"Assessment of Planned Scientific Content of the CRAF Mission"

On May 31, 1985, Drs. Robert O. Pepin and D.M. Hunten, chair and past chair, respectively, 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 March 1985 meeting, the Committee on Planetary and Lunar Exploration (COMPLEX) made a detailed assessment of the proposed Comet Rendezvous-Asteroid Flyby (CRAF) mission, which included the science objectives, the mission profiles, and engineering considerations. This assessment was made with the help of several presentations by representatives of the mission Science Working Group and the engineering study team.

As you know, it is the practice of COMPLEX to assess the scientific content of a mission, as it nears proposal as a new start candidate, in order to measure how well the agency has responded, in a mission context, to the committee's science strategy. The conclusions of the assessment are a measure of the support of the committee and the Space Science Board for the proposed planetary mission. The committee intends to make further assessments during the development period of the mission leading to launch.

ASSESSMENT SUMMARY

The committee finds that science planning for the CRAF mission is proceeding within the guidelines established by COMPLEX for in-situ cometary exploration and preliminary asteroidal reconnaissance. We feel confident that the few points of concern expressed by the committee at this stage of the mission strategy development will be addressed by the selected science teams. A later assessment will deal specifically with these concerns, noted below, and with the critical matter of measurement capabilities of the selected instrumental packages as they relate to the measurement requirements specified in the COMPLEX strategy report.

RECOMMENDATIONS OF THE COMPLEX STRATEGY REPORT

In the opinion of the committee, these initial investigations of primitive bodies of U.S. spacecraft are of the utmost significance. It is scientifically exciting to appreciate that the CRAF mission will be the first to approach and observe, over a prolonged period, an object whose dynamics and composition are fundamentally unlike any of the planets and satellites visited so far in the space exploration program. As a group, the primitive bodies provide a vital link in understanding the primordial composition and early evolutionary history of solar system material; this conclusion is based on their rich diversity and the general belief that many of these objects are composed of condensed material that has survived, essentially unaltered, since the formation of the original solar nebula.

The COMPLEX report "Strategy for the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and Meteoroids: 1980-1990" presents in details the science objectives, exploration strategy, and measurement requirements for the first phases of primitive body investigations. In order to guide the exploration of these bodies over an indefinite period, COMPLEX has recommended that the primary goal of their investigation be "to determine their composition and structure and to deduce their history in order to increase our knowledge of the chemical and isotopic composition and physical state of the primitive solar nebula and to further our understanding of the condensation, accretion, and evolutionary processes that occurred in various parts of the solar system before and during planet formation." There are three additional goals: "to determine the diversity of composition and structure" of primitive bodies; "to understand the role played by accretion of these bodies in the evolution of the crustal and atmospheric composition and the crustal structure of the terrestrial planets"; and, unique to comets, "to understand the dynamical processes responsible for the production, maintenance, and behavior of the gas, dust, and plasma envelopes of active comets". Within the framework of these goals, COMPLEX has defined the prioritized set of primary science objectives for comets and asteroids which, in the opinion of the committee, can be accomplished in a ten-year period of exploration.

Comet science objectives focus on the composition and physical state of the nucleus, processes that govern the development and behavior of the cometary atmosphere, and interactions with the solar wind, in that order of priority. The report considers several points of exploration strategy. The science objectives require rendezvous-mode investigations of comets, in which the spacecraft maneuvers near the nucleus for an extended period of time. Becuase young, active comets are thought to contain the best preserved samples of primitive materials, and because their vigorous emission of dust and gas interacts strongly with the local solar radiation and plasma environment and thus optimizes observation of dynamical interaction processes, the COMPLEX strategy recommendations point to comets of this type as optimal targets for study. However, the report noted that such comets would probably be difficult to reach, and that in-situ study of the more likely candidates—generally older, more evolved comets in intermediate or short-period orbits—would still yield a significant return in the context of overall comet science objectives. In order to allow similarities and differences between the explored comet and other remotely observable comets to be studied on a more quantitative basis, COMPLEX regards as essential a determination of the relationship between remotely observable characteristics and the detailed composition of the selected cometary body. This calibration requires simultaneous observations of the comet from remote platforms on the Earth and in Earth orbit.

Asteroid science objectives, in priority order, are to determine composition and bulk density, to investigate surface morphology and evidence for operation of endogenic and exogenic processes, and to determine internal properties of selected asteroids of diverse types. The COMPLEX report notes that although a full response to these science objectives requires rendezvous-type investigations of several selected asteroids, flybys are an available mode of spacecraft exploration. The flyby mode can meet, in part, reconnaissance-phase objectives, and is particularly useful in view of the great diversity of asteroid characteristics.

ASSESSMENT OF THE CRAF MISSION

This letter comments on the degree to which the recommendations of the COMPLEX report are followed by the more general elements of the proposed CRAF mission—rationale, science objectives, general strategy, and timeliness. Until the science instrument definition phase for CRAF is completed, detailed evaluation of the actual suite of proposed measurements, and their likely uncertainties, cannot be carried out with confidence. The committee intends to make and convey this assessment at an appropriate time, after briefings by the selected science teams.

The committee finds that the overall goals and scientific objectives for the comet rendezvous phase of the CRAF mission are fully responsive to the general COMPLEX rationale and recommendations for cometary exploration in this decade. The asteroid flyby phase of CRAF is similar to that now planned for Galileo. Both encounters will return valuable information unobtainable from Earth in these first close looks at two members of this very diverse class of bodies. They represent important first steps in a longer-term exploratory effort to respond to the primary scientific objectives, strategy, and detailed measurement requirements set out by COMPLEX for the study of asteroids.

We note the particular timeliness for return of essential cometary data embodied in the CRAF mission schedule, which allows an interval following the multiple international Halley flyby observations that is both long enough to digest and apply these data to final mission planning, and short enough to preserve continuity of scientific momentum and expertise. Timely infusion of Halley and CRAF results will also be crucially important in lifting some of the observational and experimental constraints that now limit the type and quality of data we may expect from the two current sources of information about these primitive bodies: remote sensing investigations and collection and laboratory study of putative cometary dust (Brownlee particles).

Turning to considerations of exploration strategy, the committee endorses the choice of Wild 2 as the primary mission target. The fact that there have been only two Wild 2 apparitions since its perturbation from a deep-space, long-period orbit places this comet within or close to the category of young, active comets identified by COMPLEX as first priority choices for rendezvous but considered in their 1980 report to be beyond our present reach. We emphasize the scientific importance of capitalizing on the unusual opportunity presented by the accessibility of Wild 2, and point out in this regard that the next and last candidate for rendezvous in the 1990's, if the CRAF launch slips beyond 1991-1992, is Tempel 2 on its 20th apparition.

Two additional points concerning strategy require comment. Both relate to the CRAF plan for measurement strategy. As noted above, the COMPLEX recommendations stress the necessity for simultaneous remote and in-situ cometary measurements as an essential calibration component of the strategy. The committee emphasizes the vital role of this calibration in placing remote observational data and data interpretation for other comets on a more firmly quantitative footing, and points out that simultaneous remote measurements from both the spacecraft and the earth are needed.

The second comment addresses the state-of-the-art instrumentation and measuring techniques envisioned by the CRAF Science Working Group for acquisition of primary scientific data from the cometary nucleus and from dust and ice grains. The committee fully recognizes that the challenges posed by the science objectives and the associated measurement requirements are formidable, and require significant and high-order advances in instrument concept and design, sampling strategy, and analytical techniques. We therefore strongly support the ongoing efforts to develop the pyrolysis cell-neutral gas mass spectrometer system, the scanning electron microscope and particle analyzer, and the penetrator with gamma-ray spectrometer. Rigorous certification for the spacecraft and comet environments is essential, since failure of any of these instrumental systems, all untested by previous flight experience, would seriously compromise primary mission science objectives. In this context, the committee calls particular attention to the penetrator experiment, where the intrinsic possibility of failure was judged by COMPLEX, from a 1980 perspective, to be high enough to warrant the following recommendation in their strategy report: "In the Committee's judgement, the science objectives can be met during the next decade without undertaking to land on or penetrate a comet nucleus. In the absence of any information about the physical constitution of comet nuclei, we consider that mission modes should not be attempted that rely on either landing or penetrating to meet the basic science objectives." The committee does not view this as in any way an injunction against development and thorough testing of a penetrator experiment for possible inclusion in the CRAF science payload; the importance assigned to hard-landing instrument packages is evident throughout the 1985 COMPLEX report on "Development of Instruments for Planetary Exploration Spacecraft". However, this COMPLEX reservation does sound a note of great caution about allocating sole responsibility for acquisition of primary scientific data—especially in an area as important as composition of the nucleus—to this instrument, without redundant measurements by backup techniques that ideally would require neither penetration nor landing.

Please feel free to contact either of us with any questions you may have, or for further discussion.