Voyager 1 Encounter with the Saturnian System Author(s): E. C. Stone and E. D. Miner Source: Science, New Series, Vol. 212, No. 4491 (Apr. 10, 1981), pp. 159-163 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/1685660 . Accessed: 04/02/2014 18:59 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science. http://www.jstor.org This content downloaded from 131.215.71.79 on Tue, 4 Feb 2014 18:59:21 PM All use subject to JSTOR Terms and Conditions was complicated by several factors. Sat- urn's greater distance necessitated a fac- tor of 3 reduction in the rate of data transmission (44,800 bits per second at Saturn compared to 115,200 bits per sec- Reports ond at Jupiter). Furthermore, Saturn's satellites and rings provided twice as many objects to be studied at Saturn as at Jupiter, and the close approaches to Voyager 1 Encounter with the Saturnian System these objects all occurred within a 24- hour period, compared to nearly 72 Abstract. An overview of the Voyager I encounter with Saturn is presented, hours at Jupiter. including a brief discussion of the flight, trajectory, science plan formulation, and Scientific studies included investiga- highlights of the results described in the subsequent reports. tion of the ring system, satellites, and magnetosphere of Saturn and of the at- The Voyager 1 encounter with Saturn ultraviolet occultation studies of Sat- mospheres of Saturn and Titan. Studies marked the successful accomplishment urn's atmosphere; and (iv) spacecraft of the planetary atmosphere focused on of the third step in the NASA Voyager passage through the ring plane at Dione's dynamics, composition, structure, and program to explore the interplanetary orbit in order to minimize hazards from magnetospheric effects (auroras). Stud- and planetary environments of the outer impinging ring particles. The actual Voy- ies of Titan were designed to determine solar system (1). The Voyager spacecraft ager 1 trajectory and the planned Voyag- the diameter of the solid surface, atmo- was described in detail by Draper et al. er 2 trajectory also provide complemen- spheric temperature and pressure pro- (2). Voyager 1, launched on 5 September tary sets of satellite close-approach dis- files, and atmospheric composition and 1977, had its closest approach to Jupiter tances (Table 2). to search for an intrinsic magnetic field on 5 March 1979, during a 98-day period Voyager 1 encounter activities began of Titan. The other satellites of Saturn of intensive acquisition of science data on 22 August 1980, when the spacecraft were studied to determine their size, on the Jovian system and its environs was 109,000,000 km (68,000,000 miles) density, surface geology, temperature (3). Voyager 2 was not far behind Voyag- from Saturn. Closest approach, at a dis- structure, and reflective properties. Ring er 1, with closest approach to Jupiter on tance of 126,000 km (78,000 miles) above studies included radial structure, tem- 9 July 1979 (4). With assistance from Saturn's cloud tops, occurred at 2346 perature, and particle size determina- Jupiter's gravity and a post-encounter UTC (coordinated universal time) on 12 tions and searches for azimuthal differ- trajectory correction maneuver, Voyag- November 1980. Titan closest approach ences and for a ring atmosphere. Magne- er I continued on toward Saturn. occurred before that of Saturn, at 0541 tospheric studies addressed the source of During its cruise from Jupiter to Sat- UTC on 12 November 1980. A view of Saturnian radio emissions, motions and urn, at a velocity of nearly 15 km/sec the Voyager 1 trajectory through the other variations within the magneto- (34,000 miles per hour), Voyager 1 ex- Saturnian system is shown in Figs. 1 and sphere, and interactions between the plored the interplanetary medium and 2. Figure 3 depicts the planned comple- magnetosphere and the rings and satel- selected celestial sources of interest. In mentarity of latitude coverage by Voyag- lites. addition, each of its science instruments ers 1 and 2. Some highlights from the following was recalibrated in preparation for the Design of the spacecraft sequences detailed reports are summarized below. Saturn observations. During this 20- month cruise period, Voyager personnel developed and tested the detailed space- Table 1. Voyager science investigations. craft sequences needed to perform the Abbre- planned scientific studies during the 4- Investigation viation Principalinvestigator month encounter with the multiringed science ISS B. A. planet. Imaging Smith, Universityof Arizona (team leader) The Voyager 1 scientific investigations Infraredradiation IRIS R. A. Hanel, are listed in Table 1. The locations of the GoddardSpace Flight Center instruments and essential subsystem Photopolarimetry PPS A. L. Lane, hardware are shown in the 1 Jet PropulsionLaboratory Voyager Ultraviolet UVS A. L. All instruments func- spectroscopy Broadfoot, Jupiter report (3). University of SouthernCalifornia, tioned nominally with the exception of Space Sciences Institute the photopolarimeter, which developed Radio science RSS G. L. Tyler, anomalous behavior during the Jupiter StanfordUniversity (team leader) Magneticfields MAG N. F. Ness, encounter and has remained inoperable GoddardSpace Flight Center since then. Plasmaparticles PLS H. S. Bridge, The trajectory of Voyager 1 was cho- MassachusettsInstitute of Technology sen to provide (i) a close encounter with Plasmawaves PWS F. L. Scarf, Titan before Saturn closest so TRW Defense and Space Systems Group approach radio PRA J. W. that Planetary astronomy Warwick, atmospheric occultations, high-reso- Radiophysics,Inc. lution imaging, Titan magnetic field stud- Low-energy'charged particles LECP S. M. Krimigis, ies, and solar wind and magnetospheric Johns Hopkins University, wake studies could be accommodated; Applied Physics Laboratory CRS R. E. (ii) optimum geometry for radio occulta- Cosmic-rayparticles Vogt, CaliforniaInstitute of Technology tion studies of the rings; (iii) radio and SCIENCE,VOL. 212, 10 APRIL 1981 0036-8075/81/0410-0159$01.00/0Copyright ? 1981AAAS 159 This content downloaded from 131.215.71.79 on Tue, 4 Feb 2014 18:59:21 PM All use subject to JSTOR Terms and Conditions All of these studies benefited from the emissions from molecular hydrogen marked by a few narrow features that prior ground-basedobservations of Sat- were also seen near the sunlit edges of correspond to orbital resonances with urn and from the Pioneer 11 Saturn re- the planet at very low latitudes. satellites 1980S1 and 1980S3. There is sults (5). The bulk of Saturn's atmosphere is evidence for satellite-driven density Saturn's atmosphere. One principal hydrogen. Helium accounts for only waves in the region of the Encke divi- objective of the Voyager mission was a about 11 percent of the mass of the sion. Radio transmission through the comparative study of the dynamics of atmosphereabove the clouds of Saturn rings indicated that the effective diame- the atmospheres of Saturn and Jupiter. compared to 19 percent in the same ter of A ringparticles is 10 m. The small Because Saturnis colder, the cloud lay- regionon Jupiter.This differenceis con- satellite 1980S28is just beyond the outer ers are deeper in the atmospherethan at sistent with a gravitationalseparation of edge of the A ring. Jupiter and are blander in appearance helium and hydrogen in Saturn's interi- The A and B ringsare separatedby the (5). Alternating dark belts and light or, which could generate the excess en- Cassini division, which contains five zones are visible in the clouds of Saturn ergy radiatedby Saturnover that which broad rings that have additional struc- and extend to much higherlatitudes than it receives from the sun. Methane, am- ture. The effective particle diameter on Jupiter. monia,ethane, acetylene, and phosphine from radio transmissionis about 8 m. The velocities of the clouds have been are also detected in the atmosphere, al- The middle, or B, ring is strikingly measuredwith respect to the motion of thoughthere was relativelylittle gaseous different,consisting of numerousnarrow the interior of Saturn, as indicated ammonia due to the low atmospheric ringlets with no apparentlarge-scale or- by the 10 hour 39.4 minute rotational temperature. Temperatures decrease der. It is possible that the ringlets are period of the magnetic field. Eastward from - 150K in the upperatmosphere to formedby the action of a largenumber of wind speeds in Saturn's near-equatorial a minimumof - 85 K at a pressure of moonlets embeddedwithin the ring. The clouds of up to 480 m/sec (1100 miles per 100 mbarand increase to - 160 K at 1.4 B ring region is also characterized by hour) are four times the highest speed bars. Temperatures in the southern sporadic radial markings or "spokes," measured in Jupiter's clouds. These hemisphere are warmer, apparently a perhaps the result of levitation of small winds, which are not stronglycorrelated seasonal effect. particlesabove the ringplane. The possi- with the belt and zone boundaries, de- Rings. Since first seen by Galileo in ble importanceof electrostatic charging crease smoothly to nearly zero at lati- 1610, Saturn's primary distinguishing effects in spoke formationand dynamics tudes near 40?N and 40?S. characteristic has been its rings. The is suggested by the detection at radio The nighttimeface of Saturnis illumi- distinct, broad A, B, and C rings, which wavelengths of electrostatic discharges nated by ringlight, degradingthe thresh- are easily observable from Earth, were from the rings.