A profound question for scientists, philosophers and, indeed, all humans concerns how the solar system originated and subsequently evolved. To understand the solar system's formation, it is necessary to document fully the chemical and physical makeup of its components today, particularly those parts thought to retain clues about primordial conditions and processes.1
In the past decade, our knowledge of the outermost, or trans-neptunian, region of the solar system has been transformed as a result of Earth-based observations of the Pluto-Charon system, Voyager 2's encounter with Neptune and its satellite Triton, and recent discoveries of dozens of bodies near to or beyond the orbit of Neptune. As a class, these newly detected objects, along with Pluto, Charon, and Triton, occupy the inner region of a hitherto unexplored component of the solar system, the Kuiper Belt. The Kuiper Belt is believed to be a reservoir of primordial objects of the type that formed in the solar nebula and eventually accreted to form the major planets. The Kuiper Belt is also thought to be the source of short-period comets and a population of icy bodies, the Centaurs, with orbits among the giant planets. Additional components of the distant outer solar system, such as dust and the Oort comet cloud, as well as the planet Neptune itself, are not discussed in this report.
Our increasing knowledge of the trans-neptunian solar system has been matched by a corresponding increase in our capabilities for remote and in situ observation of these distant regions. Over the next 10 to 15 years, a new generation of ground-and space-based instruments, including the Keck and Gemini telescopes and the Space Infrared Telescope Facility, will greatly expand our ability to search for and conduct physical and chemical studies on these distant bodies. Over the same time span, a new generation of lightweight spacecraft should become available and enable the first missions designed specifically to explore the icy bodies that orbit 30 astronomical units (AU) or more from the Sun. The combination of new knowledge, plus the technological capability to greatly expand this knowledge over the next decade or so, makes this a particularly opportune time to review current understanding of the trans-neptunian solar system and to begin planning for the future exploration of this distant realm.
Based on current knowledge, studies of trans-neptunian objects are important for a variety of reasons that can be summarized under five themes:
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--> Executive Summary A profound question for scientists, philosophers and, indeed, all humans concerns how the solar system originated and subsequently evolved. To understand the solar system's formation, it is necessary to document fully the chemical and physical makeup of its components today, particularly those parts thought to retain clues about primordial conditions and processes.1 In the past decade, our knowledge of the outermost, or trans-neptunian, region of the solar system has been transformed as a result of Earth-based observations of the Pluto-Charon system, Voyager 2's encounter with Neptune and its satellite Triton, and recent discoveries of dozens of bodies near to or beyond the orbit of Neptune. As a class, these newly detected objects, along with Pluto, Charon, and Triton, occupy the inner region of a hitherto unexplored component of the solar system, the Kuiper Belt. The Kuiper Belt is believed to be a reservoir of primordial objects of the type that formed in the solar nebula and eventually accreted to form the major planets. The Kuiper Belt is also thought to be the source of short-period comets and a population of icy bodies, the Centaurs, with orbits among the giant planets. Additional components of the distant outer solar system, such as dust and the Oort comet cloud, as well as the planet Neptune itself, are not discussed in this report. Our increasing knowledge of the trans-neptunian solar system has been matched by a corresponding increase in our capabilities for remote and in situ observation of these distant regions. Over the next 10 to 15 years, a new generation of ground-and space-based instruments, including the Keck and Gemini telescopes and the Space Infrared Telescope Facility, will greatly expand our ability to search for and conduct physical and chemical studies on these distant bodies. Over the same time span, a new generation of lightweight spacecraft should become available and enable the first missions designed specifically to explore the icy bodies that orbit 30 astronomical units (AU) or more from the Sun. The combination of new knowledge, plus the technological capability to greatly expand this knowledge over the next decade or so, makes this a particularly opportune time to review current understanding of the trans-neptunian solar system and to begin planning for the future exploration of this distant realm. Based on current knowledge, studies of trans-neptunian objects are important for a variety of reasons that can be summarized under five themes: Exploration of new territory. Telescopic discoveries of new Kuiper Belt objects (KBOs) are being made monthly. With continued access to suitable telescopes, this rate of discovery will likely be maintained for many years since very little of the sky (<0.1% of the ecliptic for objects brighter than 17th magnitude2, 3) has been
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--> surveyed to date. While telescopes are showing us that trans-neptunian objects are relatively common and are providing information about their disk-averaged surface composition, spacecraft missions are necessary to explore the detailed nature of these icy bodies. Reservoirs of primitive materials. While KBOs may not be pristine relics of the original solar nebula, it is in the outer solar system that we might expect to find the least-modified materials as well as samples that have suffered a range of degrees of modification. These bodies can provide the links for understanding the relationships among the interstellar medium, the solar nebula, and current materials in the solar system. Processes that reveal the solar system's origin and evolution. The observable characteristics of objects tell us about the processes they have experienced. The distribution of a population of objects in orbital phase space provides clues about their origins and the dynamical processes that control them over long periods. The distribution of sizes within a population reveals the relative importance of accretion versus collisional erosion. The wide range of sizes and different collisional histories among objects in the trans-neptunian region implies varying degrees of internal differentiation. Surface geology provides important constraints on an object's thermal history. Surface chemistry and atmospheric properties reveal processes of outgassing, photochemistry, transport, and redeposition of volatiles. Links to extrasolar planets. Studies of early stars similar to the Sun have shown that some are surrounded by disks of dust that are thought to be derived from collisions between comets. It is natural, therefore, to relate such dust disks to the Kuiper Belt. Applying knowledge of the Kuiper Belt to stellar dust disks suggests that the inner boundary exhibited by some disks may be an indication of the existence of planets. Comparisons of the Kuiper Belt with these dust disks is an important component of the new field of comparative studies of solar systems. Prebiotic chemistry. As remnants of the early solar system, trans-neptunian objects can provide critical clues about processes of prebiotic chemistry and about the materials that would have been delivered to the early Earth and may have formed the source of volatile materials from which life arose here and possibly on other planets of this and other solar systems. These five themes are not on an equal footing. The first three are well-established areas of scientific investigation and are backed up by a substantial body of observational and theoretical understanding. The last two, however, are more speculative. They are included here because they raise a number of interesting possibilities that seem particularly suited to an interdisciplinary approach uniting planetary scientists with their colleagues in the astrophysical and life science communities. Although not considered in any detail in this report, the distant outer solar system also has direct relevance to Earth and the other terrestrial planets because it is the source of comets that bring volatiles into the inner solar system. The resulting inevitable impacts between comets and other planetary bodies can play a major role in the evolution of planetary surfaces and atmospheres. Indeed, comets can also play major roles in the evolution of life as suggested by, for example, the Cretaceous-Tertiary boundary bolide and the extinction of the dinosaurs. Trans-Neptunian Objects. The five major themes described above involve general scientific issues that apply to the trans-neptunian region as a whole. Below COMPLEX summarizes the current knowledge and outstanding issues of the separate major types of objects in the trans-neptunian region. Triton Triton is by far the best-explored icy body in the distant outer solar system,4 and, as such, sets the context for the discussion of the other bodies. Triton is thought to be a planetary body that was captured by Neptune in the distant past. Voyager 2's flyby of Triton demonstrated the wealth of information available only from a spacecraft mission. Triton's density suggests that it has a rock core (70% by mass) surrounded by ice. Tidal heating due to orbital evolution and/or collision(s) with other satellites probably caused differentiation of the interior. Geological
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--> mapping indicates a youthful surface with few impact craters and with active volcanic eruptions. Its surface is uniformly cold (<38 Kelvin) and is covered with patches of volatile ices that appear to be strongly coupled to Triton's seasonally varying nitrogen atmosphere. The outstanding issues at Triton are as follows: When and by what process was Triton captured by Neptune? What is the degree of differentiation of the interior? Does Triton have an iron core and/or magnetic field? What drives the volcanism? How are the volatile ices brought to the surface and distributed? What is the distribution of surface materials, and how are they related to geological units? What are the structure and dynamics of Triton's atmosphere, and how do they vary with Triton's complex seasonal pattern? Pluto and Charon Pluto is both the smallest planet and the largest body in the outer solar system that is not in orbit around a giant planet. Our knowledge of Pluto and its satellite, Charon, is limited to telescopic observations. Other than the identification of certain ices on Pluto and Charon and the observation of strong variations in albedo on Pluto, little can be said about their surfaces or geology, beyond speculation based on knowledge of other icy satellites. As with Triton, Pluto's atmosphere is strongly coupled to the surface volatiles so that differences in their atmospheres result from the different nature of their surfaces. Pluto's warmer atmosphere and enhanced methane abundance are consistent with the ice on Pluto's surface containing 30 times more methane than Triton's ices and with large dark regions where the surface must be warmer. Charon's capture by Pluto probably involved a disruptive collision of the two bodies. The outstanding issues at Pluto and Charon are as follows: What are the bulk densities of Pluto and Charon? What are the interior composition and the state of differentiation of Pluto and Charon? What were the effects of the initial collision and subsequent tidal stresses produced in each body as a result of Charon's capture by Pluto? Is there activity on the surfaces of Pluto or Charon (e.g., plumes as on Triton)? Are the large-scale variations in albedo on Pluto due to variations in crustal structure or frost deposits? What is the structure of Pluto's atmosphere, and how does it change with time? Why is Pluto's atmosphere so different from Triton's? Kuiper Belt Objects Very little is known about the approximately 60 KBOs detected to date. Measurements of their orbits suggest that many of them are in resonance with Neptune. Variations in brightness are attributed to variations in size but cannot be quantified accurately without information on albedos. Measurement of brightness at different wavelengths gives an indication of surface color and suggests that surface compositions may vary among KBOs. Outstanding issues for Kuiper Belt objects are the following: What fraction of KBOs are in dynamically evolved orbits? What is the rate at which their orbits are perturbed sufficiently to send KBOs inward where they might interact with the giant planets? What does the size distribution of KBOs tell us about their accretion and erosion? If the range in observed colors is a true indication of diversity in surface composition, what causes this diversity?
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--> What is the degree of differentiation of these small bodies? Centaurs Other than spectroscopic observations that indicate diverse surface compositions, very little information is available about the half-dozen objects with eccentric orbits among the giant planets. Outstanding issues for the Centaurs are these: How many Centaurs are there? What are their orbits and how did these objects get where they are? How did their orbits evolve from the Kuiper Belt? What causes their color diversity? Does Chiron have a bound dust atmosphere, and, if so, what are the dynamical processes? Key Measurements The key measurements that will answer the outstanding issues for these different classes of objects can be obtained by similar methods. For example, to answer questions about dynamics researchers need to determine the objects' orbits by tracking their motions precisely over months to years. To answer questions about the processes of accretion and erosion it is necessary to determine each object's size by making separate measurements of brightness and albedo. The degree of internal differentiation is indicated by studying the surface geology and measuring gravitational and magnetic fields of larger objects. The distribution of surface volatile ices is derived by combining spectroscopic measurements and multispectral imaging. Stellar occultations of major bodies such as Pluto and Triton have provided rare opportunities to detect and study the vertical structure of their tenuous atmospheres. Characterization of the distribution of atmospheric hazes, clouds, and winds requires imaging from a spacecraft that passes close to the object. Conclusions and Recommendations. Three of the thematic rationales for the exploration of the trans-neptunian region (exploration of new territory, reservoirs of primitive materials, processes that reveal the solar system's origin and evolution) involve using methods that have proven successful in the past—telescopic observations, spacecraft missions, and harnessing new technologies and human ingenuity—to push the boundaries of our knowledge beyond 30 AU. Making links to extrasolar planet detection and studies of prebiotic chemistry will require planetary scientists to take interdisciplinary approaches and to venture with astronomers, chemists, and biologists into new fields of research. The main tasks for the next 10 to 15 years on the path to exploring the new frontier of planetary science in the distant outer solar system are to search for new objects and, more importantly, to document fully the chemical and physical makeup of the known bodies that constitute the trans-neptunian region. Spacecraft missions, telescopic observations, and research and analysis are the categories in which COMPLEX makes its highest-priority recommendations, as well as recommendations for augmenting this baseline effort. Spacecraft Missions To explore the makeup of objects in the trans-neptunian region, COMPLEX recommends an approach that combines 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. 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, end members of the KBO population. Since Pluto and Charon are barely spatially resolvable from Earth, many of the relevant properties can be measured only by robotic spacecraft.
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--> NASA's planning for a Pluto mission has undergone significant revision over the last few years. What was conceived of in the early 1990s as a Cassini-class mission requiring launch on a Titan-IV has been reformulated as a highly integrated spacecraft-payload combination capable of being launched on a Delta-II. The associated reduction in cost and the inclusion of a new start for a line of outer solar system missions in the administration's FY 1998 budget suggest that a Pluto mission is closer to realization than it has ever been since one was first conceived. Given Pluto's long rotation period (6.4 days) and the need for redundancy, COMPLEX recommends a dual spacecraft mission to Pluto. A single spacecraft would be able to observe only one hemisphere during its flyby. A second spacecraft would enable coverage of both hemispheres. Staggering the arrival times by, say, 6 months would also enable some retargeting of the second spacecraft based on results obtained during the first spacecraft's flyby. Augmentations Following a Pluto-Charon mission there are a number of future spacecraft projects that could be considered as part of a long-term program to explore the trans-neptunian solar system. These augmentations include: Adding a flyby of a Kuiper Belt object to a Pluto-Charon mission. The scientific potential of any Pluto-Charon mission would be greatly enhanced by the spacecraft continuing on to visit another Kuiper Belt object and thus providing measurements of the size and surface characteristics of two different KBOs that have different histories. Locating a suitable KBO along the trajectory of a Pluto mission should be a priority goal for search programs. This augmentation should be considered only if it has no serious cost or schedule impact on a Pluto-Charon mission. Conducting additional missions to Kuiper Belt objects. Objects in the trans-neptunian solar system are highly diverse, and the underlying causes for this diversity can be fully explored only by space missions. 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, Visiting a Centaur, and Encountering a suite of Kuiper Belt objects and/or Centaurs with different spectral and/or orbital characteristics. Spacecraft Technology Exploration of the outermost regions of the solar system is a demanding task, especially in an era of tight financial limitations. Although considerable progress has been made in the development of new-style missions to the outer solar system, particularly Pluto flyby missions, the technological obstacles of returning substantial scientific data from >30 AU remain formidable. Although considerable cost savings can be realized by reducing the size of the spacecraft and the complexity of its instruments, missions to the outer solar system still will demand a high launch energy, have a long mission duration (>10 years), be in low sunlight, and have a long telecommunications link. Advanced missions, such as those to put a spacecraft into orbit around a trans-neptunian object or to conduct multiple flybys of different objects, will almost certainly require the use of advanced propulsion techniques. Thus, the development of mission-enabling technologies (e.g., propulsion, compact power sources, autonomous operations, active fault management, radiation-hardened electronics, and long-distance communications) is an important adjunct to any program for the exploration of the trans-neptunian solar system. In addition, compact scientific instruments capable of characterizing the physical and chemical properties of cold (<40 Kelvin), icy objects in the distant outer solar system are needed.
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--> Telescopic Observations 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. Telescopes in the 2- to 4-meter class are ideally suited to searching for new KBOs. But larger telescopes (8 to 10 m) are required for spectroscopic studies of known KBOs. Augmentations Although access to suitable telescopes can provide much new data, with augmentations in a few critical areas ground-and space-based observations could provide even more information about the trans-neptunian solar system. These augmentations include: Equipping future large space telescopes to study trans-neptunian objects. 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. Developing instrumentation for ground-and space-based telescopes. Studies of the statistical properties of Kuiper Belt objects would benefit greatly from the availability of large array detectors. In addition, studies of the physical and chemical properties of all trans-neptunian objects would be enhanced by the availability of high-quantum-efficiency array detectors (~1 to 10 microns for studies of reflected light and ~10 to 100 microns for studies of thermal emission), and cooled telescopes. Research and Analysis. 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. References 1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 12–13. 2. C. Kowal, “A Solar System Survey,” Icarus 77:118, 1989. 3. D.C. Jewitt, J.X. Luu, and J. Chen, “The Mauna Kea-Cerro Tololo (MKCT) Kuiper Belt and Centaur Survey,”Astronomical Journal 455:1225, 1996. 4. D.P. Cruikshank, ed., Neptune and Triton, University of Arizona Press, Tucson, Arizona, 1995.