The Voyager imaging science experiment objectives at Saturn include exploratory reconnaissance of Saturn, its satellites and its rings. The imaging cameras are described, along with an abbreviated discussion of specific objectives

Smith, B. A.

English

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VOYAGER IMAGING SCIENCE 
-INVESTIGATIONS 
Bradford A Smith 
ABSTRACT 
The Voyager Imaging Scietue Exprriment objectives ar S c u m  ioduk 
rrm#maissnncr of Saturn. ia dEes and m rings. The 
imaging cameras are derrribed. dong with an abbreviated di~~~&ion f 
specihc objectives. 
At the time of the NASA/JPL Saturn's R h g  Workshop in 1973, 1 presented a 
description of the Mariner Jupiter/Saturn imaging experiment. Significant changes 
have taken place since that time: the Imaging Science Subsystem has evolved into one 
much better suited to meet the scientific objectives of the investigation, and the pro- 
ject has changed its name. Recently, the Voyager Imaging Science Team published a 
moderately detailed description of both the instrument and the scientific investigation 
(Smith et d , 1977). 
Figure 1 is a schematic representation of the medium resolution and high reao- 
lution cameras being flown on the taro Voyager spacecraft. For reasons of historical 
significance d y ,  these cameras are referred to as the "wick angle" and ''narrow 
anglev cameras, respectively. The focal length of the wide angle &-era is 200 mm, 
and the narrow angle camera is 1500 mm. Each camera employs a sele.:iw-sulfur, 
slaw-scan vidicon which provides an 800 x 800 array of 14-pm pixels, each digitived 
to 8 bits (256 levels of gray). The fields of viev me 332 and 0t4. Additional camera 
characteristics a r e  shown in Table 1. 
https://ntrs.nasa.gov/search.jsp?R=19790008605 2020-03-22T01:37:52+00:00Z
SHUTTER ASSY / 
SCAN PLATFOW 
Table 1. Imaging Scicuce Subsystem Characteristics 
Characteristic Narrow angle camera Wide mgie camera 
Focal length 
Focal number 
Field-of-view 
Trnumberb 
Nominal Shutter operation 
Active Vidicon Raster 
Scan lines per frame 
Picture elements per Line 
Pixels per frame 
Bits per pixel 
Bits per frame 
Nominal frame times 
Video baseband 
Video sampling frequency 
Angle subtended by scan line 
N-9-t Freque=y 
Resolution 10% Modulation at 
1499. US mma 
F/8. sa 
7.5 7.5 mrad 
T/11. ma 
0.05 bo 15.36 sec 
11.14 11.14 mm 
800 
800 
640 000 
8 
5 120 000 
48to480sec 
7.2 lrHe 
14.4 irHz 
9.25 rad 
32 line pairs/mm 
36 line pairs/mm 
~ - 
201,568 mma 
F/3.5" 
55.6 55.6 mrad 
T/4.178 
0.005 to 15.36 sec 
11.14 11.14 mm 
900 
800 
640 000 
8 
5 l20 000 
48 to 480 sec 
7.2 kHz 
14.4 lrHe 
69.4 rshd 
32 line pairsIfmm 
36 line pairs/mm 
'~ctual data from prototype camera systems, 
b ~ / ~ u m b e r  - An effective F/number which includes obscuration and transmission 
losses. 
Each camera has eight filters which cover a r;lectrd mqp from 425 to 600 nm 
inthewideapglecamem, a a d f r o m 3 4 5 t o 5 9 0 n m i n t h e ~ ~ e c r r m e r a ,  In
addition, the wide angle camera has three narrow-band filterrr centered on the methane 
absorption bgnde at 541 and 619 nm, and on the sodium D2 line at 589 nm. 
Observational sequences for Saturn have not yet been demlaped, and them 
remains unrmnwered some crucial questtons relating both to telemetry prbrmaoce 
d to on-board, data-storage management throllghont the mrn encounter. Thus, it 
is not pmsently possible to discuss obermtianal objectives and the sequences neces- 
sary to achieve them with the same degree of detail that I could now give for Jupiter. 
?lowever, it is clear that telemetry rates will be lower a t  Sttturn than at  Jupiter and 
that inter-experiment sequencing conflfcts will be much more serious, In ather words, 
lower telemetry rates mean less inEormationq no matter how clever m e  become ?n 
developiag our observational seque-. 
Arrival times at S a m  are 13 November 1980 for Voyager 1 and 27 August 
1931 for Vogager 2. In the baseline plan we will begin imaging in the "observatorJr" 
phase 80 days before each encounter; that is, late August 1980 and early June 1981, 
respectively. At that time w e  will be a little less than 100 million km from &turn and 
the resolution of our narrow angle cameras will be approximately 2000 km per liue 
r .  This is about twice as good as the very best photographs taken from the Earth. 
Throughout the observatoiy phase resolution tends to increase linearly with time, 
reaching 2 t o  3 km/lp on the disk of Saturn at the time of encounter. The best resolu- 
tion on the rhga nay  be a little higiner still, but will eventually be limited by image 
smear caused by the motion of the spacecraft. 
Scientific investigations of the satellites of Saturn will include distant imaging 
of the entire satellite system and near-encounter high resolution imaging of several 
satellites. At least some near-encounter imaging will be obtained over sub- 
hemispheric areas on all of the inner satellites: 3 km or  better on Mimas, Enceladus, 
Tethys, Rhea, Mom, and Titan with 1 km or  better on Mimas, Rhea, and Titan. 
Hyperion and Iapetus will be seen with resolutions of 10 to 20 km. The total attainable 
coverage of the surfaces of these satellites will be determined by actual data rates, 
sequencing conflicts and, to some extent, by image smear. 
TLe bUowiug are am- ths scbntifk objectivw to be pursued by the satellite 
s ~ w I t h i n t b e ~ s c ~ T e a m :  
Size and Sbqx~ The sizes aad shapes of most of the satellites will be deter- 
mtned with uncertainties between 0.1 and 1.0 percent. 
Global Rigb-Resolution M a p p e  Between 20 and 4% of the illuminated 
hemispheres will be photograpbed in eead d o r s .  In some cases, the tun space- 
craft trsrjectorie~ wtU provide tabced pdar and equatorid cove-. Tbese high- 
resolutiun obeervatiom will be used to examine the Rnescale details of major physio- 
grapb icprovbes  andthenatureof the  tmit ionregioals .  
Polar Volatiles: The polar regians will be explored for deposits and etmctures 
related to the past histories of these surfaces, including studies of the morphologies 
and interrelatfone of possible ice-related Setrtures. With equatorial-aoan and pdar 
bempelc(Ltures ranging from -100 K to -30 K, stabde ices slumld include water, certain 
sulfides, ammonia, and ammonium hydrates (Lebohrky, 1975). The polar regions, 
being at all times colder thtrn other areas, may have acted as sinks for volatile mate- 
rials originating elseffhere. These regions are, therefore, of special interest. 
Ti& 'Ebere is a lack of consensus in our understanding of Titan, and care- 
fully planned spacecraft measurements are required if w e  hope to  resolve these issues. 
The diameter and figure are the first priority. lke next step is to establish whether 
this observation refers to  the surface, o r  to a haze in a dense, molecular atmosphere, 
o r  to a uniform cloud layer. This might be accomplished by imaging observations in 
several colors. The suggestions that Titan may possess ciouds, aerosols o r  even 
dust layers requires that it possesses weather systems. Titan is a slowly rotating 
planetary body with an orbital period of about 16 days, and thus, the circulation of the 
satellite atmosphere could represent an intermediate case between the baroclinic cir- 
culation typical of the mid latitudes of the earth and Mars and the symmetric Venus 
weather patterns. 
In addition to the study of each satellite as an individual planetary M y ,  we 
will, of course, pursue an intercomparison among all observed satellites in the 
Saturn system, and with those in the Jupiter system as well. 
The Voyager mission provides a unique opportunity for the study of atmospheric 
systems very different from our own. At both Jupiter and Saturn, Voyager will achieve 
higher resolution and a longer observing time base than that obtained by Pioneer; both 
resolution and time are  improved by a factor of about 50. 
Throlrghwt the obmmitory p b e  the disk of Sturn will be photagcqhd in at 
least two colors (0- and ultraviolet) at flm equally spaced longitdm during each 
mbtion. A-ly three weelm before planetary emmmter, the dtsk of %tarn 
will exceed the Beld of view of the n a m w  angle camera and mosaicking will be am- 
essary. Tbe extent to which $lo?ml coverage with 2 x 2 and 3 x 3 moeaics can be 
mdnhhd wi l l  depend upon data rates and tape recorder use. Eventually we wtll bve 
to termhate this fall disk coverage d concentrate cw fhttms of special interest 
identifled and selected dnrtng the observatory phase. 
During the observatory phase, obgervations in both space and time will provide 
information on the synoptic development, growth, and dissipation of atmospheric sys- 
tems, and wil l  provide first opportunity to  observe the eonill velocity field at tbe 
cloud tops. Unlike Jupiter, Saturn as seen from the Earth presents a dearth of m- 
axisymmebric features necessary to measure zonal wind components. In fact, only 
9 such Beatures have ever been &served, i m  unknown the zonal velocities asso- 
ciated with most of the planet's axisymmetric structure. 
The near encounter observational sequences of Saturn will be nearly ihntical  
to those carried out at Jupiter, in that a major objective of the Voyager missian 
involves a detailed comparison of the meteorologies of these two planetary 
atmospheres As an example, the Pioneer 11 observatione of Gehrets et oL (1974) 
show that the axis-vmmetric structure terminates o t  Jupiter apprarsimately 45" from 
the equator. The Voyager spacecraft trajectories, however, require the Jovian 
encounters to take place nearly in the equatorial plane, s o  that it will not be possible 
to make highest resolution observations of the Jovian polar regions. We do not yet 
know whether this transition also occurs in the Saturn atmos@x?re and, if  so, at what 
latitudes. Fortunately, the Voyager trajectorizs make i t  pottsible to observe both the 
north and south poles of Saturn (see Figure 2) to complete the comparison of the 
Jovian and Saturnian meteorologies . 
There are several types of observations of Saturn's Rings to be made with the 
wide and narrow angle cameras, which should yield information on the photometric 
properties of the particles and predicted radfal drift displacements of particles within 
the rings. General profiles of the rings in reflection obtained during the observatory 
phase at  nearly uniform phase angle may be used to interpret those obtained during 
emounter, when phase angle will vary greatly across the profile. 
Strip sequences of images with high resolution taken across the rings will give 
information about minor divisions. If the strip is timed and p!aced s o  as to see the 
turned-up illuminated outer edge of an irsclfnatim resonance as predicted by dynamical 
theory (Cod, 1978). a u s e l l  chedr on that theory will be obtained. If minor diVisir~le 
are seen, 8pe can establish lower limits on drift displacements across the correspond- 
ing resonances. Tbe width of a division gives us the distance of drift during the age 
of the rings. Invisibility of a division gives us an upper limit on the distance of the 
drift o r  implies that the collision frequencies betwleen particles are too high to allow 
the resonance to develop. 
The existence of Rfng; D should be settled once and for all, and excellent 
profiles of Ring C and Cassini's Division will be obtained. 
REFERENCES 
Cook. A. F. (1978). G r o o i t u i d  monnm in m s  rings 11. To be p&W. 
Gehh.  T., &, D.. Tad. M.. I)oarc, L.. Swandell. W., Cstillo. N., KmhPlI. J , nnd B&r. 1. (1971). Ir?giw photo polar- 
imcm experiment on Piocllrr 10. Snma 183. 3 18-320. 
Lcbofrky. 1. A. (1975). Smb~lWy of from in the solar syscrm. lomr 25. 205-217. 
Smith. B. A.. Brig@. G. A.. h i c k ,  G. E.. Caok. A. F., h v i o .  M. E.. Hum. G. E.. M w .  H.. %&&lorn. I. A.. h, T.. 
w. C.. Pnd Suomi. V. E. (1977). Va?.~grr imaging arptrimmt. Spaa Srarsa Rev. 21. 103- 127. 
D. MORRISON: You talked about weather and clouds on Titan in a way that 
made me think you expecEed to see cloud patterns. Doesn't the large optical depth of 
ham suggest ft d d  be more like l d h g  down on Los Angelee on a smoggy day, 
*re poa see not-? 
B. SMITE Yes. ~lthough, if there a m  clouds above the haze, we'll see them, 
J. POLLACK: I don't think w e  have good numbers for tbe optical depth of dust 
o r  haze on ntan ri@t now. From that point of view Itd be a little hesitant to say we 
won't see excitiqg atmospheric features on ZPtan. 
J. CUZZI: Fnwn what George Siscoe was saying about the sputtering going on 
at the onter eclges of the rings, it m d d  be especially interesting to look at the outer 
e d g e o f t h e A r l s g .  
G, ORTON: Do you have any filters that are located in the methane red bands? 
B. SMITH: Yes, The 619 nrn methane absorption band, and the band at 541 nm. 
The 541 nm filter w a  put on for Uranus. There is no hope of seeing that band on 
Saturn. 
D. MORRISON: Is there any problem with scattered light for features like the 
D Ring or the inside of the Cassini division? Are you confident that your sys:ern has 
low enough scattered light levels, and that you will be close en*, to see into these 
regions? 
B, SMITH: Absolutely confident. 


Voyager imaging science investigations

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