MCPs with very low-defect densities to be used for final assembly. Once an MCP was available, LASP and Schlumberger scientists worked together during the final


assembly


short wavelength range. Fortunately, potassium bromide is a very robust photocathode and can be exposed to dry air for the short time required for testing and assembly. Once in the vacuum of space, the detector cover was opened, allowing light to enter the instrument. Both detectors utilize specially selected microchannel plates (MCP). MCP technology has a long history in spaceflight imaging instruments.


process. The units were then transported to NASA laboratories for final testing. Two FUV and two EUV detectors that met the stringent quality requirements for space travel to Saturn were assembled at the Schlumberger Princeton Technology Center (PTC) in New Jersey. One pair of detectors was designated as flight units while the second set was kept in reserve as a backup.


The UVIS also includes a high-speed photometer (HSP) channel, a hydrogen- deuterium absorption cell (HDAC) channel and electronic and control subassemblies. Scientists are using the HSP to make stellar occultation measurements of the structure and density of material in Saturn’s rings.


Cassini was launched on October 15 , 1997, from Cape Kennedy, Florida, aboard a Titan IVB/Centaur rocket, the most powerful launch vehicle in the US fleet (above). After Cassini was placed in orbit around Earth, the upper stage fired to send Cassini on an interplanetary trajectory that would eventually deliver the spacecraft to Saturn.


Spring 2006


5 7


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