Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object

Stern, S.A. and Weaver, H.A. and Spencer, J.R. and Olkin, C.B. and Gladstone, G.R. and Grundy, W.M. and Moore, J.M. and Cruikshank, D.P. and Elliott, H.A. and McKinnon, W.B. and Parker, J.Wm. and Verbiscer, A.J. and Young, L.A. and Aguilar, D.A. and Albers, J.M. and Andert, T. and Andrews, J.P. and Bagenal, F. and Banks, M.E. and Bauer, B.A. and Bauman, J.A. and Bechtold, K.E. and Beddingfield, C.B. and Behrooz, N. and Beisser, K.B. and Benecchi, S.D. and Bernardoni, E. and Beyer, R.A. and Bhaskaran, S. and Bierson, C.J. and Binzel, R.P. and Birath, E.M. and Bird, M.K. and Boone, D.R. and Bowman, A.F. and Bray, V.J. and Britt, D.T. and Brown, L.E. and Buckley, M.R. and Buie, M.W. and Buratti, B.J. and Burke, L.M. and Bushman, S.S. and Carcich, B. and Chaikin, A.L. and Chavez, C.L. and Cheng, A.F. and Colwell, E.J. and Conard, S.J. and Conner, M.P. and Conrad, C.A. and Cook, J.C. and Cooper, S.B. and Custodio, O.S. and Dalle Ore, C.M. and Deboy, C.C. and Dharmavaram, P. and Dhingra, R.D. and Dunn, G.F. and Earle, A.M. and Egan, A.F. and Eisig, J. and El-Maarry, Mohamed Ramy and Engelbrecht, C. and Enke, B.L. and Ercol, C.J. and Fattig, E.D. and Ferrell, C.L. and Finley, T.J. and Firer, J. and Fischetti, J. and Folkner, W.M. and Fosbury, M.N. and Fountain, G.H. and Freeze, J.M. and Gabasova, L. and Glaze, L.S. and Green, J.L. and Griffith, G.A. and Guo, Y. and Hahn, M. and Hals, D.W. and Hamilton, D.P. and Hamilton, S.A. and Hanley, J.J. and Harch, A. and Harmon, K.A. and Hart, H.M. and Hayes, J. and Hersman, C.B. and Hill, M.E. and Hill, T.A. and Hofgartner, J.D. and Holdridge, M.E. and Horányi, A. and Howard and Pelgrift, J.Y. and Pelletier, F.J. and Pineau, J.P. and Piquette, M.R. and Porter, S.B. and Protopapa, S. and Quirico, E. and Redfern, J.A. and Regiec, A.L. and Reitsema, H.J. and Reuter, D.C. and Richardson, D.C. and Riedel, J.E. and Ritterbush, M.A. and Robbins, S.J. and Rodgers, D.J. and Rogers, G.D. and Rose, D.M. and Rosendall, P.E. and Runyon, K.D. and Ryschkewitsch, M.G. and Saina, M.M. and Salinas, M.J. and Schenk, P.M. and Scherrer, J.R. and Schlei, W.R. and Schmitt, B. and Schultz, D.J. and Schurr, D.C. and Scipioni, F. and Sepan, R.L. and Shelton, R.G. and Showalter, M.R. and Simon, M. and Singer, K.N. and Stahlheber, E.W. and Stanbridge, D.R. and Stansberry, J.A. and Steffl, A.J. and Strobel, D.F. and Stothoff, M.M. and Stryk, T. and Stuart, J.R. and Summers, M.E. and Tapley, M.B. and Taylor, A. and Taylor, H.W. and Tedford, R.M. and Throop, H.B. and Turner, L.S. and Umurhan, O.M. and Van Eck, J. and Velez, D. and Versteeg, M.H. and Vincent, M.A. and Webbert, R.W. and Weidner, S.E. and Weigle, G.E. and Wendel, J.R. and White, O.L. and Whittenburg, K.E. and Williams, B.G. and Williams, K.E. and Williams, S.P. and Winters, H.L. and Zangari, A.M. and Zurbuchen, T.H. (2019) Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object. Science 364 (6441), ISSN 0036-8075.

Abstract

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. MU69 consists 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. eaaw9771INTRODUCTIONThe 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.RATIONALEThe 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.RESULTSImaging 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.CONCLUSIONBoth 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.Image of MU69 from New Horizons LORRI observations taken 1 January 2019.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.

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