The B-V colors and photometric variability of Nix and Hydra, Pluto’s two small satellites Max Mutchler (STScI) S. Alan Stern (SwRI) Hal Weaver (JHU/APL) Andrew Steffl (SwRI) DPS Meeting in Pasadena 8-13 October 2006

Hubble Space Telescope Advanced Camera for Surveys (ACS) observations of the Pluto system 15 May 2005: discovery of Nix and Hydra! 18 May 2005: preliminary orbits : pre-discovery, refined orbits, colors 15 Feb 2006: confirmation of discovery and orbits 2 March 2006: B-V colors

Photometry of Nix & Hydra ACS filters: F606W “broad V”, F435W “Johnson B” Scant data: measurements on 3 of 4 dates each Hazards of a compact system: diffraction spikes, PSF halos, artifacts, saturation…difficult to discover! Adjust for changing heliocentric/geocentric distances, and solar phase angle Photometric corrections and transformations to standard UBVRI (Sirianni et al. 2005) Still scratching the surface: more data needed to fully characterize Nix & Hydra (sizes, shapes, dynamics) Let the Hubble images convey the data quality and photometric challenges (pictures worth 1000 words)…

Discovery observations 15, 18 May 2005 ACS Wide Field Channel (WFC) covers orbital stability zone (circle) Pluto-Charon very saturated (moon fishing) Pluto-Charon near WFC chip gap Constraints on additional satellites (Steffl et al., AJ) Following images are just the central region (box)

15.05 May 2005 Hubble ACS / WFC F606W “Broad V” sum AU heliocentric d AU geocentric d 0.96 deg solar phase Nix and Hydra playing gap peek-a-boo, and data only 2 exposures deep

15.05 May 2005 Hubble ACS / WFC F606W “Broad V” AU heliocentric d AU geocentric d 0.96 deg solar phase Hydra Nix Charon Pluto

18.14 May 2005 Hubble ACS / WFC F606W “Broad V” AU heliocentric d AU geocentric d 0.88 deg solar phase Pluto Charon Nix Hydra

Pre-discovery HRC observations Hubble program by Buie & Young, Primarily designed to map surface features (albedo) of Pluto and Charon New moons marginally detected (S/N~4) Photometry implied Nix reddish (B-V=+0.91), like Pluto Refined orbit determination…

Smarter follow-up observations We know Nix and Hydra exist, and that the are close in: use smaller but higher-resolution HRC We have crude magnitudes; can detect with shorter exposures, less Pluto-Charon saturation (closer-in constraints on additional satellites) Refined orbits allow us to manage the positions of moons and “spoilers”: ~15 day interval, avoid background stars, diffraction spikes Squeeze in a B filter (F435W), get B-V colors

15.66 Feb 2006 Hubble ACS / HRC F606W “Broad V” sum AU heliocentric d AU geocentric d 1.58 deg solar phase

15.66 Feb 2006 Hubble ACS / HRC F606W “Broad V” AU heliocentric d AU geocentric d 1.58 deg solar phase Pluto Charon Nix Hydra

2.753 March 2005 Hubble ACS / HRC F606W “Broad V” AU heliocentric d AU geocentric d 1.77 deg solar phase Pluto Charon Nix Hydra

2.746 March 2005 Hubble ACS / HRC F435W “Johnson B” AU heliocentric d AU geocentric d 1.77 deg solar phase Pluto Charon Nix Hydra

Little photometric variability Effective radius variation <14%, i.e. not factor of 2 or 4 like small irregular objects Lower albedos than Charon? On the larger/rounder end of allowable range (~100 km)? Both have neutral (grey) reflectivities, like Charon: similar origins (collision event) No other satellites of similar size, to limiting magnitude V=26.8, diameters km (Steffl et al., AJ) Photometric results for Nix & Hydra IAU Circular 8686, and Stern et al., AJ (submitted), astro-ph/

Hubble ACS photometry * Charon B-V=0.710 (from Buie et al., 1997, Icarus, 125, 233) DateNix (S/2005 P2) Hydra (S/2005 P1) May 2005V = /- 0.15N/A May 2005N/AV = / Feb 2006V = /- 0.20V = / March 2006 V= /- TBD B-V = / * V= /- TBD B-V = / * Variation 1.00 : 0.79 : : 0.78 : 0.73

Co-authors planning more elaborate observations for July 2015…

Publications IAU Circulars 8625, 8676, 8686 Weaver et al. 2006, Nature Stern et al., 2006, Nature Steffl et al., 2006, AJ Stern et al., 2006, AJ, (submitted)

The B-V colors and photometric variability of Nix and Hydra, Pluto’s two small satellites Max Mutchler, Alan Stern, Hal Weaver, Andrew Steffl Pluto’s two small satellites, Nix and Hydra, were observed on four dates (15.1 and 18.1 May 2005, 15.7 February 2006, and 2.8 March 2006) using the Hubble Space Telescope’s (HST) Advanced Camera for Surveys (ACS). V-band magnitudes were obtained for all dates, and B- band magnitudes were additionally measured on the last date. We find that both satellites have essentially neutral (grey) reflectivities, like Charon. We also find that neither satellite exhibited strong photometric variation, which might suggest that Nix and Hydra are toward the large end of their allowable size range, and therefore may have lower albedos than Charon. Support for this work was provided by NASA through Grants GO and GO from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., and the New Horizons Pluto-Kuiper Belt mission.