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OUTER SOLAR SYSTEM

Initial results from the New Horizonsexploration of 2014 MU69, a smallKuiper Belt objectS. A. Stern* et al.

INTRODUCTION: The Kuiper Belt is a broad,torus-shaped region in the outer Solar SystembeyondNeptune’s orbit. It contains primordialplanetary building blocks and dwarf planets.NASA’s New Horizons spacecraft conducteda flyby of Pluto and its system of moons on14 July 2015. New Horizons then continuedfarther into the Kuiper Belt, adjusting itstrajectory to fly close to the small Kuiper Beltobject (486958) 2014 MU69 (henceforth MU69;also informally known as Ultima Thule). Stellaroccultation observations in 2017 showed thatMU69 was ~25 to 35 km in diameter, andtherefore smaller than the diameter of Pluto(2375 km) by a factor of ~100 and less massivethan Pluto by a factor of ~106. MU69 is locatedabout 1.6 billion kilometers farther from theSun than Pluto was at the time of the NewHorizons flyby.MU69’s orbit indicates that it isa “cold classical”Kuiper Belt object, thought tobe the least dynamically evolved population inthe Solar System. A major goal of flying pastthis target is to investigate accretion processesin the outer Solar System and how those pro-cesses led to the formation of the planets.Because no small Kuiper Belt object had pre-

viously been explored by spacecraft, we alsosought to provide a close-up look at such abody’s geology and composition, and to searchfor satellites, rings, and evidence of present orpast atmosphere. We report initial scientificresults and interpretations from that flyby.

RATIONALE: The New Horizons spacecraftcompleted its MU69 flyby on 1 January 2019,with a closest approach distance of 3538 km—less than one-third of its closest distance toPluto. During the high-speed flyby, made at14.4 km s−1, the spacecraft collected ~50 gigabitsof high-resolution imaging, compositional spec-troscopy, temperaturemeasurements, and otherdata on this Kuiper Belt object. We analyzedthe initial returned flyby data from the sevenscientific instruments carried on the spacecraft:the Ralph multicolor/panchromatic cameraand mapping infrared composition spectrom-eter; the Long Range Reconnaissance Imager(LORRI) long–focal length panchromatic visibleimager; the Alice extreme/far ultraviolet map-ping spectrograph; the Radio Experiment(REX); the Solar Wind Around Pluto (SWAP)solar wind detector; the Pluto Energetic Par-

ticle Spectrometer Science Investigation (PEPSSI)high-energy charged particle spectrometer;and the Venetia Burney Student Dust Counter(VBSDC), a dust impact detector.

RESULTS: Imaging of MU69 showed it to be abilobed, contact binary. MU69’s two lobes ap-pear tohave formed close to one another, becom-ing an orbiting pair that subsequently underwent

coupled tidal and orbitalevolution to merge intothe contact binary we ob-serve today. The objectrotates on its axis every15.92 hours; its rotationpole is inclined approx-

imately 98° to the plane of its heliocentric orbit.Its entire surface has a low visible-wavelengthreflectivity (albedo) but displays brighter anddarker regions across its surface, ranging from5 to 12% reflectivity. The brightest observedregions are the “neck” of MU69, where the twolobes are joined, and two discrete bright spotsinside the largest crater-like feature on the ob-ject’s surface. Although MU69’s albedo variessubstantially across its surface, it is uniformlyred in color, with only minor observed colorvariations. This coloration likely represents arefractory residue from ices and organic mol-ecules processed by ultraviolet light and cosmicrays. Spectra of the surface revealed tentativeabsorptionbanddetectionsdue towater ice andmethanol. The geology of MU69 consists ofnumerous distinct units but shows only a smallnumber of craters, providing evidence that thereis a deficit of Kuiper Belt objects smaller than~1 km in diameter, and that there is a com-paratively low collision rate in its Kuiper Beltenvironment compared to what would be ex-pected in a collisional equilibrium population.A three-dimensional shapemodel derived fromthe images showsMU69 is not simply elongatedbut also flattened. The larger lobe was foundtobe lenticular,withdimensionsof approximately22 × 20 × 7 km (uncertainty <0.6 × 1 × 2 km),whereas the smaller lobe is less lenticular, withdimensions of approximately 14 × 14 × 10 km(uncertainty <0.4 × 0.7 × 3 km). No evidence ofsatellites, rings, or an extant atmosphere wasfound around MU69.

CONCLUSION: Both MU69’s binarity and un-usual shape may be common among similarlysized Kuiper Belt objects. The observationthat its two lobes are discrete, have retained theirbasic shapes, and do not display prominentdeformation or other geological features indic-ative of an energetic or disruptive collision indi-cates thatMU69 is the product of a gentlemergerof two independently formed bodies.▪

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Stern et al., Science 364, 649 (2019) 17 May 2019 1 of 1

All authors and affiliations are available in the full article online.*Corresponding author. Email: astern@boulder.swri.eduCite this article as S. A. Stern et al., Science 364, eaaw9771(2019). DOI: 10.1126/science.aaw9771Image of MU69 from New Horizons LORRI observations taken 1 January 2019.

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OUTER SOLAR SYSTEM

Initial results from the New Horizonsexploration of 2014 MU69, a smallKuiper Belt objectS. A. Stern1*, H. A. Weaver2, J. R. Spencer1, C. B. Olkin1, G. R. Gladstone3, W. M. Grundy4,J. M.Moore5, D. P. Cruikshank5, H. A. Elliott3,6, W. B. McKinnon7, J. Wm. Parker1,A. J. Verbiscer8, L. A. Young1, D. A. Aguilar9, J. M. Albers2, T. Andert10, J. P. Andrews1,F. Bagenal11, M. E. Banks12, B. A. Bauer2, J. A. Bauman13, K. E. Bechtold2,C. B. Beddingfield5,14, N. Behrooz2, K. B. Beisser2, S. D. Benecchi15, E. Bernardoni11,R. A. Beyer5,14, S. Bhaskaran16, C. J. Bierson17, R. P. Binzel18, E. M. Birath1, M. K. Bird19,20,D. R. Boone16, A. F. Bowman2, V. J. Bray21, D. T. Britt22, L. E. Brown2, M. R. Buckley2,M.W. Buie1, B. J. Buratti16, L. M. Burke2, S. S. Bushman2, B. Carcich2,23, A. L. Chaikin24,C. L. Chavez5,14, A. F. Cheng2, E. J. Colwell2, S. J. Conard2, M. P. Conner2, C. A. Conrad1,J. C. Cook25, S. B. Cooper2, O. S. Custodio2, C. M. Dalle Ore5,14, C. C. Deboy2,P. Dharmavaram2, R. D. Dhingra26, G. F. Dunn3, A. M. Earle18, A. F. Egan1, J. Eisig2,M. R. El-Maarry27, C. Engelbrecht2, B. L. Enke1, C. J. Ercol2, E. D. Fattig3, C. L. Ferrell1,T. J. Finley1, J. Firer2, J. Fischetti13, W. M. Folkner16, M. N. Fosbury2, G. H. Fountain2,J. M. Freeze2, L. Gabasova28, L. S. Glaze29, J. L. Green29, G. A. Griffith2, Y. Guo2, M. Hahn20,D. W. Hals2, D. P. Hamilton30, S. A. Hamilton2, J. J. Hanley3, A. Harch23, K. A. Harmon16,H. M. Hart2, J. Hayes2, C. B. Hersman2, M. E. Hill2, T. A. Hill2, J. D. Hofgartner16,M. E. Holdridge2, M. Horányi11, A. Hosadurga2, A. D. Howard31, C. J. A. Howett1,S. E. Jaskulek2, D. E. Jennings12, J. R. Jensen2, M. R. Jones2, H. K. Kang2, D. J. Katz2,D. E. Kaufmann1, J. J. Kavelaars32, J. T. Keane33, G. P. Keleher2, M. Kinczyk34, M. C. Kochte2,P. Kollmann2, S. M. Krimigis2, G. L. Kruizinga16, D. Y. Kusnierkiewicz2, M. S. Lahr2,T. R. Lauer35, G. B. Lawrence2, J. E. Lee36, E. J. Lessac-Chenen13, I. R. Linscott37,C. M. Lisse2, A. W. Lunsford12, D. M. Mages16, V. A. Mallder2, N. P. Martin38, B. H. May39,D. J. McComas3,40, R. L. McNutt Jr.2, D. S. Mehoke2, T. S. Mehoke2, D. S. Nelson13,H. D. Nguyen2, J. I. Núñez2, A. C. Ocampo29, W. M. Owen16, G. K. Oxton2, A. H. Parker1,M. Pätzold20, J. Y. Pelgrift13, F. J. Pelletier13, J. P. Pineau41, M. R. Piquette11, S. B. Porter1,S. Protopapa1, E. Quirico28, J. A. Redfern1, A. L. Regiec2, H. J. Reitsema42, D. C. Reuter12,D. C. Richardson30, J. E. Riedel16, M. A. Ritterbush16, S. J. Robbins1, D. J. Rodgers2,G. D. Rogers2, D. M. Rose1, P. E. Rosendall2, K. D. Runyon2, M. G. Ryschkewitsch2,M. M. Saina2, M. J. Salinas13, P. M. Schenk43, J. R. Scherrer3, W. R. Schlei2, B. Schmitt28,D. J. Schultz2, D. C. Schurr29, F. Scipioni5,14, R. L. Sepan2, R. G. Shelton2, M. R. Showalter14,M. Simon2, K. N. Singer1, E.W. Stahlheber2, D. R. Stanbridge13, J. A. Stansberry44, A. J. Steffl1,D. F. Strobel45, M. M. Stothoff 3, T. Stryk46, J. R. Stuart16, M. E. Summers47, M. B. Tapley3,A. Taylor13, H.W. Taylor2, R. M. Tedford1, H. B. Throop15, L. S. Turner2, O. M. Umurhan5,14,J. Van Eck2, D. Velez16, M. H. Versteeg3, M. A. Vincent1, R. W. Webbert2, S. E. Weidner40,G. E. Weigle II48, J. R. Wendel29, O. L. White5,14, K. E. Whittenburg2, B. G. Williams13,K. E. Williams13, S. P. Williams2, H. L. Winters2, A. M. Zangari1, T. H. Zurbuchen29

The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the NewHorizons spacecraft flew close to (486958) 2014 MU69, a cold classical Kuiper Belt objectapproximately 30 kilometers in diameter. Such objects have never been substantially heatedby the Sun and are therefore well preserved since their formation.We describe initial resultsfrom these encounter observations. MU69 is a bilobed contact binary with a flattened shape,discrete geological units, and noticeable albedo heterogeneity. However, there is little surfacecolor or compositional heterogeneity. No evidence for satellites, rings or other dust structures,a gas coma, or solar wind interactions was detected. MU69’s origin appears consistent withpebble cloud collapse followed by a low-velocity merger of its two lobes.

The Kuiper Belt, a torus-shaped ensemble ofobjects in the outer Solar System beyondthe orbit of Neptune, was discovered in1992. This is the source region for Jupiter-family comets and contains primordial plan-

etesimals and dwarf planets [e.g., (1)]. The 2003Planetary Decadal Survey ranked exploration ofthe Kuiper Belt at the top of funding prioritiesfor NASA’s planetary program (2). The resultantNASA mission, New Horizons [e.g., (3)], flew

through and explored the Pluto dwarf planetsystem in 2015 (4, 5). The spacecraft has sincecontinued farther to explore Kuiper Belt objects(KBOs) and the Kuiper Belt radiation and dustenvironment (6).The target selected for the subsequent New

Horizons KBO flyby was (486958) 2014 MU69

(hereafter MU69, also informally referred to asUltima Thule). This KBOwas discovered in 2014when the Hubble Space Telescope (HST) wasbeing used to conduct a dedicated search forNew Horizons KBO flyby targets (7, 8). Beforethe arrival of New Horizons, the only definitivefacts regarding MU69 were its orbit (8), its redcolor (9), its size of ~30 km (7), and its lack of

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1Southwest Research Institute, Boulder, CO 80302, USA.2Johns Hopkins University Applied Physics Laboratory,Laurel, MD 20723, USA. 3Southwest Research Institute, SanAntonio, TX 78238, USA. 4Lowell Observatory, Flagstaff, AZ86001, USA. 5NASA Ames Research Center, Space ScienceDivision, Moffett Field, CA 94035, USA. 6Department ofPhysics and Astronomy, University of Texas, San Antonio, TX78249, USA. 7Department of Earth and Planetary Sciencesand McDonnell Center for the Space Sciences, WashingtonUniversity, St. Louis, MO 63130, USA. 8Department ofAstronomy, University of Virginia, Charlottesville, VA 22904,USA. 9Independent consultant, Carbondale, CO 81623, USA.10Universität der Bundeswehr München, Neubiberg 85577,Germany. 11Laboratory for Atmospheric and Space Physics,University of Colorado, Boulder, CO 80303, USA. 12NASAGoddard Space Flight Center, Greenbelt, MD 20771, USA.13KinetX Aerospace, Tempe, AZ 85284, USA. 14SETI Institute,Mountain View, CA 94043, USA. 15Planetary ScienceInstitute, Tucson, AZ 85719, USA. 16Jet PropulsionLaboratory, California Institute of Technology, Pasadena, CA91109, USA. 17Earth and Planetary Science Department,University of California, Santa Cruz, CA 95064, USA.18Massachusetts Institute of Technology, Cambridge, MA02139, USA. 19Argelander-Institut für Astronomie, Universityof Bonn, Bonn D-53121, Germany. 20Rheinisches Institut fürUmweltforschung, Universität zu Köln, Cologne 50931,Germany. 21Lunar and Planetary Laboratory, University ofArizona, Tucson, AZ 85721, USA. 22Department of Physics,University of Central Florida, Orlando, FL 32816, USA.23Cornell University, Ithaca, NY 14853, USA. 24Independentscience writer, Arlington, VT 05250, USA. 25Pinhead Institute,Telluride, CO 81435, USA. 26University of Idaho, Moscow, ID83844, USA. 27Department of Earth and Planetary Sciences,Birkbeck, University of London, London WC1E 7HX, UK.28University Grenoble Alpes, Centre National de la RechercheScientifique, Institut de Planétologie et d’Astrophysique deGrenoble, 38000 Grenoble, France. 29NASA Headquarters,Washington, DC 20546, USA. 30Department of Astronomy,University of Maryland, College Park, MD 20742, USA.31Department of Environmental Sciences, University ofVirginia, Charlottesville, VA 22904, USA. 32National ResearchCouncil of Canada, Victoria, BC V9E 2E7, Canada. 33Divisionof Geological and Planetary Sciences, California Institute ofTechnology, Pasadena, CA 91125, USA. 34Marine, Earth, andAtmospheric Sciences, North Carolina State University,Raleigh, NC 27695, USA. 35National Optical AstronomyObservatory, Tucson, AZ 26732, USA. 36NASA MarshallSpace Flight Center, Huntsville, AL 35812, USA.37Independent consultant, Mountain View, CA 94043, USA.38Independent consultant, Crested Butte, CO 81224, USA.39Independent collaborator, Windlesham GU20 6YW, UK.40Department of Astrophysical Sciences, PrincetonUniversity, Princeton, NJ 08544, USA. 41Stellar Solutions,Palo Alto, CA 94306, USA. 42Independent consultant,Holland, MI 49424, USA. 43Lunar and Planetary Institute,Houston, TX 77058, USA. 44Space Telescope ScienceInstitute, Baltimore, MD 21218, USA. 45Johns HopkinsUniversity, Baltimore, MD 21218, USA. 46Roane StateCommunity College, Oak Ridge, TN 37830, USA. 47GeorgeMason University, Fairfax, VA 22030, USA. 48Independentconsultant, Burden, KS 67019, USA.*Corresponding author. Email: astern@boulder.swri.edu

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detectable variations in its light curve (10) orlarge, distant satellites.MU69’s orbit has a semimajor axis a = 44.6

astronomical units (AU), with eccentricity e =0.042 and inclination i = 2.45°, making it amember of the cold classical KBO (CCKBO) pop-ulation (here, cold refers to low dynamical ex-citation, not surface temperature). CCKBOs arethought to be (i) distant relics formed from theSolar System’s original protoplanetary disk and(ii) more or less dynamically undisturbed bodiesthat therefore formed in situ ~4.5 billion yearsago and have since remained at or close to theircurrent, large heliocentric distances (11, 12). Rel-ative to other Kuiper Belt populations, CCKBOshave a more uniformly red color distribution [e.g.,(13)], as well as a different size-frequency distri-bution (i.e., the population of objects as a functionof object size) (14) and higher average visible al-bedos than are typical in the Kuiper Belt [e.g., (15)].Additionally, many CCKBOs have satellites (16).Because CCKBOs are dynamically undisturbed

from their formation location [or nearly so (12)],they have never been warmed above the am-bient, radiative equilibrium temperatures of 30to 60 K in the Kuiper Belt. MU69’s small equi-valent spherical diameter of ~19 km is insuffi-cient to drive internal evolution long after itsformation. Therefore, small CCKBOs likeMU69

are expected to be primordial planetesimals,preserving information on the physical, chem-ical, and accretional conditions in the outer solarnebula and the processes of planetesimal forma-tion [e.g., (1, 12)].NewHorizons flew closest toMU69 at 05:33:22.4

(±0.2 s, 1s) universal time (UT) on 1 January 2019.The closest approach distance of 3538.5 ± 0.2(1s) km was targeted to the celestial north ofMU69’s center; its relative speed past MU69 was14.43 km s–1. The asymptotic approach directionof the trajectory was approximately in the eclipticplane at an angle of 11.6° from the direction to theSun. The flyby’s observation planning details havebeen summarized elsewhere (6). This report ofinitial flyby results is based on the ~10% of all

collected flyby data that had been sent to Earthbefore 1 March 2019; full data transmission isexpected to complete in mid-2020.NewHorizons carries a suite of seven scientific

instruments (3); all were used in the flyby ofMU69. These instruments are (i) Ralph, whichconsists of the Multispectral Visible ImagingCamera (MVIC), a multicolor/panchromatic map-per, and the Linear Etalon Imaging Spectral Array(LEISA), an infrared (IR) composition mappingspectrometer; (ii) the Long Range ReconnaissanceImager (LORRI), a long–focal length panchromaticvisible camera; (iii) theAlice extreme/far ultravioletmapping spectrograph; (iv) a Radio Experiment(REX) to measure surface brightness temper-atures andX-band radar reflectivity; (v) the SolarWind Around Pluto (SWAP) charged-particlesolar wind spectrometer; (vi) the Pluto EnergeticParticle Spectrometer Science Investigation(PEPSSI)MeV charged-particle spectrometer; and(vii) the Venetia Burney Student Dust Counter(VBSDC), a dust impact detector.

General properties

MU69 (Fig. 1) has a bilobate shape with two dis-crete, unequally sized lobes. This shape demon-strates that it is a contact binary—that is, a pairof formerly separate objects now in physical con-tact with one another. These two lobes contact atan annular interface with higher than averagesurface reflectance, which we refer to as MU69’sneck. The larger and smaller lobes of MU69 havebeen informally designated “Ultima” and “Thule,”respectively; formal names will be assigned at alater date.The bilobate nature of MU69 is reminiscent of

known bilobate comets that have been imagedby spacecraft (17–22). However, MU69’s residencein the cold classical Kuiper Belt makes clear thatits bilobate shape must be primordial, makingMU69 the only unquestionably primordial con-tact binary so far explored by spacecraft.MU69’s two lobes are discrete, have retained

their basic shapes, and do not (at the availableresolution) display prominent compression frac-

tures, deformation, or other geological featuresindicative of an energetic or violent merger. Allavailable evidence indicates thatMU69 is insteadthe product of a gentle collision or merger of twoindependently formed bodies, possibly contact-ing one another at (or more slowly than) theirmutual gravitational infall speed, which we es-timate to be several meters per second (based onplausible densities; see below).The observed bilobate shape is inconsistent

with a recent formation from the collision of twoseparate, heliocentric CCKBOs because the char-acteristic relative impact speeds of CCKBOs arecurrently ~300 m s–1 (23). Such a collision wouldhave heavily deformed or destroyed the two com-ponents of the contact binary. We conclude thatthe lobes instead most likely formed and mergedin a gentle dynamical environment, such as ina localized particle cloud collapse early in SolarSystem history [e.g., (24, 25)], further indicatingthat MU69 itself is primordial. The similarity inalbedos and colors of the two lobes (discussedbelow) is further evidence for this formationhypothesis.Images taken during the approach phase show

that the rotational period of MU69 is 15.92 ±0.02 hours; this period is consistent with otherCCKBO rotational periods (9, 26, 27). MU69’srotational pole was found to point at approx-imately right ascension = 311°, declination = –25°,corresponding to an obliquity of 98° with respecttoMU69’s heliocentric orbital plane; that is, MU69

currently rotates almost face-on to the Sun. Thispole position, and the fact that the spacecraftapproach vector was only ~36° from the directionof the KBO’s spin axis, resulted in a light curve ofvery low amplitude, which was not detecteduntil shortly before the flyby despitemanyweeksof prior observations from New Horizons. Be-cause asymmetric reradiation (i.e., YORP, orYarkovsky-O’Keefe-Radzievskii-Paddack) effectsare ineffective on bodies the size ofMU69 at largedistances from the Sun (28, 29), and because (asdescribed below) the visible evidence for impactcratering onMU69 ismodest, the spin period andobliquity of MU69 are unlikely to have substan-tially changed since the binary merger.We simultaneously fitted the shape of MU69

and its pole position using a forward-modelingprocess. For each of the early-return LORRI andMVIC images in which MU69 was resolved, thisprocess rendered the shape with the relevantillumination and orientation using the photo-metric parameters described below, convolvedthat rendered image with the appropriate point-spread function (for LORRI or MVIC, as appro-priate) to simulate the angular resolution andsmearing of the real images, and then numer-ically compared the result to all of the resolvedimages of MU69 that had been returned. Theshape model was parametrically defined, sep-arately for each lobe, using the “octantoid” for-malism (30) (see methods). Our best-fitting shapemodel (Fig. 2) has overall dimensions of ap-proximately 35 × 20 × 10 km, with estimateduncertainties of less than 1 × 1 × 3 km. Thethickness (i.e., the third dimension) is the least

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Maryland

Thule

5 km Neck

Ultima

Illumination direction (32.5 ̊phase)

A B

Fig. 1. Close approach 06 (CA06) MVIC observation of MU69. (A) MVIC image made at a solarphase angle of 32.5°, a range of 6640 km, and a native pixel scale of 130 m per pixel, resampled to a2× finer pixel scale and then deconvolved using a similarly resampled MVIC point-spread function.(B) Red-cyan stereographic anaglyph of the MU69 approach hemisphere.

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well constrained because the spacecraft ap-proach was at a high negative (i.e., southern)latitude on the object, so little of the positive (i.e.,northern) half of MU69 was directly observed.From this shape model, the Ultima lobe was

found to be lenticular with dimensions of ap-proximately 22 × 20 × 7 km (uncertainty < 0.6 ×1 × 2 km), whereas the Thule lobe is more equi-dimensional, approximately 14 × 14 × 10 km(uncertainty < 0.4 × 0.7 × 3 km). The modelvolumes of Ultima and Thule are 1400 km3

(± <600 km3) and 1050 km3 (± <400 km3), re-spectively. The centers of the lobes are 16 kmapart. The principal axes of the lobes are roughlyparallel to one another (likely <7° misalignment),which strongly suggests that the lobes tidallylocked prior to contact (see below).The apparently lenticular shape of the Ultima

lobe is unlike any other known heliocentrically

orbiting Solar System body, but is reminiscent ofsome small ring satellites of Saturn, such as Atlasand Pan, which have accreted equatorial ridgesof fine-grained material (31). The origin of thisflattened shape of Ultima is indeterminate; pos-sible explanations include accretion from a thin(i.e., flattened) particle sheet during pebble cloudcollapse (24), the head-on collision of two verysimilarly sized bodies within a narrow rangeof speeds (32), or deformation arising fromrapid rotation or tidal forces prior to its mergerwith Thule. These possibilities are discussedbelow.Because no satellites of MU69 were detected

(see below), MU69’s density is poorly constrained.Cometary nuclei are the Solar System bodieslikely to be most similar to MU69; they are gen-erally found to have low density (<103 kg m–3)and high porosity (>50%) (21). The most pre-

cisely determined comet nucleus density is thatof 67P/Churyumov-Gerasimenko, at 533±6kgm–3

(33), although it is unknown how much thatcomet’s density has evolved from its primordialvalue. Adopting the shape model in Fig. 2, alower bound on MU69’s density of ~280 kg m–3

can be derived by assuming thatMU69’s two lobesare only marginally bound by self-gravity andthere is no tensile strength in the neck. Alterna-tively, assuming a characteristic cometary densityof 500 kg m–3 (21) gives a breakup rotation ratefor MU69 of ~12 hours. At this density, if the twolobes merged from a mutual circular orbit, sub-stantial loss of angular momentum would havebeen required to reach their current, 15.92-hourperiod. Because mutual gravity would exceedcentrifugal acceleration, densities greater than~280 kg m–3 also imply that the neck region iscurrently under compression.

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A B

C

+y

+x

1 km

D

+y

+z

+x

+z

E

spin axis

Fig. 2. Shape model for MU69. (A) The same MVIC image as Fig. 1A for comparison. (B) Shape model of MU69 based on approach rotationalcoverage and close flyby observations, seen at the same viewing and illumination conditions as (A). (C to E) Shape model seen along MU69’s z (C),x (D), and y (E) axes, with the object’s spin axis indicated in red. The arrow of the spin axis is the positive (i.e., north) pole. The shape of the +zhemisphere, which was mostly in darkness during the encounter, is the least well constrained part of the shape model.

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We calculated the gravitational and rotationalpotential across the surface of MU69 assumingthe shape model in Fig. 2, a uniform density of500 kg m–3, and its 15.92-hour rotation period.The surface acceleration ranges between 0.5 and1 mm s–2 (and is nowhere negative). The localacceleration slopes (i.e., the gradients of the gra-vitational and rotational potential across thesurface) are low except in the neck, where theycan exceed 35°. The equators of Ultima andThule are gravitational and rotational potentialhighs.

Surface reflectivity, color,and composition

Figure 3A shows a visible-wavelength I/F (re-flected intensity divided by incident flux) con-tour map from the close approach 04 (CA04)LORRI observation (13° solar phase angle) andits corresponding I/F histogram. At this solarphase angle, the modal I/F of MU69 is 0.078.The I/F distributions of MU69’s two lobes havethe same mode within the measurement errorof ±0.05 I/F. I/F variations ranging from ~0.02to ~0.12 are seen on both lobes (including theirterminator regions). However, away from theterminator (i.e., the surface boundary betweendaylight and nighttime where lighting effectsbias results), the minimum I/F is ~0.05. Thisoccurs on a portion of the wall of the large de-pression on Thule, which we informally refer toas Maryland (surface features on Ultima Thuleare informally named afterU.S. stateswithmajorcontributions to New Horizons); see Fig. 1. Themaximum I/F of ~0.12 occurs at both of thebright spots inMaryland and on the neck region.The I/F distribution of the Ultima lobe has asharper peak than that of Thule (Fig. 3B).The brighter material on Ultima and Thule is

mostly segregated into three kinds of surfacemanifestations: (i) nearly circular spots that be-come more numerous with decreasing size (rang-ing from a few kilometers across to the resolutionlimit of several tens of meters); (ii) curvilinear and

quasi-linear features that are narrow relative totheir length; and (iii) broad patches, which aremore prominent on the Thule lobe. Darker thanaverage surface units on the binary also come intypes (i) and (iii), but not (ii).It is unclear how these three types of com-

paratively bright terrain originated and how theymay differ. However, initial stereographic analy-sis of New Horizons images using establishedtechniques (34, 35) shows that many of the ob-served bright areas on MU69 are located eitherwithin topographic depressions (e.g., the neck,the bright spots in Maryland, and the floors ofpits and troughs) or at the bases or inflectionpoints of slopes. Therefore, a possible explana-tion is that bright, fine-grainedmaterial has beentransported downslope to these locations, inwhich case the higher brightness could be dueto predominantly smaller particle size (36). How-ever, other interpretations including composi-tional or thermal effects, space weathering, andcold trapping are also possible.The neck feature at the Ultima-Thule merger

zone might not have the same origin as otherbright locales. Possibilities include (i) surfaceprocesses such as fine particle accumulation, asdiscussed above; (ii) processes related to the lobemerger, such as the extrusion of preexistingbright surface material on one lobe during theimpact; (iii) post-impact thermal extrusion ofices due to changes in thermal properties or con-ditions at the merger zone after contact; and (iv)evolutionary processes such as preferential space-weathering effects or thermal effects created bythe geometry surrounding the neck.Analyses of the approach and departure images

have allowed us to characterize the solar phasecurve of MU69 at phase angles up to 153°, con-siderably higher than possible for Earth-basedobservations of this object, which are limited to~2° (Fig. 4). The derived phase coefficient, or slope,is b = 0.038 ± 0.014 magnitudes per degree, be-tween 1.3° and 32.5°. This slope is consistent withthat measured for other low-albedo small Solar

System bodies, including comet nuclei [e.g., (37)].Applying a Hapke photometric model (36) to theentire phase curve yields the nominal photometricproperties of MU69’s surface: single scatteringalbedo wo = 0.24, mean topographic slope angleq = 33°, and single-particle phase function param-eters using a McGuire-Hapke formalism (37) ofb = 0.32 and c = 0.75. These results give MU69’svisible (0.55-mm V-band) geometric albedo (i.e., thealbedo at 0° solar phase) of pV = 0.165 ± 0.01. Thisis a typical value forCCKBOs, the geometric albedosof which range from 0.09 to 0.23, with a mean of0.15 (38). MU69’s phase integral is q = 0.37 ± 0.16,yielding a spherical (Bond) albedo of 0.061 ± 0.026.MVIC color images reveal a globally averaged

visible-wavelength reddish reflectance slope of31.1 ± 0.5% per 100 nm, computed usingMVIC’sblue, red, and near-IR filters (Fig. 5, A to C),where the quoted uncertainty here is statisticalonly. This color is consistent with that of otherCCKBOs (39–41). Only subtle color (and spectral;see below) differences (and spectral differences;see below) are detected between the two lobesof MU69, despite their distinct shapes and ap-pearances. The two lobes are clearly resolved fromone another at the resolution of the color data.Remote observations of KBOswith satellites shownear-equal colors of the orbiting bodies, inter-preted as resulting from co-accretion from a locallyhomogeneous portion of the nebula (42).The clearest regional color and spectral sig-

natures across MU69’s surface appear (i) at theneck between the two lobes, and (ii) at the Thulelobe bright spots in Maryland, which displayspectral slopes of 28.2 ± 0.2% per 100 nm and30.8 ± 0.2% per 100 nm, respectively. At leasttwo locations on the Ultima lobe also show lessred than its average color. Principal componentsanalysis shows that 97% of the variance in MVICcolor data is attributable to shading and albedo,whereas image noise and true color contrastsaccount for just 3%. The subtle color differencesseen could be indicative of compositional dif-ferences, although differences in particle size,porosity, etc., can also produce differences inspectral slopes.LEISA spectral observations (Fig. 5D; see

methods) show that MU69 is brighter in thenear-IR than it is in the visible spectrum, dem-onstrating that the red slope observed usingMVIC extends into the IR. From 1.2 to 2.5 mm,the observed I/F after radiometric calibrationranges from 0.15 to 0.2. The colorant responsiblefor the redness ofMU69 and other CCKBOs couldbe tholin-like complex organic macromolecules,produced from simpler species through radio-lytic and photolytic breaking of bonds leading torecombination into progressively heavier mol-ecules [e.g., (43, 44)]. Space weathering of silicatescan also produce a red coloration, but no directevidence of silicates is seen on MU69. Principalcomponents analysis shows that more than 90%of the variance in the LEISA data is also due toshading and albedo, with relatively little varianceattributable to regional spectral variability.As shown in Fig. 5D, there are key color slope

similarities between the spectrum of MU69 and

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those of the KBO (55638) 2002 VE95 (45) and theescaped KBO 5145 Pholus [e.g., (46)]. Also, theseobjects all exhibit an absorption band near 2.3 mm,tentatively attributed to methanol (CH3OH) orperhaps more complex organic molecules inter-mediate in mass between simple molecular icesand tholins (47). Similar spectral features are alsoapparent on the large, dark red equatorial regionof Pluto informally called Cthulhu [e.g., (48)], sug-gesting similarities in the chemical feedstock andprocesses that could operate there and on MU69.Broad spectral absorption features on MU69

near 1.5 and 2.0 mm indicate the presence of H2Oice. However, the shallowness of these featuressuggests that water ice may have a relatively lowabundance inMU69’s uppermost surface, at leastcompared with water ice–rich planetary satellites,and even on Pholus and in Cthulhu where H2Oice ismore clearly detected. In this regard, opaquespecies such as complex organics are known fortheir ability tomask the spectral signature of H2Oice in the near-IR (49). No unambiguous spectralsignatures of silicates, or of volatile ices like thosethat were observed on Pluto (CO, N2, NH3, orCH4) (4), have been detected forMU69, butmostof these supervolatile species are not expectedat MU69 owing to the ready thermally drivenescape of such ices from this object over time. Anapparent absorption at 1.8 mm has not yet beenassigned a molecular identification.

Thermal considerations

Temperatures on MU69 are set by the balancebetween the absorption of sunlight and thermal

emission back to space. For the 6% Bond albedoestimated above, and an assumed 90% emissivity,absorption and emission atMU69’smeandistancefrom the Sun are balanced at 42 K. For plausiblevalues of thermal conductivity in the 10–5 J m–1

s–1 K–1 range, we estimate the characteristic skindepths to which diurnal and seasonal wavespropagate to be ~0.001 m and ~1 m, respectively(see methods). Therefore, diurnal and seasonaltemperature variations likely only affect the outerfewmillimeters tometers ofMU69’s surface, so theaverage temperature pertains to the vast majorityofMU69’s interior. At this 42K temperature, frozenvolatile species such as CO,N2, andCH4not trappedin clathrates would sublimate and escape relativelyrapidly compared to the age of the Solar System,but amorphous H2O ice would not crystallize andso could survive over the age of the Solar System.Near the surface of MU69, temperature varies

on seasonal and diurnal time scales. As a result ofMU69’s low orbital eccentricity, insolation onlydiffers by 17% over the course of its orbit. How-ever, summer/winter hemisphere insolation variesconsiderably over MU69’s 293-year orbit becauseof the 98° obliquity of its pole, resulting in longpolar days and nights during which regions re-ceive continuous sunlight, or none at all, formanydecades. Around equinox, the 15.92-hour rotationperiod would also produce strong diurnal varia-tions in insolation. Changing insolation gener-ates thermal waves as heat is conducted intoand out of the subsurface.We expect summer surface temperatures to

approach a maximum instantaneous equilib-

rium temperature of ~60 K, whereas wintertemperatures (as on the unilluminated face ofMU69 during the New Horizons flyby) are muchlower, down to the seasonal skin depth. Analysisof the REX radiometry data at the 4.2-cmX-bandradio wavelength indicates a brightness temper-ature in the 20 to 35 K range on thewinter (night)side of MU69. Thermal emission at centimeterwavelengths emerges from a range of depths,potentially up to many tens of wavelengths be-low the surface, so the warmer subsurface is animportant source of flux. For typical icy satelliteemissivity at centimeter wavelengths (50, 51), theobserved REX brightness temperature range ap-pears to be consistent with the expected low ther-mal inertia values estimated for other KBOs (52).

Geophysical and geological properties

We measured the limb profile of MU69 usingpreviously published techniques (53). These limbprofiles (Fig. 6, A and B) were derived from theCA04 LORRI (characteristic 140m per pixel) andCA06 MVIC (characteristic 130 m per pixel) ob-servations and were compared with best-fittingellipses for the projected shape of each lobe. Theelliptical axes remove the long-wavelength signaland are not necessarily representative of the 3Dbody shape (i.e., they are not measured withrespect to the shape model in Fig. 2).On the larger lobe Ultima, the limb topography

shows total (i.e., minimum to maximum devia-tion) relief of ~1 km, whereas the relief on thesmaller lobe Thule is more muted at ~0.5 km.Assuming a density r = 500 kg m–3, a surfacegravity g = 0.001 m s–2, and topography of ver-tical scale h = 0.5 to 1 km implies stresses on theorder of rgh/3, or ~100 Pa. Such modest stressescan be supported by friction. Excessively steepregions (>35°) near MU69’s neck likely requireadditional internal strength to remain stable.Cohesion of several hundreds of pascals wouldbe capable of supporting these slopes. Suchstrengths are thought to be normal for comet-like bodies (22) and so are plausible for KBOs ofMU69’s size as well.A geomorphological map of MU69 is shown

in Fig. 6C. The two lobes have somewhat dif-ferent surface geology. Thule’s surface is domi-nated by Maryland, a depression of probableimpact origin (unit labeled lc; see Fig. 6C forkey to these abbreviations), ~7 km in diameter(see above). Stereographic analysis shows thatthe depth of Maryland is <2 km; this depth isconsistent with the observed limb topographyvariations. Maryland’s interior shows no unam-biguous signs of horizontal layering but doescontain two prominent bright spots of similarsize and albedo. Two distinct, kilometer-scale,possible impact craters occur on Maryland’s rimcrest. Separately, four distinct troughs appearnear the terminator of Thule in unit um.Apart from Maryland, the rest of Thule’s ob-

served surface is characterized by broad (fewkilometers wide), dark swaths (unit dm) that se-parate lighter-toned,mottled units (units pm, um,and rm). In some places, these dark swaths con-tain bright spots and a bright-floored, quasi-linear

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Fig. 4. The visible (0.55 mm) solar phase curve of MU69 obtained by combining HSTand NewHorizons data.The solid line is the Hapke photometric model (82) fitted to the total integrated I/Fof MU69. Data are from New Horizons close approach images acquired at solar phase angles of12°, 13°, 32.5°, and 153° and from observations by the HSTat low phase angles (<1.3°). Error bars on the153° observation primarily arise from uncertainty in the shape of MU69’s night side. Magnitudes arenormalized to the geometric albedo (pV = 1) at opposition (a = 0°) in the V band (0.55 mm); nocorrections have been made to account for rotational variation in reflectance (i.e., the light curve),but MU69’s light curve amplitude upper limit is low [≤0.15 magnitudes (10)], so this neglected effect issmall. For comparison, the dashed line is the solar phase curve of 103P/Hartley 2 (83), a bilobateJupiter family comet visited by the Deep Impact spacecraft (84). Although Hartley 2 is darker (pV =0.045 ± 0.009) than MU69 at 0.55 mm, the phase curves of these two bodies have similar shapes.

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trough. The crenulated boundaries of unit dmmay be a decrescence morphology, whereby thisunit is partly bounded by scarps that have re-treated. Unit dm may be a deposit of volatile icewith bounding scarps forming as a result of sub-

limation at its periphery, with the upper surfaceof the deposit being protected from sublimationby a dark, refractorymantle, perhaps derived fromthe deposit itself (54, 55). Portions of unit um thatare proximal to Maryland may be ejecta from the

crater, or related to ejecta, but this cannot beconfirmed with the current analysis. A distinct,relatively bright region (unit rm) at the equato-rial, distal end of Thule exhibits roughness at thescale of a few hundred meters; some of the fea-tures there appear to be pits, craters, or mounds.A lightly cratered surface of MU69 was pre-

dicted by a recent cratering model (23); indeed,few definitively impact-related scars are identifiedonMU69. We have considered several hypothesesfor the origin of the many pits seen near the ter-minator in Fig. 2A. These include structurallycontrolled collapse pits, outgassing pits, sublima-tion pits, and impact craters (56); they likely arenot all created by the same process. Our assess-ment is that the chains of similarly sized pits aremore likely to be formed by internal processesthan by cratering, but the isolated pits that showapproximately circular planform outlines, bowl-shaped interior depressions, and, in some cases,raised rims aremore consistent with impact cratermorphology. There are no obvious crater candi-dates that are intermediate in size between these~1-km-diameter pits (unit sp) andMaryland (unitlc, ~7 km).On Ultima, eight similarly sized (~5 km scale)

units of rolling topography (based on subtlealbedo gradations and limb profiles) dominateits observed landscape (units ma to mh). Thealbedo and texture of these units are generallysimilar to one another, although each containsbrighter material to differing degrees. These unitsabut each other, typically bordered with distinct,curvilinear, andgenerallyhigher-reflectancebound-ary regions (particularly unitmh, which is ringedbya brighter annulus). In one instance (near the ter-minator), a trough and a short pit chainmark sucha boundary, but the low solar incidence anglesprevailing acrossmuch ofUltimamake it unclearwhether the boundaries are always associatedwith topographic features. Stereographic anal-ysis indicates thatmost of these units have broadlypositive relief, although the central unit mh isrelatively flatter. It is also unclear whether theseboundaries necessarily imply superposition rela-tionships among the components. One unit,at the limb of Ultima (unitmd), appears to bemore angular in stereography and is either moreelevated or tilted with respect to the othercomponents.The apparent similarity in the size of Ultima’s

units (ma through mh) is likely a clue to theirorigin. Whether they are a relic of Ultima’s for-mation or a result of a later evolution is unclear.One formation-related hypothesis is that the in-dividual units onUltima are accretional subunitsof smaller planetesimals that formed Ultima.This apparent assemblage of units on Ultima isconsistent with observation of and proposalsfor the formation of layers on comets such as9P/Tempel and 67P/Churyumov-Gerasimenko(20, 57). However, challenges to this hypothesisare the apparently (i.e., at the available resolu-tion) nearly unimodal size of these units andtheir apparent absence from Thule, although thelatter could be the result of resurfacing causedby the Maryland cratering event. Also, at the

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Fig. 5. MU69’s color and near-IR spectral reflectance. (A) MVIC enhanced color image at a scale of1.5 km per pixel. (B) CA04 LORRI image at 140 m per pixel. (C) (A) overlaid on (B). (D) MVIC colormeasurements (colored points) and a LEISA near-IR spectrum of MU69 (black points). Data at wavelengthsshorter than 1 mmare from theMVIC visible/near-IR color imager at a phase angle of 11.7°; data atwavelengthslonger than 1.2 mm are from the LEISA IR spectrograph at a phase angle of 12.6° and a mean spatial scaleof 1.9 km per pixel.The MVIC data are split into multiple terrain units (Ultima and Thule lobes, the brightneck region, and a combination of all other bright spots identified in LORRI data); the LEISA spectrumis a global average. All LEISA data points illustrate an estimated 1s uncertainty; MVIC data points illustrate anestimated 1s uncertainty relative to the red channel flux.The data are compared to Hapke model spectrashown as the brown dot-dashed line of 2002 VE95 (45) and the magenta dashed line of 5145 Pholus (46).Those curves are scaled by0.45 and0.84, respectively, tomatch the averagenear IR I/FofMU69.The apparentwavelength shifts of some features in the MU69 spectrum relative to the dashed models are likely due tounmodeled temperature, particle size, and temperature effects.Tentative identifications of absorption bands ofwater and methanol ices are marked, along with an unknown feature at 1.8 mm (see text).

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expected impact speeds during accretion, suchas in a local particle cloud collapse resulting ina body like MU69 (perhaps no more than a fewmeters per second, based on the mutual escapespeed of the lobes), these accretional subunitswould likely not be expected to merge into ascompact a body as Ultima unless they were ex-

tremely weak (i.e., cohesionless and friction-less) at the time of their accretion (22).

Satellites and orbiting rings/dust search

Both satellites and rings have been detectedaround KBOs larger than MU69 and aroundCentaurs (16, 58, 59), which have escaped the

Kuiper Belt and now orbit among the giantplanets. Although no KBO as small as MU69 isknown to have rings, satellites around KBOsare common, particularly in the CCKBO popu-lation (16).We searched for satellites and rings of MU69

using co-added stacks of LORRI images acquiredduring approach to MU69 at exposure times of10 to 30 s. High-resolution images taken nearclosest approach provide additional constraintson satellites close to MU69. No satellites weredetected in our data. Figure 7A shows the quan-titative limits on satellites obtained from thesedata as a function of distance from MU69.Additionally, small particles orbiting MU69

could form ring or other dust structures withunusual geometries (60) because MU69’s veryweak gravity is of similar magnitude to the solarradiation pressure. However, larger grains, whichare less affected by solar radiation pressure, couldform a conventional equatorial ring (60).Approach images constrain any ring or dust

assemblages located≳250 km fromMU69 to haveI/F ≲ 2 × 10–7 (for a 10-km-wide ring) at a phaseangle of 11° (Fig. 7B). This is equal to or less thanthe I/F of many faint outer rings of the giantplanets, which have ring widths of >20 km(61, 62). A single downlinked high-phase (165°),forward-scattered observation also shows no evi-dence for rings or dust farther than ~400 kmfrom MU69 at I/F > 10–4. The VBSDC dustcounter experiment reported zero dust impactdetections during the passage through the grav-itational stability sphere of MU69, consistent witha lack of extant rings or other dust assemblages.

Exosphere and heliosphericinteraction searches

Because of MU69’s small size, it is likely thathighly volatile ices that might once have beenpresent at its surface would have escaped tospace long ago (6, 56). However, less volatile ices(e.g., methanol, acetylene, ethane, and hydrogencyanide) could be retained over geological timescales, and irradiation of these species couldresult in reddening over time as longer-chaintholins are produced (63, 64). This implies a slowloss of hydrogen atoms to space as surface icesare converted into tholins. In addition to this es-caping H flux, occasional large impacts couldprovide a source for a transient atmosphere.We searched for evidence of both a coma of

escaping gas and charged particle emissions fromMU69. The searches included use of the Aliceultraviolet spectrograph to search for resonanceline emission from a coma, as well as in situsearches for emitted MU69 ions with SWAP andPEPSSI.An Alice count rate spectrum is shown in

Fig. 8A. This observation wasmade over a periodof 300 s, ~90min before closest approach, from arange r ~ 80,000 km. We fitted a model to thespectrum including background emissions frominterplanetary hydrogen plus four nearby stars.No coma emissions fromMU69were detected. Atthe brightest likely coma emission of the hy-drogen 121.6-nm line, we find a 3s upper limit

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Fig. 6. Approach observation limb profiles and geomorphological map of MU69. (A andB) Limb topography profiles of Ultima and Thule, respectively, measured using the LORRI CA04 andMVIC CA06 observations, after subtracting the best-fitting projected elliptical figures. The semimajor(A) and semiminor (B) axes of the best-fitting ellipses are indicated. Error bars represent thedifference in estimated limb positions between two independent workers; the median difference was~0.3 pixels for MU69 as a whole. The low solar phase angle of the CA04 observation (~12°) allowsthe limb to be measured more reliably around a larger portion of the perimeter of MU69 than for CA06(solar phase 32.5°). (C) Geomorphological map of MU69. The base map is the MVIC image fromFig. 2A. The positive spin axis of MU69 is pointing approximately into the page. The mapped boundariesare preliminary. Note that this mapping is physiographic in nature and is not intended to rigorouslyconvey stratigraphic relations between units.

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source rate of <3 × 1024 H atoms s–1 released byMU69. Scattering of sunlight by H atoms at thissource rate would produce a detectable emis-sion, assuming a distribution that falls off as r–2

from MU69.No structured magnetic interaction between

the solar wind andMU69was expected because, ata typical interplanetary magnetic field of 0.2 nT,the gyro-radius of a proton picked up by the so-lar wind is ~2 × 104 km; this distance is largerthanMU69 by a factor of ~1000 and is alsomuchlarger than the flyby closest approach distance of~3500 km. Shown in Fig. 8, B to E, are SWAP datataken during the time of the flyby. The bulk solarwind density and speed measured by SWAP nearMU69 are ~2000 protons m–3 and ~425 km s–1,respectively, for a solar wind flux of 8.5 × 108

protons m–2 s–1. Variations are attributable tochanges in spacecraft orientation relative to thesolar wind, but there is no signature of any de-tected interaction of the solar wind with MU69.Shown in Fig. 8, F to I, are PEPSSI data takenduring the flyby, which also showno evidence ofanyMU69-related signature. All PEPSSI variationsin count rate are associated with spacecraft at-titude changes, as with SWAP, and are consistentwith an unperturbed interplanetary medium.We can estimate the interaction that might be

detected by SWAP or PEPSSI using the upperlimit from Alice. For an outflow source rate of Qparticles s–1, the density at r is given by n ~ Q/4pr2v. For the Alice upper limit ofQ ~ 3 × 1024 Hatoms s–1, the density at closest approach range is~2.4 × 107 H atoms m–3 (assuming the H atomshave a radial velocity v ~ 800m s–1, their thermalspeed at 40 K). A fraction g of those H atoms canbecome ionized and picked up by the solar windto be detected by SWAP or PEPSSI. Then thecount rate is R = eGnvSWg/4p, where e is thedetection efficiency near the solar wind speed, Gis the instrument geometric factor, and vSW is thesolar wind speed. This indicates a PEPSSI countrate of R ~ 8000 g counts s–1. We estimate g ~ 2 ×10–7 from the fraction of H atoms that could bephotoionized by sunlight during the travel timet ~ dCA/v for H atoms to get from MU69 to theclosest approach distance ofNewHorizons (dCA);the expected PEPSSI count rate of 1.6 × 10–3 Hz

would thus be smaller than the typical back-ground count rate (~1 Hz) by a factor of ~600. Asimilar result is found for SWAP.Hence, the upperlimit found with Alice data is more constrainingthan the SWAP and PEPSSI upper limits.For comparison, we estimate the loss rate

from photosputtering of water ice onMU69 to be~1019 H atoms s–1, much less than our detectionupper limit. This estimate is based on the com-bined direct solar and interplanetary medium H121.6-nm flux atMU69 of 1.6 × 1012 photonsm–2 s–1

and4.0× 1011 photonsm–2 s–1, respectively (65, 66),and a yield of 0.003 for H 121.6-nm sputtering ofwater ice (67–69). Projected and total surface areasfor MU69 of ~4.1 × 108 m2 and 1.7 × 109 m2,respectively, were used in this estimate. Over4.5 billion years, the water ice lost by this processwould reduce the size of MU69 by a negligible~0.01 m. Water ice is also eroded by solar windions (mainly protons), but this process is esti-mated to remove only about the same amount ofmaterial over time as the photosputtering (70).

Implications for formation

The New Horizons flyby has revealed manyproperties of MU69 but has also raised somepuzzles. The latter include the origin of its twononspherical and markedly different lobe shapes;the provenance of its brighter spots, zones, andlinear/curvilinear features; the nature of the sim-ilarly sized surface units on Ultima; the degree towhich the object is cratered; the origin of itsbright neck; and how its two lobes formed andthen merged to create the contact binary weobserve.Despite these puzzles, MU69 has already pro-

vided information on the ancient accretion pro-cesses that operated in the distant protosolarnebula and the Kuiper Belt (71, 72). For example,MU69 lends weight to model predictions thatbinaries in the Kuiper Belt may have formed inlocal, low- to medium-velocity accretion cloudsas in pebble cloud gravitational collapse models(24). The lack of strong surface albedo, color, andcomposition heterogeneity between the two lobessupports this hypothesis, because in a local pebblecloud collapse,MU69’s two lobeswould form froma single source of material. However, it is also

possible that material accreted throughout thecold classical Kuiper Belt may have been com-positionally homogeneous to begin with.The binary size ratios produced in existing

pebble cloud collapse models (24), while focusedon the formation of muchmoremassive (100-kmscale), co-orbiting binaries, are also consistentwith the size ratio of the two lobes of MU69 (sizeratio ≈ 0.75). Such models also produce appro-priately low merger speeds (24, 25) if they scalewith the virial mass to much smaller clouds.However, MU69’s pole is highly inclined to theecliptic, which is not a common outcome in cloudcollapse models, given that the cloud’s initialrotation state is set by heliocentric Keplerianshear. In contrast, turbulent particle concentra-tion models, such as an overdense collection ofswirling particles collapsing under the influenceof aerodynamic drag in the protosolar nebula(73), have no preferred initial swirl (mean angularmomentum) orientation.Mechanisms fundamentally different from local

pebble or particle cloud collapse have also beenproposed for the formation of small-body binaries.However, some such mechanisms, such as YORPspin-up and fission (28, 29), apply only in the innerSolar Systemwhere thermal radiation forces aresufficiently strong. In the outer Solar System, bi-nary systems may instead form via three-bodyexchange capture (74), but such mechanisms re-quire heliocentric encounter velocities generallynear or lower than theHill speed [the heliocentricKeplerian shear speed at the limit of the primarybody’s gravitational sphere of influence (24)],which for MU69 would have been an implausiblylow ~1 cm s–1; hence, suchmodels are disfavored.We also do not favor this three-body formationmechanism because binaries formed by suchthree-body exchange would likely rotate eitherprograde or retrograde (24, 75), not highly oblique-ly like MU69.For MU69’s two lobes to reach their current,

merged spin state, they must have lost angularmomentum if they initially formed as co-orbitingbodies. The lack of detected satellites of MU69

may imply ancient angular momentum sink(s)via (i) the ejection of formerly co-orbiting smallerbodies by Ultima and Thule, (ii) gas drag, or

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Fig. 7. Upper limits on satellitesand ring/dust structuresaround MU69. (A) Satellitesearch limiting diameters,assuming that satellites havereflectivity similar to that ofMU69. The Hill radius (i.e.,maximum orbit stability radiusagainst solar tides) shownhere assumes an effectivespherical-equivalent radius of7.5 km for MU69 (approximating thelobes as ellipsoids) and a density of500 kg m–3. (B) Profiles of limitingring detectability for I/F versusdistance from MU69, assuming a 10-km-wide ring, obtained using LORRI observations made at 22 hours (blue) and 6.5 hours (orange) before closestapproach. The increasing I/F constraints inward of ~250 km are due to stray light from MU69 within LORRI’s optics, not material around MU69.

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Fig. 8. MU69 atmosphericand plasma search results.(A) Alice ultraviolet airglowspectrum. A modeledbackground (blue) includesinterplanetary H emissionsand four nearby stars.A small model signal at116.6 nm (gold) indicates theH 121.6 nm count rate thatwould be expected if MU69

were outgassing H atomsat Q ~ 3 × 1024 atoms s–1;the 121.6-nm H signalappears at this wavelengthbecause the observationswere offset to avoid alow-sensitivity region of thedetector. No MU69 comaemission is detected in theobserved spectrum (black).(B and C) SWAP low-energyplasma spectrometer datashowing light (blue) andheavy (orange) secondarychannel electron multiplier(SCEM) count rate energy/charge spectra, where light(i.e., H+ and He++) and heavyions are distinguished bytheir low and highsecondary/primary electronratios as they pass throughSWAP’s carbon foil (65), andcoincidence rates (COIN)spectra. The heavy ions aresparse and are not associ-ated with close approach.(D and E) SWAP orientationwith respect to the Sun inaltitude (q) and azimuth (ϕ)angles (black, full range; red,zoomed). All the changesobserved in the coincidencerate spectra in (B) and (C)are associated with changesin spacecraft orientation; nochanges related to the pres-ence of MU69 were observed.(F to I) PEPSSI energeticparticle spectrometer data.Count rates are shown forthree products for ~80 minnear closest approach toMU69 (indicated by thevertical black line). Datawere acquired in 1-s binsbut averaged over 2-minintervals to improve thesignal-to-noise ratio. (F) Suprathermal particles at >2 keV nucleon–1 (dominated by interstellar pickup He+); (G) energetic protons (30 keV to 1 MeV);(H) galactic cosmic rays (mostly >100 MeV protons); (I) the angle between PEPSSI aperture and the Sun. Changes in instrument orientation account forall the observed count rate variability; all particle rates shown are typical for the undisturbed interplanetary medium, with an increased photonbackground when the Sun is in the PEPSSI field of view (FOV).

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both. This suggests that contact binaries maybe rare in CCKBO systems with orbiting satel-lites. Another possibility, however, is that thelobes Ultima and Thule impacted one anothermultiple times, sheddingmass alongwith angularmomentum before making final contact. Butthe alignment of the principal axes ofMU69’s twolobes tends to disfavor this hypothesis. In con-trast, tidal locking could quite plausibly have pro-duced the principal axis alignment we observe,once the co-orbiting bodies were close enoughand spin-orbit coupling was most effective (76).Gas drag could also have played a role in foster-ing the observed coplanar alignment of the Ul-tima and Thule lobes (Fig. 2). Post-mergerimpacts may have also somewhat affected theobserved, final angular momentum state.

MethodsShape model fitting process

Because the pole and rotation rate of MU69 werenot known before the NewHorizons flyby, it wasnecessary to simultaneously fit the shape andpole of the object. The data available to ac-complish this were the resolved images of MU69

that were obtained from a few days prior toclosest approach up to closest approach itself.The problem was broken into parameters forpole, rotation rate, mean surface albedo, and aparametrically defined shape. Each of the twolobes of the KBO was defined with its own pa-rametric model; the separation between the lobeswas made an additional free parameter. The “oct-antoid” formalism was used for the lobes (30),which is similar to spherical harmonics but isprecisely an ellipsoid at the lowest order. Startingby assuming that both lobes are ellipsoids, in-creasing complexity and spatial resolution werethen added to the shape model by increasing theharmonic orders of the octantoids. Images wereconverted to radiometrically calibrated I/F spaceto minimize the work the fitting program wouldhave to do.To test this parameter set against the data,

synthetic versions of the resolved images weregenerated as follows. First, the parametric shapewas rastered onto a 3D mesh for each lobe, andthe surface normals for each polygon in themeshwere calculated. Next, the geometry for the imagewas calculated, using the navigation SPICEkernels (77) for distance to MU69 and the WorldCoordinate System (WCS) for pixel scale andimage rotation. The mesh was then renderedin OpenGL using the derived geometry. Therenders were performed twice, once to providea depth buffer and a second time that used thedepth buffer to allow self-shadowing on theobject. The second rendering also calculatedthe brightness for each pixel of the image in I/Fspace, using the Hapke photometric modeldescribed in the text. Performing the renderingin OpenGL allowed the work to use GPUs tospeed up the shape-fitting process. The renderedimages from each New Horizons camera werethen convolved with the point-spread functionof that camera. This smeared out the renderedimages to the same angular resolution as the

real images, allowing direct comparison of theimages on a pixel-by-pixel basis, even in the earlyimages in whichMU69 was only a few pixels long.Finally, the sum of the square of the differenceof each real image was compared to its sim-ulated version; a sum of all of those differencesprovides an estimate of the c2 for any derivedparameter set.With the problem now reduced to a param-

eter set and a c2 function, standard function-minimization techniques were used to find theoptimal shape and pole. As noted above, thisstarted with simple ellipsoidal shapes for eachlobe, which were used to find initial solutionsfor the pole, rotation, and obliquity. Next, thecomplexity of the model shapes was graduallyincreased as new images were incorporated.The number of images fitted and the complexityof the model both increased the time to calculatec2 for any parameter set, and thus increased thetime required for the optimization process. Thisin turn limited the spatial resolution of the shapemodel.

LEISA data processing

The IR spectral imaging spectroscopy capabilityon New Horizons is provided by the LEISA in-strument (78). LEISA’s focal plane consists of a256 × 256 HgCdTe detector array with a linearvariable filter affixed to it such that each row ofthe array is sensitive to a different IR wavelengthbetween 1.2 and 2.5 mm. Its average spectral re-solving power (l/Dl) is close to 240. LEISA isoperated by sweeping its field of view acrossthe target scene while images are recorded ata frame rate of approximately 1 frame per pixelmoved, so that each part of the scene is recordedin each of LEISA’s wavelengths. The highest spa-tial resolution LEISA scan of MU69 was desig-nated CA04_LE. This scan can also be identifiedby its uniqueMission Elapsed Time (MET) num-ber, 0408624118; it was obtained about 4:58 UTon 1 January, approximately 35min before closestapproach, and from a mean spacecraft range of31,000 km. LEISA’s pixel scale maps to 1.9 km onMU69 at that range. Without knowledge of theexact shape or spin state of MU69, we could notmap the signal recorded in each of LEISA’s spec-tral pixels to their exact locations on the surfaceof the body. Instead, we mapped them to a tan-gentplane orientedperpendicular to the spacecraft-MU69 line of sight, located at the distance of MU69.Attitude data reported by the spacecraft wereused to account for the motion of the targetthrough the LEISA field of view. This motionwas complex because spacecraft pointing wascontrolled by frequent thruster firings to main-tain pointing within a specified amount. Overthe course of the scan, the spacecraft also closedrange with the target, from approximately 33,000to 29,000 km. This changing geometry resultedin a wavelength-dependent scale in the spectralimage cube. To correct for this effect in extract-ing the spectrum shown in Fig. 5D, the region ofinterest for spectral extraction was constructedusing the same wavelength-dependent scale.The diminished solar flux at 43 AU, combined

with the low albedo of MU69, produces a signallevel similar to the noise in a single LEISA pixel,so multiple pixels had to be averaged togetherto produce the spectrum in Fig. 5D. The scatterof the points gives an indication of the noise inthe resulting average spectrum. Spectral modelsof granular combinations of H2O and CH3OHiceswith tholins are able to reproduce the overallalbedo and features in the LEISA spectrum,except for the unidentified band at 1.8 mm.

Thermal models

Incident sunlight varies across MU69’s surfaceover its 293-year orbit around the Sun and alsoits 15.92-hour rotation about its spin axis, causingtime-variable temperatures. The local radiativebalance is also affected by topography, especiallyin the neck region, where greater shadowing re-duces incident sunlight but also reduces the solidangle of dark sky into which thermal emissioncan be radiated. Heat propagating inward oroutward moderates insolation-driven surfacetemperature variations, with thermal inertiaG (the square root of the product of heat capacityc, density r, and conductivity of the material k,with units of J m–2 s–1/2 K–1) controlling the de-gree of moderation. The thermal waves penetrateto a characteristic skin depth, dskin = (2k/wrc)1/2,where w is the angular frequency of the temper-ature forcing. Conductivity is the least certain ofthese parameters, being sensitively dependenton the material texture and temperature. It isgenerally low for cold, granular materials invacuum, and the expectation for MU69 is forvery loosely consolidated material. This followsfrom the low expected bulk density of MU69, aswell as from thermal observations of other KBOs.Herschel and Spitzer space telescope observa-tions have shown amean diurnal thermal inertiafor Centaurs and KBOs of ~2.5 J m–2 s–1/2 K–1 (52).Assuming a bulk density of 500 kgm–3 and a heatcapacity of 350 J kg–1 K–1 of H2O-ice at 40 K (79), aG of 2.5 J m–2 s–1/2 K–1 implies a very low con-ductivity of approximately 3.6 × 10–5 J m–1 s–1 K–1.If instead we follow (80, 81) and assume that iceIh is the dominant constituent, such a low con-ductivity would require a porosity of ~65%, notunreasonable for a bulk density of 500 kg m–3.Low thermal inertia and low conductivity im-

ply that surface temperatures on MU69 are closeto instantaneous equilibrium between absorbedinsolation and thermal emission. The low conduc-tivity implies that the thermal waves driven bydiurnal and seasonal insolation variations affectonly the very outermost layers, withdskin = 0.001mand ~1 m, respectively. The seasonal skin depthcould be somewhat greater if conductivity in-creases with depth below the surface. MU69

could have an even lower thermal inertia thanthe much larger objects in the Herschel andSpitzer sample, given that its lower mass andsparseness of impact craters imply less colli-sional compaction over time and that its smallersize limits the possibility of early heating byshort-lived radionuclides that might have led tosintering and thus higher thermal conductivityin larger objects.

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ACKNOWLEDGMENTS

We thank all ~2500 present and past New Horizons teammembers, NASA and its Deep Space Network, KinetX AerospaceCorporation, the Caltech Jet Propulsion Laboratory, and theEuropean Space Agency Gaia and NASA HST space missions fortheir contributions to making the flyby of MU69 successful; ourNASA Headquarters Program Scientist, C. Niebur; and J. Andersonof the Space Telescope Science Institute. We also thank NASAAdministrator J. Bridenstine for his key support during theDecember 2018 to January 2019 partial U.S. governmentshutdown; SwRI President A. Hamilton and Johns Hopkins AppliedPhysics Laboratory Director R. Semmel for many years of valuableproject support; and three anonymous referees for their helpfulcontributions to this paper. We acknowledge the contributions ofour late team members Thomas Coughlin, Robert Farquhar,

William Gibson, Lisa Hardaway, and David C. Slater. Funding:Supported by NASA’s New Horizons project via contractsNASW-02008 and NAS5-97271/TaskOrder30. Also supported bythe National Research Council of Canada (J.J.K.). Authorcontributions: S.A.S., J.R.S., J.M.M., O.L.W., W.B.M., W.M.G.,G.R.G., and H.A.E. were responsible for drafting this manuscript.S.A.S. is the principal investigator of the New Horizons mission.All other authors participated in mission planning, missionoperations, mission engineering, mission management, missionpublic affairs, or science data reduction or analysis, and/or providedinputs and critique to this manuscript. Competing interests: Wedeclare no competing interests. Data andmaterials availability: Allimages, spacecraft data, and the shape model used in this paperare available at http://dx.doi.org/10.6084/m9.figshare.7940630.Additional fully calibrated New Horizons MU69 data and higher-orderdata products will be released by the NASA Planetary Data Systemat https://pds-smallbodies.astro.umd.edu/data_sb/missions/newhorizons/index.shtml in a series of stages in 2020 and 2021, owingto the time required to fully downlink and calibrate the dataset.

20 February 2019; accepted 16 April 201910.1126/science.aaw9771

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, a small Kuiper Belt object69Initial results from the New Horizons exploration of 2014 MU

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DOI: 10.1126/science.aaw9771 (6441), eaaw9771.364Science 

, this issue p. eaaw9771Sciencethat process.type of Kuiper Belt object is mostly undisturbed since the formation of the Solar System and so will preserve clues about

consists of two lobes that appear to have merged at low speed, producing a contact binary. This69January 2019. MU on 169 present the initial results from the New Horizons flyby of MUet al.orbiting in the outer Solar System. Stern

is part of the Kuiper Belt, a collection of small icy bodies69much smaller body about 30 kilometers in diameter. MU, a69After flying past Pluto in 2015, the New Horizons spacecraft shifted course to encounter (486958) 2014 MU

69New Horizons flies past MU

ARTICLE TOOLS http://science.sciencemag.org/content/364/6441/eaaw9771

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