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Observations of Kuiper belt objects (KBOs) in Neptunes 5:2 resonance present two puzzles: this third order resonance hosts a surprisingly large population, comparable to the prominent populations of Plutinos and Twotinos in the first order 3:2 and 2:1 resonances, respectively; secondly, their eccentricities are concentrated near $0.4$. To shed light on these puzzles, we investigate the phase space near this resonance with use of Poincare sections of the circular planar restricted three body model. We find several transitions in the phase space structure with increasing eccentricity, which we explain with the properties of the resonant orbit relative to Neptunes. The resonance width is narrow for very small eccentricities, but widens dramatically for $egtrsim0.2$, reaching a maximum near $eapprox0.4$, where it is similar to the maximum widths of the 2:1 and 3:2 resonances. We confirm these results with N-body numerical simulations, including the effects of all four giant planets and a wide range of orbital inclinations of the KBOs. We find that the boundaries of the stable resonance zone are not strongly sensitive to inclination and remain very similar to those found with the simplified three body model, with the caveat that orbits of eccentricity above $sim0.53$ are unstable; higher eccentricity orbits are phase-protected from destabilizing encounters with Neptune but not with Uranus. These results show that the 5:2 resonant KBOs are not more puzzling than the Plutinos and Twotinos; however, detailed understanding of the origins of eccentric, inclined resonant KBOs remains a challenge.
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Here we report WFPC2 observations of the Quaoar-Weywot Kuiper belt binary. From these observations we find that Weywot is on an elliptical orbit with eccentricity of 0.14 {pm} 0.04, period of 12.438 {pm} 0.005 days, and a semi-major axis of 1.45 {pm} 0.08 {times} 104 km. The orbit reveals a surpsingly high Quaoar-Weywot system mass of 1.6{pm}0.3{times}10^21 kg. Using the surface properties of the Uranian and Neptunian satellites as a proxy for Quaoars surface, we reanalyze the size estimate from Brown and Trujillo (2004). We find, from a mean of available published size estimates, a diameter for Quaoar of 890 {pm} 70 km. We find Quaoars density to be rho = 4.2 {pm} 1.3 g cm^-3, possibly the highest density in the Kuiper belt.
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The central objective of the New Horizons prime mission was to make the first exploration of Pluto and its system of moons. Following that, New Horizons has been approved for its first extended mission, which has the objectives of extensively studying the Kuiper Belt environment, observing numerous Kuiper Belt Objects (KBOs) and Centaurs in unique ways, and making the first close flyby of the KBO 486958 2014 MU69. This review summarizes the objectives and plans for this approved mission extension, and briefly looks forward to potential objectives for subsequent extended missions by New Horizons.
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