"On the Scientific Viability of a Restructured CRAF Science Payload"

On August 10, 1990, Dr. Larry W. Esposito, chair of the Committee on Planetary and Lunar Exploration, and 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.

In response to cost growth in the Comet Rendezvous Asteroid Flyby (CRAF) program and insufficient reserves for fiscal years 1991 and 1992, you requested that the Space Studies Board assess the scientific viability of a restructured CRAF science payload. Your letter of 2 July 1990 to the SSB Chairman, Louis J. Lanzerotti, indicated that NASA would propose several options for descoping the CRAF mission and requested the Board's Committee on Planetary and Lunar Exploration (COMPLEX) to

The Board's responding letter of 9 July 1990 indicated that it was requesting COMPLEX to review the descoping options proposed by NASA and also to explore additional options, together with related issues, that might preserve acceptable combinations of the primary scientific objectives for CRAF. COMPLEX accepted this larger task, of which the assessment of options proposed by the CRAF Project science Group (PSG) can now be considered a subset. In addition to evaluating the proposed options, COMPLEX has considered all the experiments on the baseline CRAF mission both individually and in groups for their responsiveness to the Committee's existing scientific strategy for the study of primitive bodies in the solar system.

CRAF is a mission designed to meet the highest scientific objectives for the study of comets and asteroids. The baseline payload has been judged by COMPLEX to be fully responsive to these scientific objectives. The CRAF mission has been combined programmatically with the Cassini (Saturn Orbiter/Titan Probe) mission, whose mutual objective has been described as "understanding the birth and evolution of our planetary system." CRAF is the first U.S. spacecraft mission dedicated primarily to the study of primitive bodies and is thus important in maintaining a balanced approach to exploration of the various elements central to the evolution of the solar system, as described in previous Space Studies Board and COMPLEX reports.

Two general points should be made about this review. The first is that scientific objectives for the study of asteroids do not figure strongly in the evaluation of CRAF. Although the asteroid flyby is a significant part of the mission and was discussed by the Committee in its deliberations, the matters related to the scientific objectives for the study of asteroids were not as strong in determining the scientific return of the mission because the majority of the mission duration and effort is directed toward the comet rendezvous. Secondly, COMPLEX noted that since the CRAF mission is a part of the combined CRAF/Cassini program, the scientific evaluations would be conducted in the absence of direct information about the Cassini scientific capabilities because the Cassini payload has not yet been selected. Therefore, in light of the many commonalities between CRAF and Cassini, this evaluation must be considered incomplete. The NASA program managers for CRAF have assured the Committee, however, that CRAF science will not be compromised in favor of Cassini, nor vice versa.

COMPLEX met on 12-13 July 1990 to hear presentations by personnel from NASA headquarters and from the Jet Propulsion Laboratory's (JPL) CRAF Project. These presentations outlined the background and the magnitude of the CRAF cost overrun. The presenters identified and discussed with the Committee the actions planned to reduce and spread out costs for the other major elements of the mission, including the spacecraft. The remaining budgetary difficulties were found to require some descoping of the science payload. COMPLEX was consequently provided with two options developed by the PSG for descoping the science payload (see Appendix A). Option 1 was a deletion of the CRAF penetrator and its suite of instruments. Option 2 consisted of a specific list of descoping options, whose total cost savings in FY91 and FY92 would still leave a substantial shortfall. This option also included another list of cost savings that could be achieved by deleting entire instruments. No priority among these instruments was presented to COMPLEX.

The scientific impacts of these options for descoping were evaluated against the scientific objectives identified by COMPLEX in its report, Strategy for the Exploration of Primitive Solar-System Bodies: Asteroids, Comets and Meteoroids: 1980-1990 (National Academy of Sciences, 1980), and as discussed in three subsequent letter reports (dated 31 May 1985, 27 May 1987, and 1 September 1988). The Committee's primary objectives for comet exploration, established in the 1980 strategy in order of priority, are

Based on a comparison of the two proposed options with these objectives, COMPLEX does not recommend either option. Instead, the Committee has developed a third option as described below.

COMPLEX recommends that CRAF's budgetary shortfall be met by the following actions: (1) cap the Penetrator-Lander (PENL) experiment's costs in FY91 and FY92. as proposed by the experiment's Principal investigator; (2) delete the PENL's heat shield cover; and (3) delete from the selected mission payload the Scanning Electron Microscope and Particle Analyzer (SEMPA), the Magnetometer (MAG), the Coordinated Radio, Electron, and Wave Experiment (CREWE), and the Cometary Retarding Ion Mass Spectrometer (CRIMS) instruments. In arriving at this recommendation COMPLEX was guided by the belief that more science would be lost by making extensive cuts spread throughout the whole instrument package than by selectively removing a few instruments. Selecting which experiments to delete was of course difficult, because all the experiments have the potential to yield unique and important information.

In making this recommendation, COMPLEX compared all the CRAF instruments' anticipated capabilities to the Committee's existing science strategy and to recommendations set forth in its letter reports. In the 1988 letter report, COMPLEX strongly supported SEMPA because it is "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." COMPLEX also noted in its 1987 letter report that "the compositional diversity seen in micron and submicron Halley dust suggests that individual particle measurements from the SEMPA and COMA instruments on CRAF will provide critical data for comparison of cometary matter with chondritic and interstellar grain models." The SEMPA experiment must now be evaluated in the context of how it contributes to the mission's overall goals in competition with other experiments that also analyze the composition of cometary dust.

In the Committee's opinion SEMPA does not address the objective of determining the composition as c6mpletely and as effectively as the CIDEX and COMA experiments do. In addition to supplying information on elemental composition, CIDEX provides information on the composition of volatiles and organics, while COMA provides information on the light elements (hydrogen, lithium, carbon) and on isotopic composition. COMPLEX does not consider the petrologic and mineralogic information provided by SEMPA to have as high a priority as this information on bulk composition. The measurement of composition by X-ray fluorescence in SEMPA is electron-excited, giving lower sensitivity due to bremsstrahlung. Thus COMPLEX now recommends deleting SEMPA from the CRAF payload.

The recommendation to delete the MAG, CREWE, and CRIMS experiments is motivated in part by the fact that they address primarily the objective of characterizing the interaction of the comet with the solar wind, which is a lower priority than the study of the comet's nucleus and coma. Furthermore, the design of the baseline CRAF mission largely precludes obtaining data that would substantially characterize the comet's interaction with the solar wind, because CRAF remains very close to the nucleus throughout most of the period of significant activity in the nucleus. COMPLEX places a higher priority on the Suprathermal Plasma Investigation of Cometary Environments (SPICE) than on the other experiments for the study of particles and fields because SPICE, which measures newly injected cometary ions (pickup ions), is a very sensitive monitor of activity in the nucleus, and will also measure the electron densities and velocity distributions that are essential to understanding the chemical processes of the coma. The SPICE instrument, even without MAG, CREWE, and CRIMS, can identify newly injected cometary ions both by their unique velocity distribution and by their composition.

Deletion of the MAGI CREWE, and CRIMS instruments entails a painful loss of scientific return. The MAG would provide knowledge of the solar wind magnetic field during the tail excursion portion of the nominal mission; loss of the MAG would jeopardize the science return from the tail excursion to such an extent that the mission design should be reconsidered. The deletion of CREWE would mean that important plasma physical processes, such as wave generation caused by mass loading, would not be studied. Loss of CRIMS would mean that the mass and velocity distributions of low-energy ions would be less well determined, but these measurements are substantially duplicated by the Neutral Gas and Ion Mass Spectrometer (NGIMS) and SPICE. Overall, the science objectives for the study of particles and fields are judged to be of lower priority than the objectives established for the comet's nucleus and for the coma as stated in the COMPLEX science strategy for primitive bodies.

Based on the numbers presented at our recent meeting by the CRAF project scientist, Dr. Marcia Neugebauer, Table 1 shows the savings that are expected to result from the recommended deletions of SEMPA, MAG, CREWE, and CRIMS plus the repricing and descoping of PENL.

Table 1 Anticipated Savings from Recommended Actions to Reduce CRAF Costs

NOTE: Numbers from Appendix B

Appendix C indicates that NASA anticipates a budgetary shortfall of approximately $12M in FY91 and $20M in FY92. COMPLEX notes that its recommendation solves the FY91 shortfall completely and allows some carryover to FY92. The savings, plus the carryover, do still leave a shortfall of $3.4M in FY92. COMPLEX further recommends that NASA overcome the small FY92 shortfall through a combination of less drastic measures, including shifting of some costs into later years. Although a $32M cost reduction appears to be necessary, the Committee's recommendation would actually result in a total development runout savings of $43.7M. Therefore, COMPLEX is confident that its recommendation is satisfactory for solving the budgetary problem.

The Committee also notes that if the selection of experiments for the Cassini mission does not include the Visual and Infrared Mapping Spectrometer (VIMS) experiment, this will place an additional burden on CRAF of approximately $7M. If the VIMS is not selected for the Cassini mission, COMPLEX recommends removing VIMS from CRAF, and restoring SEMPA.

Deletion of the penetrator would severely compromise the ability of the CRAF mission to address the highest-priority goals identified by COMPLEX. For this reason, COMPLEX recommends against Option 1 (the deletion of PENL) as presented by the CRAF PSG. The Committee noted in its 1988 letter report "the unique ability of the penetrator instruments to address in situ the highest priority science objective ... namely, determination of the dust and volatile composition, state, and physical properties of the [comet] nucleus."

With regard to characterizing bulk elemental composition within the comet nucleus, the penetrator's Gamma-Ray Spectrometer (GRS) will make elemental in situ measurements of approximately one metric ton of the material in which the penetrator is embedded. The sensitivity of the GRS appears to be adequate to assess the degree of chemical fractionation (e.g., distinguishing refractory from volatile elements, iron from silicon) to allow comparison with known types of primitive meteorites.

With regard to ices, the penetrator's Differential Scanning Calorimeter and Evolved Gas Analyzer (DSC/EGA) combination is the only experiment capable of determining the phase structure of the icy component of the comet's nucleus and how the volatile molecules are trapped in the ices. It is crucial to understand the mechanism for trapping volatiles in cometary nuclei because such trapping processes provide constraints on the conditions leading to the formation of comets. While other instruments measure the abundances of parent molecular species in the coma, these measurements are ambiguous with respect to the abundances of these species in the environment of cometary formation. The DSC/EGA experiment provides the means to resolve such ambiguities by measuring the ice structure and trapped gases. In its 1987 letter report, COMPLEX recognized that the data obtained in the DSC/EGA analysis "does not ... appear accessible by any other means."

The Committee notes that a repricing of PENL to yield the savings shown in Table 1 has been proposed by the principal investigator, but not confirmed by NASA. If such a repricing is, in fact, not entirely feasible, the cap on the penetrator cost will require a descoping of the penetrator. COMPLEX finds that the accelerometers and thermal probes have the lowest scientific priority of the penetrator experiments. The Committee received no information concerning either the management or technical risk of the penetrator, but the principal investigator's statement that it is not inherently more risky than several of the other experiments was not contested by the JPL or NASA representatives at the COMPLEX review.

As indicated above, Option 2 proposed by the CRAF PSG involves a mandatory list of descoping options (Table A in Appendix A) and an optional list of possible deletions (Table B in Appendix A), only some of which need to be selected. COMPLEX recommends against Option 2 altogether because the Table A deletions taken in total would significantly and unnecessarily increase the technological risk in developing many of the science instruments, and would jeopardize scientific preparation that is necessary for the mission. In particular, the following actions listed in Table A would result in significant loss of scientific return and/or unnecessary technological risk: deletion of the Thermal Infrared Radiometer Experiment's (TIREX) engineering unit; delay of gas/dust modeling; deletion of the SPICE electron measurements; deletion of JPL's calibration target for TIREX, VIMS, and the Imaging Science System (ISS); deletion of the X-Ray Fluorescence (XRF) function from the Cometary Ice and Dust Experiment (CIDEX); and general descoping of prelaunch science activities.

COMPLEX expresses its dismay that such a major reduction in mission scope has had to be undertaken so early in an approved mission. The Committee strongly believes that the fully configured CRAF mission is responsive to the goals and strategies articulated in previous reports and that the deselection of instruments is not conducive to excellence in planetary scientific exploration. Although the proposed descoped CRAF would also remain responsive to these science goals, COMPLEX notes that any further reduction of mission scope may irreparably impair CRAF's ability to respond adequately to the Committee's stated science goals and objectives.