The New Horizons and Hubble Space Telescope Search For Rings, Dust, and Debris in the Pluto-Charon System

摘要

We searched for dust or debris rings in the Pluto-Charon system before,during, and after the New Horizons encounter. Methodologies included searchingfor back-scattered light during the approach to Pluto (phase $\sim15^\circ$),in situ detection of impacting particles, a search for stellar occultationsnear the time of closest approach, and by forward-scattered light duringdeparture (phase $\sim165^\circ$). A search using HST prior to the encounteralso contributed to the results. No rings, debris, or dust features wereobserved, but our detection limits provide an improved picture of theenvironment throughout the Pluto-Charon system. Searches for rings inback-scattered light covered 35,000-250,000 km from the system barycenter, azone that starts interior to the orbit of Styx, and extends to four times theorbital radius of Hydra. We obtained our firmest limits using the NH LORRIcamera in the inner half of this region. Our limits on the normal $I/F$ of anunseen ring depends on the radial scale of the rings: $2\times10^{-8}$($3\sigma$) for 1500 km wide rings, $1\times10^{-8}$ for 6000 km rings, and$7\times10^{-9}$ for 12,000 km rings. Beyond $\sim100,000$ km from Pluto, HSTobservations limit normal $I/F$ to $\sim8\times10^{-8}$. Searches for dust fromforward-scattered light extended from the surface of Pluto to the Pluto-CharonHill sphere ($r_{\rm Hill}=6.4\times10^6$ km). No evidence for rings or dustwas detected to normal $I/F$ limits of $\sim8.9\times10^{-7}$ on $\sim10^4$ kmscales. Four occulation observations also probed the space interior to Hydra,but again no dust or debris was detected. Elsewhere in the solar system, smallmoons commonly share their orbits with faint dust rings. Our results suggestthat small grains are quickly lost from the system due to solar radiationpressure, whereas larger particles are unstable due to perturbations by theknown moons.