Pluto Open Night Han

The discovery of two new satellites of Pluto

Max Mutchler Space Telescope Science Institute Open Night 3 January 2006

Andy Lubenow 1956 - 2005

Hubble Pluto Satellite Search Team Overview reporting the discovery to the New Horizons Science Team on November 2, 2005 at the Kennedy Space Center • History: discovery of Pluto, Charon, and the Kuiper Belt • Early Hubble observations of Pluto • Hubble mission support for New Horizons, and Deep Impact • Discovery of two new satellites with Hubble • Confirming the discovery: checklist, ground- based and Hubble follow-up Left to Right: • Implications, and recent related discoveries: 10th Hal Weaver (JHU/APL), Andrew Steffl (SwRI), S. Alan Stern (SwRI), planet with a satellite, etc. Leslie Young (SwRI), John Spencer (SwRI), Marc Buie (Lowell Observatory), Bill Merline (SwRI), Max Mutchler (STScI), and…Eliot Young (SwRI) • Questions? Door prizes!

The discovery of The search for “Planet X” Pluto in 1930, Lowell Observatory, Flagstaff, Arizona and confirmation

Percival Lowell Vesto Slipher Clyde Tombaugh

1 Announcing the discovery of a “trans-Neptunian” The discovery of Pluto’s moon Charon in 1978 planet (Pluto) James Christy and Robert Harrington, U.S. Naval Observatory, Washington, D.C.

Circular No. 3241

Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION The slowly emerging Postal Address: Central Bureau for Astronomical Telegrams Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. TWX 710-320-6842 ASTROGRAM CAM Telephone 617-864-5758 picture of Pluto

1978 P 1

Capt. J. C. Smith, U.S. Navy, reports: "Elongation of the photographic image of Pluto has been detected by J. W. Christy on plates taken with the U.S. Naval Announcing the Observatory's 155-cm astrometric reflector on 1978 Apr. 13, 20 and May 12; 1970 June 13, 15, 16, 17 and 19; and 1965 Apr. 29 and May 1. The maximum elongation is ~0"9 in p.a. 170o-350o. Observed position angles are consistent with a uniform revolution/rotation period equal to the lightcurve period of 6.3867 days. The data discovery of suggest that there is a satellite, 2-3 magntiudes fainter than Pluto, revolving around Pluto in this period at a mean distance of ~20 000km; the implied sun-Pluto mass ratio is 140 000 000:1. The other orbital elements are: e ~ 0, Node = 350o, 1978 P 1 Incl = 105o (with respect to the plane of the sky), T(Node) = 1978 May 12.2 UT. The probable satellite was confirmed on exposures with the 155-cm reflector on 1978 July 2 and 5 and by J. A. Graham with the 400-cm reflector at Cerro Tololo Interamerican Observatory on July 6. Further observations are very desirable; a (Charon) brief ephemeris follows." 1978 UT p.a. Sep. 1978 UT p.a. Sep. July 8.2 357o 0"8 July 12.2 165o 0"9 9.2 342 0.8 13.2 129 0.3 10.2 277 0.2 14.2 5 0.6 11.2 181 0.7 15.2 384 0.9

The designation 1978 P 1 conforms with the temporary nomenclature system for announcing discoveries of new satellites. This system was approved at the meeting in June of the IAU Working Group for Planetary System Nomenclature and has been submitted for endorsement at the forthcoming meeting of the IAU Executive Committee.

Charon 1200 km

Earth Pluto Moon 12,800 km 2400 km 3000 km

2 Everything we know about Pluto 1 Pluto has • 1930 Pluto discovered; eccentric orbit * not given up • 1955 rotation period 6.4 days Is Pluto… • 1965 stable 3:2 resonant orbit with Neptune it’s secrets • 1973 obliquity > 90 deg * very easily. • 1976 methane ice on surface; size constrained • 1978 Charon discovered; “binary planet” * • 1980 Occultation reveals Charon radius to be 600 km Everything • 1985 Pluto-Charon mutual events begin learned about it ? since 1930 still Everything we know about Pluto 2 fits on a couple • 1986 Pluto & Charon radii, albedos, colors • 1987 Pluto density is 2 g/cm3 of 3x5 cards! • 1988 Pluto orbit chaotic; occultation reveals atmosphere and polar caps • 1989 Pluto and Triton similar, thermal structure in * And much of it atmosphere • 1992 Nitrogen and CO ice, density disparity seems, well, weird! • 1994 Discovery of the Kuiper Belt • 2001 Binary Kuiper Belt objects (KBOs)

3 Pluto satellite searches

• Kuiper & Humason, 1950 (didn’t find Charon) • Stern 1991: beyond 6 arcsec • Christy & Harrington (serendipitous) • Buie & Young (albedo maps, too shallow) Early Hubble • Weaver 2005 (almost rejected!) observations • DD or GO in 2006? of Pluto and Charon

Satellite discovery observations

• HST proposal submitted by Weaver, Stern, et al., was rejected, then accepted when STIS died • Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) covers entire orbital stability zone, puts Pluto-Charon near interchip gap • 4 long exposures on May 15 and May 18, 2005 • An informal request to inspect the data from Hal Weaver on June 13, while preparing for Deep Impact Hubble ACS imaging

Calibrating and drizzling ACS images

Hubble Servicing Mission 3B

Advanced Camera for Surveys The Whirlpool Galaxy M51

4 Pluto WFC image with full orbital stability zone (circle)…

Chip gap: peek-a-boo!

…although subsequent images are cropped to the center box 15 May 2005, frame 1

15 May 2005, frame 2 15 May 2005, frame 3

15 May 2005, frame 4 15 May 2005, sum 4 frames

5 15 May 2005, median 4 frames 18 May 2005, frame 1

18 May 2005, frame 2 18 May 2005, frame 3

18 May 2005, frame 4 18 May 2005, sum 4 frames

6 18 May 2005, median 4 frames 15 May 2005, median 4 frames

Charon orbital radius: 19,400 km Finding needles in the haystack… P1 orbital radius: 55,000 km P2 orbital radius: 45,000 km

Charon period: 6.4 days P1 period: 25.5 days P2 period: 38.2 days P1

Charon

15 and 18 May 2005, sum 8 frames 15 and 18 May 2005, median 8 frames

Excerpts of the e-mail that got things rolling….well, eventually…

Date: Thu, 16 June 2005 11:22:23 -0400 From: Max Mutchler < [email protected] > Subject: Re: Pluto data To: Hal Weaver < [email protected] >

Hal,

I now have…images for all 10 Pluto-Charon frames…for each epoch… useful for blinking against other images to decide if something is real or just… artifacts ….still need visual verification. I think visually scanning / blinking among the files above is the way to go…However, I found myself wishing Pluto-Charon were not placed so close to the WFC interchip gap.

I saw two interesting candidate 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 Press release image: the discovery was (orbital period ~36 days)? Since the gap coincides with the Pluto-Charon orbital plane (and the orbital surprisingly easy for Hubble with ACS… plane of any additional moons?), unfortunately we are only 2-exposures deep but not quite as easy as it looks here. there (easier to be tricked by cosmic rays)… More later, Max

7 My summer vacation or Confirmation “life is what happens to you while you are busy making other plans” • Independent discovery in Aug 2005 by Andrew • June 15: Pluto moons discovery Steffl (SwRI in Boulder, Colorado) • Checklist of alternate explanations (proceed with • June 27: Supernova SN2005cs in M51 extreme paranoia): detector artifacts (show)? • July 4: HST imaging of Deep Impact Plutinos? KBO? Neptune Trojans? •Aug: HST Moon observations • Search other existing data: Hubble, Subaru • Our own ground-based attempts to image the • Aug 26: return from vacation; e-mail reply new moons: Keck, VLT, Gemini (bad timing) from Hal Weaver • Hubble followup in Feb 2006 (2 gyros!) • Aug 28: Hubble enters 2-gyro mode! • Confident enough to announce on 31 Oct 2005

Attempts to image the new satellites with ground-based observatories The “checklist” of possible explanations • Artifacts from the detector or optics: hot pixels, ghosts, scattered light (show some; Hartig)? • Background binary KBOs? Binary KBO? • Plutinos or Neptune Trojans? • Expecting moons much farther out; hiding in plain view, front and center; MT pipeline! • Predicted small chance of one KBO in entire WFC field; odds of two vanishingly small • New moons of Pluto! Gemini (Hawaii) Very Large Telescope (Chile)

Preliminary assumptions and conclusions • Orbits are co-planar with Charon, and nearly circular • Probably formed primordially with Charon (collision), not later (captured) • Possible dust arcs • No other moons of similar magnitude (unless artifacts hid them in June)

8 The “quadruple planet” Pluto Pre-discovery observations in 2002 Visual Diameter Orbital Orbital • Hubble program 9391 by Buie & magnitude radius * period * Young (barycentric) • ACS / HRC with blue and visual Pluto 2400 km 6.4 days filters (9999 mi) • Primarily designed to map surface features of Pluto and Charon 1200 km 6.4 days Charon (999 mi) • New moons marginally detected • Further observations will S/2005 P 1 22.93 30-160 km 64,700 km 38.2 days definitively determine orbits; and +/- 0.12 (99 mi) (9999 mi; 3.7x (~6x Charon) hopefully confirm these Charon) detections: are the satellites where they should be? S/2005 P 2 23.38 30-160 km 49,400 km 25.5 days +/- 0.17 (99 mi) (9999 mi; 2.8x (~4x Charon) Charon

* These numbers assume co-planar and circular orbits for P1 and P2

What does a “quadruple planet” Announcing the look like? discovery of S/2005 P 1 and S/2005 P 2 on Oct 31, 2005 with an IAU Circular

The name game Should we call Pluto a planet?

• I’m neutral: a semantic “parlor game”. But consider… • Pluto: god of the underworld, picked by a • Is Pluto just the first of many Kuiper Belt “ice dwarfs”? • Ceres was called a planet for 50 years, then “demoted” to asteroid little girl, Disney’s dog (add cartoon); (a precedent) symbolizes Percival Lowell • Is larger Xena the 10th planet? • Charon: ferries the dead across the river • Are slightly smaller Sedna, Quaoar planets too? Styx; mispronounced like James Christy’s • Will we have only 8 planets, or hundreds of them? wife Sheryl • Is this a problem…or is it progress? • The IAU is working on it…maybe they shouldn’t? • P1 and P2: we’ll decide soon; decided not • A healthy unresolved “controversy”: let the healthy debate (and to temporarily use informal names people’s interest) rage on! • IAU guidelines (Greco-Roman mythology); • A rose by any other name… these objects are fascinating approval takes awhile

9 Recent discoveries & implications

• Xena & Gabrielle: 10th planet, with a moon • Buffy • Pluto: ugly duckling, or the norm in a new reality? • Are the other 8 planets the oddballs?

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

Xena P1

2400 km 1200 km ~100 km

Pluto Moon Earth

Hitch a ride on the fastest rocket ever… http://pluto.jhuapl.edu

13 17 00 00

Launch currently set for: January 17, 2006 2:11 PM EST

10 Voyagers Another Voyage of Discovery to Launched in 1977 inspire the next generation of Introduce Hal here math and science students -- for them to literally grow up with -- • Add his 4-5 slides here and where we should expect the unexpected…

Good luck New Horizons!

Questions?

… AND TWO LITTLE MOONS !