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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.