RESEARCH ◥ ticle Spectrometer Science Investigation (PEPSSI) RESEARCH ARTICLE SUMMARY high-energy charged particle spectrometer; and the Venetia Burney Student Dust Counter (VBSDC), a dust impact detector. OUTER SOLAR SYSTEM RESULTS: Imaging of MU69 showed it to be a ’ Initial results from the New Horizons bilobed, contact binary. MU69 stwolobesap- pear to have formed close to one another, becom- inganorbitingpairthatsubsequentlyunderwent exploration of 2014 MU , a small coupled tidal and orbital 69 ◥ ON OUR WEBSITE evolution to merge into Kuiper Belt object the contact binary we ob- Read the full article at http://dx.doi. serve today. The object S. A. Stern* et al. org/10.1126/ rotates on its axis every science.aaw9771 15.92 hours; its rotation .................................................. pole is inclined approx- INTRODUCTION: The Kuiper Belt is a broad, viously been explored by spacecraft, we also imately 98° to the plane of its heliocentric orbit. torus-shaped region in the outer Solar System sought to provide a close-up look at such a Its entire surface has a low visible-wavelength beyond Neptune’s orbit. It contains primordial body’s geology and composition, and to search reflectivity (albedo) but displays brighter and planetary building blocks and dwarf planets. for satellites, rings, and evidence of present or darker regions across its surface, ranging from NASA’s New Horizons spacecraft conducted past atmosphere. We report initial scientific 5 to 12% reflectivity. The brightest observed Downloaded from a flyby of Pluto and its system of moons on results and interpretations from that flyby. regions are the “neck” of MU69, where the two 14 July 2015. New Horizons then continued lobes are joined, and two discrete bright spots farther into the Kuiper Belt, adjusting its RATIONALE: The New Horizons spacecraft inside the largest crater-like feature on the ob- ’ ’ trajectory to fly close to the small Kuiper Belt completed its MU69 flyby on 1 January 2019, ject s surface. Although MU69 s albedo varies object (486958) 2014 MU69 (henceforth MU69; with a closest approach distance of 3538 km— substantially across its surface, it is uniformly also informally known as Ultima Thule). Stellar less than one-third of its closest distance to red in color, with only minor observed color http://science.sciencemag.org/ occultation observations in 2017 showed that Pluto. During the high-speed flyby, made at variations. This coloration likely represents a −1 MU69 was ~25 to 35 km in diameter, and 14.4 km s , the spacecraft collected ~50 gigabits refractory residue from ices and organic mol- therefore smaller than the diameter of Pluto of high-resolution imaging, compositional spec- ecules processed by ultraviolet light and cosmic (2375 km) by a factor of ~100 and less massive troscopy, temperature measurements, and other rays. Spectra of the surface revealed tentative 6 than Pluto by a factor of ~10 .MU69 is located data on this Kuiper Belt object. We analyzed absorption band detections due to water ice and about 1.6 billion kilometers farther from the the initial returned flyby data from the seven methanol. The geology of MU69 consists of Sun than Pluto was at the time of the New scientific instruments carried on the spacecraft: numerous distinct units but shows only a small Horizons flyby. MU69’sorbitindicatesthatitis the Ralph multicolor/panchromatic camera number of craters, providing evidence that there a “cold classical” Kuiper Belt object, thought to and mapping infrared composition spectrom- is a deficit of Kuiper Belt objects smaller than ~1 km in diameter, and that there is a com- be the least dynamically evolved population in eter; the Long Range Reconnaissance Imager the Solar System. A major goal of flying past (LORRI) long–focal length panchromatic visible paratively low collision rate in its Kuiper Belt on June 4, 2019 this target is to investigate accretion processes imager; the Alice extreme/far ultraviolet map- environment compared to what would be ex- in the outer Solar System and how those pro- ping spectrograph; the Radio Experiment pected in a collisional equilibrium population. cesses led to the formation of the planets. (REX); the Solar Wind Around Pluto (SWAP) A three-dimensional shape model derived from Because no small Kuiper Belt object had pre- solar wind detector; the Pluto Energetic Par- the images shows MU69 is not simply elongated but also flattened. The larger lobe was found to be lenticular, with dimensions of approximately 22 × 20 × 7 km (uncertainty <0.6 × 1 × 2 km), whereas the smaller lobe is less lenticular, with dimensions of approximately 14 × 14 × 10 km (uncertainty <0.4 × 0.7 × 3 km). No evidence of satellites, rings, or an extant atmosphere was foundaroundMU69. CONCLUSION: Both MU69’s binarity and un- usual shape may be common among similarly sized Kuiper Belt objects. The observation that its two lobes are discrete, have retained their basic shapes, and do not display prominent deformation or other geological features indic- ative of an energetic or disruptive collision indi- cates that MU69 is the product of a gentle merger of two independently formed bodies.▪ All authors and affiliations are available in the full article online. *Corresponding author. Email: [email protected] Cite this article as S. A. Stern et al., Science 364, eaaw9771 Image of MU69 from New Horizons LORRI observations taken 1 January 2019. (2019). DOI: 10.1126/science.aaw9771 Stern et al., Science 364, 649 (2019) 17 May 2019 1of1 RESEARCH ◥ through and explored the Pluto dwarf planet RESEARCH ARTICLE system in 2015 (4, 5). The spacecraft has since continued farther to explore Kuiper Belt objects (KBOs) and the Kuiper Belt radiation and dust OUTER SOLAR SYSTEM environment (6). The target selected for the subsequent New Initial results from the New Horizons Horizons KBO flyby was (486958) 2014 MU69 (hereafter MU69, also informally referred to as Ultima Thule). This KBO was discovered in 2014 exploration of 2014 MU69, a small when the Hubble Space Telescope (HST) was being used to conduct a dedicated search for Kuiper Belt object New Horizons KBO flyby targets (7, 8). Before the arrival of New Horizons, the only definitive facts regarding MU69 were its orbit (8), its red S. A. Stern1*,H.A.Weaver2,J.R.Spencer1,C.B.Olkin1,G.R.Gladstone3,W.M.Grundy4, color (9), its size of ~30 km (7), and its lack of 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, 11 12 2 13 2 F. Bagenal ,M.E.Banks ,B.A.Bauer ,J.A.Bauman ,K.E.Bechtold , 1Southwest Research Institute, Boulder, CO 80302, USA. C. B. Beddingfield5,14,N.Behrooz2,K.B.Beisser2,S.D.Benecchi15,E.Bernardoni11, 2Johns Hopkins University Applied Physics Laboratory, 5,14 16 17 18 1 19,20 Laurel, MD 20723, USA. 3Southwest Research Institute, San R. A. Beyer , S. Bhaskaran ,C.J.Bierson ,R.P.Binzel ,E.M.Birath,M.K.Bird , 4 16 2 21 22 2 2 Antonio, TX 78238, USA. Lowell Observatory, Flagstaff, AZ D. R. Boone ,A.F.Bowman ,V.J.Bray ,D.T.Britt ,L.E.Brown ,M.R.Buckley , 86001, USA. 5NASA Ames Research Center, Space Science Downloaded from M. W. Buie1,B.J.Buratti16,L.M.Burke2,S.S.Bushman2,B.Carcich2,23,A.L.Chaikin24, Division, Moffett Field, CA 94035, USA. 6Department of 5,14 2 2 2 2 1 Physics and Astronomy, University of Texas, San Antonio, TX C. L. Chavez ,A.F.Cheng ,E.J.Colwell ,S.J.Conard ,M.P.Conner ,C.A.Conrad, 7 25 2 2 5,14 2 78249, USA. Department of Earth and Planetary Sciences J. C. Cook ,S.B.Cooper ,O.S.Custodio ,C.M.DalleOre ,C.C.Deboy , and McDonnell Center for the Space Sciences, Washington P. Dharmavaram2,R.D.Dhingra26,G.F.Dunn3,A.M.Earle18,A.F.Egan1, J. Eisig2, University, St. Louis, MO 63130, USA. 8Department of 27 2 1 2 3 1 Astronomy, University of Virginia, Charlottesville, VA 22904, M. R. El-Maarry , C. Engelbrecht ,B.L.Enke,C.J.Ercol ,E.D.Fattig ,C.L.Ferrell, 9 1 2 13 16 2 2 USA. Independent consultant, Carbondale, CO 81623, USA. T. J. Finley , J. Firer ,J.Fischetti ,W.M.Folkner ,M.N.Fosbury ,G.H.Fountain , 10Universität der Bundeswehr München, Neubiberg 85577, 2 28 29 29 2 2 20 http://science.sciencemag.org/ J. M. Freeze , L. Gabasova ,L.S.Glaze ,J.L.Green , G. A. Griffith ,Y.Guo ,M.Hahn , Germany. 11Laboratory for Atmospheric and Space Physics, D. W. Hals2,D.P.Hamilton30,S.A.Hamilton2,J.J.Hanley3,A.Harch23,K.A.Harmon16, University of Colorado, Boulder, CO 80303, USA. 12NASA 2 2 2 2 2 16 Goddard Space Flight Center, Greenbelt, MD 20771, USA. H. M. Hart ,J.Hayes ,C.B.Hersman ,M.E.Hill ,T.A.Hill ,J.D.Hofgartner , 13 14 KinetX Aerospace, Tempe, AZ 85284, USA. SETI Institute, 2 11 2 31 1 M. E. Holdridge ,M.Horányi ,A.Hosadurga ,A.D.Howard ,C.J.A.Howett, Mountain View, CA 94043, USA. 15Planetary Science S. E. Jaskulek2,D.E.Jennings12,J.R.Jensen2,M.R.Jones2,H.K.Kang2,D.J.Katz2, Institute, Tucson, AZ 85719, USA. 16Jet Propulsion 1 32 33 2 34 2 Laboratory, California Institute of Technology, Pasadena, CA D. E. Kaufmann , J. J. Kavelaars ,J.T.Keane ,G.P.Keleher ,M.Kinczyk ,M.C.Kochte , 17 2 2 16 2 2 91109, USA. Earth and Planetary Science Department, P. Kollmann ,S.M.Krimigis ,G.L.Kruizinga ,D.Y.Kusnierkiewicz ,M.S.Lahr , University of California, Santa Cruz, CA 95064, USA.