An assessment study of a Saturn-Orbiter plus Titan-probe mission was made. The NASA Solar System Exploration Committee (SSEC) had recommended two separate missions to the Saturn system, in keeping with its charter to design low-cost, dedicated planetary missions. These were a Titan probe, to be carried by a small spacecraft that would include some type of radar mapping device, and a Saturn Orbiter that would be a more sophisticated spacecraft, to be launched separately. A Saturn orbiter and a Titan probe are combined in a single mission that would be carried out in collaboration with NASA. It is this proposal, called the Cassini Project, which was approved by NASA and ESA for an assessment study to be carried out over the next year and a half. Details of the Cassini mission are discussed

Owen, Tobias

English

NASA Technical Reports Server

THECASSINIMISSION
N87-17638
Tobias Owen
State University of New York at Stony Brook
This is an interim report on an assessment study of a Saturn-Orblter plus
Titan-probe mission. The study was jointly undertaken by the European Space
Agency (ESA) and NASA in response to recommendations from a committee estab-
lished by the European Science Foundation and the U. S. Space Science Board.
This committee relied in part on earlier work by European and U.S. scientists.
The NASA Solar System Exploration Committee (SSEC) had recommended two sepa-
rate missions to the Saturn system, in keeping with its charter to design
low-cost, dedicated planetary missions. These were a Titan probe, to be
carried by a small spacecraft that would include some type of radar mapping
device, and a Saturn Orbiter that would be a more sophisticated spacecraft,
to be launched separately. This proposal became part of the SSEC Core
Program of missions planned to occur before the year 2000. A competitive
plan, to fly a duplicate of the Galileo spacecraft (including a Saturn probe)
to Saturn had floundered because of budgetary considerations. At about this
time (fall of 1983), a group of European scientists led by D. Gautier and
W.-H. Ip had proposed to ESA to combine a Saturn orbiter and a Titan probe in
a single mission that would be carried out in collaboration with NASA. Each
agency would assume responsibility for one of the major components. It is
this proposal, called the Cassini Project, which was approved by NASA and ESA
in January 1984 for an assessment study to be carried out over the next year
and a half.
Obviously, to do better than Voyager did at Saturn, we must either get more
specialized or more sophisticated, or both at once if we can possibly do it.
Having recovered from the extreme conservatism that dominated mission plan-
ning in the early stages of the SSEC work, the Cassini Science Study Team
perhaps went overboard in assembling a strawman payload. That meant adding
instruments that Voyager didn't have, and taking instruments that Voyager did
have and making them better, or more suited to the Saturn-Titan system. We
certainly wanted to fly a radar in order to study the surface of Titan through
the satelllte's ubiquitous aerosol cover, and to do much better studies of the
rings. We can get closer to the various components of the system by using an
orbiter, and we can do so several times. Of course the probe of Titan's
atmosphere is an enormous step forward. We can spend up to four years in a
series of orbits around Saturn, exploring much more of the magnetosphere than
Voyager could. This would include measurements from orbits at high inclina-
tion angles. Yet we must keep expenses down. This requires collaboration
between at least two partners to divide up the cost of the mission.
The Cassini Project is easy to divide. NASA is taking on the commitment to
build the orbiter, which will be one of the new Mariner Mark II spacecraft
the agency is planning. The Europeans are building a new lightweight probe,
especially configured for the atmosphere of Titan. Europeans and Americans
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will share in the experiments to be included in both the probe and the orbi-
ter in various ways that have not yet been formally defined.
At the present time, the launch date is set for 1994, with a Delta-Vega tra-
jectory that leads to arrival at Saturn in the year 2002. This puts a strain
on most people's calendars, but that's the best we can do if everything goes
exactly as planned at present. (Since this preview was presented in May
1985, the Assessment Study was completed on schedule in June. Copies may be
obtained by writing to Mr. John Beckman at JPL. The formal evaluation of
this study by the ESA review panels occurred in January 1986. The panel
recommended initiation of a Phase A study in January of 1987.)
A llst of the people involved in the Study is given in Table I. In addition
to these individuals, we have invited a number of other scientists to attend
our meetings in Europe and in the U. S. The spacecraft configuration is
still being changed in response to science and engineering requirements. A
major driver for the orbiter is to achieve overall compatibility with other
Mariner Mark II missions. The June 1985 version of the spacecraft is shown
in Fig. I.
Table 1
Participants in Cassini Assessment Study
ESA NASA
G. Haskell
D. Gautier
W.-H. Ip
S. Bauer
M. Fulchigonl
W. Piotrowskl
T. Owen
M. Allison
J. Cuzzl
D. Hunten
T. Johnson
H. Masursky
R. Samuelson
E. Sittler
F. Scarf
Unlike the Jupiter system that Galileo is visiting, Saturn has only one large
satellite, so the spacecraft repeatedly encounters Titan to "power" the orbi-
tal tours. We get over 20 encounters with Titan at about 1000 km, so we can
actually use the orbiter as an upper atmosphere probe. Therefore, the aeron-
omy instruments will be on the orbiter rather than on the probe. We'll also
get two close encounters with Iapetus and frequent close visits to the inter-
esting inner satellites. Radar mapping of Titan is one of the key experi-
ments being proposed for the orbiter. The level of sophistication of this
device is still being studied. The magnetospherlc scientists would like the
orbiter to move up to a high inclination orbit toward the end of the nominal
four-year mission.
232
Figure  1. The conf igu ra t ion  of t h e  C a s s i n i  s p a c e c r a f t  as of December 1985. 
233 
DR. HUNTEN: I think planetary people would like a high inclination too.
DR. ALLISON: But not in the magnetotail.
DR. OWEN: The consensus of the group is remarkable as you can see. The
Titan probe entry and descent is very different from the Galileo entry probe
profile. We have nearly four hours of descent time through Titan's atmos-
phere, which means that one can capture samples and analyze them almost at
leisure. There are proposals for imaging in the near infrared, and possibly
with radar as well. The probe will land or splash-down at relatively low ve-
locities, 3 to 4 meters per second. Byron Swenson, who has worked this out,
says the impact is similar to a drop from a height of 18 inches on Earth. So
if you can jump 18 inches, you'll find out what the impact on Titan will be
like.
DR. HAPKE: Can the probe float?
DR. OWEN: The probe can indeed float. A quick study of the splash-down
indicates that the probe would be completely submerged on impact, but would
then bob up to the surface. However, immersion in this 94 K ocean poses a
tough thermal problem, and no effort is being made to solve it. The guide-
lines we're working with are that surface science must not drive the mission,
but if the probe can carry out some useful measurements after the landing
with no extra effort, it will certainly do so.
Figure 2 provides an idea as to what the satellite encounters are like. It
is based on a presentation by Walter Flury of ESOC for one of the orbital
tours that has been studied. Voyager 1 and Voyager 2 miss distances are com-
pared with Cassini. It is apparent that except in the case of the Voyager I
encounter with Titan, we get much better satellite encounters than Voyager
was able to achieve. And we get more of them. In place of Voyager's single,
intensely interesting Titan encounter, Cassini will provide over 30 such
visits.
Highlights of the proposed orbiter payload include the fact that we are
insisting on both a long-focus and a short-focus camera in order to provide
adequate resolution at various distances. The radar is very much under dis-
cussion, and there is a battery of infrared instruments which is also not yet
well-deflned. More detailed definition of these instruments will be a prime
early task for the Phase A studies. What are the trade-offs between using a
microwave radiometer to study Titan's surface, and using the radar instrument?
What combination of infrared instruments gives the greatest amount of infor-
mation about the atmospheres of Titan and Saturn? And so on.
Most Of the instruments on the probe have been flown before. There is a pro-
posal to try to combine a gas chromatograph and a mass spectrometer, but this
requires careful evaluation. An aerosol collector should be included as one
of the inlet systems for this instrument if its efficacy can be demonstrated.
Imaging on the way down, perhaps combined with some new infrared capability
is also under review. Just to give a feeling for what kind of "free" surface
science can be done, note that a GCMS on a probe that lands in an ocean can
do some pretty nice experiments just by having a suitably designed, heated
inlet.
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CLOSEST
APPROACH
(103 Km)
9OO
8OO
700
60O
5OO
4OO
500
200
I00
---.I VGR 1
--...o VGR 2
-.---I CASSINI
\
\
I
/ \
/
/
4
I
I
I
I
?
I
I
I
I
I
I
I
I
I
I
MIMAS TETHYS RHEA HYPERION
ENCELADUS DIONE TITAN IAPETUS
Figure 2. A comparison of flyby distances in the Saturn system for
Voyagers 1 and 2 and the Cassini Orbiter.
235
The four-year lifetime of the orbiter mission means that there will be an
opportunity to do quite a lot of surveying on various aspects of this system.
We expect many new phenomena to be discovered, since the Voyager encounters
were very brief. The details of the tour have not been worked out, but we
are very, very early in the design of the mission and orbital tours are one
of many aspects that will receive lots of additional study in the years
ahead. I am sure that many readers of this short report will become parti-
cipants in the future studies and in the mission itself, if it is approved.
All suggestions for improving the science return from this exciting enter-
prise will be very welcome.
DR. ROSSOW: We have time for a few comments.
DR. HUNTEN: Let me make one comment about the prospects. A couple of weeks
ago the meeting of the Space Science Board was attended by ESA top manage-
ment--namely the Director General and the Scientific Director. They seemed
to be taking it for granted that the Cassini study is going to go into Phase
A. They don't even regard it as an issue.
DR. POLLACK: I'd like to make a couple of quick comments on something that's
germane to Cassini as well as some other missions, and has to do with the
possibility of gaining very significant meteorological information by measur-
ing horizontal winds by Doppler tracking of the probes. Some of you may know
that I've been working very hard in terms of examining the feasibility of
this for the Galileo probe. The good news is that if we can do it for Jupiter,
and I am optimistic that we can, then it's much easier to do so for the other
planets including Saturn, Uranus and Neptune as well as for Titan. I just
want to quickly give people a feeling for what the status of that is. First,
why is measuring the horizontal winds as a function of altitude by Doppler
tracking of the probe important? The point is that depending upon whether the
winds are driven by deep convection as Andrew Ingersoll indicated, or driven
by water condensation or solar energy deposition, you have distinctly different
altitude profiles for the resulting zonal winds. Consequently, there is a
significant amount of useful information to be gained for any outer planet or
any satellite that has an appreciable atmosphere by Doppler tracking a probe.
In the case of Galileo, the reason that this is such a tremendous challenge is
that unless you know quite accurately what the longitude of the probe is (and
I degree is really not good enough because of the large rotation), you may
potentially have a serious problem in determining the wind profile. In the
case of Saturn and even more so for Titan, it would be a somewhat easier
situation. Even in the case of Galileo going into Jupiter, we are optimistic
that we really can recover wind information. Work that I've done with David
Atkinson at Ames indicates that given significant probe uncertainties in
longitude for even a relatively difficult test case profile, we would in fact
have been able to obtain a very good recovery on wind profile and the longi-
tude of entry with a linear least squares technique. The bottom line is that
we think Doppler tracking of probes has something very relevant to offer in
terms of putting good constraints upon dynamical theories of Jupiter, Titan,
Saturn and the rest of the outer planets.
236
qDR. BELTON: I was very glad to see that you have two cameras, i.e., compli-
mentary optical systems on Cassini. I hope that you will pursue this, and
maintain this right through the whole exercise.
DR. OWEN: That was the voice of Galileo imaging, which was unfortunately
denied the possibility of two cameras through budgetary constraints. It is
certainly our intent to maintain this dual camera capability, if at all
possible.
237


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