The Galileo Jupiter orbiter and its science investigations are briefly described as a baseline design for a Saturn orbiter

Morrison, D.

English

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. GALILEO ORBITER SPACECRAFT 
INsTRCrMEIqTA TIOM 
ABSTRACC 
Sht Galilee Jupirrr orbiter and its r~ienrc inwstigaciom arc brp4. 
d e s c n i  as a W i n e  design fw a S a m  orbiter. 
2 An essential element of the proposed SOP mission is a long-lived Saturn 
orbiter. The orbiter spacecraft will release the atmosphere probes % both Sakrn and 
Titan and will serve a s  the relay point for transmitting the probe data to Earth; it wil l  
.then remain in orbit for twc or  more pears investigating the planet, ita rings and 
satellites, and its magnetospherr. 
2 A s  presently envisioned, the SOP orbiter is modeled clwely on the GaIileo 
2 Jupiter orbiter, just a s  the SOP p r o k s  a r e  expected to be derivatives of the Galileo 
probe (Colin, 1979). In t\is paper, I briefly describe the Galileo orbiter and its 
scientific investigations as examples of the type of spacecraft and associated science 
that cmld be launched to Saturn in the mid-1980's. It i s  important to note, however, 
that these Galileo designs are  not fixed a t  the time of this writing (spring 1978), and 
rhat in any case signficant modifications, particularly in scientific experiments, could 
2 be accommodated [or SOP . 
https://ntrs.nasa.gov/search.jsp?R=19790008610 2020-03-22T01:37:59+00:00Z
The C ' i e o  orbiter i s  a new d q m l a m  in planetary spacemaft. In the past, 
tbese mecraft have been either three-axis stabilized (the Marinexhroyager class) 
or spin stabilized (Ule Pioneer class). The Mariner vehicles are optimised for remote 
sensing, since they are highly stable platforms from which pointed experiments, such 
as those iavohring cameras and spectrometers, can be carried out. They are a b o  
capable of highly cantroiled manewers when necessary, The simp& spirming space- 
cr- in contrast, provide! a superior base for particles and fielde experiments, which 
need to scan many directions rapidly to characterize the space euvircmment of tbe 
spacecraft. The W e o  orbiter is designed to incorporate both spinnhg and stable 
mounts for scientific experiments. 
The bulk of the Calileo orbiter is designed to spin around an Eartb-oriented 
axits at several rpm. Included in this spinning section are tbe large telemetry antenna, 
the selenide radioisotope power generators, the retropropulsian system, and most of 
the spmecdt structure and elecbroplics. Attached to this spinning s p a c e c d t  is a 
despun platform for remote sensing instruments, with a connecting bearing across 
which power and telemetry signals can be transmitted During cruise o r  when the 
propulsion engines are being used, this despun section can be spun-up to roiate with 
the rest of the spcecraft. 
The Galileo communication system makes use of an S- band (about 12 cm 
wavelength) uplink, or cu.nmand system, and both S-band and X-band (about 3 cm 
wavelength) dawnlink capabilities. The bulk of the science data will be transmitted a t  
X-band using an Earth-directed 4. $-meter antenna with a beam-width of less than a 
degree. The maximum data rate from Jupiter is 115.2 kilobits per second. With 
even modest improvements in transmitting and receiving capabilities over the next few 
2 years, a t  least a comparable data rate should be achieved by SOP from Saturn. 
The Galileo retropropulsion system used to insert the spacecraft in orbit and 
to modify the orbit subsequently i s  being provided by the Federal Republic of Germ-. 
This engine has a thrust of 400 Newtons, and a substar tial fraction of the orbiter mass 
consists of fuel for orbit insertion and navigaaon needed for close flybys of the 
2 Galilean satellites. If SOP uses an ion drive engine, the burden on the chemical 
retropulsim system is  somewhat decreased. 
Science experiments a r e  mounted on both the spinning and despun sections of 
the Galileo orbiter. The magnetometer and plasma wave experiments a r e  on a i ong 
spinning boom, while other particles and fielcls experiments are mounted closer to the 
spin axis. On the despun section, a scan platform provides the base for the imaging 
camera and the *r remote sensing telescopes that are bore-sighted with it. The 
epeckd absolute pointing a a m m q  i s  0.2 degree, with short-term jitter of less than 
0.1 degree. 
Galileo Scientific Imestigatious 
A science payload for Galileo was tentatively selected in August 1977, with 
determination of tbe final selection scheduled for October of 1978. Tbe description 
given here zefers to the tentative selection and may not represent the find Galileo 
py1-A 
Sevenfeen investigations were selected for Galileo, sixteen involving instru- 
ments, and one dealing with the scien=c capabilities of the radia-telemetry system. 
Six of tbese are probe investigations, as described in the companion paper by Colin 
(1978). The other tm are on the orbiter. These ewriments are listed in Table 1 and 
briefly described below, beginning with those Dn the e n  platform. 
Table 1. Tentatively Selected Science Investigations for the 
Galileo Jupiter Orbiter 
PI Investigation 
- 
31. Belton, Kitt Peak 
R. Carlson, JPTA 
C. Hord, U. Coimsdo 
A. Lacis, Goddard Institute 
M. Kivelson, UCTA 
1). Gurnett, U. Iowa 
L. Frank, U. Iowa 
D. Williams, NOAA 
R, Grard, ESTEC 
E. Gruen, MPI Kernphysik 
Imaging (CCD, 1500-mm focal length) 
Xear Infrared Yapping Spectrometer 
1-ltravi olet Speckom e ter 
Photopolar imeaier ./Radiometer 
?.Iagnetometer 
Plasma l a v e  Spectrometer 
Plasma 
Energetic Particles 
Electron Emitter 
Dust 
The Imaging Iuvestigatian is actually a series of rated investigatiars being 
carr ied out by nn Imaging Science Team consisting of thirteen individually selected 
scientists. The camera is provided by NASA. The -tics, a 1500-mm focal lengtb 
catadioptric telescope and 8-position fil ter wheel, are Voyager brdware. However, 
Galileo will be the f i r s t  planetary spacecraft to use  the new CCD (charge carpled 
d?vice) detectors, with their high quantum efficiency, liaearity, large dynamic raw, 
a& wctend4 infrared respmsis;ty. The Wile0 camera will use an $0 x 600 pixel 
frame, with a resolution of 20 microradians per line pair. 
Also on the scan platform ' -- the X?US, or Sear-Infrared Mapping Spectrometer. 
This instrument i s  des!yaed primarily to identify and map mineralogical units oa the 
Galilean satellites, but i t  wil l  also be used for cloud studies and temperature sounding 
of the Jmian atmosphre. Its spectral sensirivity covers most of the infrared reflec- 
tance region \\-here diagnostic spectral feature:: of ices and silicate minerals appear. 
The spectral r e s o W i  power is about 100 and the angular resolution is 0.5 nil l i -  
radians. Galileo is the f i r s t  spacecraft to ca r ry  a NMSStype instrument  
A Fastie-Etn-t ultraviolet spectrometer (ITS) is bore-sieted with the imaging 
and hXl!S telescopes. This C75 is designed primarily to study the composition and 
structure of the upper atmosphere of Jupiter a d  the tenuous atmospheres of the 
satellites. The u-auelength rang? is fmm 110 to 430 nm, with s p e c t n l  resolution of 
about 1 nm. Similar spectrometers have been flmm before on Pirmeer Venus and 
Voyager. 
The final scan-platform instrument is a photopolari me ter  'radiometer, designed 
primarily for stud\- of cioud and haze properties on Jupiter. -.I 10-cm telescope and 
1Gposition filter wheel allow measurements in a number of spectral hands in the 
visible and near-IR. -4 similar  instrument is on Pioneer Venus, \chile other photo- 
polarimeters have fimn to the outer planets on Pioneers 10 and 11 and Voyager. 
The n c d  four instruments in Table 1 &bin  coordirated data a t  u ~ d e r -  
standing the physicai ~ m a r n i c s  of the Jovian magnetospl-ere and the plasl-a processes 
that affect i t ,  -411 have major inheritance from Voyager, Pioneer Venus, ~ n d  other 
planetary and Earth-orbiting spacecraft. F i r s t  of these is the Xfagnetometer. mounted 
on a boom on the spinning part  of the spacecraft. Dual triaxial fluxgate magne:orn- 
eters a r e  used, sensitive to magnetic fields over thc dynamic range from near 1 
milligamma t o  a s  high a s  16 kilogamn~a. 
Also maunted on the magnetometer boom a r e  the antenuas for  the Plasma Wave 
Spectrometer, which directly measures the varying electric and magnetic fields in the 
Javian plasma. Frequency range i s  from 6 He to 300 kHz, 
Tbe Plasma investigation will measure both positive ions and electrons in the 
plas:llra we= an energy range from 1 eV to 50 keV. Energy spectra will be determined 
as a function of dkection wer essentially the entire celestial sphere. Miniature mass 
spcctrurneters uill  also identify several major ionic species, inchding several c o m p  
nents of the extended atmosphere of lo. 
=In Energetic hrticles experiment, also mamted on the spinning section of the 
Galileo orbiter, consists of a series of particle telescopes to determine the energy and 
angular distribution of protons, eIcct.mrs, and ions u a w  xn the Jovian magneto- 
sphere. The energy ranges up to 11 Me\' for electxws and 55 MeV per nucleon f o r  
ions. This experiment can also measure the composition of the trapped ions from He 
through Fe. 
The final two experiments are also mounted on the spiming part of the orbiter. 
The Electron Emitter i s  designed to clamp tbe spscecraft potential to that of the 
surrurmding piasma, The r)ust investigation has the g o d  of determining the physical 
and dynarnical properties of small dust particles or microme+morites in t-he -3a-im 
environment 3!ass, velocih-, and charge can 'be determined over the mass range from 
lo-'' to gram. This is a much higher sensitivity than has been achieved in past 
experiments to measure dust in the mter solar system. 
Possible hlodi ficatians for Saturn 
If the Calileo spacecraft proves satisfactory for the Jupiter missior., it will 
probably need only slight modificaticns for Saturn. The interfaces \\ill need substantial 
alteration to accommodate the ion drive propulsion system and the seccnd probe, but 
the basic spacecraft need not change significantly. 'less radiation shielding ~ t ~ i l l  be 
r e q u i d ,  but the spacecraft power sources and telemetry rate may need enhancement 
if the performance a t  Saturn is to equal that expected of Calileo at  Jupiter. 
Nore substantial changes may be needed in the scientific payload. On Galileo, 
tiere Is a heavy cmphasis on particles and fields n-zasurements: we do not yet hxow if 
Seturn has a substantial magnetosphere, and it is possible that after the Pioneer 11 
and Voyager flyoys we will not conclude that a similar emphasis in this area is needed 
for a Saturn orbiter. 
In the remu* sensing arerr, however, our present l i m i W  knowledge suggests 
two additions to the payload. Interest in atmospheric dynamics on Saturn and perhaps 
also on Titan argues for the addition of a second, wide-angle CCD camera to obtain 
synoptic gl@A-scale images. Such coverage will he very limited on Galileo, but in the 
case of Jupiter many goals in atmospheric dynamics can perhaps be carried out by the 
Space Telescope. For Saturn, in contrast, imaging from an orbiter seems to be 
required. The second new instrument suggested by discussions a t  &is workshop is a 
s-&millimeter radiometer for studies of the engs. A great deal of informaticm on 
particle sizes and bulk composition may reside in radiation emitted at wavelengths from 
100 pm to 1 mm, and no existing spacecraft instrument operates in this area. 
At present, of course, all of these suggestions concerning the detailed space- 
2 
craft design and science payload of a SOP mission are highly speculative. Only after 
the Pioneer 11 and Voyager flybys will we have the information needed to assess these 
issues prqerlq-. The purpose of the present exercise is to show that even the unmodi- 
fied Galileo orbiter and science instruments are reasonably well suited to a Saturn 
mission. 
REFERENCES 
Colsn. L. (1978). O;rtrr PI- Robc Mmara.  t k q ~ m .  ad Scracr. In &a duar 


Galileo orbiter spacecraft and instrumentation

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