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OCR for page 3
Chapter 1
INTRODUCTION
In June 1968 the Space Science Board made a study of Planetary
Exploration 1968-1975~ in which the principal conclusion was
that lithe planetary exploration program be presented~ not in
terms of a single goal, but rather in terms of the contribu-
tion that exploration can make to a broad range of scientific
disciplines."
This view was further expressed in a series of recommen-
dations for missions to all targets that seemed practicable
at that time, with measurements to forward the objectives of
all disciplines involved in the space program. In particular
the need for exploration of Venus was brought out.
To achieve such a broad and flexible program, the study
emphasized the need for modest and relatively low-cost mis-
sions. A prime recommendation ,vas "that NASA initiate now
a program of Pioneer/IMP-class spinning spacecraft for orbit-
ing Venus and Mars at each opportunity~ and for exploratory
missions to other targets."
While developments since 1968 tend to modify specific
points made in the 1968 study, the spirit of the recommenda-
tions is as important today as it was then. Nevertheless~
the program of the National Aeronautics and Space Administra-
tion contains no significant Venus missions (the Venus/Mercury
flyby is essentially a Mercury mission with only a small con-
tribution to Venus science) and no small missions to the inner
planets in the Pioneer/IMP class (which includes the Planetary
Explorer discussed in this report).
At the same time a strong Mars program has developed,
with a very ambitious program funded for the early 1970's.
There is also an ambitious program for Grand Tour missions
to the outer planets proposed in the 1969 NAS study The Outer
Solar System: A Program for Exploration.
The present study was convened to re-examine the ques-
tion of Venus exploration in the light of the proposed NASA
programs and the scientific developments since 1968. In the
event that an imbalance in planetary-exploration strategy
should be found, the study had, as a second objective, the
task of recommending appropriate Venus missions.
3
OCR for page 4
4
The study was concerned with relative priorities within
the NASA planetary-exploration program. There are, however,
larger issues: the relative priorities between different parts
of NASA's unmanned science program (astronomy, applications,
lunar exploration, space physics, space biology); the relation-
ships between manned and unmanned programs; and the effect of
changing national priorities upon both programs. These issues
are the concern of an extensive Space Science Board study, also
carried out during the summer of 1970, under the chairmanship
of Herbert Friedman. The two studies came together because a
preliminary version of our recommendations was available to the
Friedman study and influenced its conclusions.
The reason for the exploration of Venus is related to the
justification for exploring the solar system, as well expressed
in the NAB studies: Space Research/Directions for the Future;
Planetary Exploration 1968-1975; and The Outer Solar System:
A Program for Exploration. Nevertheless, it is appropriate
briefly to address the question: Why explore Venus now?
On the most general level, the answer to the question is
tied to the remarkable evolution of planetary studies during
the 1960's. Not long ago planetary studies were a part of
astronomy, often accorded a different treatment and status
from solar physics and astrophysics but not yet related to
any other discipline. The advent of the space probe in the
1960's provided the tool and some of the data for the detailed
investigations and model building characteristic of, and essen-
tial to, terrestrial studies. Very rapidly studies of plane-
tary meteorology, planetary aeronomy, planetology, and plane-
tary biology emerged which involved, in the main, research
workers from the parallel terrestrial disciplines. Earth and
planetary studies suddenly merged and simultaneously diverged
from astronomy. In some major universities, departmental and
research center organization was changed to meet this develop-
ment.
From an intellectual point of view we have moved to the
position in which terrestrial studies have expanded to include
the whole of the solar system. Problems such as the origin of
the solar system, the origin of life, and the large-scale pro-
cesses that control man's environment will in the future be
considered in terms of many rather than a single object. The
detail will obviously differ from one planet to another: from
the immense detail required on earth for environmental pur-
poses and made possible by the comparative accessibility to
the very small amounts of information on Uranus, Neptune, and
Pluto and the difficulty of substantially increasing this
information.
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5
Thus the call for a balanced approach to planetary explo-
ration has a firm logical basis, although the word "balanced"
is not clearly defined in terms of the relative priority of
new objectives and the investigation in depth of more familiar
problems. In whichever way the situation is viewed, however,
the absence of almost any firm information about the solid sur-
face and lower atmosphere of Venus is the most striking \.veak-
ness in our information about the inner planets.
If we consider the need for Venus research in terms of
particular examples we find striking examples of how knowledge
of this planet can illuminate our knowledge of earth. The sur-
prise effect of the unexpected ionospheric data on both Mars
and Venus has led to timely re-examination of fixed ideas about
the earth's ionosphere. Studies of atmospheric circulations in
slowly rotating atmospheres have provided new ideas applicable
to tropical meteorology. The extensive cloud systems of Venus
have led to investigations of the coupling between clouds and
motions not yet undertaken on earth. The question of why Venus
has a complete cloud cover and a high surface temperature is
beginning to interest those concerned with possible environmen-
tal changes on earth. This is not so much in the belief that
earth might go the way Venus if the atmosphere is suffi-
ciently polluted but rather with the thought that Venus is an
observable example of a class of problem that concerns our own
environment.
One final answer to the question, !I\.Jhy explore Venus now?"
is that events have conspired to produce a situation in which
exploration of Venus is unusually timely and rewarding. De-
spite Venera entry probes and Mariner flybys, we know very
little about the lower atmosphere of Venus, and yet we have
many competing theories the merits of which can be tested with
relatively simple measurements. For example, we do not know
how many and how thick are the cloud layers, let alone the
composition of the uppermost. We know that lower-atmosphere
motions are of central importance to all aspects of the phy-
sics and chemistry of the atmosphere, but we know nothing
about them. At least three independent hypotheses exist with
regard to the high surface temperatures.
In each of these major areas rather elaborate theories
have been developed, generally of a quantitative nature, so
that they can be tested by means of a few well-defined mea-
surements with proven instruments. This is a situation that
occurs but rarely and should be exploited when it does. It
provides a further and powerful reason for a strong program
of exploration of Venus in the coming decades.
OCR for page 6
6
The Study Group was able to determine at an early stage
that the developments in Venus science since 1968 heightened
the need for substantial effort to explore the planet. Con-
siderable attention was therefore directed to the question of
the appropriate exploration strategy.
Although since 1968, no programs have been approved or
funded involving missions to Venus or small spacecraft, it has
been a period of planning activity by NASA, its centers, and
its contractors. This planning has taken place in close con-
tact with scientific groups from outside the agency, and the
result has been a particularly rewarding interaction among
scientists, engineers, and planners, which in this group's
opinion represents a significant advance in the planning of
space missions. This Study Group was, therefore, presented
with a series of detailed and careful studies of the explora-
tion of Venus by means of Planetary Explorer spacecraft (see
Chapter 6).
Our study benefited greatly from access to these NASA
plans. In general, we found them to be well conceived and
almost ideally suited to the requirements of scientific in-
vestigation in the 1970's and 1980's. As a consequence, our
recommendations are detailed and circumstantial and often
presented in terms of this particular spacecraft.
