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NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL
June 18, 2004 Current Operating Status
On September 1, 1988, Dr. Robert O. Pepin, chair of the Committee on Planetary
and Lunar Exploration, sent the following letter to Dr. Geoffrey A. Briggs, director of
NASA's Solar System Exploration Division.
At its June 1988 meeting, the Space Science Board's Committee on Planetary and
Lunar Exploration (COMPLEX) carried out a detailed assessment of the combined
Comet Rendezvous-Asteroid Flyby (CRAF) and Titan Probe-Saturn Orbiter
(Cassini) missions, which are proposed for new start status in FY1990. This
assessment included science objectives, mission profiles, and engineering
considerations for both missions, and was made with the assistance of
presentations by representatives of the mission Science Working Groups and the
engineering study team.
As you know, it is the practice of COMPLEX to assess the science content of
missions, as they near proposal as new start candidates, in order to measure how
well the agency has responded, in a mission context, to the committee's relevant
exploration strategies. The conclusions of these assessments are measures of the
support of the committee and the Space Science Board for the proposed planetary
missions. The committee also carries out further reviews during the development
periods of the missions leading to launch.
In two previous letter reports, dated May 31, 1985, and May 27, 1987, COMPLEX
has commented on the mission plans and the strawman and selected instrument
payloads for the CRAF mission, and evaluated these plans and instrumental
capabilities in the light of results from investigation of Comet Halley. The present
report again reviews the CRAF mission, this time in the context of the combined
CRAF/Cassini initiative. It also contains our first assessment of the agency's
mission strategy and strawman payload for Cassini, at a time prior to the issuance
of the Announcement of Opportunity for Cassini instrument selection, now
scheduled for December 1989, and instrument confirmation, anticipated in March
1991. This report does not review the CRAF and Cassini missions in the broader
context of NASA's overall priorities.
ASSESSMENT SUMMARY
The committee has not revised its earlier positive assessment of the overall ability
of the CRAF mission as currently designed to implement the COMPLEX strategy
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for exploration of comets and reconnaissance of asteroids. In the discussion that
follows we address a number of specific points, including progress in CRAF
instrument development, the prime and backup cometary targets, and changes in
spacecraft capabilities resulting from commonality in the Mariner Mark II design
that now supports both the CRAF and the Cassini components of the combined
program.
The committee finds that the science planning for the Cassini mission is
proceeding within the guidelines recommended by COMPLEX in its 1986 strategy
for the next phase of outer planet exploration. The strawman science instrument
payload and the preliminary mission profile are fully responsive, within the defined
scope of the mission, to the first priority of that strategy, a balanced
multidisciplinary investigation of the Saturnian planet-ring-magnetosphere-satellite
system. Within this balance, there is appropriate emphasis on Titan. Only in the
area of detailed characterization of Saturn's atmosphere, which lies beyond the
scope of Cassini, does the mission fail to address the primary science objectives
set out by the committee. We note that advantage of the opportunity for primitive
body reconnaissance has been taken by the inclusion of flyby measurements on at
least one asteroid in the current mission plan.
In the view of COMPLEX, the current status of Cassini as a joint NASA-ESA
venture is one of its major strengths. European fabrication of the Titan probe and
sponsorship of central scientific instruments and teams represent an
unprecedented and effective international sharing of the science payload, data
analysis, mission operation, and fiscal responsibilities of a planetary exploration
mission.
In the body of this report, we comment specifically on certain aspects of Cassini
science planning, and emphasize that attainment of mission objectives ultimately
depends on adequate support for development of capable science instruments. A
later assessment will readdress these issues, and deal in more detail with the
objectives and capabilities of the selected instruments in the context of the
measurement requirements recommended in the COMPLEX strategy report.
COMET RENDEZVOUS-ASTEROID FLYBY (CRAF)
A detailed summary of the rationale and goals of the COMPLEX strategy for
primitive body exploration, the specific comet and asteroid science objectives
identified by the committee, and the types and precision of measurements required
to implement these objectives, is given in our earlier letter reports on the CRAF
mission and will not be repeated here. In this assessment of the mission, the
committee reiterates its perception that the baseline objectives and measurement
capabilities of the selected science instrument payload are both congruent with the
prioritized scientific objectives recommended by COMPLEX for in situ comet
exploration and asteroid reconnaissance, and responsive to the specific
measurement requirements established by the committee to meet these objectives.
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We have previously focused some specific attention on two of the payload
instruments, the penetrator and the scanning electron microscope and particle
analyzer (SEMPA), both of which represent high-order advances in flight
instrument concept and design, sampling and measurement strategy, and analytic
techniques. Neither of these instruments has flight inheritance, and for these
reasons they were regarded as involving a relatively high degree of risk in fulfilling
primary mission science objectives. This concern has been addressed in part, as
expressed in our May 1987 letter report, by the identification of appropriate backup
capabilities in other instruments for characterizing solid materials in or from the
cometary nucleus.
The committee is pleased to note the following developments since its last review
of the CRAF mission:
The penetrator experiment. COMPLEX recognizes the unique ability of the
penetrator instruments to address in situ the highest priority science objective
identified in its strategy report, namely determination of the dust and volatile
composition, state, and physical properties of the nucleus. For this reason we
endorsed continued development of this experiment in our 1985 and 1987 letter
reports, while cautioning that substantial development work remained to be
performed during the accommodation study to demonstrate the reliability of
penetrator deployment and instrument function. We are informed that specific tests
have now been carried out to demonstrate the ability of the penetrator to implant
successfully in a range of materials representative of cometary nucleus models,
and that the project has switched from a solid to a liquid fueled penetrator rocket to
allow adjustment of penetration velocity based on assessment of nucleus material
properties derived by remote sensing from the main spacecraft. Additional tests
show that the gamma-ray spectrometer crystal can withstand the penetration
acceleration. In view of these results we extend our endorsement of penetrator
development for the mission. We are also informed that the project is building a
second penetrator for inclusion in the payload if spacecraft mass and performance
margins permit. The advantages of a second unit are evident, both as a fully
competent backup in the event of penetration or instrument failure in the first
deployment and as a means to assess the heterogeneity of the nucleus if both are
successful.
Scanning electron microscope and particle analyzer (SEMPA). The committee's
perceptions of this instrument and of the penetrator have been similar. Both are
technologically advanced, sophisticated, untested by flight experience, and in
concept uniquely capable of satisfying primary cometary science objectives, in
SEMPA's case the detailed chemical, mineralogic, and morphologic
characterization of individual cometary dust particles emitted from the nucleus. We
have endorsed the effort to develop SEMPA and to demonstrate its viability in the
spacecraft and cometary environments during accommodation study, and are
encouraged to learn that an engineering/research prototype of the flight instrument
has been constructed and is operating successfully in the laboratory.
Target comets and asteroids. Adoption of the CRAF/Cassini plan and the current
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schedule for the CRAF mission have necessitated a change in target comet from
P/Tempel 2 to P/Wild 2. In addition, several backup flight opportunities have been
identified, involving both P/Wild 2 and P/Kopff as potential targets. We have
previously commented (May 1985) on the suitability of P/Wild 2 for comet
investigations. From available data, the properties of both P/Wild 2 and P/Kopff
appear to be appropriate for achieving the scientific objectives of the CRAF
mission. The identified asteroid encounter for the prime mission, with
Eunomia—the largest S-type object in the belt, with unusual metal-rich and metal-
poor surface exposures according to Earth-based observation—appears admirably
suited for asteroid reconnaissance studies. It is our understanding that planned
flyby measurements include imaging at 165-m/pixel resolution, IR spectral mapping
and thermal mapping, and mass and density determinations. Although the full suite
of COMPLEX science objectives for asteroid exploration can be satisfied only by
multiple rendezvous missions, these measurements are in accord with objectives
for the reconnaissance phase.
Mariner Mark II spacecraft. We note that the only major engineering changes in the
previously planned CRAF mission resulting from the cRAF/Cassini combination are
in the area of spacecraft capabilities, and that they are very positive changes for
CRAF. Enhancements to the CRAF spacecraft resulting from commonality of
spacecraft design for CRAF and Cassini include addition of a second RTG,
doubling the power originally available at aphelion, and use of a larger Voyager-
class antenna in place of the smaller Viking-orbiter antenna. Resulting increases in
power and data rate will provide markedly superior measurement capabilities
during the initial reconnaissance, rendezvous, and penetrator deployment phases
of the mission. The committee wishes to commend the efforts of the engineering
development team in implementing a Mariner Mark II design philosophy of high
inheritance, judicious use of new technology, standardization, and modular design,
which we may expect will minimize both cost and risk without compromising
mission science objectives.
TITAN PROBE-SATURN ORBITER (Cassini)
Recommendations of the COMPLEX Strategy Report
In its 1986 report A Strategy for Exploration of the Outer Planets: 1986-1996,
COMPLEX has identified the highest priority for outer planet exploration within this
period to be the intensive study of Saturn—the planet, satellites, rings, and
magnetosphere—as a system. Major science objectives, unprioritized by the
committee, are exploration and intensive study of Titan's atmosphere and surface
and of Saturn's atmosphere, rings, small satellites, and magnetosphere. Specific
objectives for Titan include measurement of the composition, structure, and
circulation of the atmosphere, and characterization of atmosphere-surface
interactions; and reconnaissance of the physical properties and geographic
variability of the surface (solid or liquid, rough or smooth), with emphasis given to
any information needed ot guide the design of a lander vehicle. Specific objectives
for the Saturn system include determination of atmospheric elemental composition,
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dynamics, and cloud composition and structure, to a level well below the H2O
cloud base; measurement of ring particle composition and variety, spatial
distribution of ring particles, and the evolution of dynamic structures; comparative
determinations of satellite surface compositions, densities, geologic histories, and
geomorphological processes; and specification of magnetospheric structure,
dynamics, and processes, and of the mutual interactions of the magnetosphere
with Saturn's atmosphere, rings, icy satellites, Titan, and the solar wind.
The committee's preferred strategy was to implement most of these major
objectives in a single mission that would comprehensively address the top priority
of intensive, multidisciplinary study of the Saturn system and the interrelationships
among its individual phenomena. For missions of lesser scope—a category that
includes Cassini since it does not carry the probe required to address the first-
order objectives of Saturn atmosphere investigations—COMPLEX emphasized that
the interrelated nature of the system must be kept in mind in defining mission
objectives, and that the first such mission should include a reconnaissance of
Titan's surface for guidance of future exploration as well as for its scientific
importance.
Assessment of the Cassini Mission
The committee's preliminary assessment, based on expected Mariner Mark II
design and performance characteristics, the proposed mission profile, and the
objectives and measurement capabilities of the strawman science payload
instruments on both the Titan probe and the orbiter, is that the Cassini mission as
currently planned (1) reflects well the intrinsically high degree of importance,
excitement, and potential for major scientific return associated with the opportunity
to explore the Saturn system, and (2) is responsive to most of the specific
COMPLEX science objectives and to the necessity for a balanced approach,
across the broad range of multidisciplinary phenomena offered by the individual
components of the system, in meeting these objectives. The only primary objective
set out by COMPLEX that is not implemented by Cassini is detailed in situ
investigation of the composition, structure, and dynamics of Saturn's atmosphere
and clouds by an instrumented probe; we note, however, that remote atmospheric
investigations are effectively addressed by planned payload instruments, for
example by the capability of a new (non-Voyager) microwave radiometer to enable
studies of ammonia distribution and atmospheric temperatures near the ammonia
cloud level. Opportunities for satellite science measurements during the currently
planned 4-year orbital tour appear excellent: some 30-40 passes of Titan within
3000 km, and encounters with most satellites close enough to enable achieving
primary magnetospheric and ring science objectives by careful planning of the
orbital tour. Titan atmosphere objectives appear to be well addressed by current
plans for probe instrumentation and deployment. Specific comments relating to
current planning for asteroid science, Titan surface science, ring and plasma
investigations, and the science instrument payload are given below.
Asteroid science. We are pleased to note that the prime Cassini mission plan
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includes encounter prior to Earth flyby with 66 Maja, a 39-km C-type asteroid, in
keeping with the policy of the planetary exploration program to construct flight
profiles of outer solar system mission that enable close flybys of asteroids for
reconnaissance measurements. The project is encouraged to continue its efforts to
identify other possible encounter opportunities on the Earth-Saturn outbound leg of
the mission.
Titan surface science. COMPLEX was informed of the project's plans to conduct
surface science experiments on Titan in the event the probe survives landing. With
the exception of the possible addition of a 3-axis accelerometer package to
measure impact signature and assess surface properties, the current surface
science plan utilizes instruments already on the prove for atmospheric
measurements during descent, and is therefore achievable with minimal
incremental cost. The planned surface investigations—mass spectrometer and gas
chromatograph analyses of surface material, in addition to the accelerometer
measurements—could provide much more definitive information on surface
composition than that inferred from orbiter or probe-descent measurements. such
investigations exceed the 1986 report requirements for remote reconnaissance of
the surface from the orbiter or descending probe, and anticipate lander studies
envisioned for an advanced post-Cassini phase of exploration. Consequently, we
endorse the project's plan for surface measurements of this nature, but on an
opportunistic basis (i.e., if the probe survives, and if surface materials are
successfully introduced into the GC/MS and analyzed during the at most 15
minutes of available post-landing communication with the orbiter). At this time
CoMPLEX does not advocate major modifications to the probe to ensure survival,
nor addition of instruments beyond the accelerometer package specifically to do
surface science. Probe survival in its present design appears sufficiently likely, and
its instrument complement sufficiently competent, to carry out the project's present
surface science plan with some reasonable chance of success.
Ring and plasma studies. The committee finds that the objectives for ring science
will be effectively addressed by the strawman payload on the Cassini orbiter, with
substantial improvements over Voyager capabilities. Major direct contributions will
come from imaging, near and mid-far IR spectrometers, the UV imaging
spectrometer, the microwave radiometer/spectrometer, and the high-speed
photometer. Auxiliary but important ring-related data will be provided by the dust
analyzer and plasma instruments, and perhaps by the Titan radar.
The time period around Saturn orbit insertion provides a unique opportunity to
achieve important science objectives that will not recur later in the mission profile
as currently designed. The committee recognizes that the primary mission
responsibility at this time is the successful execution of the orbit insertion
maneuver, and that spacecraft configuration and power resource allocation must
be dictated by this priority. We recommend, however, that the mission plan include
science observations to the maximum extent feasible during the time when the
spacecraft is inside 2.3 Saturn radii, particularly on magnetic field ring and plasma
measurements may prove to be possible. Ideally these would include imaging the
F, A, and B rings at resolutions that may approach 100 m, spectroscopic
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measurements at otherwise unattainable spatial resolutions, and in situ
measurements of the plasma and the ring atmosphere. Also worthy of
consideration, although probably more difficult to implement, would be radar
observations of the rings using the Titan imaging radar and/or sounder. The period
of time around orbital insertion is also the most favorable for measurements of the
cosmic-ray albedo neutron environment and the atomic hydrogen ring atmosphere.
It additionally provides the only possible opportunity to study the plasma
environment of the main rings, which bears on mechanisms for spoke formation
and has implications for Saturn's ionosphere. The ring plane crossings also
represent a unique opportunity to study dust and plasma populations and
electrostatic plasma waves that are known to be localized near the ring plane.
Science instruments. The selected instrument complement for CRAF and the
strawman payload for Cassini are comprehensive and well balanced, and can be
expected to implement the science objectives for both missions as set out in the
relevant COMPLEX exploration strategies. The committee urges that appropriate
levels of funding be made available to support development of these science
instruments for flight. We emphasize that our scientific assessments of both
missions are predicated, in the final analysis, on the actual availability of the
instruments and associated measurement capabilities upon which those
assessments have been based.
Please feel free to contact the committee with any questions you may have
regarding this report, or for further discussion.
Last update 5/18/00 at 10:02 am
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