[Source: Science, full page: (LINK). Abstract, edited.]
Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object
S. A. Stern 1,*, H. A. Weaver 2, J. R. Spencer 1, C. B. Olkin 1, G. R. Gladstone 3, W. M. Grundy 4, J. M. Moore 5, D. P. Cruikshank 5, H. A. Elliott 3,6, W. B. McKinnon 7, J. Wm. Parker 1, A. J. Verbiscer 8, L. A. Young 1, D. A. Aguilar 9, J. M. Albers 2, T. Andert 10, J. P. Andrews 1, F. Bagenal 11, M. E. Banks 12, B. A. Bauer 2, J. A. Bauman 13, K. E. Bechtold 2, C. B. Beddingfield 5,14, N. Behrooz 2, K. B. Beisser 2, S. D. Benecchi 15, E. Bernardoni 11, R. A. Beyer 5,14, S. Bhaskaran 16, C. J. Bierson 17, R. P. Binzel 18, E. M. Birath 1, M. K. Bird1 9,20, D. R. Boone 16, A. F. Bowman 2, V. J. Bray 21, D. T. Britt 22, L. E. Brown 2, M. R. Buckley 2, M. W. Buie 1, B. J. Buratti 16, L. M. Burke 2, S. S. Bushman 2, B. Carcich 2,23, A. L. Chaikin 24, C. L. Chavez 5,14, A. F. Cheng 2, E. J. Colwell 2, S. J. Conard 2, M. P. Conner 2, C. A. Conrad 1, J. C. Cook 25, S. B. Cooper 2, O. S. Custodio 2, C. M. Dalle Ore 5,14, C. C. Deboy 2, P. Dharmavaram 2, R. D. Dhingra 26, G. F. Dunn 3, A. M. Earle 18, A. F. Egan 1, J. Eisig 2, M. R. El-Maarry 27, C. Engelbrecht 2, B. L. Enke 1, C. J. Ercol 2, E. D. Fattig 3, C. L. Ferrell 1, T. J. Finley 1, J. Firer 2, J. Fischetti 13, W. M. Folkner 16, M. N. Fosbury 2, G. H. Fountain 2, J. M. Freeze 2, L. Gabasova 28, L. S. Glaze 29, J. L. Green 29, G. A. Griffith 2, Y. Guo 2, M. Hahn 20, D. W. Hals 2, D. P. Hamilton 30, S. A. Hamilton 2, J. J. Hanley 3, A. Harch 23, K. A. Harmon 16, H. M. Hart 2, J. Hayes 2, C. B. Hersman 2, M. E. Hill 2, T. A. Hill 2, J. D. Hofgartner 16, M. E. Holdridge 2, M. Horányi 11, A. Hosadurga 2, A. D. Howard 31, C. J. A. Howett 1, S. E. Jaskulek 2, D. E. Jennings 12, J. R. Jensen 2, M. R. Jones 2, H. K. Kang 2, D. J. Katz 2, D. E. Kaufmann 1, J. J. Kavelaars 32, J. T. Keane 33, G. P. Keleher 2, M. Kinczyk 34, M. C. Kochte 2, P. Kollmann 2, S. M. Krimigis 2, G. L. Kruizinga 16, D. Y. Kusnierkiewicz 2, M. S. Lahr 2, T. R. Lauer 35, G. B. Lawrence 2, J. E. Lee 36, E. J. Lessac-Chenen 13, I. R. Linscott 37, C. M. Lisse 2, A. W. Lunsford 12, D. M. Mages 16, V. A. Mallder 2, N. P. Martin 38, B. H. May 39, D. J. McComas 3,40, R. L. McNutt Jr. 2, D. S. Mehoke 2, T. S. Mehoke 2, D. S. Nelson 13, H. D. Nguyen 2, J. I. Núñez 2, A. C. Ocampo 29, W. M. Owen 16, G. K. Oxton 2, A. H. Parker 1, M. Pätzold 20, J. Y. Pelgrift 13, F. J. Pelletier 13, J. P. Pineau 41, M. R. Piquette 11, S. B. Porter 1, S. Protopapa 1, E. Quirico 28, J. A. Redfern 1, A. L. Regiec 2, H. J. Reitsema 42, D. C. Reuter 12, D. C. Richardson 30, J. E. Riedel 16, et al.
1 Southwest Research Institute, Boulder, CO 80302, USA. 2 Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. 3 Southwest Research Institute, San Antonio, TX 78238, USA. 4 Lowell Observatory, Flagstaff, AZ 86001, USA. 5 NASA Ames Research Center, Space Science Division, Moffett Field, CA 94035, USA. 6 Department of Physics and Astronomy, University of Texas, San Antonio, TX 78249, USA. 7 Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA. 8 Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA. 9 Independent consultant, Carbondale, CO 81623, USA. 10 Universität der Bundeswehr München, Neubiberg 85577, Germany. 11 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA. 12NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. 13 KinetX Aerospace, Tempe, AZ 85284, USA. 14 SETI Institute, Mountain View, CA 94043, USA.
15Planetary Science Institute, Tucson, AZ 85719, USA. 16 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA. 17 Earth and Planetary Science Department, University of California, Santa Cruz, CA 95064, USA. 18 Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 19 Argelander-Institut für Astronomie, University of Bonn, Bonn D-53121, Germany. 20 Rheinisches Institut für Umweltforschung, Universität zu Köln, Cologne 50931, Germany. 21 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. 22 Department of Physics, University of Central Florida, Orlando, FL 32816, USA. 23 Cornell University, Ithaca, NY 14853, USA. 24 Independent science writer, Arlington, VT 05250, USA. 25 Pinhead Institute, Telluride, CO 81435, USA. 26 University of Idaho, Moscow, ID 83844, USA. 27 Department of Earth and Planetary Sciences, Birkbeck, University of London, London WC1E 7HX, UK. 28 University Grenoble Alpes, Centre National de la Recherche Scientifique, Institut de Planétologie et d’Astrophysique de Grenoble, 38000 Grenoble, France. 29 NASA Headquarters, Washington, DC 20546, USA. 30 Department of Astronomy, University of Maryland, College Park, MD 20742, USA. 31 Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA. 32 National Research Council of Canada, Victoria, BC V9E 2E7, Canada. 33 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. 34 Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA. 35 National Optical Astronomy Observatory, Tucson, AZ 26732, USA. 36 NASA Marshall Space Flight Center, Huntsville, AL 35812, USA. 37 Independent consultant, Mountain View, CA 94043, USA. 38 Independent consultant, Crested Butte, CO 81224, USA. 39 Independent collaborator, Windlesham GU20 6YW, UK. 40 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA. 41 Stellar Solutions, Palo Alto, CA 94306, USA. 42 Independent consultant, Holland, MI 49424, USA. 43 Lunar and Planetary Institute, Houston, TX 77058, USA. 44 Space Telescope Science Institute, Baltimore, MD 21218, USA. 45 Johns Hopkins University, Baltimore, MD 21218, USA. 46 Roane State Community College, Oak Ridge, TN 37830, USA. 47 George Mason University, Fairfax, VA 22030, USA. 48 Independent consultant, Burden, KS 67019, USA.
*Corresponding author. Email: email@example.com
Science 17 May 2019: Vol. 364, Issue 6441, eaaw9771 / DOI: 10.1126/science.aaw9771
New Horizons flies past MU69
After flying past Pluto in 2015, the New Horizons spacecraft shifted course to encounter (486958) 2014 MU69, a much smaller body about 30 kilometers in diameter. MU69 is part of the Kuiper Belt, a collection of small icy bodies orbiting in the outer Solar System. Stern et al.present the initial results from the New Horizons flyby of MU69 on 1 January 2019. MU69consists of two lobes that appear to have merged at low speed, producing a contact binary. This type of Kuiper Belt object is mostly undisturbed since the formation of the Solar System and so will preserve clues about that process.
Science, this issue p. eaaw9771
The Kuiper Belt is a broad, torus-shaped region in the outer Solar System beyond Neptune’s orbit. It contains primordial planetary building blocks and dwarf planets. NASA’s New Horizons spacecraft conducted a flyby of Pluto and its system of moons on 14 July 2015. New Horizons then continued farther into the Kuiper Belt, adjusting its trajectory to fly close to the small Kuiper Belt object (486958) 2014 MU69 (henceforth MU69; also informally known as Ultima Thule). Stellar occultation observations in 2017 showed that MU69 was ~25 to 35 km in diameter, and therefore smaller than the diameter of Pluto (2375 km) by a factor of ~100 and less massive than Pluto by a factor of ~106. MU69 is located about 1.6 billion kilometers farther from the Sun than Pluto was at the time of the New Horizons flyby. MU69’s orbit indicates that it is a “cold classical” Kuiper Belt object, thought to be the least dynamically evolved population in the Solar System. A major goal of flying past this target is to investigate accretion processes in the outer Solar System and how those processes led to the formation of the planets. Because no small Kuiper Belt object had previously been explored by spacecraft, we also sought to provide a close-up look at such a body’s geology and composition, and to search for satellites, rings, and evidence of present or past atmosphere. We report initial scientific results and interpretations from that flyby.
The New Horizons spacecraft completed its MU69 flyby on 1 January 2019, with a closest approach distance of 3538 km—less than one-third of its closest distance to Pluto. During the high-speed flyby, made at 14.4 km s−1, the spacecraft collected ~50 gigabits of high-resolution imaging, compositional spectroscopy, temperature measurements, and other data on this Kuiper Belt object. We analyzed the initial returned flyby data from the seven scientific instruments carried on the spacecraft: the Ralph multicolor/panchromatic camera and mapping infrared composition spectrometer; the Long Range Reconnaissance Imager (LORRI) long–focal length panchromatic visible imager; the Alice extreme/far ultraviolet mapping spectrograph; the Radio Experiment (REX); the Solar Wind Around Pluto (SWAP) solar wind detector; the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) high-energy charged particle spectrometer; and the Venetia Burney Student Dust Counter (VBSDC), a dust impact detector.
Imaging of MU69 showed it to be a bilobed, contact binary. MU69’s two lobes appear to have formed close to one another, becoming an orbiting pair that subsequently underwent coupled tidal and orbital evolution to merge into the contact binary we observe today. The object rotates on its axis every 15.92 hours; its rotation pole is inclined approximately 98° to the plane of its heliocentric orbit. Its entire surface has a low visible-wavelength reflectivity (albedo) but displays brighter and darker regions across its surface, ranging from 5 to 12% reflectivity. The brightest observed regions are the “neck” of MU69, where the two lobes are joined, and two discrete bright spots inside the largest crater-like feature on the object’s surface. Although MU69’s albedo varies substantially across its surface, it is uniformly red in color, with only minor observed color variations. This coloration likely represents a refractory residue from ices and organic molecules processed by ultraviolet light and cosmic rays. Spectra of the surface revealed tentative absorption band detections due to water ice and methanol. The geology of MU69 consists of numerous distinct units but shows only a small number of craters, providing evidence that there is a deficit of Kuiper Belt objects smaller than ~1 km in diameter, and that there is a comparatively low collision rate in its Kuiper Belt environment compared to what would be expected in a collisional equilibrium population. A three-dimensional shape model derived from 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 found around MU69.
Both MU69’s binarity and unusual 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 indicative of an energetic or disruptive collision indicates that MU69 is the product of a gentle merger of two independently formed bodies.
The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU69, a cold classical Kuiper Belt object approximately 30 kilometers in diameter. Such objects have never been substantially heated by the Sun and are therefore well preserved since their formation. We describe initial results from these encounter observations. MU69 is a bilobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color 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 with pebble cloud collapse followed by a low-velocity merger of its two lobes.
Keywords: Space; Solar System; Kuiper Belt.