The New Horizons and Hubble Space Telescope search for rings, dust, and debris in the Pluto-Charon system

Abstract We conducted an extensive search for dust or debris rings in the Pluto-Charon system before, during, and after the New Horizons encounter in July 2015. Methodologies included attempting to detect features by back-scattered light during the approach to Pluto (phase angle α  ∼ 15°), in situ detection of impacting particles, a search for stellar occultations near the time of closest approach, and by forward-scattered light imaging during departure ( α  ∼ 165°). An extensive search using the Hubble Space Telescope (HST) prior to the encounter also contributed to the final ring limits. No rings, debris, or dust features were observed, but our new detection limits provide a substantially improved picture of the environment throughout the Pluto-Charon system. Searches for rings in back-scattered light covered the range 35,000–250,000 km from the system barycenter, a zone that starts interior to the orbit of Styx, the innermost minor satellite, and extends out to four times the orbital radius of Hydra, the outermost known satellite. We obtained our firmest limits using data from the New Horizons LORRI camera in the inner half of this region. Our limits on the normal I / F of an unseen ring depends on the radial scale of the rings: 2 × 10 − 8 (3 σ ) for 1500 km wide rings, 1 × 10 − 8 for 6000 km rings, and 7 × 10 − 9 for 12,000 km rings. Beyond  ∼ 100, 000 km from Pluto, HST observations limit normal I / F to ∼ 8 × 10 − 8 . Searches for dust features from forward-scattered light extended from the surface of Pluto to the Pluto-Charon Hill sphere ( r Hill = 6.4 × 10 6 km). No evidence for rings or dust clouds was detected to normal I / F limits of ∼ 8.9 × 10 − 7 on  ∼ 10 4 km scales. Four stellar occulation observations also probed the space interior to Hydra, but again no dust or debris was detected. The Student Dust Counter detected one particle impact 3.6 × 10 6 km from Pluto, but this is consistent with the interplanetary space environment established during the cruise of New Horizons. Elsewhere in the solar system, small moons commonly share their orbits with faint dust rings. Our results support recent dynamical studies suggesting that small grains are quickly lost from the Pluto-Charon system due to solar radiation pressure, whereas larger particles are orbitally unstable due to ongoing perturbations by the known moons.