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Chapter 2
RECOMMENDATIONS
SCIENTIFIC RETURN FROM VENUS MISSIONS
In this report we examine the state of knowledge of the atmo-
sphere of Venus, the important questions that can now be asked,
and the relatively straightforward means by which these ques-
tions can be answered.
Noteworthy in this context is the extraordinary flow of
ideas since the U.S. Mariner 5 and the Soviet Venera 4, 5, and
6 missions. Compared to the range of questions of first mag-
nitude that remain to be answered the contribution from these
space missions was small. Nevertheless, the few pieces of in-
formation that they contributed made the investment of time
and effort into the complex process of geophysical modeling
worthwhile, and we now have stimulating quantitative theories
about the atmosphere and surface of Venus.
A strong scientific interest in Venus now prevails among
geophysicists and astrophysicists. This interest can best be
maintained by a series of missions that will permit broad par-
ticipation by scientists in the U.S. planetary program.
Another important reason to support the exploration of
Venus is that we must have more information on that virtually
unknown planet in order to obtain some of the data necessary
to unravel the puzzle of the origin of the solar system--a
question of major interest to laymen and scientists alike.
Furthermore, a better understanding of the atmosphere of
Venus will make important contributions to our understanding
of generalized atmospheric problems and, hence, lead to added
understanding of our own atmosphere. The problems of Hadley
circulations and interactions between clouds and atmospheric
dynamics are two cases in point. The long-range climatic ef-
fects of pollutants on the transfer of solar and infrared ra-
diation is another.
We believe that the combination of scientific goals and
the feasibility of contributing to these goals makes the ex-
ploration of Venus one of the most important objectives for
planetary exploration in the 1970's and 1980's. And we,
7
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therefore, recommend that exploration of Venus be prominent
in the program of the National Aeronautics and Space Adminis-
tration during the 19701s and 19801s.
THE VALUE OF SMALL MISSIONS
The idea of small, minimum-cost exploratory missions has immedi-
ate appeal to large sectors of the scientific community. This
is in part accounted for by a feeling of responsibility toward
the taxpayer and society. It also results, however, from the
real advantages that accrue from projects involving minimum
funding and therefore minimum complexities of planning and or-
ganization, personnel, and collaboration between organizations.
Exploration 1968-
The Space Science Board study,. PZanetar>y..
19?5~ took place at a time when the Mariner program had estab-
lished some of the fundamental physical parameters of the inner
planets and most of the technology needed for future missions.
As a result of this experience, the study chose, as the pre-
ferred course for the early 19 IS to go in the direction of
smaller, lower-cost missions. The first priority recommended
by the study was lIaprogram of Pioneer/IMP-c.1ass [Planetary
Explorer] spinning spacecraft for orbiting Venus and Mars at
each opportunity, and for exploratory missions to other tar-
gets.1i That entry probes were not included in this statement
merely reflects. the prevailing climate of engineering thought
at that time: entry probes did not come to mind as practical,
inexpensive spacecraft. Since then, the success of the Venera
probes and a number of engineering studies have greatly changed
the situation; moreover, it has become clear that the entry
probe is an ideal vehicle for attacking a number of prime
scientific questions.
There are many contributing reasons for this support for
small missions:
1. If the cost is sufficiently low, a series of missions
can be envisaged. This opens upm~ny opportunities, novel to
planetary exploration. One is that relatively high-risk ex-
periment.s can be undertaken if a high return of scientific
information can be foreseen. Initiative is encouraged if the
cost of failure is reduced.
2. A series of missions can be planned, Missions can
build upon the observations of previous ones and upon the new
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theories and speculations to which they may give rise. Early
observations can often be kept much simpler if there is an ex-
pectation of more elaborate measurements to follow, based upon
the preliminary results. It is, of course, necessary to de-
velop organizational techniques to take advantage of this pos-
sibility. It is barely possible to make some changes in in-
strumentation from One VenuS opportunity to the next. It
should, however, be relatively easy to ensure complete payload
redesign, if necessary, every second opportunity (i.e., at
approximately 3-year intervals).
3. A series of missions can stimulate the active parti-
cipation and collaboration of many scientists and scientific
disciplines. Moreover, with many opportunities, the value of
the program to the educational process is enhanced because it
becomes possible to involve less-senior experimenters--even
to make use of graduate students under supervision.
4. A series of missions can, in times of fiscal strin-
gency, be reduced in frequency without a complete cutoff of
the program and the attendant loss of experimenters. Because
of the nature of NASA funding, projects are not always funded
as predicted, and it is therefore healthy to maintain the max-
imum flexibility in this respect.
5. It is difficult to plan science far in advance. On
the other hand, a very expensive mission is often inflexible.
This can lead to a mismatch between the desire for unfettered
and innovative thinking On the one hand and responsible tech-
nical planning on the other. This gap closes as the cost of
the mission decreases, although it cannot ever be expected to
disappear. Scientists, engineers, and administrators each
have to modify their attitudes when collaborating in a large
project, but the Planetary Explorer concept should ensure that
the difficulties are minimized.
6. The Planetary Explorer concept makes it possible to
use probes, landers, and orbiters in combination, each sup-
porting the other, in planetary exploration. There are ex-
amples in this report in which orbiter science can prepare
for better probe science and vice versa, and lander science
can benefit from both. Thus, atmospheric-probe measurements
can establish the wind-noise spectrum that might be encoun-
tered by a landed seismometer--a parameter essential to de-
fine sensitivity.
When the above advantages of small missions are combined,
the value of this concept becomes clear. We are not without
previous experience of this kind of development. The
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exploration of the earth's atmosphere increased greatly in pacEc
and became a different kind of program when the low-cost Aero-
bee rocket replaced the V2 and Viking rockets in the early
1950's. Although the Aerobee has only arrow flight stability
and a relatively small payload, and it was a long step back-
wards in sophistication, it has been a workhorse for aeronomy
and astronomy observations with which generations of experi-
menters have been well satisfied. We may expect that the
Planetary Explorer will playa comparable role in NASA's plane-
tary program.
The rest of this report summarizes the results of our ex-
amination of the scientific questions posed by Venus, of the
potential for rapid advance in our knowledge of the planet,
and of the Planetary Explorer as the prime vehicle for this
advance in the next 10 years.
We peoommend the Planetary Explorer as the prime vehicle
for the initial exploration of Venus by orbiters, atmospheric
probes, and landers, because this inexpensive, Delta-launched
spacecraft has the capability and versatility to obtain an-
swers to most of our prime questions about Venus and to aid
in defining the environment of Venus for possible bigger mis-
sions in the future.
OF MINIMUM COST
ACHIEVEMENT
As discussed below, the cost of a given set of hardware can
depend greatly on the philosophy adopted, especially the
amount of testing required. Most of the benefits discussed
in the previous section are a direct consequence of small
project size, which implies a proportionately low cost. Every
effort should be made to keep testing and paperwork to the
minimum required for a reasonable assurance of success. As
the total cost of a mission diminishes, the ideas of what is
reasonable can be relaxed. Recent practice in the planetary
program has been to require extra flight instruments just for
testing and to undertake environmental and life tests whose
total cost may far exceed that of the instrument itself. In
most rocket work, on the other hand, reliance is placed on
the use of high-quality parts and careful design, with only
brief tests in a simulated environment. The quality of the
product is still high. For a given mission the philosophy
chosen should lie somewhere between these extremes, but we
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suggest a considerable relaxation of the standards previously
applied to the large planetary missions.
Another area that can greatly affect costs is the choice
of experiments. For example, complex mechanisms may need to
be developed for some purposes~ but they tend to be expensive
and unreliable. In general, such equipment should be avoided
unless there is an overriding scientific requirement.
We recommend that NASA be prepared to accept a somewhat
higher risk for the Planetary Explorer than has been its prac-
tice for previous planetary missions) if substantial over-all
cost savings can be achieved.
Because of our minimal experience in carrying out un-
manned experiments on a remote planetary surface, and because
of the high temperature and pressure of the surface of Venus,
we recommend that experiments to be carried on early lander
missions emphasize simplicity of operation and) where possible,
avoid complex manipulation and processing of surface material.
INSTRUMENT DEVELOPMENT
Missions to Venus, particularly those entering the atmosphere,
NASA
will require a wide range of novel scientific equipment.
has already shown its willingness to support the development
of new instruments~ for example, those needed for cloud studies.
To publicize this fact, we recommend that NASA announce the
existence of a series of opportunities listing some of the more
crucial needs. Such an announcement, coupled with a general de-.
scription of the expected spacecraft and their capabilities~
should stimulate much of the necessary work.
Current technology appears to be unable to provide a
landed mission with a lifetime of more than a day or two, be-
cause of the need to keep down the temperature of the instru-
ments and batteries. Instruments such as seismometers can
reap their greatest benefit only with a much longer period of
operation. Therefore, we recommend that NASA encourage en-
gineering research and development to provide~ if possible,
lander systems and experiment components able to operate at
temperatures up to 800 K.
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QUARANTINE REQUIREMENTS
could
A slight possibility exists that terrestrial organisms
grow on airborne particles near to the cloud tops of Venus.
The problem was discussed at the 1970 CaSPAR meeting, and some
interest was expressed in investigations of airborne life.
Life on Venus is no more than a remote contingency, but the
possibility of contamination by terrestrial organisms must be
considered.
The saving feature of all Venus missions is that there
is no longer any doubt that a temperature of about 700 K pre-
vails over the entire surface of the planet. There is no pos-
sibility that terrestrial organisms can grow at such tempera-
tures, and we are therefore at worst concerned with a short
period of transit through the cooler regions of the atmosphere.
According to the CaSPAR agreements, the cumulative proba-
bility up to 1988 of contaminating the planet must be less than
10-3. With 20 miss~gns, the probability per mission must then
be less than 5 x 10 We are satisfied that this constraint
.
is readily met, even if the bus or orbiter should enter the
atmosphere. These unshielded vehicles will mostly vaporize
in the upper atmosphere, and at most a few charred members may
fall rapidly through the temperate region of the cloud tops.
For numerical estimates we may start with the figures given
in the Planetary Explorer, Phase A Report (Goddard Space Flight
Center, October .1969, Section 6 and Appendix C). The number of
spores is taken as 104. The probability of release in the at-
mosphere under the above circumstances is estimated to be less
than 10-3; we regard the Goddard figure of 0.3 as far too high
for atmospheric release, because it was based
on a hard-surface
of growth was given as 10-4,
impact. The probability but this
assumes the presence of a stable particle or droplet to grow
on. However, droplets are subject to evaporation, while solid
particles must be subject to rapid mixing to support them a-
gainst fallout; they will therefore reach a hot region in a
short time. We believe that the probability gf growth in the
atmosphere should be amended to less than 10- for a total
probability of contamination per impact of less than 10-5.
We therefore see no reason why the bus or orbiter should
not be permitted to impact the planet whenever a scientific
benefit is to be gained thereby. Low-periapses orbiters should
also be open to consideration. Surface-sterilized entry
probes, hermetically sealed and with a fully sterilized heat
shield, present a far lower probability of contamination than
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do the bus or orbi~er, and risk of contamination from them
may be neglected.
We therefore recommend that, with some precautions, space-
craft be allowed to impact the planet when scientific benefit
is to be gained thereby.
CONTINUING PLANNING GROUP
The fullest possible benefit of the Planetary Explorer con-
cept can only be realized by an integrated series of missions,
as discussed in Chapter 6. At any given time there will be a
set of prime questions requiring answers; but some of them
cannot be asked at present. Also, certain importantexperi-
ments will require a further definition of the environment by
a previous mission before they can operate to full efficiency.
(For example, television is useless if there is dense fog at
the surface.) The best available mechanism to assure that the
scientific requirements are met is the planning group, which
has already been used successfully for the Venus-Mercury Mari-
ner and the Viking programs. But for a Planetary Explorer
series it is.not enough to have such a group for each indi-
vidual mission: at least some continuity is necessary over
the series.
In addition, the highly integrated nature of the experi-
ments on a particular mission suggests that the traditional
payload concept--a collection of separate experiments indi-
vidually conceived--is not valid. Rather, each mission should
be regarded as a single experiment with the individual experi-
mental subsystems carefully chosen to complement one another
and to maximize the scientific return within the mission con-
Therefore, we reaommend that NASA set up and main-
straints.
tain a continuing planning group for the exploration of Venus
which will advise on strategy for the mission series and on
conceptual payloads for each mission.
FUTURE MISSIONS
The strength of the Planetary Explorer concept as discussed
in this report is due in great part to quite recent improve-
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ments in launch capability~ communications~ and science instru-
mentation. We have been able~ therefore~ to demonstrate how
most of the important first-order questions about Venus can be
answered by a series of Planetary Explorer missions. In a
longer view, questions arise regarding the continuation of this
strategy.
As far as we wish to predict~ the study of planetary aer-
onomy and particles and fields can be continued indefinitely
within the Planetary Explorer concept. The two requirements
for a full investigation of the first-order questions are (1)
measurements within the upper atmosphere from entering vehi-
cles and from low-periapsis orbiters and (2) measurements of
particles and fields from orbit, preferably with two vehicles
at the same time.
Atmospheric studies made during the recommended Planetary
Explorer missions will be extremely fruitful in defining the
chemical and physical makeup of the atmosphere. The poten-
tially great complexity of the cloud physics and of the cir-
culation, however, leaves the possibility that these processes
may remain poorly understood. Considering their great rele-
vance to the determination of the high surface temperatures,
it may become desirable, on the basis of the first missions,
to have a more advanced effort to answer these questions. No
guess as to the appropriateness of Planetary Explorer technol-
ogy to advanced work is possible, although the recent rate at
which capabilities have improved leaves ground for optimism.
Most basic questions about the solid planet itself can be
answered only by measurements made on the surface. It appears
possible to make some important measurements of this kind with
missions of the modest scale proposed. However, the lander
mission suggested in this report could be as far as currently
defined Planetary Explorer technology can go in this direction.
If passive seismology proves viable, future visits to the sur-
face ought to be made at three sites simultaneously. Analysis
of surface materials can be definitive only if several sites
are visited and if several fairly complicated experiments are
carried out, involving, for example~ long lifetime, prepara-
tion of samples, and vacuum conditions. As far as we wish to
foresee, this sort of thing goes beyond the current Planetary
Explorer concept in cost and complexity. We think that such
a program will not be desirable until the 1980's~ and we sus-
pect that improvements in launch vehicle and instrument tech-
nology in the interim will greatly increase the viability of
any more ambitious type of mission.
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INTERNATIONAL COLLABORATION
International collaboration projects in geophy~ics--the Inter-
national Geophysical Year, the Upper Mantle Project, and the
International Year of the Quiet Sun--have contributed greatly
to the development of knowledge of man's environment. Further,
the International Geodynamics Project has recently been launched
by the International Council of Scientific Unions to study the
fundamental dynamical processes within the earth's interior
which are responsible for crustal movements--e.g., earthquakes.
Development of international collaboration in planetary explo-
ration is a natural extension of these successful scientific
policies in geophysics, in the development of which the United
States has played a leading role. The proposed program of in-
vestigation of Venus is scientifically the broadest yet proposed
for a planetary investigation involving most of the subdisci-
plines of geophysics. Thus it is most suitable for collabora-
tion with scientists of other nations.
We therefore recommend that NASA actively seek the col-
laboration of other national space organizations in planning
and carrying out these investigations.
EARTH-BASED STUDIES
Optical and radio (including radar) studies of Venus have been
valuable in the past and are expected to remain so. We rec-
ommend that NASA continue to support and develop earth-based
studies, and we commend its past efforts in this respect. Im-
provement of radio facilities can be expected to give better
thermal maps, as well as radar maps of the surface comparable
to earth-based optical photographs of the moon. Optical work
at the cloud tops can give long-term information on inhomo-
geneities and on the possible four-day atmospheric rotation.
The techniques complement one another and give valuable sup-
plementary information for the Planetary Explorer missions.
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VENUS/MERCURY FLYBY
We understand that the Venus/Mercury flyby scheduled for launch
in 1913 is intended primarily for the exploration of Mercury.
However, the opportunity exists for valuable measurements dur-
ing the flyby of Venus. These measurements, particularly high-
resolution, high-contrast imaging of the cloud layer, would
play an important role in advance planning for the Planetary
Explorer program. Specifically, imaging is not contemplated
on the early Planetary Explorer missions. Results from the
Venus/Mercury flyby could help to optimize the payloads for
later Explorer orbiters. For these reasons, we endorse the
Venus/Mercury mission for the contribution it can make to
Venus science.
