5
Conclusions and Recommendations

Three of the themes for the scientific exploration of the trans-neptunian region (exploration of new territory, reservoirs of primitive materials, and processes that reveal the solar system's origin and evolution) involve using methods that have proven successful in the past. The methods required to push the boundaries of our knowledge beyond 30 AU are telescopic observations, spacecraft missions, and harnessing new technologies and human ingenuity. Addressing the remaining two themes—making links to extrasolar planet detection and studies of prebiotic chemistry—will require planetary scientists to take interdisciplinary approaches and to venture into new fields of research in cooperation with astronomers, chemists, and biologists.

For the next 10 to 15 years, the prime task on the path to exploring this new frontier of planetary science is to document fully the chemical and physical makeup of the objects that compose the trans-neptunian region. COMPLEX recommends an approach that combines remote, telescopic observations of the bulk properties of a large sample of Kuiper Belt objects with close-up, spacecraft studies of the detailed properties of a few specific objects. Ground-and space-based telescopic studies and spacecraft missions are ideally suited to surveys and detailed investigations, respectively. Since planetary missions are expensive and thus likely to be few in number, they must be directed toward specific objects likely to provide the greatest amount of information about the trans-neptunian region.

Subsequent sections outline how spacecraft, telescopic, and research and analysis programs can each contribute to the exploration of the trans-neptunian region. For each area, COMPLEX makes a baseline recommendation and then recommends one of more possible augmentations.

Spacecraft Missions

The highest scientific priority for the exploration of the trans-neptunian solar system is extensive and detailed measurement of the fundamental physical and chemical properties of the Pluto-Charon system. Pluto is unique in that it is the largest known object in the trans-neptunian region and thus represents one of the end members of the Kuiper Belt population. Because Pluto and Charon are barely spatially resolvable from Earth, many of the relevant properties can be measured only by a robotic spacecraft. The evolution of NASA's thinking about the scope of a Pluto mission from the Cassini-class approach envisioned in the early 1990s to the “sciencecraft” concept formulated more recently by the Pluto Express Science Definition Team1 (see Box 4.1) strongly suggests that the initiation of such a mission is consistent with NASA's budgetary expectations for the early years of the next decade.



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--> 5 Conclusions and Recommendations Three of the themes for the scientific exploration of the trans-neptunian region (exploration of new territory, reservoirs of primitive materials, and processes that reveal the solar system's origin and evolution) involve using methods that have proven successful in the past. The methods required to push the boundaries of our knowledge beyond 30 AU are telescopic observations, spacecraft missions, and harnessing new technologies and human ingenuity. Addressing the remaining two themes—making links to extrasolar planet detection and studies of prebiotic chemistry—will require planetary scientists to take interdisciplinary approaches and to venture into new fields of research in cooperation with astronomers, chemists, and biologists. For the next 10 to 15 years, the prime task on the path to exploring this new frontier of planetary science is to document fully the chemical and physical makeup of the objects that compose the trans-neptunian region. COMPLEX recommends an approach that combines remote, telescopic observations of the bulk properties of a large sample of Kuiper Belt objects with close-up, spacecraft studies of the detailed properties of a few specific objects. Ground-and space-based telescopic studies and spacecraft missions are ideally suited to surveys and detailed investigations, respectively. Since planetary missions are expensive and thus likely to be few in number, they must be directed toward specific objects likely to provide the greatest amount of information about the trans-neptunian region. Subsequent sections outline how spacecraft, telescopic, and research and analysis programs can each contribute to the exploration of the trans-neptunian region. For each area, COMPLEX makes a baseline recommendation and then recommends one of more possible augmentations. Spacecraft Missions The highest scientific priority for the exploration of the trans-neptunian solar system is extensive and detailed measurement of the fundamental physical and chemical properties of the Pluto-Charon system. Pluto is unique in that it is the largest known object in the trans-neptunian region and thus represents one of the end members of the Kuiper Belt population. Because Pluto and Charon are barely spatially resolvable from Earth, many of the relevant properties can be measured only by a robotic spacecraft. The evolution of NASA's thinking about the scope of a Pluto mission from the Cassini-class approach envisioned in the early 1990s to the “sciencecraft” concept formulated more recently by the Pluto Express Science Definition Team1 (see Box 4.1) strongly suggests that the initiation of such a mission is consistent with NASA's budgetary expectations for the early years of the next decade.

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--> A Pluto-Charon mission should be capable of characterizing the following: Precise sizes, masses, and shapes of Pluto and Charon to constrain models of their interior structures; Global geology and geomorphology to investigate endogenic and exogenic processes and to determine crustal evolution; Distribution and chemical composition of surface materials to elucidate the processes that probably led to outgassing, transport, redeposition, and chemical alteration of volatiles; Structure, composition, and escape rate of the atmosphere of Pluto; Nature of the solar wind interaction with Pluto's atmosphere and/or surface; and Magnetic field that might be remnant or generated in Pluto's interior. Two spacecraft are essential for redundancy and to provide full coverage of the surfaces of Pluto and Charon. Spacing the encounters by a period of, say, 6 months would allow retargeting of the second spacecraft in response to observations made during the first encounter. Augmented Spacecraft Program A possible augmentation to the baseline Pluto-Charon mission described above is to consider the possibility of extending it to include flybys of one or more other Kuiper Belt objects (KBOs). The scientific potential of any Pluto-Charon mission would be greatly enhanced by the spacecraft continuing on to visit another KBO and thus providing measurements of the size and surface characteristics of two different KBOs that have different histories. While the probability of finding a KBO within range of a Pluto-Charon mission may be small, high priority should be given to telescopic searches for candidate targets along the trajectory of a Pluto mission. Such an augmentation should be considered only if it has no serious cost or schedule impact on a Pluto-Charon mission. The outer solar system contains a wide variety of objects. Some of the underlying causes of this diversity (such as differentiated vs. homogeneous interiors, degrees of surface processing, and so on) can be fully explored only by space missions. COMPLEX hopes that experience with the Rosetta mission to a comet and Discovery-class missions to the inner solar system will pave the way for affordable spacecraft missions to outer solar system objects. Scientific priorities for spacecraft missions to the trans-neptunian region in the more distant future, after the successful conduct of a Pluto-Charon mission and a KBO flyby, are, in rank order, as follows: Returning to Triton, one of the largest Kuiper Belt objects, to complete the characterization of the Pluto-Charon-Triton triad. Goals of such a mission should include exploring the unmapped hemisphere, constraining models of the interior with measurements of gravitational and magnetic fields, and investigating the expected temporal variations in Triton's atmosphere. Comparison of Triton with Pluto is especially important because these objects are of similar size and are thought to have similar origins, but Triton has probably experienced a very different thermal history owing to its capture by Neptune. Visiting a Centaur, those icy objects that have orbits among the giant planets and are thought to be Kuiper Belt objects whose orbits have been perturbed by Neptune. Thus, close comparison of a Centaur object with objects that continue to reside beyond Neptune is important for understanding the processes that occur when an icy object is brought closer to the Sun. Encountering a suite of Kuiper Belt objects and/or Centaurs with different spectral and/or orbital characteristics. Spectroscopic and photometric observations suggest that a wide variety of objects exist in the trans-neptunian region, and their different orbits indicate a range of sources. Each flyby provides ground truth for disk-integrated, telescopic observations. To fully interpret statistical studies of the properties of many Kuiper Belt objects, the range of object types needs to be sampled and studied in detail with spacecraft missions.

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--> Spacecraft Technology The technical challenges posed by returning substantial scientific data from the distant outer solar system remain formidable. Thus, the development of mission-enabling technologies is an important adjunct to any program for the exploration of the trans-neptunian solar system. To achieve the high launch energy required to get to distances >30 AU with current launch vehicles requires that the spacecraft mass be considerably reduced if outer solar system missions are to be affordable. The >10-year duration of missions demands high reliability of components and efficient mission operations. Spacecraft power and communication systems will require new technologies to improve efficiencies to allow the spacecraft to operate and transmit data back from >30 AU. The development of spaceflight instruments capable of characterizing the physical and chemical properties of cold (<40 Kelvin) icy objects at distances >30 AU is another important adjunct to a trans-neptunian exploration program. Lightweight, multiwavelength cameras have been designed for inner solar system missions. The low temperatures of the trans-neptunian region require instruments that can image at longer (far-infrared) wavelengths (~100 microns). In addition to building larger detector arrays with very low power consumption, passive and active cooling systems need to be developed. Telescopic Observations Even in an era of spacecraft missions to the distant outer solar systems, Earth-and space-based telescopic studies will remain an important source of information about Kuiper Belt objects. Thus, continued support for both ground-and space-based telescopic studies is an essential aspect of a program for the exploration of the trans-neptunian solar system. The highest priority for both ground-and space-based studies is significant access to existing and future moderate-to large-aperture telescopes equipped with modern instrumentation designed to meet the needs of planetary observers. The primary source of information about the number and spatial distribution of outer solar system objects continues to be systematic use of ground-based telescopes to search for KBOs near the ecliptic plane. An efficient survey requires a modest telescope (2-meter class) with a large, efficient electronic detector. Detection of small, distant objects requires continued access to ~2-meter telescopes and, for the faintest objects, substantial access to 4-meter-class (and larger) telescopes in the future. In the absence of numerous dedicated spacecraft missions, high-resolution infrared and millimeter spectroscopy and far-infrared radiometry are the only means of determining the chemical composition and albedo of outer solar system objects. Thus, continued support of NASA's Infrared Telescope Facility and, in the near future, the Stratospheric Observatory for Infrared Astronomy for planetary studies is essential. Observations of the distant outer solar system should be a primary goal of the next generation of space-based telescopes. These facilities will likely have higher spatial resolution, wider spectral range, and/or greater photometric sensitivity than either HST or the current generation of ground-based telescopes. To be capable of making the critical measurements of trans-neptunian objects, future large space telescopes should be designed from the outset to incorporate the ability to track moving targets and to measure the thermal emission from small, cold (<40 Kelvin) objects. While access to telescopes is essential, the provision of adequate instrumentation is also important. Support for the development of instruments that enhance telescopic observations of the trans-neptunian region is an important augmentation to a program of ground-and space-based observations of the distant outer solar system. Efficient large detectors arrays are needed to increase the discovery rate of KBOs to allow the determination of statistical properties of KBO populations. Advancing our knowledge regarding the physical and chemical properties of the KBOs using both Earth-and space-based telescopes to determine accurate photometry and radiometry is directly related to having large, high-quantum-efficiency detector arrays, especially in the far infrared.

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--> Research and Analysis. A combination of spacecraft missions and telescopic observations will provide much new data relevant to the trans-neptunian solar system, but this does not necessarily equate to new understanding. Only after the raw data have been thoroughly analyzed and placed in the context of existing research is new knowledge likely to arise. Continued support for research and analysis programs and for relevant theoretical and laboratory studies is an essential component of a program of spacecraft and telescopic observations of the trans-neptunian solar system. Theoretical and laboratory studies of the physical and chemical processes that influence the structure and evolution of cold (<40 Kelvin), icy bodies located in the trans-neptunian region should be fully supported to enhance the scientific return from spacecraft missions and telescopic observations. The trans-neptunian solar system presents a range of environmental extremes quite unlike those found in more familiar parts of the solar system. As a result, common materials can exist in exotic states about which very little is known. Volatiles such as CH4 and CO can, for example, exist as rocklike ices or as gases with chemistries more akin to that of the interstellar medium than that of any well-studied planetary atmosphere. Enhancing the facilities available for laboratory studies of the properties of planetary materials at low temperatures (<40 Kelvin) and low pressures will be a useful augmentation to existing research efforts. Reference 1. Pluto Express Science Definition Team, J.I. Lunine (chair), Pluto Express: Report of the Science Definition Team, NASA, Washington, D.C., 1995.