The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, is composed of primitive objects preserving information about Solar System formation. In January 2019, the New Horizons spacecraft flew past one of these objects, the 36-kilometer-long contact binary (486958) Arrokoth (provisional designation 2014 MU69). Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters in diameter) within a radius of 8000 kilometers. Arrokoth has a lightly cratered, smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.

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 Origins Survey (OSSOS), a wide-field imaging program in 2013-2017 with the CanadaFrance-Hawaii Telescope, surveyed 155 deg 2 of sky to depths of m r = 24.1-25.2. We present 838 outer solar system discoveries that are entirely free of ephemeris bias. This increases the inventory of trans-Neptunian objects (TNOs) with accurately known orbits by nearly 50%. Each minor planet has 20-60 Gaia/Pan-STARRS-calibrated astrometric measurements made over 2-5 oppositions, which allows accurate classification of their orbits within the trans-Neptunian dynamical populations. The populations orbiting in mean-motion resonance with Neptune are key to understanding Neptune's early migration. Our 313 resonant TNOs, including 132 plutinos, triple the available characterized sample and include new occupancy of distant resonances out to semimajor axis a ∼ 130 au. OSSOS doubles the known population of the nonresonant Kuiper Belt, providing 436 TNOs in this region, all with exceptionally high-quality orbits of a uncertainty σ a 0.1%; they show that the belt exists from a  37 au, with a lower perihelion bound of 35au. We confirm the presence of a concentrated low-inclination a ; 44 au "kernel" population and a dynamically cold population extending beyond the 2:1 resonance. We finely quantify the survey's observational biases. Our survey simulator provides a straightforward way to impose these biases on models of the trans-Neptunian orbit distributions, allowing statistical comparison to the discoveries. The OSSOS TNOs, unprecedented in their orbital precision for the size of the sample, are ideal for testing concepts of the history of giant planet migration in the solar system.

Using data from the Deep Ecliptic Survey (DES), we investigate the inclination distributions of objects in the Kuiper Belt. We present a derivation for observational bias removal and use this procedure to generate unbiased inclination distributions for Kuiper Belt objects (KBOs) of different DES dynamical classes, with respect to the Kuiper Belt Plane. Consistent with previous results, we find that the inclination distribution for all DES KBOs is well fit by the sum of two Gaussians, or a Gaussian plus a generalized Lorentzian, multiplied by sin i. Approximately 80% of KBOs are in the high-inclination grouping. We find that Classical object inclinations are well fit by sin i multiplied by the sum of two Gaussians, with roughly even distribution between Gaussians of widths 2.0 € −0.5

We report resolved photometry of the primary and secondary components of 23 transneptunian binaries obtained with the Hubble Space Telescope. V-I colors of the components range from 0.7 to 1.5 with a median uncertainty of 0.06 magnitudes. The colors of the primaries and secondaries are correlated with a Spearman rank correlation probability of 99.99991%, 5 sigma for a normal distribution. Fits to the primary vs. secondary colors are identical to within measurement uncertainties. The color range of binaries as a group is indistinguishable from that of the larger population of apparently single transneptunian objects.Whatever mechanism produced the colors of apparently single TNOs acted equally on binary systems. The most likely explanation is that the colors of transneptunian objects and binaries alike are primordial and indicative of their origin in a locally homogeneous, globally heterogeneous protoplanetary disk. IntroductionTransneptunian objects (TNOs) have long been known to display a surprisingly diverse range of colors (Jewitt & Luu, 1998). In some cases these colors have been correlated with the wavelengths relevant to this work (V through J bands), TNOs have been found to be spectrophotometrically indistinct (i.e. they don't display strong spectroscopic features), but they differ from one another in slope (Boehnhardt et al. 2002;Peixinho et al. 2004). Attempts to model the spectrum shortward of 1000nm have generally assumed a variable mixture of red organic material and a spectrally gray material (Brunetto et al. 2008;Grundy 2008c).Unresolved colors have been obtained for about half of the more than 50 known transneptunian binaries (TNBs), but colors of resolved binary components have been reported for only 8 TNBs . For many of the resolved measurements, the uncertainties are large (Noll et al. 2002Osip 2003) or unstated (Margot et al. 2005). No clear trends are identifiable from this small sample, although in most cases the colors of the components are similar.PREPRINT 4In this work we present resolved color photometry of 23 TNBs using images from the Hubble Space Telescope (HST). We report photometry derived from both newly acquired observations and reanalyzed archival data. By using an improved point-spread-function-fitting technique we can achieve smaller uncertainties than previous work. With this new, uniformly reduced data we are able, for the first time, to definitively address the question of the colors of TNBs. As we show, these colors have implications for TNOs as a whole and their origins in the protoplanetary disk. ObservationsIn this work we report color photometry from the Wide Field Planetary Camera 2 (WFPC2) and the Advanced Camera for Surveys High Resolution Camera (HRC) from seven HST programs. The colors of 13 TNBs reported here were measured as part of program 11178 . It used the WFPC2 camera to measure TNB colors during a single epoch (normally spanning two 96-minute HST orbits, occasionally spanning three) using the 'wide V' F606W filter and the 'wide I' F814W filter. The...