Once the Pioneer 10 and 11 spacecraft had demonstrated that spacecraft missions to the giant planets were possible and of high scientific interest, more sophisticated spacecraft were built. The Voyager spacecraft were managed by the NASA Jet Propulsion Laboratory (JPL) and were considerably larger and more complex than Pioneers 10 and 11 (Figure 7.31). The spacecraft weigh 825 kg each and the main

Figure 7.31. Voyager 1 and 2 spacecraft. Courtesy of NASA.
Figure 7.32. Voyager 1 and 2 trajectories. Courtesy of NASA.

communications dish is 3.7 m in diameter. The spacecraft are three axis-stabilized and are powered by three RTGs, which together generated 470 W at launch.

Voyager 1 was launched from Cape Canaveral on September 5, 1977 and flew past Jupiter on March 5, 1979 at an altitude of 2.9 Rj (206,700 km). The spacecraft then flew by Saturn on November 12, 1980 at an altitude of 1.1 RS (64,200 km) and the trajectory also provided Voyager 1 with a close encounter with Titan. Subsequently, Voyager 1 left the ecliptic plane and is now heading out to interstellar space at an angle of 35° to it (Figure 7.32). On February 17, 1998, Voyager 1 passed Pioneer 10 to become the most distant man-made object in space (Figure 7.33).

Voyager 2 was actually launched before Voyager 1 on August 20,1977 from Cape Canaveral and subsequently made the "Grand Tour" of all four giant planets. Voyager 2 flew past Jupiter on July 9, 1979 at an altitude of 8 Rj (570,000 km) and then flew on to Saturn, which it passed on August 25, 1981 at an altitude of 0.7 RS (41,000 km). The flyby of Uranus took place on January 24,1986 at an altitude of 3.2 Rv (81,500 km) and Voyager 2 made a very close flyby of Neptune on August 25, 1989 at an altitude of just 0.2 RN (5,000 km). The resulting trajectory is taking Voyager 2 south out of the ecliptic plane at an angle of 48°. Both spacecraft have

Figure 7.33. Current position of Pioneer and Voyager spacecraft. Courtesy of NASA.

sufficient power (currently ^280 W/spacecraft), and attitude control propellant to keep them operational until probably 2020.

The Voyager spacecraft carried four instruments of relevance to atmospheric remote sensing.

Imaging Science Subsystem (ISS)

The Voyager ISS consisted of a high-resolution narrow-angle (NA) video camera and a more sensitive, lower resolution wide-angle (WA) camera. Each camera had an 800 x 800 element detecting array and the FOV was 7.4 x 74mrad for the narrowangle camera, and 55.31 x 55.31 mrad for the wide-angle camera, giving pixel sizes of 10 ^rad and 70 ^rad, respectively. Each camera was equipped with a set of eight filters covering various wavelengths across the visible and UV.

Ultraviolet spectrometer (UVS)

The Voyager UVS was a reflection diffraction grating spectrometer, which dispersed the spectrum from 50 to 170 nm onto an array of 128 adjacent detectors. Hence, the spectral resolution of the instrument was 1 nm. The instrument had two fields of view: (1)1.7 x 15 mrad boresighted with the camera (ISS); and (2) 4.4 x 15 mrad offset from the boresight by 20°.

Photopolarimeter Subsystem (PPS)

The Voyager PPS was a photoelectric photometer and used a 15 cm telescope and a set of eight filters between 235 nm and 750 nm, eight polarizers, and four field stop apertures, each located on a separate wheel to allow any combination of filter, polarizer, and field stop. The allowed FOVs had diameters of 2.1 mrad, 5.8 mrad, 17 mrad, and 61 mrad, respectively.

Infrared Spectrometer and Radiometer (IRIS)

The Voyager IRIS comprised two instruments sharing a single large-aperture telescope system and is shown in Figure 7.34. The large primary mirror of diameter 0.5 m was needed to record the extremely low thermal emission and reflected flux from the more distant giant planets. Shortwave radiation (0.3-2 ^m) was monitored

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