The discovery of two new satellites of Pluto Overview Max Mutchler Space Telescope Science Institute mutchler@stsci.edu New Horizons Educator Workshop 16 January 2006 Hubble s Advanced Camera; Discovery team Discovery of Pluto, Charon, and the Kuiper Belt Hubble mission support for New Horizons: discovery of two more Pluto satellites Confirming and following-up the discovery Implications, and recent related discoveries More information via the web Questions? Door prizes! Hubble Pluto Satellite Search Team reporting the discovery to the New Horizons Science Team on November 2, 2005 at the Kennedy Space Center Calibrating, pointing, and drizzling Hubble Servicing Mission 3B in March 2002: ACS installed Advanced Camera for Surveys (ACS) Left to Right: Hal Weaver (JHU/APL), Andrew Steffl (SwRI), S. Alan Stern (SwRI), Leslie Young (SwRI), John Spencer (SwRI), Marc Buie (Lowell Observatory), Bill Merline (SwRI), Max Mutchler (STScI), and Eliot Young (SwRI) The discovery of Pluto in 1930, and confirmation The discovery of Pluto s moon Charon in 1978 Clyde Tombaugh James Christy & Robert Harrington U.S. Naval Observatory Washington, D.C.
Discovery of the Kuiper Belt in 1992 Discovery of two new moons of Pluto Press release image for new moons: the discovery was surprisingly easy for Hubble with ACS but not quite as easy as it looks here. New satellite discovery observations Notice the star trails, cosmic rays, chip gap Hubble proposal designed by Weaver, Stern, et al., initially rejected, then accepted when STIS died Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) covers entire orbital stability zone Pluto-Charon near chip gap: peek-a-boo! 4 long exposures on May 15 and May 18, 2005, using only 2 orbits Discovery on June 15: try it yourself 15 May 2005, frame 1 Notice the star trails, cosmic rays, chip gap Dithering across the chip gap now see anything? 15 May 2005, frame 2 15 May 2005, frame 3
Dithering across the chip gap now see anything? Looking for real objects among all the artifacts 15 May 2005, frame 4 15 May 2005, sum 4 frames Looking for real objects among all the artifacts Do it again 3 days later where are the moons? 15 May 2005, median 4 frames 18 May 2005, frame 1 Do it again 3 days later where are the moons? Dither across the gap where are the moons? 18 May 2005, frame 2 18 May 2005, frame 3
Do it again 3 days later where are the moons? 18 May 2005, frame 4 18 May 2005, sum 4 frames Clean image Clean image 18 May 2005, median 4 frames 15 May 2005, median 4 frames New moons are roughly 3-4x farther out than Charon, with possible 6:4:1 orbital resonances S/2005 P 1 Charon S/2005 P 2 15 and 18 May 2005, median 8 frames New moons are roughly 3-4x farther out than Charon, with possible 6:4:1 orbital resonances
Initial thoughts Why is Pluto suddenly going so easy on us?!? Well-designed program: long exposure times (but not too long), two epochs the gap is OK Two objects! They somewhat validate each other, and assumptions about their orbits Surprised they are so close to Pluto and Charon: expecting any moons to be farther out, but they don t violate dynamical constraints (Stern, 1994) Could they be something other than moons? Confirmation and follow-up Independent discovery in Aug 2005 by Andrew Steffl Search other existing data: Hubble, Subaru Hubble follow-up: impossible until 15 Feb 2006 (2 gyros) Ground-based attempts to image the new moons in Sep/Oct: Keck, VLT, Gemini (difficult until spring 2006) Checklist of alternate explanations: rule them out? Confident enough to announce on 31 October 2005 The checklist of possible explanations Detector artifacts? Optical ghosts or scattered light? Overlapping cosmic rays or star trails? Real, but asteroids? KBOs (Plutinos)? New moons of Pluto! Preliminary assumptions and implications Relative sizes of Pluto, Charon, and new moons (P1 and P2) Orbits are co-planar with Charon, nearly circular, possibly in stable resonances with each other No other moons of similar magnitude (unless artifacts hid them); very compact system Pluto first KBO with multiple satellites: implies there are probably many more Probably formed primordially with Charon (collision), not later (captured) 2300 km 1200 km New moons are roughly 12x smaller than Charon, and 5000x fainter than Charon P1 P2 ~100 km
What does a quadruple planet look like? http://www.stsci.edu/~mutchler/pluto_50.html Xena & Gabrielle Animation produced with Celestia The 10 th planet? Should we call Pluto a planet? Xena Pluto Moon Earth I m neutral. But some things to consider Is Pluto just the first of many Kuiper Belt ice dwarf planets discovered? Is larger Xena the 10 th planet? Are slightly smaller Sedna, Quaoar planets? Ceres was called a planet for ~50 years, then re-classified as an asteroid (a precedent) Will we have only 8 planets, or hundreds of them? Is this a problem? Seems like progress to me. The IAU is working on it in the meanwhile, it is a harmless and healthy non-controversy Good luck to New Horizons, the next great Voyage of Discovery It will surely inspire the next generation of math and science students. It s greatest discoveries will surely be the unexpected ones. Voyagers Launched in 1977 Questions? AND TWO LITTLE MOONS! http://www.boulder.swri.edu/plutonews http://www.stsci.edu/~mutchler/pluto_50.html http://pluto.jhuapl.edu http://hubblesite.org hubblesite.org