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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 the CRAF/Cassini Mission"
On March 30, 1992, Dr. Louis J. Lanzerotti, chair of the Space Studies Board, sent
the following letter to Dr. Lennard A. Fisk, associate administrator for NASA's
Office of Space Science and Applications.
At its meeting on February 18 and 19, 1992, a subpanel of the Space Studies
Board's Committee on Planetary and Lunar Exploration (COMPLEX) chaired by
Professor Peter H. Stone, of the Massachusetts Institute of Technology, carried out
a detailed review of the CRAF (Comet Rendezvous - Asteroid Flyby) and Cassini
(Titan Probe-Saturn Orbiter) missions. This review was part of COMPLEX's
continuing advisory program to assess the responsiveness of NASA missions to
science objectives given in COMPLEX's published strategies for exploring the solar
system. The results of this review were presented to the Space Studies Board at its
meeting on February 26-28 for consideration in the broad context of the status and
outlook of the U.S. civil space research program.
COMPLEX's review was planned well before the release of the President's budget
message on January 29. That message proposed cancellation of CRAF and called
for a reassessment of the technical and schedule risks in the Cassini program.
Nevertheless, COMPLEX proceeded with its review to assist those who must
respond to the President's proposal. We recognize that scientists, as others, are
subject to the effects of large budget deficits and that research must compete with
other national needs. We feel obligated, however, to articulate the consequences
of budgetary decisions. This letter summarizes the Board's overall assessment of
the two missions and is accompanied by a summary providing COMPLEX's
detailed scientific evaluation.
In brief, the Board recognizes that the current and near-term national budget
environment severely constrains the conduct of the nation's space research
program. Therefore, the Board recommends that NASA carefully reevaluate the
Cassini spacecraft and instrument complement with the objective of ensuring the
mission's prospects for adequate and stable funding leading to the scheduled 1997
launch, while retaining the maximum science content possible. This reevaluation
should take into account cancellation of CRAF, if this mission is indeed canceled
as proposed in the President's FY 93 budget message. Based on COMPLEX's
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evaluation, it is the strong recommendation of the Board that a scientifically
responsive Cassini mission, reconfigured if necessary, proceed to development
and launch on the present schedule.
The Board is dismayed by the proposed cancellation of the CRAF mission, which
would be of great scientific merit even without the comet penetrator experiment.
However, the Board recognizes that present and anticipated resources are not
likely to be adequate to successfully undertake both missions and to meet the
science objectives of both at this time.
The U.S. program of outer solar system exploration has brilliantly demonstrated
American vision and technical mastery. The United States, alone, has undertaken
and completed the initial reconnaissance of the major planets of the outer solar
system, visiting in turn Jupiter, Saturn, Uranus, and Neptune and obtaining
revolutionary data about these planets and their atmospheres, moons and rings,
and plasma environments. The Board believes that a vigorous program of outer
solar system exploration is an essential part of a national space exploration
agenda. Because of the very long travel times to the outer solar system, seven or
more years, it is important not to interrupt development of our next mission or delay
its launch. The Saturn system, with its complex interacting system of magnetic
fields, plasmas, rings, and moons, is an ideal laboratory for many of the physical
processes believed to be important in the formation and present-day dynamics of
our solar system and of planetary systems of other stars. It is for these reasons
that the Board believes that high priority within the broad civil space agenda should
be attached to the ongoing U.S. Saturn exploration program. We further believe
that the Cassini mission should proceed without delay in order to benefit from the
extremely favorable orientation of Saturn's rings at the spacecraft's projected
arrival in 2004.
This review of the Cassini mission was COMPLEX's first since the initial selection
of the instrument payload and has been completed prior to the final confirmation of
these instruments. Thus, COMPLEX's conclusions are based on the current state
of definition of the mission. Given the pending confirmation of the payload, and the
programmatic changes that could result from the proposed cancellation of CRAF
and technical reassessment of Cassini, COMPLEX plans to reexamine Cassini at a
later time.
Scientific Assessment of the CRAF And Cassini Missions
March 30, 1992
Summary
At its meeting on February 18 and 19, 1992, a subpanel of the Space Studies
Board's Committee on Planetary and Lunar Exploration (COMPLEX) chaired by
Professor Peter H. Stone, of the Massachusetts Institute of Technology, carried out
a detailed review of the CRAF (Comet Rendezvous-Asteroid Flyby) and Cassini
(Saturn Orbiter-Titan Probe) missions. This review was part of COMPLEX's
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continuing advisory program to assess the responsiveness of NASA missions to
science objectives given in COMPLEX's published strategies for exploring the solar
system.
It is COMPLEX's opinion that Cassini is highly responsive to the scientific priorities
set out in its report, A Strategy for Exploration of the Outer Planets: 1986-1996. 1
The instrument payload that has been tentatively selected, the mission plan that
has been outlined, and the spacecraft that is being developed together provide an
excellent opportunity to advance our understanding of Saturn and its satellites,
rings, and magnetosphere. The Saturn system is unique within the solar system
because of the wide variety of interactions—electrodynamical, hydrodynamical,
and gravitational—among the system's different components. Improving our
understanding of these interactions is important for developing better theories of
evolution of the early solar system and of planetary and satellite systems in
general. In addition, study of Titan's atmosphere is of high priority because it has a
composition and chemistry that may be similar to Earth's early atmosphere. The
Cassini mission as currently configured is extremely responsive to the objective of
studying the Saturn system as a whole.
COMPLEX notes with concern that present budget constraints are jeopardizing all
of the planetary program's large missions, including Cassini. The recent
reconfiguration of the Earth Observing System into a series of small spacecraft
might be thought to provide a guide for the achievement of science goals outside
the context of large missions. Such an analogy is inappropriate for Cassini. The
long travel times between Earth and the outer solar system require long-lived
components, specialized power systems, and long-distance communications
fundamentally different from those required for Earth-orbital missions. With current
technology, any mission sent past the asteroid belt must be more than a Discovery-
class mission. While intermediate-size missions (larger than Discovery class, but
smaller than Cassini) could undoubtedly achieve some of COMPLEX's objectives
for the Saturn system, they could not achieve many others. For example, studies of
the interactions between the different components of the system, and concurrent
coordinated observations of Titan's atmosphere by both the Huygens probe and by
remote sensing instruments, require large suites of instruments that place heavy
demands on the spacecraft's resources. Thus COMPLEX believes that the Cassini
exploration of the Saturn system cannot be fully accomplished by reconfiguration
into one or more small spacecraft.
COMPLEX views with dismay the proposal to cancel the currently approved CRAF
mission. This mission is the outcome of many years of planning by numerous
groups of distinguished scientists, NASA centers, and competitively selected
scientific instrument teams. In proposing and planning CRAF, the research
community fully recognized the importance of assessing priorities in choosing to
pursue this major endeavor.
COMPLEX has long articulated the unique scientific opportunities provided by the
in situ study of cometary nuclei, believed to be the best-preserved relics of the
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earliest history of our solar system. The report Strategy for the Exploration of
Primitive Solar System Bodies—Asteroids, Comets, and Meteoroids: 1980-1990, 2
assigned highest priority to reconnaissance and initial exploration of comets, with
special emphasis on the rendezvous mode planned for CRAF. This mode is
essential for studying the sequence of events that occurs as a comet approaches
and recedes from the sun. The CRAF mission has been developed in full
accordance with the science objectives and recommendations of COMPLEX. In
addition, the mission incorporates an excellent set of asteroid flybys, another high-
priority recommendation of COMPLEX.
COMPLEX recognizes that budget constraints have forced significant changes in
CRAF since its last review in July 1990. These changes were the deletion of the
penetrator experiment (PENL) and of the Scanning Electron Microscope and
Particle Analyzer (SEMPA) experiment, a launch delay, a change in the mission's
targets, and an increase in the required lifetime of the mission. However, in
COMPLEX's opinion, these changes do not invalidate its earlier judgments. CRAF
remains a scientifically sound mission, responsive to COMPLEX's most important
near-term priorities for the exploration of primitive solar system bodies.
Cancellation of CRAF will not lessen the importance of these scientific objectives,
which should be pursued at the earliest possible opportunity.
Cassini
COMPLEX's 1986 report, A Strategy for Exploration of the Outer Planets: 1986-
1996, states that the highest priority for outer planet exploration in the next decade
is intensive study of Saturn—the planet, satellites, rings, and magnetosphere—as
a system. Specifically, the recommended exploration and intensive study of the
Saturn system include the following objectives:
Titan's atmosphere: Measure the composition, structure, and circulation
q
of Titan's atmosphere, and characterize the atmosphere-surface
interaction;
Titan's surface: Carry out a reconnaissance of the physical properties
q
and geographic variability of Titan's surface;
Saturn's atmosphere: Determine the elemental composition, dynamics,
q
and cloud composition and structure, to a level well below the H2O cloud
base;
Saturn's rings: Measure particle composition and spatial distribution,
q
determine the evolution of dynamic structures, and search for
shepherding satellites;
Saturn's small satellites: Make comparative determinations of surface
q
composition, density, geologic history, and geomorphological processes;
Saturn's magnetosphere: Specify the structure, dynamics, and
q
processes, and the interactions of the magnetosphere with Saturn's
atmosphere, rings, icy satellites, Titan, and the solar wind.
These objectives can be met with an appropriately chosen mission profile and
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complement of scientific instruments, mounted on a spacecraft with sufficient
power and communications capabilities. COMPLEX is favorably impressed with the
progress made by the Cassini Project in the design of such a spacecraft and
mission. The spacecraft accommodates the Cassini science requirements, while
being flexible to mission changes. It appears to be a robust and capable carrier for
the Cassini investigations. The level of maturity in the design is high for the current
phase of development. It is clear that a number of difficult problems have been
solved while maintaining prudent engineering margins. (In addition to adequately
meeting the Cassini requirements, the spacecraft will also serve the needs of
CRAF.)
The instrument payload selected for Cassini is highly responsive to most of the
important science objectives for the Saturn system. Following is a summary of the
information that the currently configured mission will be able to obtain for each of
the major components of the system.
Titan
Many of the scientific goals for Titan will be addressed by the Huygens probe, the
component of the Cassini mission supplied by the European Space Agency (ESA).
Its current suite of instruments, complemented by spectroscopic and radar
observations from the Cassini orbiter, will do an excellent job of fulfilling these
goals by providing a first characterization of Titan's atmosphere and surface. The
probe's instruments include gas and haze-particle analyzers, capable imaging and
spectral radiometers, as well as atmosphere profilers to determine temperature
and pressure. Doppler tracking of the descent will provide the first direct
measurement of Titan's atmospheric circulation. Near the surface the probe
instruments will measure the composition of the atmosphere, the shock of
landing—different for a solid as opposed to a liquid surface—and the density and
refractive index of the liquid surface, if present.
Although not in orbit around Titan, the Cassini orbiter will repeatedly pass over
Titan's surface and will directly measure the composition of the upper atmosphere.
The orbiter's infrared spectrometer will determine temperature and composition
globally and as functions of time, complementing the measurements made during
the probe's descent. Orbiter imaging at visible and infrared wavelengths will
determine haze structure and variability. Properties of the upper atmosphere will be
measured during Titan flybys by the orbiter's ion and neutral mass spectrometer.
The complementarity of obtaining orbiter data coincident with Huygens probe data
is an important advantage of the Cassini mission as currently configured.
As the Cassini orbiter repeatedly passes over Titan, its radar will yield further
information on the nature of the surface in high-resolution strip-scans. This will
allow imaging of a significant fraction of the surface at a resolution of 1 km or
better. The radar will provide information on the composition of the surface and the
depth of hydrocarbon oceans or lakes, if they are present. The radar will also
operate in a radiometer mode and map surface dielectric constant variations over
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the entire surface.
In sum, Cassini will represent a major step in achieving COMPLEX's objectives for
Titan.
Saturn's Atmosphere
The Cassini orbiter will determine properties of Saturn's atmosphere at all latitudes
and will monitor dynamical changes. Infrared spectra will yield composition and
temperature throughout the stratosphere and upper troposphere and, when
combined with near-infrared and imaging measurements of reflected sunlight, will
determine the thermal energy balance both locally and globally. Cloud structure
and horizontal atmospheric motions within the upper troposphere will be obtained
from temporal imaging sequences. Temperature, pressure, and ammonia
abundance will be determined with excellent vertical resolution to a depth
corresponding to a pressure of approximately 1 bar by radio occultations. When
combined with infrared spectra, radio occultations will also provide an improved
determination of the helium abundance.
The Cassini instruments, together with an orbital tour that includes high-latitude
coverage, will address all the Saturn atmospheric objectives outlined by COMPLEX
except those for inert gas composition and isotopic abundances.
Rings
Throughout the Cassini mission, the orbiter will take images of Saturn's rings at a
full range of viewing angles in both reflected sunlight and the thermal infrared.
These images will form the database for tracking dynamical effects, wave motions,
and spoke kinematics. The orientation of the rings as seen from Earth is
particularly favorable for the proposed orbital tour. According to current mission
plans, the rings will occult the orbiter 25 times, providing excellent measurements
of the rings' transmission and scattering properties at three distinct radio
wavelengths. The radio science experiment and complementary ultraviolet stellar
occultation data will determine the particle size and mass distributions in all of
Saturn's rings as a function of their distance from the planet.
Cassini's complement of selected instruments is sufficient to achieve all the
objectives for Saturn ring science set down by COMPLEX.
Saturn's Small Satellites
Besides Titan, Saturn's satellite system includes several small icy bodies, each
displaying a variety of surface landforms and evolutionary histories. They have
been affected by internal activity, possibly including tectonism and ice volcanism,
and external processes, such as impact. The morphology and stratigraphy of the
satellites will be evident from the data provided by the imaging system. These data
will advance the understanding of satellite surface processes and history, in
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addition to addressing the thermal history and state of satellite interiors. The
spectroscopic instruments will determine the chemical and mineralogical
composition of satellite surfaces. The distribution of various compositional units
defined by both spectroscopy and imaging will permit the three-dimensional
reconstruction of the configuration of the outer crusts of the icy satellites. Cassini's
current instrument payload and mission configuration will provide an
unprecedented view of the composition, state, and geological evolution of the small
icy satellites of Saturn.
The proposed Cassini mission configuration is fully responsive to COMPLEX's
goals for the exploration of Saturn's small satellites.
Magnetosphere
The particles-and-fields instruments will be able to measure particle fluxes with
good coverage and good resolution of energy, spatial orientation, mass, and time.
Measurements of plasma waves will enable the determination of the sources and
sinks of magnetospheric plasma. The particles-and-fields instruments also have an
excellent capability to characterize the interaction between Titan and Saturn's
magnetosphere. Cassini's magnetometer will be able to determine the
configuration of Saturn's nearly axially symmetric magnetic field. This, when
combined with the directional capabilities of the radio receiver, will allow
determination of the origin of the kilometric radio emission modulation and, hence,
the characterization of the nonsymmetric components of Saturn's magnetic field.
The temporal and spatial (both radial and latitudinal) coverage of Saturn's
magnetosphere during the Cassini mission should clearly establish the nature and
origin of temporal variations in the magnetosphere.
The Cassini payload and mission design appear to be fully capable of achieving
the major scientific objectives of studying Saturn's magnetosphere.
COMPLEX's overall conclusion is that the Cassini mission, as currently configured,
is extremely responsive to the highest-order priority for exploring the outer planets,
i.e., intensive study of Saturn as a system.
CRAF
The primary objective of CRAF since its inception has been a comet rendezvous.
Comets represent some of the least-altered material left from the formation of the
solar system. Thus the study of comets yields important constraints on conditions
in the early solar nebula. However, ground-based observations of comets are
limited by interference from Earth's atmosphere and by the generally poor viewing
geometry for comets when they are near the Sun. Furthermore, the presence of a
cometary coma makes viewing the nucleus difficult. The first close observations of
a comet were obtained in 1985 when the International Cometary Explorer
encountered Comet Giacobini-Zinner. Later, in 1986, spacecraft from Europe,
Japan, and the Soviet Union completed fast flybys of Comet Halley. COMPLEX
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concluded in a letter report 3 that these encounters left COMPLEX's objectives for
the exploration of comets largely unchanged. These objectives, given in
COMPLEX's 1980 report, Strategy for the Exploration of Primitive Solar System
Bodies—Asteroids, Comets, and Meteoroids: 1980-1990, are as follows (in order of
priority):
1. To determine the composition and physical state of the nucleus
(determination of the composition of both dust and gas is an important
element of this objective);
2. To determine the processes that govern the composition and distribution
of neutral and ionized species in the cometary atmosphere; and
3. To investigate the interaction between the solar wind and cometary
atmosphere.
COMPLEX has reviewed CRAF four times, and each time concluded 4 that the
mission as configured at the time of the review was responsive to the above
objectives. The present review considers whether the changes in the mission since
the last review, in July 1990, invalidate earlier conclusions. The significant changes
were the descoping of the instrument payload, in the fall of 1990, and the change
in the mission profile, in the fall of 1991, which delays the date of launch.
The descoping of the instrument payload in the fall of 1990 was forced by a new
NASA assessment of the costs and risks involved with the development of the
CRAF penetrator experiment (PENL). This assessment led to a programmatic
decision to remove PENL and the Scanning Electron Microscope and Particle
Analyzer (SEMPA) from the instrument payload. In its July 1990 review,
COMPLEX had identified SEMPA as not being as effective as the Comet Ice and
Dust Experiment (CIDEX) and the Cometary Matter Analyzer (COMA) in
addressing the most important science objective of CRAF, namely, determining the
composition and physical state of the nucleus. Thus at the time, COMPLEX
stated5 that SEMPA had lower priority than PENL and that, in spite of the loss of
SEMPA, CRAF would remain responsive to COMPLEX's previously stated science
goals.
The loss of the penetrator experiment is much more serious. As stated in the same
letter, "deletion of the penetrator would severely compromise the ability of the
CRAF mission to address the highest-priority goals identified by COMPLEX."
PENL was the only experiment that would have sampled the comet nucleus in situ.
As important as the loss of PENL was, however, COMPLEX continues to hold the
view, stated in its 1980 report Strategy for the Exploration of Primitive Solar
System Bodies—Asteroids, Comets, and Meteoroids: 1980-1990, that comet
"science objectives can be met during the next decade without undertaking to land
on or penetrate a comet nucleus." This view is based on the fact that the descoped
CRAF will still be able to contribute many things to comet science in response to
COMPLEX's primary, near-term objectives. Following is a list of what CRAF would
still be able to do:
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Measure the shape and size of the nucleus;
q
Measure the mass to better than one percent;
q
Accurately compute the bulk density from the mass and volume;
q
Measure the mass distribution of the nucleus;
q
Map the active and dormant regions of the nucleus to determine why
q
they are different;
Study the morphology and evolution of craters;
q
Characterize the surface energy budget;
q
Determine the surface composition;
q
Determine the dust composition;
q
Determine the gas composition—both neutral and ion species;
q
Gain information about the onset of activity in the comet and the
q
formation of the coma;
Characterize jet features and the relation between dust and gas in jets;
q
Study the magnetic field in the coma; and
q
Study the tail, including the interaction with the solar wind.
q
Many of these anticipated results directly address the primary objective of
characterizing the nucleus. COMPLEX therefore believes that the descoped CRAF
mission is still responsive to its highest-priority near-term goals for comet science.
At the same time, COMPLEX reiterates its comments in Assessment of Solar
System Exploration Programs: 1991: "The 1990 deselection of the CRAF
penetrator requires continuing attention to alternative means of directly sampling a
comet nucleus. The completion of this goal may still require acquiring and returning
a sample of a cometary nucleus in some future mission." 6
The change in the CRAF mission profile was forced by the congressional budget
decisions for FY 92. The change caused the CRAF launch to be delayed from
February 1996 to April 1997. This slippage delays the comet rendezvous from
2003 to 2006, but accommodated a cut in the proposed funding for the
CRAF/Cassini program in FY 92. This change does not per se cause any loss in
the mission's anticipated return for comet science.
The launch slippage does, however, enhance significantly the results anticipated
for asteroid science. The primary goals of asteroid exploration, set forth in
COMPLEX's 1980 primitive bodies strategy, are to determine the composition, bulk
density, and surface morphology of asteroids. Elucidation of the diversity of
asteroids is an essential aspect of these goals. The earlier mission profile would
have included flybys of only one or two small asteroids, whereas the new profile
includes flybys of two large asteroids, 88 Thisbe and 19 Fortuna, and at least one
small asteroid, 1084 Tamariwa. The large asteroids are particularly important since
they are likely to be primitive, undifferentiated bodies that would provide
information about processes of planet formation in the early solar system.
In addition to the three asteroids, the baseline mission now includes a gravity
assist from Mars. This will afford an important opportunity to augment knowledge
about Mars, using excellent ultraviolet, visible, and near-infrared imaging
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capabilities not included on currently planned Mars missions. Along with the
opportunity to study Mars itself comes the chance to study Phobos and Deimos, its
two satellites. CRAF will be able to compare and contrast these irregular satellites
with the three asteroid targets.
The slippage of the launch by one year and the comet rendezvous by three years
does not come without drawbacks. In addition to the added run-out costs, there is
additional risk of component failure. However, the lifetime of the reconfigured
mission still falls within the design criterion originally specified for CRAF. Although
reduction of power from the radioisotope thermoelectric generators is a concern,
the rendezvous nature of this mission, with the spacecraft in prolonged close
proximity to the comet, relaxes some of the constraints on power usage and makes
power usage less a concern for CRAF than for a more rapid flyby mission.
Therefore, COMPLEX concludes that the complications arising from the one-year
launch delay for CRAF are more than offset by the enhanced opportunities
resulting from NASA's redesign of the mission profile.
REFERENCES
1. Space Science Board, A Strategy for Exploration of the Outer Planets: 1986-
1996, National Academy Press, Washington, D.C., 1986.
2. Space Science Board, Strategy for the Exploration of Primitive Solar System
Bodies—Asteroids, Comets, and Meteoroids: 1980-1990, National Academy of
Sciences, Washington, D.C., 1980.
3. Space Science Board, letter to Geoffrey A. Briggs, May 27, 1987.
4. Space Science Board/Space Studies Board, letter to Geoffrey A. Briggs, May
31, 1985; letter to Geoffrey A. Briggs, May 27, 1987; letter to Geoffrey A. Briggs,
September 1, 1988; letter to Lennard A. Fisk, August 10, 1990.
5. Space Studies Board, letter to Lennard A. Fisk, August 10, 1990.
6. Space Studies Board, Assessment of Solar System Exploration Programs: 1991,
National Academy Press, Washington, D.C., 1991.
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