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.

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU 69 ) has been largely undisturbed since its formation. We study its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through radiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/ or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, suggesting Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29 ± 5 K.The New Horizons spacecraft flew past (486958) Arrokoth at the beginning of 2019 (1). Arrokoth rotates with a 15.9 hour period about a spin axis inclined 99.3° to the pole of its 298 year orbit at a mean distance from the Sun of 44.2 AU (2, 3). Its near-circular orbit, with a mean eccentricity of 0.03 and inclination of 2.4° to the plane of the Solar System, makes it a Kuiper belt object (KBO) and more specifically, a member of the "kernel" sub-population of the cold classical KBOs (CCKBOs) (4). CCKBOs have distinct origins and properties from KBOs on more excited orbits, which are thought to have formed closer to the Sun before being perturbed outward by migrating giant planets early in Solar System history (5). CCKBOs still orbit where they formed in the protoplanetary nebula, the accretion disk of gas and dust around the young Sun. They have a high fraction of binary objects (6), a uniformly red color distribution (7, 8), a size-frequency distribution deficient of large objects (9, 10), and higher albedos (11,12). These properties arise from the environment at the outermost edge of the protoplanetary nebula, from a distinct history of subsequent evolution of CCKBOs compared to other KBOs, or of some combination of these two. Arrokoth provides a record of the process of forming planetesimals, the first generation of gravitationally bound bodies, that has been minimally altered by subsequent processes such as heating and impactor bombardment (3). Its distinctive bi-lobed, 35 km-long shape with few impact craters favors formation via rapid gravitational collapse, rather than scenarios involving more gradual accretion via piece-wise agglomeration of dust particles to assemble incrementally larger aggregates (13). We study Arrokoth's color, composition, and thermal environment using data from the New Horizons flyby, and discuss the resulting implications for its formation and subsequent evolution.

The New Horizons spacecraft observed Jupiter's icy satellites Europa and Ganymede during its flyby in February and March 2007 at visible and infrared wavelengths. Infrared spectral images map H2O ice absorption and hydrated contaminants, bolstering the case for an exogenous source of Europa's "non-ice" surface material and filling large gaps in compositional maps of Ganymede's Jupiter-facing hemisphere. Visual wavelength images of Europa extend knowledge of its global pattern of arcuate troughs and show that its surface scatters light more isotropically than other icy satellites.