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The discovery of two new
satellites of Pluto
Max Mutchler
Space Telescope Science Institute
FIT visit
18 January 2006
T-minus 4 hours and counting…
Hubble’s Advanced Camera, Discovery Team
Discovery of Pluto, Charon, and the Kuiper Belt
Early Hubble observations of Pluto
Hubble mission support for New Horizons: discovery of
two more Pluto satellites
Confirming and following-up the discovery
Implications, and recent related discoveries
New Horizons mission to Pluto and beyond
More information via the web
Calibrating, pointing, and drizzling
Hubble Servicing Mission 3B in March 2002: ACS installed
Advanced Camera
for Surveys (ACS)
Hubble Pluto Satellite Search Team
reporting the discovery to the New Horizons Science Team
on November 2, 2005 at the Kennedy Space Center
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
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
• 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…
Notice the star trails, cosmic rays, chip gap…
15 May 2005, frame 1
Notice the star trails, cosmic rays, chip gap…
15 May 2005, frame 2
Dithering across the chip gap now…see anything?
15 May 2005, frame 3
Dithering across the chip gap now…see anything?
15 May 2005, frame 4
Looking for real objects among all the artifacts…
15 May 2005, sum 4 frames
Looking for real objects among all the artifacts…
15 May 2005, median 4 frames
Do it again 3 days later…where are the moons?
18 May 2005, frame 1
Do it again 3 days later…where are the moons?
18 May 2005, frame 2
Dither across the gap…where are the moons?
18 May 2005, frame 3
Do it again 3 days later…where are the moons?
18 May 2005, frame 4
“Clean” image
18 May 2005, median 4 frames
“Clean” image
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
S/2005 P 2
15 and 18 May 2005, median 8 frames
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 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? KBO (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)
2300 km
1200 km
New moons are roughly 12x smaller than Charon,
and 5000x fainter than Charon
~100 km
What does a “quadruple planet” look like?
with Celestia
Announcement and publications
Weaver et al, 2005, IAU Circular 8625
Weaver et al., 2006, Nature (accepted)
Stern et al., 2006, Nature (accepted)
Steffl et al., Astronomical Journal (submitted)
Pre-prints available online at:
“Xena & Gabrielle”
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”
Kuiper Belt
July 2015
March 2007
Jan 2006
New Horizons mission
Pluto-Charon Encounter Geometry
Arrival July 14, 2015
Charon-Earth Occultation
Pluto-Earth Occultation
Charon-Sun Occultation
Pluto-Sun Occultation
S/C trajectory time ticks: 10 min
Charon orbit time ticks: 12 hr
Occultation: center time
Position and lighting at Pluto C/A
Distance relative to body center
Charon C/A
26,937 km
13.87 km/s
Pluto C/A
11,095 km
13.77 km/s
00 00
Launch currently set for:
January 18, 2006
1:16 PM EST
http://hubblesite .org