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n 1977, twin spacecraft lifted off from the same launch pad at the Kennedy Space Center in Florida, only a few months apart. These spacecraft, Voyager 1 and Voyager 2, were Earth's first emissaries to the outer solar system. The information that they returned about the giant planets and their moons revolution- ized our understanding of the solar system. Voyager stands as one of the greatest missions of planetary discov- ery in the space age. For all the Voyager program's success, however, Voyager 1 and 2 left the reconnais- sance of the outer solar system incom- plete. To date, no missions have explored the ninth planet, Pluto, and its satellite, Charon. At the time of the Voyager mis- sions, Pluto was thought to be less important than other planetary bodies, such as Neptune and Saturn's moon Titan, which were also easier to reach. In 1992, however, scientists found the first direct evidence for the existence of the Kuiper Belt a region of icy planets in orbit around other stars. Many of these extrasolar planets reside in large disks of material orbiting young stars, which scientists believe resemble our solar system in its youth. If current theory holds, objects in these extrasolar disks are similar in composition to objects in the Kuiper Belt. Most KBOs are thought to have undergone relatively little change since their formation in the solar neb- ula some 4.6 billion years ago. If that is the case, they would be relics of the building blocks from which the planets were formed. Thus, analyzing the structure and composition of these objects will provide new insights into the early history of planetary systems and help scientists understand the processes that formed Earth and other planets. Studies of the surface composi- tions of KBOs undertaken with Earth- based telescopes suggest that these objects likely contain organic matter and volatile materials such as water. This is an exciting discovery because . . . l . ~ I_ Even the most detailed map of Pluto's surface, assembled from observations made with the Hubble Space Telescope, reveals little about the most distant planet from the Sun. The nature of the surface features remains unknown. planetary debris on the edge of the solar system. The discovery of the Kuiper Belt refocused attention on Pluto. Instead of an afterthought, Pluto became the largest and most important member of an entirely unexplored type of body the Kuiper Belt objects (KBOs). Another exciting discovery came within a few years of the detection of the first KBO the identification of ~ ~ Faze {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~ researchers believe that the water and carbon-rich materials essential for the development of life on Earth are not indigenous to our planet. In other words, these life-giving materials were delivered to the primordial Earth dur- ing impacts with objects originating elsewhere in the solar system. A likely source of the impactors, and thus of the building blocks of life, may have been the Kuiper Belt. The first exploratory voyage to an unexplored region always turns up new and unexpected findings. As the first spacecraft specifically targeted to study this distant part of the solar sys- tem, the Kuiper Belt-Pluto Explorer will undoubtedly lead to a new understanding of the KBOs and, per- haps, inform us about the role played by KBOs in the origin and evolution of other parts of the solar system. Collisions, for example, are a ubiq- uitous process in planetary formation and in shaping planetary environ- ments. But next to nothing is known about the 4.6 billion-year-old colli- sional history of the Kuiper Belt. A comparison of the density of craters on Pluto, its moon Charon, and sever- al KBOs will provide our first hard data on the history of impacts in the extreme outer solar system. If indeed KBO material is as ancient and relatively unaltered as sci- entists believe, characterizing the composition will provide an impor- tant reference for comparison with the surface materials on other related bod- ies, including the Centaurs, the nuclei of comets, and certain near-Earth asteroids. Such observations may pro- vide information on whether comets are fragments of large KBOs or are themselves primordial bodies. Kuiper Belt-Pluto Explorer data will also allow researchers to compare the surface compositions of KBOs with Pluto, Charon, and Triton a satellite of Neptune suspected of being a captured KBO. This may allow us to determine how primitive material in the outer solar system is changed over the course of planetary evolution. Telescopic observations to date indicate that KBOs have diverse and sometimes unexpected characteris- tics most display wide color varia- tions from object to object, some have rapid rotation rates, a few exist in loosely bound double systems, and Pluto even has a tenuous atmos- phere. Because of the incredible variety in color, size, composition, and orbit among KBOs, the value of the Kuiper Belt-Pluto Explorer mis- sion increases as it observes more KBOs and, thus, samples more of the

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Kuiper Belt-Pluto Explorer physical and chemical diversity dis- played by these objects. NASA has been working for more than a decade to develop a mission to Pluto, and much of the planning and design has already been done. The compelling scientific investigations outlined above and the body of exist- ing technology and planning led the SSE Survey to conclude that a flyby mission to multiple KBOs, including Pluto and Charon, should be NASA's highest priority for medium-size mis- sions in the decade 2003-2013. Profile Kuiper Be~-P~uto Explorer Mission Type: Muiti-object Flyby Cost Class: Medium Priority Measurements: Determine the dimensions and shapes of the KBOs visited. Assess their crater density. Measure their surface composition through imaging spectroscopy. Detect their atmospheres. Search for evidence of ongoing geological activity (e.g., geysers). Assess the dust density with increasing distance into the Kuiper Belt. Guiding Themes Addressed Important Planetary Science Questions Addressed Artist's impression of New Horizons, the Kuiper Belt-Pluto Explorer.