A Retrospective on the Discovery
of Nix and Hydra
Hal Weaver
JHU Applied Physics Laboratory
Outline
• The long and winding road for the Hubble Pluto Companion
program
– Rejection, Rejection, and (finally!) Acceptance
• Preparing to observe the Pluto System
– Prospects not so promising anymore?
• Hubble observations and their aftermath
– Successful observations on 2005 May 15 and May 18
– Analysis of data stretched out for several months
• NH in Thermal Vac, Deep Impact, Delivering NH to KSC
– Sleuthing what’s going on: artifacts or real objects?
– Victory at last:
• S/2005 P1 and S/2005 P2 discoveries announced on 2005 Oct 31
• Follow-up Hubble observations made in Feb-Mar 2006
• Satellites get their official names (Nix and Hydra)
2
Motivation for Pluto Companion Search
• New Horizons became a “real” mission in early 2003
− NASA selects NH in Nov 2001
− NH passes Preliminary Design Review in Oct 2002
− NASA approves NH for Phase C/D in Mar 2003
− Time to get serious about Pluto!
• NH Principal Investigator Alan Stern asserts:
− Since NH is going to Pluto, we need to take inventory of
the bodies in the Pluto system
− No theoretical reasons why Pluto can’t have more
satellites; KBO binaries are fairly common
− Current limits on Pluto satellites are not very sensitive
− Why not use Hubble to perform a deep search for
Pluto companions?
3
Putting Together the First Proposal (DDT)
• In late-February 2003:
– Alan asks Hal to lead a Hubble search proposal
– Hal agrees Hubble proposal is a great idea and volunteers to do
most of the proposal work, but suggests either Alan, John
Spencer, or Marc Buie serve as PI because Hal was on recently
submitted Director’s Discretionary Time (DDT) proposal
• DDT needed because next Pluto opposition in June 2003:
Cycle 11 deadline was Sep 2002, Cycle 12 deadline was
Jan 2003, and Cycle 13 deadline not till Jan 2004
– Alan declines, suggests John as PI with Marc as CoI
– Hal contacts John and Marc, and lays out plan
• In March 2003:
– Hal performs feasibility study, circulates 1st draft on 3/23
– Hal circulates 2nd draft on 3/28, hands-off to John
• In early-April 2003:
– John submits DDT proposal on 4/9
4
Hubble DDT Pluto Companion Proposal
• Key arguments to justify proposal:
–
–
–
–
–
Search needed to support New Horizons
Detection of more Pluto satellites interesting in its own right
Hubble could provide 100x sensitivity improvement over previous searches
Opposition in Jun 2003 is best time because Pluto moving into dense star field
Modest request (only 2 orbits!) for potential big return
Proposal Rejected
on 2003 April 22
5
Cycle 13 GO Proposal
• Hal took over as PI because John
moved from Lowell to SwRI
• Elliot Young and Leslie Young
added as CoIs
• Submitted in Jan 2004
• Rejection by Solar System Panel
in April 2004, but ranked in 2nd
quartile
6
Rejection of Cycle 13 Proposal
Panel Review Comments:
Strengths: Proposed observations might enhance the New Horizons mission
return, providing a small TNO (captured by Pluto-Charon) at the opposite
end of the size spectrum from Pluto and Charon themselves. Discovery of
captured outer satellites would be an interesting result in itself.
Weaknesses: Because of stability considerations, any satellite discovered
in this program would probably be at a relatively large distance from New
Horizons at the time of flyby. Proposers have not pushed ground-based
techniques that can get to about mag 26 (recent discoveries of planets
around gas giants from ground based telescopes go to >25). Additional
science for New Horizons is not large because focus of close observations
will be Pluto and Charon and any discovered satellites will likely be
small, suggesting a small number of resolution elements when spacecraft
can be pointed at the newly discovered target.
Reasonableness of Resources: Observations at the confusion minimum in 2009
would still be 6 years before the Pluto encounter (July 2015), allowing
plenty of time to make follow-up observations to determine the orbit and
plan an observing sequence.
7
One Door Shuts, Another Door Opens
• Failure of STIS power supply in Aug 2004 leaves large gap in
Hubble observing calendar
– STScI initiates review of submitted Cycle 13 programs searching for
possible replacements for previously approved STIS programs
• STScI notifies us on 2004 Sep 28 that the Pluto Companion
program can be added to the Cycle 13 pool
– Pluto Companion program becomes GO 10427
– Phase 2 proposal submitted in late October 2004:
• Two orbit program with visits separated by ~5 days
– Time separation long enough to detect Plutocentric motion
• Observing window requested is 2005 May 15 – Jun 30
– But not May 16-17 (cluster of stars near Pluto) or May 26 (bright star)
– Maximize Pluto’s apparent motion to enable removal of stars
• Four long exps (475 s) each orbit to go deep (V ~ 27)
– Observations scheduled for 2005 May 15 and May 18 by Alison Vick
8
But has our program lost its Mojo?
• On May 3, Alan sends Hal and John a preprint from Nicholson &
Gladman reporting failed search for Pluto companions from
Palomar Observatory
– Claimed no companions down to mR ≤ 25.0
• Note: Nix and Hydra have mR ≈ 22.7 – 23.1
– Hubble program no longer 100x better than previous searches!
• Elliott Young completes his analysis of previously obtained
Hubble Pluto data (PI = Marc Buie; ACS/HRC from 2002-2003)
and distributes preprint (Young & Buie) on May 14
– Claimed no companions down to mB ≤ 25.9 (5σ)
• Hal finds error and corrects limit to mB ≤ 24.2 (6/19/2005)
• Note: Nix and Hydra have mB ≈ 23.8 – 24.2
• Hal decides Pluto companion search will be extremely difficult
and will require careful analysis over entire orbital stability zone
9
Hubble Advanced Camera for Surveys (ACS), Wide Field Camera (WFC)
Two epochs, same strategy: 1 short exp, 2 long exps, dither, 2 long exps
10
Full Frame ACS/WFC Images : UT 2005 May 15
4,408,000 km
202"
Pluto
Combine First Pair of Images
Companions in Gap
202"
Pluto
Combine Second Pair of Images
Companions in Upper Chip
11
Preliminary Analysis: May 21 – Jun 14
• On May 21 (Saturday):
– Hal looked at short exp images (0.5 s) taken on May15 and May 18 to
verify that Pluto and Charon positions and photometry were consistent
with expectations
• Hal spends most of next 2 weeks on:
– New Horizons testing at Goddard (systems tests, mission sims, I&T prep)
– Preparations for Hubble observations of 9P/Tempel 1 (Deep Impact)
• During Jun 10 – 14:
– Hal creates composites from long exp images (475 s) to mitigate effects
of stars, cosmic rays, and CCD pixel artifacts
• Some images clearly show Nix & Hydra, but Hal did not notice
– Hal creates a movie allowing viewer to blink through all 4 images taken
during May 15 in 512 x 512 frames (entire FOV is 8 x 8 frames)
• But image shift used to create movie deleted companions!
– Hal puts all the above products on a website and sends email to proposal
team on Jun 14 inviting them to peruse the results
• John Spencer replies that he sees Pluto & Charon in movie
12
The frames to the
left are a composite
of all four long exps
taken on 2005
May 15
WFC1 is at the top
and WFC2 is at the
bottom
The combo
technique removed
regions that weren’t
common to all four
images, so no
Pluto & Charon, and
no Nix & Hydra
Pluto is in a dense
star field and the
residuals of many
stars are still evident
throughout the FOV
13
What I missed on 2005 June 11
Frame to right is combo of last two
images taken on May 15 showing full
WFC1 FOV 4096 x 2048 pixels
Frame below is expanded view of
region near Pluto & Charon;
Nix & Hydra are clearly seen!
14
Enter Deep Impact and Max Mutchler (STScI)
• Hal was CoI on Hubble program to observe comet
9P/Tempel 1, the target of the Deep Impact mission
• Warm-up observations on Jun 14 showed unexpected
outburst in activity
• Hal asks Max for help with future ACS data on comet
and suggests looking at Pluto data for practice
• DI event on July 4; Hubble observed daily from Jul 2-5,
Jul 11, and Jul 16
15
Max spots two objects near Pluto-Charon
Email from Max to Hal on June 16:
I found myself wishing Pluto-Charon were not placed so close to the WFC
interchip gap. I saw two interesting candidates near Pluto-Charon in epoch
1: one near 12-o-clock and another near 10-o-clock, then at a similar
distance in epoch 2, something at 9-o-clock: could it be an object in orbit
around Pluto (orbital period ~36 days)? Since the gap coincides with the
Pluto-Charon orbital plane (and the orbital plane of any additional moons?),
unfortunately we are only 2-exposures deep there (easier to be tricked by
cosmic rays).
May 15
May 18
16
What Next?
• Hal says “sounds interesting Max” but…
– Can’t be that easy!
– Let’s put this on the back burner for now
• NH T/V testing starts on Jun 20
• Need to focus on Deep Impact observations
• In mid-July Alan says “I have a new post-doc and
want him to work on the Hubble Pluto data”
– Hal says nothing to Alan about Max’s findings because
he wants independent assessment of Hubble images
• Andrew Steffl joins the team
– Meets with Hal at APL on Jul 27
– Hal doesn’t mention Max’s results
– Andrew starts work during first week of August but
doesn’t see any companions during initial search
– Andrew goes to meeting in England during second
week in August
• Hal goes on vacation to Israel for Aug 9-23
17
The Plot Thickens
• Andrew finds two candidates in May 18 images and sends email to
Hal and Alan on Aug 19
– Objects “seem real” but sometimes fall in inter-chip gap
– He does not see them in May 15 images
• Alan calls Hal on Aug 24 to relate this news
– Hal then tells Alan about Max’s previous findings (from mid-June)
• After further work, Andrew sees objects in May 15 images and sends
email to Hal and Alan on Aug 24
18
Andrew’s Independent Detections
May 18
May 15
19
Have we really found two new satellites?
• Could these objects be image artifacts?
– New objects are ~3000x fainter than Pluto and ~500x
fainter than Charon: Optical ghosts?
– Both look real (PSF-like), but …
• Could they be background objects?
– If so, must be plutinos; binary plutinos?
– Highly unlikely, but …
• Why weren’t these objects detected previously?
Recent null searches:
-- Palomar (Nicholson
– Bright enough (V ~ 23) and separated enough from
& Gladman)
Pluto (~1.9 – 2.4”) to be seen by big telescopes
-- Hubble (Young & Buie)
– Are we inconsistent with previous null results?
-- Subaru (Aug-Sep 2003)
• Keep quiet until results are verified by independent
observations!
20
The Quest for Confirmation
• We spent the next 2 months (late-Aug to late-Oct) trying to
confirm the Hubble results
• First try Hubble itself:
– Prepared DDT proposal and submitted on Aug 30 (from KSC)
– No go: Hubble entered 2-gyro ops on Aug 29 so no Pluto
observations possible until February 2006
• Next try Keck (bring in Bill Merline):
– NIRC observations attempted on Sep 9-10
– Bad weather spoils our program, but Mike Brown discovers
Eris satellite (Dysnomia) and observes Pluto too!
– Good data obtained on Sep 13, but no companions!
• Try VLT (with help from Christophe Dumas):
– Use SINFONI on Sep 28, 29, 30, but no companions!
• Try Gemini North (with help from Matt Mountain):
– Use NIRI/Altair on Oct 11, 13, 14, 15, but no companions!
• Re-Analyze Subaru data taken 2003 Aug 31 and Sep 23
– Deep SuprimeCam images, but no companions!
21
Now What?
• Securing ground-based confirmation in 2005 deemed futile
– Observing conditions simply too poor (short visibility, high air mass)
• But possibility of ground-based detections in early 2006 certainly
seems feasible
– Diminished Hubble role in discovery, if we sit on our result
• Hal tracks down George Hartig (ACS h/w expert) for consultation
– Originally contacted George on Aug 31, and he sent Hal a paper on ACS
straylight and ghosts on Sep 2
– Hal talks to George on Oct 18 and sends him figures of reduced images.
Max meets with him on Oct 19 to review the data in person. In an email
to Hal and Max later that day, George declares:
I've looked at the images you sent, and also reviewed the data again with
Max. The evidence looks very solid to me, and I can conceive of no
instrumental artifact that would be at all likely to produce the two PSF-like
images that appear to revolve about Pluto as they do.
22
Bring in other CoIs
• Telecon with entire team convened on Oct 20
– Is everyone convinced by the data already in hand?
• On Oct 24, Marc Buie presents his own independent analysis of 2005
data and says he has no doubts objects are Pluto satellites
• John Spencer agrees there is an exceptionally small probability that
the objects are not in orbit about Pluto (i.e., that they could be other
Plutinos that happen to lie along the same line of sight)
• Leslie Young suggests making some simple assumptions about the
orbit solution to permit testing satellite hypothesis
23
Orbit Solution Works!
On Oct 24, Bill Merline
computed model orbits for
satellites assuming:
Nix
Charon
• Circular orbits (e = 0)
• All objects in same orbital plane
Hydra
Voila!
24
Confirmation from Hubble 2002 ACS Images
• Marc Buie detects objects
in 2002 Hubble data after
several days of intensive
data analysis in late-Oct
• Low S/N, but results from
2002 and 2005 are
consistent
25
Two New Satellites!
12.8"
Combination of 4 Images
279,000 km
Combination of 4 Images
IAU Circular on 2005 Oct 31
27
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)
Still there on Feb 15th !
286,000 km
12.5"
ACS/HRC Image on 2006
Feb 15.6 UT
P1
P2
Charon
Pluto
Combination of Four Images
And in color on March 2nd !
291,000 km
ACS/HRC F606W on 2006 Mar 2.7 UT
12.8"
ACS/HRC F435W on 2006 Mar 2.7 UT
B-V colors: Pluto=0.87, Charon=0.71, P2=0.65, P1=0.65 (Solar=0.66)
Naming Pluto’s New Satellites: Nix & Hydra
• Long, involved process taking many months
• Many names considered, but it all boiled down to:
− Must have connection to Greek mythology, preferably to the “underworld”
− Must acknowledge role played by the New Horizons mission
Kaare Aksnes, President of the IAU Planetary System Nomenclature Working Group,
states: “In Greek mythology, Nyx is the goddess of the night, but since asteroid
3908 already bears the Greek name Nyx, we changed Nyx to its Egyptian equivalent,
Nix. Hydra was a nine-headed serpent with poisonous blood that had its den at the
entrance to Hades, where Pluto and his wife Persephone entered the Underworld.”
2006 June 21
31
Backup
32
Movie of Images from 2005 May 15
33
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
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