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The Role of Small Missions in Planetary and Lunar Exploration: Chapter 1
The Role of Small Missions in
Planetary and Lunar Exploration
1
Background and Current Environment
In the 1960s and early 1970s, planetary science was carried out by small
spacecraft in modest programs, such as Mariner and Pioneer. These programs
were highly successful and revolutionized our knowledge and under- standing of
the planets. As a result, more complicated spacecraft with more ambitious
objectives were constructed. Many of these (e.g., Viking, Voyager, Magellan, and
others) were also spectacularly successful. However, the evolution from small to
large missions, coupled with shrinking budgets, led to increased time between
missions and thus fewer opportunities for innovation and discovery.1
The excessive reliance on large missions in the late 1970s and 1980s has
been unhealthy for planetary science. Missions are few and far between, and
failures-when they occur-threaten the entire planetary science program.
REPORT MENU Moreover, the budgetary challenges facing our nation mandate that the future
NOTICE exploration of the solar system will be increasingly constrained. Large and
MEMBERSHIP complex missions that address a broad range of scientific objectives, such as
PREFACE Galileo and Cassini, despite their virtues, will be flown less often, if ever. To
EXECUTIVE SUMMARY
address these problems, NASA is seeking to establish small-mission programs,
CHAPTER 1
two examples of which are Discovery and Mars Surveyor (see Box 1.1 for a
CHAPTER 2
discussion of the relative cost of large and small planetary missions). The
CHAPTER 3
implementation of a program of focused, small planetary missions in the context
CHAPTER 4
of a balanced approach to solar system exploration has advantages not only for
CHAPTER 5
science, but also for education and technological development.
APPENDIX
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Box 1.1 Mission Cost
Comparing the cost of one space science mission to another is an endeavor
replete with pitfalls. Besides relatively simple factors such as changing inflation rates,
what should and should not be included in a mission budget? Among the costs that
could, and perhaps should, be legitimately included are those for spacecraft
development, the launch vehicle, tracking, mission operations, and data analysis, but
this approach has not always been taken. Indeed, practices have changed over the
years, and ft is difficult for non-experts to know if they are comparing apples with
apples or apples with apples and oranges.
COMPLEX, a committee with expertise in the planetary sciences, clearly
identifies itself as non-experts when it comes to discussions of mission costs. The
committee recognizes, however, hat no consideration of the merits of small missions
is complete without a clear understanding of what "small" actually means. The basic
question, of course, is how many Discovery missions are equivalent, financially, to a
Magellan or a Galileo?
With no special expertise in mission economics, COMPLEX has instead
gathered data from people who are experts. Between 1988 and 1994, for example, the
General Accounting Office and the Congressional Research Service issued a number
of reports analyzing the costs of NASA planetary missions.* Reconciling the data in
the-se disparate reports would, however, be a study in its own right. The same could
be said about any attempt to reconcile other independent analyses of mission costs. To
avoid these problems, COMPLEX requested relevant data directly from NASA's
Office of Space Science. The figures shown in Table 1.1 are all in 1994 dollars and
are calculated in such a way that the cost of one mission can be compared directly
with the costs of others. Mission operations costs are also given for each of these
missions.
____________________
*General Accounting Office, Space Exploration: Cost, Schedule, and Performance of
NASA's Magellan Mission to Venus, NSIAD-88-13OFS, Washington, D.C., May
1988; General Accounting Office, Space Exploration: Cost, Schedule, and
Performance of NASA's Mars Observer Mission, NSIAD-88-137FS, Washington,
D.C., May 1988; General Accounting Office, Space Exploration: Cost, Schedule, and
Performance of NASA's Galileo Mission to Jupiter, NSIAD-88-138FS, May 1988;
General Accounting Office, Space Exploration: Cost, Schedule, and Performance of
NASA's Ulysses Mission to the Sun, NSIAD-88-129FS, Washington D.C., May 1988;
General Accounting Office, Space Science: Causes and Impacts of Cutbacks to
NASA's Outer Solar System Exploration Missions, NSIAD-94-24, Washington, D.C.,
December 1993; General Accounting Office, NASA Program Costs: Space Missions
Require Substantially More Funding Than Initially Estimated, NSIAD-93-97,
Washington; D.C., December 1992; and Congressional Research Service, Library of
Congress, Big Science and Technology Projects: Analysis of 30 Selected U.S.
Government Projects, 94-687 SPR, Washington, D.C., December 7, 1994.
Other disciplines such as astrophysics, space physics, and earth science
have successfully established small-mission programs. Perhaps because
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The Role of Small Missions in Planetary and Lunar Exploration: Chapter 1
interplanetary (deep-space) missions are often more challenging than Earth-
orbiting missions, attempts to develop a comparable line of small planetary
missions have not succeeded. The Planetary Observer program of the early
1980s was the most public attempt by NASA to initiate a continuing, small-
mission planetary line, but it was not approved by the Office of Management and
Budget and Congress. Mars Observer, which was to be the first of this series,
overran its initial budget by a large factor, for various reasons, including a
changing external environment and NASA mismanagement,2,3 and no
subsequent Observer missions were flown.
TABLE 1.1 Mission Cost
Total Cost
Program (1994 $ million) Mission Summary Launch Date
Mariner Mars '71 489.7 2 orbiters (4 instruments May 1971
Operations 94.1
Mariner Mercury-Venus 366.1 1 spacecraft (6 instruments) March 1973
'73
Operations 27.2
Pioneer 10, 11 351.6 2 spacecraft (11 instruments) March 1972 and April
1973
Operations 50.8
Viking 3282.6 2 orbiters (4 instruments), 2 August 1975 and
landers (13 instruments) September 1975
Operations 116.3
Voyager 807.7 2 spacecraft (13 instruments) August 1977 and
September 1977
Jupiter and Saturn 160.1
operations
Uranus operations 159.9
Neptune operations 135.7
Pioneer Venus 444.7 1 orbiter (12 instruments), May 1978 and August
1 probe bus (2 instruments), 1978
1 large probe (7
instruments),
and 3 small probes (3
instruments)
Operations 35.1
Magellan 625.1 1 orbiter (3 instruments) May 1989
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Operations 118.3
Galileo 1478.2 1 orbiter (11 instruments), October 1989
1 probe (7 instruments)
Operations 491
Mars Observer 617 1 orbiter (7 instruments) September 1992
Operations 41.8
Near-Earth Asteroid 145 1 spacecraft (6 instruments) February 1996
Rendezvous
Operations 38.4
Mars Global Surveyor 131.2 1 orbiter (6 instruments) November 1996
Operations 22.7
Mars Pathfinder 167.5 1 lander (3 instruments) December 1996
Operations 14.9
Cassini 1424.1 1 orbiter (12 instruments October 1997
plus 2 instruments for
European Space Agency
probe)
Operations 490
SOURCE: Data from Wesley Huntress, Jr., Associate Administrator, Office of Space Science,
NASA.
A primary problem of a planetary program that consists only of large
missions is that risk becomes unmanageable. The great cost and importance of
any single mission encourage NASA to apply expensive procedures (e.g.,
redundancy, complex "fail-safe" software, backup spacecraft, superfluous tests,
and excessive review) in an endeavor to mitigate risk. Such attempts to eliminate
risk have greatly increased the cost of large missions, without clearly increasing
their reliability, as Galileo and Mars Observer have demonstrated. The avoidance
of risk also leads to engineering conservatism, which has delayed the
introduction of some promising technical advances into planetary missions and
may have been partly responsible for the 1992 cancellation of the Comet
Rendezvous/Asteroid Flyby (CRAF) mission.
The scarcity of opportunities, which is a direct consequence of the
increased cost and high risk of large missions, leads to even more complex
missions. If there will be only one mission to a given planet in many years,
scientists insist that it answer as many questions as possible. Thus missions are
ambitious, with an array of instruments, high data-transmission rates, and
complicated mission profiles. This in turn raises risk, complexity, and cost,
ensuring that future mission opportunities will be scarce.
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As mentioned above, the idea of establishing a line of small, low-cost
planetary probes is far from new. It is worthwhile to recall the circumstances of
three previous attempts to develop small missions (Planetary Explorer, Planetary
Observer, and Lunar Scout) and three current examples of low-cost deep-space
missions (Clementine, Discovery, and Mars Surveyor).
PLANETARY EXPLORER
In 1968, the Space Science Board recommended "that NASA initiate now
a program of Pioneer/Interplanetary Monitoring Platform-class spinning
spacecraft for orbiting Venus and Mars at each opportunity, and for exploratory
missions to other targets."4 Subsequent studies conducted by NASA, industrial
contractors, and the planetary science community led to the concept of a low-
cost, spin-stabilized Planetary Explorer spacecraft. This universal bus, following a
Delta launch, could deploy a variety of scientific payloads, including atmospheric
probes, landers, or orbiters.
Planetary Explorer received a major endorsement in Venus: Strategy for
Exploration (the so-called "Purple Book"), issued by the Space Science Board in
June 1970. That report recommended that the Planetary Explorer concept be
used "as the prime vehicle for the initial exploration of Venus . . . ."5
Simple, low-cost missions have obvious attractions. Those enunciated by
the 1970 report included the following:
A series of missions can be planned;
high-risk experiments offering high-scientific return can be undertaken;
The participation of, and collaboration between, many scientists and
scientific disciplines would be enhanced;
International cooperation would be promoted;
Education would be furthered through the participation of "less senior
experimenters-even . . . graduate students under supervision"; and
Programmatic flexibility would be strengthened in times of rapid
scientific advance and/or fiscal uncertainty.
Low cost would be encouraged by:
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The Role of Small Missions in Planetary and Lunar Exploration: Chapter 1
Keeping paperwork to a minimum;
Avoiding "complex mechanisms" unless there is a definite scientific
requirement;
Standardizing hardware to the maximum extent possible; and
Having NASA accept a greater level of risk than in previous planetary
missions.
The Purple Book outlined a series of Planetary Explorers to be sent to
Venus at every launch opportunity in the period from 1975 to 1980. The first
mission would consist of a bus carrying four (one large and three small)
atmospheric probes and would be followed by an orbiter, a lander, and finally an
atmospheric probe equipped with balloons. In November 1971, NASA
discontinued work on Planetary Explorer at Goddard Space Flight Center for
programmatic reasons and transferred it to Ames Research Center, where it
continued under the new name Pioneer Venus. No subsequent Planetary
Explorers were built.
Unfortunately, what the Space Science Board had envisaged as a low-
cost program using tried and true instruments and an innovative approach to
management, rapidly "crystallized as a single opportunity mission-a Multiprobe
and an Orbiter that reflected significant and major advances in the sophistication
of spacecraft and their instrumentation. . . .6 These missions received a new start
in FY 1975, and, after surviving a number of development problems, serious
budgetary crises,7 and threats of cancellation, Pioneer Venus I (Orbiter) and
Pioneer Venus 2 (Multiprobe) were launched on May 20 and August 8, 1978,
respectively.
The atmospheric probes returned data on December 9, 1978, as they
descended through Venus's atmosphere for almost an hour. Five days earlier,
the Pioneer Venus Orbiter had settled into an elliptical orbit about the planet.
Although designed to operate for only 2 years, it continued radioing data back to
Earth until October 8, 1992.
PLANETARY OBSERVER
The most prominent attempt to initiate a series of low-cost planetary
science missions was made by NASA's Solar System Exploration Committee
(SSEC) in the early 1980s as part of an effort to develop a stable and affordable
mission strategy during a period of budgetary scarcity.8 These small missions
were to be called "Observers" and were intended to have much the same
character as today's Discovery missions.
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These missions (primarily planetary orbiters) would take advantage of
modified versions of production-line Earth-orbital platforms and use mature
instrumentation. The missions were to be managed by a NASA center and the
science payloads to be selected in the traditional manner in response to a
competitive Announcement of Opportunity. The launch vehicle designated by the
national launch policy of the time was the space shuttle.
In practice, what seemed like a modest innovation in approach turned out
to be a continuing problem and, ultimately, a technical failure. The payload
selected was too ambitious for a low-cost mission. The competitively selected
spacecraft for Mars Observer required much more modification from production-
line, commercial, Earth-orbital systems than anticipated. At the same time, the
fixed-price contract lacked the flexibility to allow the changes needed as the
mission evolved, primarily due to external factors. A switch in launch vehicle (to a
Titan III), resulting from the Challenger disaster, caused major technical
revisions, changes in risk mitigation policies, painful descoping of the payload,9
and a costly schedule delay that compounded all of the above problems. Further,
the budget available to the project was driven significantly by external factors
during the several years in which NASA's space science program was
reconstituted in the post-Challenger years.
The first Observer mission-the Mars Geoscience/Climatology Orbiter,
later renamed Mars Observer-was also the last. This outcome had become clear
as costs escalated during the period of mission redesign following the loss of
Challenger. Plans to follow the first mission with a Lunar Observer using a similar
payload were shelved (see the section "Lunar Scout" below), as were proposals
to employ this Moon-bound spacecraft as a backup for the Mars mission.
Standing on its own, the Mars Observer project was required to acquire spare
systems to provide insurance against the possible failure of the (as yet untested)
upper-stage launcher or of the spacecraft itself; this requirement added
significantly to costs. Ironically, when Mars Observer was in fact lost just before
its scheduled insertion into Mars orbit in August 1993, NASA decided not to use
the spares to rebuild the spacecraft (although major components of Mars
Observer will be used by Mars Surveyor missions).
The Mars Observer Failure Review Board's report10 listed many general
concerns (some of which are mentioned above), including top-level systems
engineering and management inadequacies, as well as specific technical
problems, that were probable sources of the loss. One of the principal lessons
learned was that too much reliance was placed on the heritage of the
spacecraft's hardware, software, and procedures. This approach was
inappropriate, given that the spacecraft was fundamentally different from the
Earth-orbiting weather satellites from which Mars Observer and, thus, its heritage
were derived. Another lesson learned was the failure of Mars Observer's
manufacturer to make the best use of the Jet Propulsion Laboratory's experience
in building planetary spacecraft. The Review Board also criticized the static
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nature of the mission's acquisition and management strategy even as Mars
Observer evolved from the first in a series of Planetary Observers to a unique
mission. The Review Board recommended that "many of these concerns should
be carefully considered by NASA management, since they have the potential to
affect future spacecraft developments and operations." Indeed, it is clear that the
numerous lessons learned-by NASA, by industry, and by the science community-
from the failed Observer experiment should be applied to all new deep-space
programs, and especially to those that are substantially cost- constrained.
LUNAR SCOUT
The detailed exploration of the composition and geophysics of the Moon
has long been of high priority in the planetary science community. Besides being
an object of purely scientific interest, the Moon is an obvious target for future
voyages by astronauts. Political and budgetary complications arising from
interactions between human exploration and scientific goals profoundly
influenced the Ranger, Surveyor, and Lunar Orbiter programs in the 1960s11 and,
more recently, led to the collapse of a proposed series of small lunar science
missions.
In the 1970s, the preferred way to augment the data gathered by the early
lunar robotic probes and by the Apollo missions was to place a satellite equipped
with sophisticated remote-sensing instruments into polar orbit about the Moon.
The proposed mission, the Lunar Polar Orbiter, was complex and expensive, and
it never was flown for a variety of programmatic reasons, including cost. The
Planetary Observer program (see above) offered a new opportunity to initiate
more modest lunar science missions. When the Observer line ended with Mars
Observer, a proposal was made to create a Lunar Observer using spare Mars
Observer hardware. This craft, like its martian counterpart, was, however, in the
$500-million-plus price range.
The Space Exploration Initiative (SEI), put forward by then-President
Bush in 1989, created new pressures to gather the lunar science data needed to
support the planned extensive program of human exploration. Adding the
necessary instruments to the proposed Lunar Observer raised its cost to $1
billion or more. A more modest plan, utilizing a series of small, low-cost orbiters
(Lunar Scout) and less well defined landers (Artemis), was adopted by NASA's
Office of Exploration. COMPLEX assessed the proposed payloads of the first two
Lunar Scouts and found them generally responsive to the priorities for lunar
science stated in the committee's past reports.12 Although the Lunar Scouts had
a sound scientific foundation, political support for an expensive human
exploration program and anything relating to it was absent. The Lunar Scout and
Artemis programs collapsed when funding for the SEI failed to materialize and
the Office of Exploration was disbanded.
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Clementine
The only recent, completed example of an approach to substantially
reduce the cost of deep-space probes has been Clementine, a technology-
demonstration mission sponsored by the Ballistic Missile Defense Organization.13
NASA participation in the mission was limited to communications support,
scientific advice on the mission design, and analysis of the data collected.
Clementine was built by the Naval Research Laboratory and carried an
instrument payload of 7 kilograms, including ultraviolet-visible imagers, near- and
long-wavelength infrared cameras, and a laser altimeter. Following launch on a
refurbished Titan 2G intercontinental ballistic missile on January 25, 1994,
Clementine was placed into a polar orbit around the Moon a month later.
Although it successfully completed three months' worth of lunar
measurements, along with many of its technical goals, a software error triggered
an uncontrollable spin and Clementine was unable to make the transfer
maneuvers necessary to fly past the near-Earth asteroid 1620 Geographos. As a
result, Clementine did not have the opportunity to attempt one of its major
technical goals, the autonomous acquisition of a moving target. Despite this
failure, the mission was viewed in the popular press as a success; it is interesting
to speculate whether a NASA mission that did not achieve an important objective
would have been treated as favorably.
Clementine must in COMPLEX's view be counted as a practical
demonstration of a quick, low-cost mission, even though it was not driven by
science. The lessons learned in this mission need to be probed to a greater depth
than is possible in this report; only then can they guide the success of other low-
cost missions.14
DISCOVERY AND MARS SURVEYOR
Two programs for a series of planetary missions with limited objectives,
Discovery and Mars Surveyor, are currently under development. NASA received
new starts in FY 1994 for a pair of small missions: Near-Earth Asteroid
Rendezvous (NEAR) and Mars Pathfinder, which were called "Discovery"
missions although they do not now satisfy current guidelines for this program.
NEAR, which is to be launched in February 1996, will rendezvous with the near-
Earth asteroid 433 Eros for a year starting in December 1998. This mission will
carry 55 kilograms of remote-sensing instruments, including a visible imager, a
near-infrared spectrograph, a laser altimeter, and x- and gamma-ray
spectrometers. These will be used to determine the asteroid's surface geology
and, insofar as possible without in situ measurements, its bulk properties and
composition. The spacecraft is being built and managed by the Johns Hopkins
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University's Applied Physics Laboratory.
Mars Pathfinder, which is being constructed by NASA's Jet Propulsion
Laboratory, will serve as a science and engineering test of the entry, descent,
landing, and deployment systems for future small Mars landers. Its payload
includes a microrover (supplied by NASA's Office of Space Access and
Technology), an imaging system, and devices for assaying the local soil. The
instrument packages for both Clementine and NEAR are all facility instruments
and were preselected, as was the Pathfinder concept, and so none of these
missions fully satisfies the mission concept that the committee considers below.
NASA has proposed that NEAR and Mars Pathfinder form the start of a
new program, the Discovery program, whose stated goals are to "increase flight
rates and launch certainty, complement large missions to keep a steady rate of
incoming planetary data, broaden university and industry participation in solar
system exploration missions, and increase public awareness of solar system
exploration missions."15 This proposed program envisages a range of missions
and targets. NASA's FY 1996 budget proposes a new start for a third Discovery
mission, Lunar Prospector. The status of this mission was uncertain at the time
this report was written.
The Mars Surveyor program, which received approval in the FY 1995
budget, is conceived to be a series of low-cost missions (both orbiters and
landers) that are concentrated on the Red Planet, a particularly high-priority
target. According to current plans, the first phase of this program will involve the
launch of a low-cost orbiter, Mars Global Surveyor, equipped with a subset of
Mars Observer's instruments, in November 1996. Some 10 months later, the
spacecraft will employ aerobraking to place itself in a Sun-synchronous orbit
about Mars. The second phase of the Mars Surveyor program will occur some 26
months later, with the launch of an additional orbiter possibly carrying some of
the remainder of Mars Observer's payload, and a small lander, possibly derived
from Mars Pathfinder. Subsequent missions, involving a separate orbiter and
lander, will follow at each possible launch opportunity. The landers, possibly
deployed in concert with international partners, will address strongly focused
science objectives, such as assessing the distribution of water at the surface of
Mars, determining the planet's interior structure, or seeking clues of past climatic
changes in the mineralogy of weathered products.
For this report, COMPLEX did not further consider the Mars Surveyor
program because, at least in its initial stages, it will not be run by principal
investigators (Pls), as COMPLEX believes other small programs should be. In
addition, its later (i.e., lander) stages are not currently defined in sufficient detail
to assess fully. Nevertheless, many of the observations made below in this
document also apply to this program. COMPLEX recommends that the option of
using a lead-PI approach to later Mars Surveyor missions be studied. The
question of whether Discovery missions to Mars should compete within the Mars
Surveyor program needs additional review.
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REFERENCES
1. NASA Advisory Council, Space and Earth Science Advisory
Committee, The Crisis in Space and Earth Science, NASA, Washington, D.C.,
1986.
2. Polk, Charles, Mars Observer Project History, NASA-Jet Propulsion
Laboratory, Pasadena, Calif., 1990.
3. Travis, John, and Cohen, Jon, "Mars Observer's Costly Solitude,"
Science 261, pages 1264-1267, September, 3 1993.
4. Space Science Board, National Research Council, Planetary
Exploration 1968-1975, National Academy of Sciences, Washington, D.C., 1967,
page 5.
5. Space Science Board, National Research Council, Venus: Strategy for
Exploration, National Academy of Sciences, Washington, D.C., 19'70, page 1.
6. Fimmel, Richard O., Colin, Lawrence, and Burgess, Eric, Pioneer
Venus, NASA SP-416, NASA, Washington, D.C., 1983.
7. Comptroller General of the United States, National Aeronautics and
Space Administration Should Provide the Congress with More Information on the
Pioneer Venus Project, General Accounting Office, PSAD-77-65, Washington,
D.C., 1977.
8. National Aeronautics and Space Administration, Solar System
Exploration Committee, Planetary Exploration Through Year 2000: A Core
Program, Government Printing Office, Washington, D.C., 1983.
9. Space Science Board, National Research Council, Letter report
regarding an assessment of the impact on integrated science return from the
1992 Mars Observer mission, from the Committee on Planetary and Lunar
Exploration to Geoffrey A. Briggs, NASA, July 12, 1988.
10. Mars Observer Failure Investigation Board, Report of the Mars
Observer Failure Investigation Board, NASA, Washington, D.C., 1994.
11. Space Studies Board, National Research Council, Science
Management Strategies for the Human Exploration of Space, in preparation.
12. Space Studies Board, National Research Council, Scientific
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Assessment of Proposed Robotic Lunar Missions of NASA's Office of Exploration
Letter report from Committee on Planetary and Lunar Exploration to Michael D.
Griffin (NASA), August 21, 1992.
13. Nozette, S., et al., "The Clementine Mission to the Moon: Scientific
Overview," Science 266, pages 1835-1839, December 16,1994.
14. Space Studies Board, National Research Council, Lessons Learned
from the Clementine Mission, National Academy Press, Washington, D.C., 1995,
in preparation.
15. National Aeronautics and Space Administration, Solar System
Exploration Division, Discovery Program Handbook, NASA, Washington, D.C.,
1992, page 1.
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