A Retrospective on the Discovery of Nix and Hydra

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

3

• 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?

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

5

Proposal Rejected on 2003 April 22

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 ProposalPanel 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

202"

202"

Full Frame ACS/WFC Images : UT 2005 May 15

Combine First Pair of ImagesCompanions in Gap

Combine Second Pair of ImagesCompanions in Upper Chip

Pluto

Pluto

4,408,000 km

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

13

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

What I missed on 2005 June 11

14

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!

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-CharonEmail 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).

16

May 15 May 18

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 9-2317

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

19

May 15

May 18

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?– Bright enough (V ~ 23) and separated enough from

Pluto (~1.9 – 2.4”) to be seen by big telescopes

– Are we inconsistent with previous null results?

• Keep quiet until results are verified by independent observations!

20

Recent null searches: -- Palomar (Nicholson & Gladman) -- Hubble (Young & Buie) -- Subaru (Aug-Sep 2003)

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!

24

Hydra

Nix

Charon

On Oct 24, Bill Merline computed model orbits for satellites assuming:

• Circular orbits (e = 0)• All objects in same orbital plane

Voila!

Confirmation from Hubble 2002 ACS Images

25

• 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

Two New Satellites!

Combination of 4 Images Combination of 4 Images

12.8" 279,000 km

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 !

Combination of Four Images

12.5

"

ACS/HRC Image on 2006 Feb 15.6 UT

Pluto

Charon

P2P1

286,000 km

And in color on March 2nd !

B-V colors: Pluto=0.87, Charon=0.71, P2=0.65, P1=0.65 (Solar=0.66)

12.8

"

291,000 km

ACS/HRC F606W on 2006 Mar 2.7 UTACS/HRC F435W on 2006 Mar 2.7 UT

Naming Pluto’s New Satellites: Nix & Hydra

31

• 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 asteroid3908 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 theentrance to Hades, where Pluto and his wife Persephone entered the Underworld.”

2006 June 21

Backup

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Movie of Images from 2005 May 15

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15 May 2005, frame 1

Notice the star trails, cosmic rays, chip gap…


Notice the star trails, cosmic rays, chip gap…


Dithering across the chip gap now…see anything?


Dithering across the chip gap now…see anything?


Do it again 3 days later…where are the moons?




Dither across the gap…where are the moons?



Documents

A Retrospective on the Discovery of Nix and Hydra