A brief review of the objectives and capabilities of the Voyager mission at Saturn is provided. In addition to a description of the eleven Voyager investigations and the Saturn encounter geometry, the scientific capabilities are discussed in the areas of atmospheric, satellite, magnetospheric, and ring studies

Stone, E. C.

English

NASA Technical Reports Server

VOYAGER INVESTIGATIONS OF 
THE, SATURNIAN SkXI'EM 
California Institute of Technology 
Pasadena. Califomin 91 125 
This paper provides a brief review of the objectives and capabilities of the 
Voyager Mission at Saturn. In addition to a brief description of the eleven 
Voyager Investigations and the Saturn encounter geometry, the scientific 
capabilities are discussed in tke areas of Atmospheric, Satellite, Magne- 
tospheric, and Ring studies. 
INTRODUCTION 
In 1977 two Voyager spacecraft were launched toward encounters with Jupiter, 
Saturn, and possibly Uranus. In this paper the eleven scientific investigations and the 
encounter geometry a t  Saturn will f i rs t  be briefly reviewed, followed by a discussion 
of the planned studies of the Saturnian system, This discussion, which will be 
organized into the four broad categories of Atmospheres, Satellites, Magnetosphere, 
and Rings, will necessarily be preliminary, since design of the sequence of observa- 
tions will not begin until 1979. More detailed information on the mission and each of 
the eleven investigations is available in two special' issues of Space Science Reviews 
(Stone, 1977). 
Voyager Project Scientist 
https://ntrs.nasa.gov/search.jsp?R=19790008604 2020-03-22T01:37:43+00:00Z
The eleven investigations and the corresponding Principal Investigators and 
Team Leaders a r e  listed in Table 1. Although not shown, a total of -100 scientific 
investigators from 38 institutions a r e  involved in the Voyager mission. A brief sum- 
mary of the nominal characteristics of the instruments is  contained in Table 2, while 
Figure 1 illustrates their location on the Voyager spacecraft. The four boresighted 
remote sensing instruments (ISS, IRIS, PPS, and WS)  share the scan platform which 
has two axes of articulation for nearly complete angular coverage. 
ENCOUNTER CHARACTERISTICS 
The heliocentric trajectories of the Voyager spacecraft a re  illustrated in Fig- 
ure  2, and details of the Saturn encounters a r e  summarized in Table 3. 
Table 1. Voyager Science Investigations 
Investigation Area Principal Investigator/Institution 
Imaging Science (ISS) ~ m i t h / ~ n i v .  Arizona (Team Leader) 
Infrared Spectroscopy and Radiometry (ZRIS) ~ a n e l / ~ s ~ C  
Photopolarimetry (PPS) 
Ultraviolet Spectroscopy (UVS) 
~illie/G'niv. Colorado 
~roadfoot/K PNO 
Radio Science (RSS) ~shleman/~tanford  Univ. (Team Leader) 
Magnetic Fields (MAG) Ness/GSFC 
Plasma (PIS) 
Plasma Wave (PWS) 
Planetary Radio Astronomy (PRA) ~ a r w i c k / ~ n i v .  Colorado 
Low Energy Charged Particles (LEC P) ~rimigis/JHU/APL 
Cosmic Rays (CRS) Vogt/Caltech 
Table 2, Instrument C h a m ~ ~ t i c p  
nus 
MAG 
PIS 
Two Se-S vidiam cameras (f = 1508 mm and f = 200 mln); . 
Narrow angle camera: 19 prad/Jine pair, 2900 - 6480 A 
MicbeIson i n W e ~ ~ m e t e r  (3.3 - 50 pm) and radiometer 
(0.33 - 2 pm); 51 cm telescope; 0. So FOV 
Pbotomultiplter. with 15 cm teleseape; 2350 - 7500 i; 
3.5". lo, 2 /4", 1/16", F W ;  3 linear polariaera 
Grating spe-tromder: 500 - 1700 with 10 resoh- 
ti= airglow (lo x 0.1" FOV) and occultation 
(lo x 0.5" FOV) 
S-Baad (2.3 GHs) and X-band (8.4 GAn); Ultra Stable 
Oscillatar (<4 x 10-12 short term drift! 
ho la. field ( ~ 1 0 ~ ~  - 0.5 0) &I Ro high field 
(5 x 10-4 - 20 G) magrle3meters; 13 m boom; 
0 - 16.7 He 
Earth-pointing sensor (13 eV - 6 keV ions) and lateral 
sensor (10 ZV - 6 keV iaus, 4 eV - 6 key electrons) 
Sixteen channels (10 He - 55.2 kHz); waveform 
malyser (150 He - 10 kHz); share FW4 antams 
Steppi i  receiver (l. 2 k)fe a d  20.4 kHg - 40.5 MHZ); 
right and left circular polarization; a r h g o d  
10 m monopole antennas 
Two soIid state detector systems on rotating platfarm; 
13 Ir3V - 10 MeV electrons; 10 keV/nuc - 15Q M e v / m  
ions 
CRS Multiple solid state detector *Aescopes; 3 - 1IQ MloV 
electrons; -1 - 500 h\T/mc wclei; 3-dimensional 
anisotropies - 
drwl-hrqrPw) sJwmi+ tRuumitm, aa 
Ultra shbir osrillmar, a d  tkt 12' par* 
anwnsa. 
The trajectory of the first-arriving spacecraft at Saturn i s  labeled JST 
(Jupiter-Saturn-Titan) because the trrzjectory includes a close encounter with Titan. 
Since the second arriving spacecraft can be target& to either the t ~ r a n u s  aim point at 
Sabm or to a close Titan flyby, the trajectory is referred ta as JSX Figwe 2 illus- 
trates both the Uranus option (X=iJ) and the Titan option (X=TB). 
The tnrgeting of JSX will affect the nature of the investigations possible a t  
Saturm. A5 shown in Table 3, for exampie, the JSX(X=T!) trajectory does not have zm 
Table 3. Saturn Encounters 
Date, Closest Approach 
Radius, Closezt Approach 
- I 1 JST 
Radius, Ring Plane 
Crossing 
Distance (lo3 km), 
Earth Occultation 
JSX(X=U) 
mstance (lo3 km), 
Sun Occultation 
J%YQC=T) -
Saturn 
I Titan 
1 235 1 158 / 219 
Rings (-4, B) -300 -C 190 1 Titan 1 21 1 -  1 2 5  
occultatim of the sun and Earth by either the Rings o r  Titan. Thus, the Uranus option 
will be seiected only if JST aecompiishes the major scientific objectives at Saturn, 
Titan, and the Rings, and only if the Voyager 2 spacecraft appears healthy enough to 
be sent on a2 ndditional 4-year mission to C'ranus. The geometries of the JST and 
JSX,S=U) flybys is  illustrated in Figures 3 and 4. 
ATMOSPHERIC STL'DIES 
There a r e  several interrelated aspects of the Saturnian and Titanian atmo- 
spheres which will be studied by Voyager, including dynamics, structure, and composi- 
tion. Some of the general characteristics of these studies will be discussed below, 
with subsequent vapers by B. A. Smith, Re A. Hanel, and 6. L. Tyler providirg rn ore 
specific details for the Imaging, Infrared, and Radio Science investigations. 


Since the stady of atmospheric dynamics Fequims abematiuns wer an e x b d d  
per icddt ime,  owof thekeyfactors istbedmerviugtimearaileMeataspecified 
spatial resohrtfaz FigaFe 5 illustrates the aiwmrbg time as a functfar d resolution 
at %burn far tbe angle (NA) caune~a nd tbe 1/4" field-of-vim ( F O  a4 IRlS 
andPPS. S t r r e e ~ e a a r e ~ g g o o d r e s o l u t i m o f l ~ ~ f r o m t h e g r a m d ~ b o  
-7000 km at Satnra, the Imaging System will have -cantly better reso?ution than 
gmmd-baujed telescopeg timmgkut tb 80 days (--2000 hn) prior to encamter, and 
there will be 30 hours during which IRIGS ind PPS will have resolution better tban 
-7000 lan. 
Lati- cwerage is also impartant to a study of tbe atmcmpkm aa~ a @oFwl 
scale. In fact, in a previous paper Tdnmega mqmrtd evidence that the temperature 
inversion is hatter in the Sauth polar region than at tk equator. As shwn in Figure 6, 
die q m m x d t  latitude of tbe tvovallrager flybys ranges £ran appraximrbsly -40. to 
+30°, providing useful coverage! of higher latitudes in both bemispkms. 
Studies of airnospheric stnicture require detailed observations of tbe pressme 
temperature profile. The expected capability of the Voyager investigations is shown 
in Fire ? which has been adapted from the Voyager Atmaspheres Working Group 
Repart. The figure indicates both the pressure range over which a given technique is 
aqx?ct& to be useful and the expected scale height resolution of the measurement. 
The FcSS measurement e m p l g s  the dual-frequency radio ocarlhtioa, while the IRlS 
measurement uses the pressu-broadened H2 band at 250 to 500 cm-I d the CHI 
band at 1306 cm'l as temperature s d e r s .  Tbe UVS measurement in the upper 
stmosphere is a solar occultation measurement Khich sbauld provide an altitu& pro- 
file for Hi CH4, CtH6, ard C2H, absorption* Stellar occultations and terminator 
" 
scans with PPS uill  also contribute to the determination of the pressure-temperature 
protile, 
The same techniques will be applied in the study of Titan's atmosphere. Both 
R. A. Hanel and G. L. Tyier will report in subsequent papers on recent analyses of the 
expected capability a t  Titan. 
SATELLITE STUDIES 
A characterization of the sl~rfaces of the satellites will be accomplished with a 
combinatilon of high-res olution imaging, infrared spectral studies, and polmimetric 

daubs over a wide range of phse angles. The closest approach distances Q the 
various satellites are indicated in Table 4 for all three possible trajectories. 
Imaging resdution of the satellites S I  through SVI will be significantly better 
than the 5 km/he pair meolution which is characteristic of the well-- mosaics of 
the half-lit disk of Mercury obfahd Mariner 10. 
TBe densities of the satellites is also an impartant characteristic which will be 
studied by combining mass detmminations from precise radio-mvigation techniques 
with size determinations from the ha ,g  System. Figure 8, which i s  adapted from 
Eehleman el UL (1977). illustrates the expected capability. The uncertainty in the mass 
estimates is indicated by the labels on the diagonal solid lines, whija the ullcertainty 
ia the volume estimates are indicated by the dashed lines. For this estimate, it has 
been rammed that the best estimate of the volume is uncertain to 0.4%. corresponding 
to a 1 pixel uncertainty in the diameter of a satellite image which fills the narrow-angle 
camera field-of-view. Thus, the mass of Titan 66) will be determined to g 0.02%, but 
Table 4. Satellite Encounters 
Closest Approach* 
SateUib (103 h) 
JST JSX(X=t? JSXOr;=TB) 
W Rhea 74 648 
SVI Titan 7 667 
SIX Phoebe 
-4'. 13500 
*Narrow angle camera resolution is 2 km/line pair at 100000 km. 
the density will be uncertain to -0.4%. For all other satellites, the density mcerWnQ 
will be dominated by the mass uncertainty wbicb ranges from -3% b - 15% for Rhea 
0, D i m  @4), Iapetus @a), and Tethys @3), and is > 50% for all the others. 
The inferaction of the satellites with the magnetosphere is also an im- 
objective which will be best studied during the close Titan flyby (7000 krn) illustrated in 
Figure 9. The close flyby not only improves the detectability d any magnetic field at 
Titan, but also maximizes the probability that the spacecraft atill intercept any mag- 
netospheric wake, whether due to the corotatian of the rnagmtmphere or due to a radial 
uutstreaming (planetary wind). 
MAGNETOSPHERIC STUDIES 
In addition to studyin3 the interaction a t  Titan, Voyapr will perform detailed 
measurements of the Saturnfan maptosphere,  the evidence f o r  which has been dis- 
cussed by J o  W. Warwick in a preceding paper. The spatial coverage of a possible 
magnetosphere is illustrated in Figure 10 which hrra been adapted from Scarf and 
Gurnett (1977). As indic8tAl in the figure, the l d  times of the exit legs of JST and 
Jsx(X=U) are quite different. Although not indicated, the latitudes of the exit legs are 
also quite different, with JST exiting at approximately +20° and JSX(X=U) a t  -30' (see 
Figure 6). Thus, JST and -JSX(X=U) provide imporPat complementary coverage of the 
magnetosphere. 
The Voyager imstigations provide broad coverage of the particle and wave 
phenomena expected a t  Saturn. Figures 11 a d  12 from the Voyager Magnetospheres 
Working Group Repart summarize the coverage. Nde &at the PRA frequency range 
includes the 300 IsHz to 1 MHz interval in which Saturn signale have been detected by 
Brown (1975) and the PWS range includes the electron and ion plasmic and cyclotron 
frequencies expected at Saturn (Scarf and Gurnett, 1977). 
Ftgm 11. Tk iutmtt) and nu*g, i~twagc of e l a r m  md W ~ S  6) tbr 
Vqager inutnmmt~. Tk &a @AII ;dl-ate tBc mntmsi~w~ obscnad b) 
P.+ 10 at Jnpitcr. Tbms jigurn IS /..a I& Vqagw Magw~u~rplmu 
WwLing Grwp R w .  
10% 
-160 
lo-m 
- Zo 
lomz2 
- 0 
FfEOlltntv. HI 
Fmgun 12. Thrfwqumq and rnrtnstt) rotrragr of u ~ t w  rk V o y a p  
t ~ ~ m W ? I I J .  Adap~edf..m the Voyager Magmtosphmr W~rk;q~G*o~lp 
Report. 
WNG STUDIES 
The study of Saturn's Rings i s  also a primary Voyager objective which will be 
studied by several of the investigations. The Radio Science Team will use the attenua- 
tion and scattering of the chlal-frequency radio to study particle size distribution and 
total amount of material in the ring a s  a functioa of radial distance from the planet. 
The JST trajectory was chosen s o  that the flyby geometry optimizes the high radial 
resolution wKck is possible with Doppler techniques as will be described by G. L. Tyler 
in a subsequert paper. Figure 13 illusirates the flybys a s  seen from Earth. Note that 
a radio occultation of the Rings occurs only on the outbound leg of JST. 
Although the JSX(X=U) trajectory does not provide a ring occultation, it does 
provide a good viewing geometry for the scan platform instruments. IRIS will be use6 
to study the eclipse cooling of the ring particles and to look for long wavelength cutoffs 
in the emitted radiation, while the PPS will view scattered light from the Rings a t  
various phase angles and will observe stellar occultations by the Rings. Of course, 
the Imaging System will directly view the Rings to  search for large objects ( 31 Irm) 
and to search for structure which may be related to Ring dynamics. 
The estimated volume density sensitivity of the various observations is illus- 
trated in Figure 14 which has been adapted from the Voyager Saturn's Rings Working 
Group Report. Assuming the upper limit to the E-Ring optical depth discussed by 
B. A. Smith in a preceding paper, the E-Ring will be - 3 orders of magnitude below the 
PPS sensitivity if it is composed of mm-sized particles, but may just be detectable in 
the less likely event that the particles a r e  micron-sized. 
C O W  LUSION 
This is necessarily a rather brief overview of the Voyager capabiiities for the 
study of the Saturnian system. Detailed planning of the observational strategies will 
begin a year from now, s o  that experience with the JST encounter a t  Jupiter can be 
folded into the plans for the Saturn encounter. In addition, results from the Pioneer 11 
encounter with Saturn in September 1979 may significantly affect the Voyager observa- 
tional strategy. Thus, the detailed scientific objectives of the Voyager Mission will 
continue to evolve within the general capability described in this paper. 
JSX ( X =  U) 
Figure 17. Earthtlieus of the Voyager encounter a t  Saturn 
298 
I i 0 RAOlOMElER l R l ~  :;Om n RADIOMETER c Q . 0 IhTERFLRnMnER / S ECLIPSE 
a' 
(RP,DIUS,~ O *. 
I S S  0 @ -  0 h1INIMbM SIZE SINGLE 
" I PARTICLE I 
1 0 - 1 8  I d 
1 0 ' ~  10. 1C lo0 loZ 
PARTICLE RADIUS. m 
Figure 14. The utimatcd mrnrmum r i r r s ~ t ~ b i ~  tolume dcffsrrirt 6s  djunct~on of 
particle radius /or 1 arious measumnrntJ of Saturn'r Rrngr. Tk pharc angla of 
thc d ; f f m t  ob~orr~t jonr arc rdrcated. Note that the mrnlmum drtafable 
telanrr dcffsrty ~ c a l a  linearly uvth tbe partrcie radnur and rnzrrrefy ulith rnng 
thickness. 
OR1GffVAL PAGE 
OF POOR Cwel OTV 
ACKNOWLEDGEMENTS 
The Voyager Project i s  being carried out for the National Aeronautics and 
Space Administration by the Je t  Propulsion Laboratory of the California Institute of 
Technology under contract NAS 7-100. 
REFERENCES 
Brown. L. W. ( 1975). Saturn radio ernlssion nedr 1 MHz. Arrroph)~. J. 198, L89- L92. 
Eshleman. V. R.. Tyler. G .  L.. Anderson, J .  D , Flcldbo, G . .  Levy, G S.. Wood. G. E . ,  .md Croft. T. .?. (1777). Radio science 
investigations with Voyager. Spat SCI. Rev. 21. 207-232. 
Kohlhase, C. E. ,  and Penzo, P. A. (1977). Voyager miss~on description. Spacr SCI. Rn.. 21, 77- 102. 
Scarf, F. L., and Gurnctt, D. A. (1977). A plasma wave lnvest~gation for the Voyager Miss~on. SpDnSo. Rev. 21, 289- 308. 
Stone, E. C.. ed. (1977). The Voyager Project. Span Srr. Rev. 21, 75-376. 


Voyager investigations of the Saturnian System

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