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The discovery of two new satellites of Pluto
Max Mutchler Space Telescope Science Institute mutchler@stsci.edu

Overview
· 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!

New Horizons Educator Workshop
16 January 2006

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
Left to Right:

Hal W eaver (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)

Advanced Camera for Surveys (ACS)

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.

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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
· Hubble proposal designed by W eaver, Stern, et al., initially rejected, then accepted when STIS died · Advanced Camera for Surveys (ACS) W ide Field Channel (W FC) 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...

Notice the star trails, cosmic rays, chip gap...

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

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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

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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

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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
· 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)

Relative sizes of Pluto, Charon, and new moons (P1 and P2)

P1 P2
2300 km 1200 km ~100 km

New moons are roughly 12x smaller than Charon, and 5000x fainter than Charon

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What does a "quadruple planet" look like?
http://www.stsci.edu/~mutchler/pluto_50.html

"Xena & Gabrielle"

Animation produced with Celestia

The 10th planet ?

Should we call Pluto a planet?
· 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 10th 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"

Xena

Pluto

Moon

Earth

Voyagers
Launched in 1977

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.

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

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