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We present flux measurements of Uranus observed at phase angles of 43.9{\deg}, 44.0{\deg}, and 52.4{\deg} by the Multispectral Visible Imaging Camera (MVIC) on the New Horizons spacecraft during 2023, 2010, and 2019, respectively. New Horizons imaged Uranus at a distance of about 24-70 AU (2023) in four color filters, with bandpasses of 400-550 nm, 540-700 nm, 780-975 nm, and 860-910 nm. High-phase-angle observations are of interest for studying the energy balance of Uranus, constraining the atmospheric scattering behavior, and understanding the planet as an analog for ice giant exoplanets. The new observations from New Horizons provide access to a wider wavelength range and different season compared to previous observations from both Voyager spacecraft. We performed aperture photometry on the New Horizons observations of Uranus to obtain its brightness in each photometric band. The photometry suggests that Uranus may be darker than predicted by a Lambertian phase curve in the Blue and Red filters. Comparison to simultaneous low-phase Hubble WFC3 and ground-based community-led observations indicates a lack of large-scale features at full-phase that would introduce variation in the rotational light curve. The New Horizons reflectance in the Blue (492 nm) and Red (624 nm) filters does not exhibit statistically significant variation and is consistent with the expected error bars. These results place new constraints on the atmospheric model of Uranus and its reflectivity. The observations are analogous to those from future exoplanet direct-imaging missions, which will capture unresolved images of exoplanets at partial phases. These results will serve as a "ground-truth" with which to interpret exo-ice giant data., Comment: Accepted for publication in the Planetary Science Journal, 11 pages, 6 figures

Using the New Horizons LORRI camera, we searched for satellites near five Kuiper belt objects (KBOs): four cold classicals (CCs: 2011 JY31, 2014 OS393, 2014 PN70, 2011 HZ102) and one scattered disk object (SD: 2011 HK103). These objects were observed at distances of 0.092-0.290 au from the New Horizons spacecraft, achieving spatial resolutions of 136-430 km (resolution is ~2 camera pixels), much higher than possible from any other facilities. Here we report that CC 2011 JY31 is a binary system with roughly equal brightness components, CC 2014 OS393 is likely an equal brightness binary system, while the three other KBOs did not show any evidence of binarity. The 2011 JY31 binary has a semi-major axis of 198.6 +/- 2.9 km, an orbital inclination of 61.34 +/- 1.34 deg, and an orbital period of 1.940 +/- 0.002 d. The 2014 OS393 binary objects have an apparent separation of ~150 km, making 2011 JY31 and 2014 OS393 the tightest KBO binary systems ever resolved. Both 2011 HK103 and 2011 HZ102 were detected with SNR~10, and our observations rule out equal brightness binaries with separations larger than ~430 km and ~260 km, respectively. The spatial resolution for 2014 PN70 was ~200 km, but this object had SNR~2.5-3, which limited our ability to probe its binarity. The binary frequency for the CC binaries probed in our small survey (67%, not including 2014 PN70) is consistent with the high binary frequency suggested by larger surveys of CCs (Fraser et al. 2017, Noll et al. 2020) and recent planetesimal formation models (Nesvorny et al. 2021), but we extend the results to smaller orbit semi-major axes and smaller objects than previously possible., Comment: 30 pages, 15 figures

Sputnik Planitia, Pluto's gigantic ice glacier, hosts numerous scientific mysteries, including the presence of thousands of elongated pit structures. We examine various attributes of these pit structures in New Horizons data sets, revealing their length, aspect ratios, and orientation properties; we also study their interior reflectivities, colors, and compositions, and compare these attributes to some other relevant regions on Pluto. We then comment on origin mechanisms of the pits and also the fate of the missing volatiles represented by the pits on Sputnik Planitia., Comment: 36 pages, 14 figures

We present the first measurements of Charon's far-ultraviolet surface reflectance, obtained by the Alice spectrograph on New Horizons. We find no measurable flux shortward of 1650 A, and Charon's geometric albedo is $<0.019$ ($3\sigma$) at 1600 A. From 1650--1725 A Charon's geometric albedo increases to $0.166\pm0.068$, and remains nearly constant until 1850 A. As this spectral shape is characteristic of H$_2$O ice absorption, Charon is the first Kuiper belt object with a H$_2$O ice surface to be detected in the far-ultraviolet. Charon's geometric albedo is $\sim3.7$ times lower than Enceladus' at these wavelengths, but has a very similar spectral shape. We attribute this to similarities in their surface compositions, and the difference in absolute reflectivity to a high concentration or more-absorbing contaminants on Charon's surface. Finally, we find that Charon has different solar phase behavior in the FUV than Enceladus, Mimas, Tethys, and Dione, with a stronger opposition surge than Enceladus and a shallower decline at intermediate solar phase angles than any of these Saturnian satellites., Comment: Accepted for publication in the Planetary Science Journal; 9 pages, 7 figures, 2 tables

One of the most striking and curious features of the small Kuiper Belt Object (KB), Arrokoth, explored by New Horizons, is the bright, annular neck it exhibits at the junction between its two lobes. Here we summarize past reported findings regarding the properties of this feature and report new findings regarding its dimensions, reflectivity and color, shape profile, and its lack of identifiable craters. We conclude by enumerating possible origin scenarios for this unusual feature. New results include a new measurement of the observed neck area of 8+/-1.5 km2, a total neck surface area of 32 km2, a 12.5:1 ratio of neck circumference to height, a normal reflectance histogram of the observed neck, and the fact that no significant (i.e., >2 sigma) color units were identified, meaning the neck's color is generally spatially uniform at the 1.5 km/pixel scale of the best color images. Although several origin hypotheses for the bright material in the neck are briefly discussed, none can be conclusively demonstrated to be the actual origin mechanism at this time; some future tests are identified., Comment: 19 Pages 08 Figures 01 Tables

In 2015, the New Horizons spacecraft flew past Pluto and its moon Charon, providing the first clear look at the surface of Charon. New Horizons images revealed an ancient surface, a large, intricate canyon system, and many fractures, among other geologic features. Here, we assess whether tidal stresses played a significant role in the formation of tensile fractures on Charon. Although presently in a circular orbit, most scenarios for the orbital evolution of Charon include an eccentric orbit for some period of time and possibly an internal ocean. Past work has shown that these conditions could have generated stresses comparable in magnitude to other tidally fractured moons, such as Europa and Enceladus. However, we find no correlation between observed fracture orientations and those predicted to form due to eccentricity-driven tidal stress. It thus seems more likely that the orbit of Charon circularized before its ocean froze, and that either tidal stresses alone were insufficient to fracture the surface or subsequent resurfacing remove these ancient fractures., Comment: 12 pages, 5 figures, 2 tables

The New Horizons mission has returned stunning images of the bilobate Kuiper belt object (486958) Arrokoth. It is a contact binary, formed from two intact and relatively undisturbed predecessor objects joined by a narrow contact region. We use a version of pkdgrav, an N-body code that allows for soft-sphere collisions between particles, to model a variety of possible merger scenarios with the aim of constraining how Arrokoth may have evolved from two Kuiper belt objects into its current contact binary configuration. We find that the impact must have been quite slow (less than 5 m/s) and grazing (impact angles greater than 75 degrees) in order to leave intact lobes after the merger, in the case that both progenitor objects were rubble piles. A gentle contact between two bodies in a close synchronous orbit seems most plausible., Comment: 34 pages, 9 figures

The New Horizons spacecraft's encounter with the cold classical Kuiper belt object (486958) Arrokoth (formerly 2014 MU69) revealed a contact-binary planetesimal. We investigate how it formed, finding it is the product of a gentle, low-speed merger in the early Solar System. Its two lenticular lobes suggest low-velocity accumulation of numerous smaller planetesimals within a gravitationally collapsing, solid particle cloud. The geometric alignment of the lobes indicates the lobes were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly due to dynamical friction and collisions within the cloud or later gas drag. Arrokoth's contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper belt, and so informs the accretion processes that operated in the early Solar System., Comment: Published in Science 28 Feb 2020 (First release 13 Feb 2020)

Pluto's surface is geologically complex because of volatile ices that are mobile on seasonal and longer time scales. Here we analyzed New Horizons LEISA spectral data to globally map the nitrogen ice, including nitrogen with methane diluted in it. Our goal was to learn about the seasonal processes influencing ice redistribution, to calculate the globally averaged energy balance, and to place a lower limit on Pluto's N2 inventory. We present the average latitudinal distribution of nitrogen and investigate the relationship between its distribution and topography on Pluto by using maps that include the shifted bands of methane in solid solution with nitrogen to more completely map the distribution of the nitrogen ice. We find that the global average bolometric albedo is 0.83 +\- 0.11, similar to that inferred for Triton, and that a significant fraction of Pluto's N2 is stored in Sputnik Planitia. Under the assumption that Pluto's nitrogen-dominated 11.5 microbar atmosphere is in vapor pressure equilibrium with the nitrogen ice, the ice temperature is 36.93 +/- 0.10 K, as measured by New Horizons. Combined with our global energy balance calculation, this implies that the average bolometric emissivity of Pluto's nitrogen ice is probably in the range 0.47 - 0.72. This is consistent with the low emissivities estimated for Triton based on Voyager, and may have implications for Pluto's atmospheric seasonal variations, as discussed below. The global pattern of volatile transport at the time of the encounter was from north to south, and the transition between condensation and sublimation within Sputnik Planitia is correlated with changes in the grain size and CH4 concentration derived from the spectral maps. The low emissivity of Pluto's N2 ice suggests that Pluto's atmosphere may undergo an extended period of constant pressure even as Pluto recedes from the Sun in its orbit.

The New Horizons spacecraft provided near global observations of Pluto that far exceed the resolution of Earth-based data sets. Most Pluto New Horizons analysis hitherto has focused on the encounter hemisphere of Pluto (i.e., the antiCharon hemisphere containing Sputnik Planitia). In this work, we summarize and interpret data on the far side (i.e., the non-encounter hemisphere), providing the first integrated New Horizons overview of the far side terrains. We find strong evidence for widespread bladed deposits, evidence for an impact crater about as large as any on the near side hemisphere, evidence for complex lineations approximately antipodal to Sputnik Planitia that may be causally related, and evidence that the far side maculae are smaller and more structured than the encounter hemisphere maculae., Comment: 45 pages 12 figures

Several trace constituents can reach saturation vapor pressure in Pluto's present day atmosphere. As such, we describe a search for discrete could features in Pluto's atmosphere using New Horizons data obtained on 14-15 July 2015, during the Pluto flyby closest approach. We report that Pluto's present day atmosphere is at least largely (>99% by surface area) free of discrete clouds. We also report a handful of features that may plausibly be clouds, all of which were detected obliquely and at high phase observing geometry. No cloud candidates were identified away from the terminators or in low phase (backscattering geometry) images., Comment: 18 pages, 7 figures, 3 tables, The Astronomical Journal, 2017, in press

A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed to organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location of this material on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons. The escape of Pluto's atmosphere provides a potential feed stock for production of complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation on the basis of an image of Charon's northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase showing the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon, the supply and temporary cold-trapping of material escaping from Pluto, and, while cold-trapped, its photolytic processing into more complex and less volatile molecules. The model results are consistent with the proposed mechanism producing the observed colour pattern on Charon.

The centerpiece objective of the NASA New Horizons first Kuiper Extended Mission (KEM-1) was the close flyby of the Kuiper Belt Object KBO) 2014 MU69, nicknamed Ultima Thule. On 1 Jan 2019 this flyby culminated, making the first close observations of a small KBO. Initial post flyby trajectory reconstruction indicated the spacecraft approached to within ~3500 km of MU69 at 5:33:19 UT. Here we summarize the earliest results obtained from that successful flyby. At the time of this submission, only 4 days of data down-link from the flyby were available; well over an order of magnitude more data will be down-linked by the time of this Lunar and Planetary Science Conference presentation in 2019 March. Therefore many additional results not available at the time of this abstract submission will be presented in this review talk., Comment: 2 pages, 2 figures

Recent observations of interplanetary medium (IPM) atomic hydrogen Lyman-{\alpha} (Ly{\alpha}) emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons (NH), are presented. The observations include regularly spaced great-circle scans of the sky and pointed observations near the downstream and upstream flow directions of interstellar H atoms. The NH Alice data agree very well with the much earlier Voyager UVS results, after these are reduced by a factor of 2.4 in brightness, in accordance with recent re-analyses. In particular, the falloff of IPM Ly{\alpha} brightness in the upstream-looking direction as a function of spacecraft distance from the Sun is well-matched by an expected 1/r dependence, but with an added constant brightness of ~40 Rayleighs. This additional brightness is a possible signature of the hydrogen wall at the heliopause or of a more distant background. Ongoing observations are planned at a cadence of roughly twice per year., Comment: 21 pages, 5 figures, 1 table, accepted for publication in Geophys. Res. Lett

The central objective of the New Horizons prime mission was to make the first exploration of Pluto and its system of moons. Following that, New Horizons has been approved for its first extended mission, which has the objectives of extensively studying the Kuiper Belt environment, observing numerous Kuiper Belt Objects (KBOs) and Centaurs in unique ways, and making the first close flyby of the KBO 486958 2014 MU69. This review summarizes the objectives and plans for this approved mission extension, and briefly looks forward to potential objectives for subsequent extended missions by New Horizons.

Pluto's landscape is shaped by the cycles of the volatile ices covering its surface. In particular, the Sputnik Planitia (SP) ice sheet displays a large diversity of terrains, with bright and dark plains, pits, topographic depressions and evidences of recent and past glacial flows. Outside SP, New Horizons also revealed numerous N2 ice deposits, in Tombaugh Regio and at mid-northern latitudes. These observations suggest a complex history involving volatile and glacial processes on different timescales. We present numerical simulations of volatile transport on Pluto performed with a model able to simulate the N2 cycle over millions of years (Myrs), taking into account the changes of obliquity and orbital parameters as experienced by Pluto. Results show that over one obliquity cycle, the latitudes of SP between 25{\deg}S-30{\deg}N are dominated by N2 condensation, while the latitudes between 30-50{\deg}N are dominated by N2 sublimation. We find that a net amount of 1 km of ice has sublimed at the northern edge of SP during the last 2 Myrs. By comparing these results with the observed geology of SP, we can relate the formation of the pits and the brightness of the ice to the ice flux occurring at the annual timescale, while the glacial flows at its eastern edge and the erosion of the water ice mountains all around the ice sheet are related to the astronomical timescale. We also perform simulations with a glacial flow scheme which shows that SP is currently at its minimum extent. We also explore the stability of N2 ice outside SP. Results show that it is not stable at the poles but rather in the equatorial regions, in particular in depressions, where thick deposits may persist over tens of Myrs, before being trapped in SP. Finally, another key result is that the minimum and maximum surface pressures obtained over the simulated Myrs remain in the range of mm-Pa and Pa, respectively., Comment: 49 pages, 16 figures

A search for temporal changes on Pluto and Charon was motivated by (1) the discovery of young surfaces in the Pluto system that imply ongoing or recent geologic activity, (2) the detection of active plumes on Triton during the Voyager 2 flyby, and (3) the abundant and detailed information that observing geologic processes in action provides about the processes. A thorough search for temporal changes using New Horizons images was completed. Images that covered the same region were blinked and manually inspected for any differences in appearance. The search included full-disk images such that all illuminated regions of both bodies were investigated and higher resolution images such that parts of the encounter hemispheres were investigated at finer spatial scales. Changes of appearance between different images were observed but in all cases were attributed to variability of the imaging parameters (especially geometry) or artifacts. No differences of appearance that are strongly indicative of a temporal change were found on the surface or in the atmosphere of either Pluto or Charon. Limits on temporal changes as a function of spatial scale and temporal interval during the New Horizons encounter are determined. The longest time interval constraint is one Pluto/Charon rotation period (~6.4 Earth days). Contrast reversal and high-phase bright features that change in appearance with solar phase angle are identified. The change of appearance of these features is most likely due to the change in phase angle rather than a temporal change. Had active plumes analogous to the plumes discovered on Triton been present on the encounter hemispheres of either Pluto or Charon, they would have been detected. The absence of active plumes may be due to temporal variability (i.e., plumes do occur but none were active on the encounter hemispheres during the epoch of the New Horizons encounter ..., Comment: Accepted for publication in Icarus

We searched for dust or debris rings in the Pluto-Charon system before, during, and after the New Horizons encounter. Methodologies included searching for back-scattered light during the approach to Pluto (phase $\sim15^\circ$), in situ detection of impacting particles, a search for stellar occultations near the time of closest approach, and by forward-scattered light during departure (phase $\sim165^\circ$). A search using HST prior to the encounter also contributed to the results. No rings, debris, or dust features were observed, but our detection limits provide an improved picture of the environment throughout the Pluto-Charon system. Searches for rings in back-scattered light covered 35,000-250,000 km from the system barycenter, a zone that starts interior to the orbit of Styx, and extends to four times the orbital radius of Hydra. We obtained our firmest limits using the NH LORRI camera in the inner half of this region. Our limits on the normal $I/F$ of an unseen ring depends on the radial scale of the rings: $2\times10^{-8}$ ($3\sigma$) for 1500 km wide rings, $1\times10^{-8}$ for 6000 km rings, and $7\times10^{-9}$ for 12,000 km rings. Beyond $\sim100,000$ km from Pluto, HST observations limit normal $I/F$ to $\sim8\times10^{-8}$. Searches for dust from forward-scattered light extended from the surface of Pluto to the Pluto-Charon Hill sphere ($r_{\rm Hill}=6.4\times10^6$ km). No evidence for rings or dust was detected to normal $I/F$ limits of $\sim8.9\times10^{-7}$ on $\sim10^4$ km scales. Four occulation observations also probed the space interior to Hydra, but again no dust or debris was detected. Elsewhere in the solar system, small moons commonly share their orbits with faint dust rings. Our results suggest that small grains are quickly lost from the system due to solar radiation pressure, whereas larger particles are unstable due to perturbations by the known moons., Comment: Submitted to Icarus, 38 pages, 24 figures

We report on a variety of standard techniques used by New Horizons including a solar ultraviolet occultation, ultraviolet airglow observations, and high-phase look-back particulate search imaging to search for an atmosphere around Pluto's large moon Charon during its flyby in July 2015. Analyzing these datasets, no evidence for a present day atmosphere has been found for 14 potential atomic and molecular species, all of which are now constrained to have pressures below 0.3 nanobar, as we describe below, these are much more stringent upper limits than the previously available 15-110 nanobar constraints (e.g., Sicardy et al. 2006); for example, we find a 3$\sigma$ upper limit for an N$_2$ atmosphere on Charon is 4.2 picobars and a 3$\sigma$ upper limit for the brightness of any atmospheric haze on Charon of I/F=2.6x10$^{-5}$. A radio occultation search for an atmosphere around Charon was also conducted by New Horizons but will be published separately by other authors., Comment: 17 pages, 7 figures. Submitted to Icarus, in revision

NASA's New Horizons spacecraft observed (15810) 1994 JR$_1$, a 3:2 resonant Kupier Belt Object (KBO), using the LOng Range Reconnaissance Imager (LORRI) on November 2, 2015 from a distance of 1.85 AU, and again on April 7, 2016 from a distance of 0.71 AU. These were the first close observations of any KBO other than Pluto. Combining ground-based and Hubble Space Telecope (HST) observations at small phase angles and the LORRI observations at higher phase angles, we produced the first disk-integrated solar phase curve of a typical KBO from $\alpha$=0.6-58$^\circ$. Observations at these geometries, attainable only from a spacecraft in the outer Solar System, constrain surface properties such as macroscopic roughness and the single particle phase function. 1994 JR$_1$ has a rough surface with a 37$\pm$5$^\circ$ mean topographic slope angle and has a relatively rapid rotation period of 5.47$\pm$0.33 hours. 1994 JR$_1$ is currently 2.7 AU from Pluto; our astrometric points enable high-precision orbit determination and integrations which show that it comes this close to Pluto every 2.4 million years (10$^4$ heliocentric orbits), causing Pluto to perturb 1994 JR$_1$. During the November spacecraft observation, the KBO was simultaneously observed using HST in two colors, confirming its very red spectral slope. These observations have laid the groundwork for numerous potential future distant KBO observations in the New Horizons-Kuiper Belt Extended Mission., Comment: 7 pages, published in ApJL

On July 14th 2015, NASA's New Horizons mission gave us an unprecedented detailed view of the Pluto system. The complex compositional diversity of Pluto's encounter hemisphere was revealed by the Ralph/LEISA infrared spectrometer on board of New Horizons. We present compositional maps of Pluto defining the spatial distribution of the abundance and textural properties of the volatiles methane and nitrogen ices and non-volatiles water ice and tholin. These results are obtained by applying a pixel-by-pixel Hapke radiative transfer model to the LEISA scans. Our analysis focuses mainly on the large scale latitudinal variations of methane and nitrogen ices and aims at setting observational constraints to volatile transport models. Specifically, we find three latitudinal bands: the first, enriched in methane, extends from the pole to 55deg N, the second dominated by nitrogen, continues south to 35deg N, and the third, composed again mainly of methane, reaches 20deg N. We demonstrate that the distribution of volatiles across these surface units can be explained by differences in insolation over the past few decades. The latitudinal pattern is broken by Sputnik Planitia, a large reservoir of volatiles, with nitrogen playing the most important role. The physical properties of methane and nitrogen in this region are suggestive of the presence of a cold trap or possible volatile stratification. Furthermore our modeling results point to a possible sublimation transport of nitrogen from the northwest edge of Sputnik Planitia toward the south., Comment: 43 pages, 7 figures; accepted for publication in Icarus

The New Horizons spacecraft mapped colors and infrared spectra across the encounter hemispheres of Pluto and Charon. The volatile ices CH$_4$, CO, and N$_2$, that dominate Pluto's surface, have complicated spatial distributions resulting from sublimation, condensation, and glacial flow acting over seasonal and geological timescales. Pluto's H$_2$O ice "bedrock" is also mapped, with isolated outcrops occurring in a variety of settings. Pluto's surface exhibits complex regional color diversity associated with its distinct provinces. Charon's color pattern is simpler, dominated by neutral low latitudes and a reddish northern polar region. Charon near infrared spectra reveal highly localized areas with strong NH$_3$ absorption tied to small craters with relatively fresh-appearing impact ejecta., Comment: in Science 351, aad9189 (2016)

Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N$_2$) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH$_4$), acetylene (C$_2$H$_2$), ethylene (C$_2$H$_4$), and ethane (C$_2$H$_6$) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state--over seasonal or geologic time scales., Comment: in Science 351, aad8866 (2016)

Approach images taken by the LORRI imaging system during the New Horizons spacecraft encounter have been used to determine the mean radii and shapes of Pluto and Charon. The primary observations are limb locations derived using three independent approaches. The resulting mean radii of Pluto and Charon are 1188.3 +/- 1.6 km and 606.0 +/- 1.0 km, respectively (2-sigma). The corresponding densities are 1854 +/- 11 kg m-3 and 1701 +/- 33 kg m-3 (2-sigma). The Charon radius value is consistent with previous Earth-based occultation estimates. The Pluto radius estimate is consistent with solar occultation measurements performed by the ALICE and Fine Sun Sensor instruments on New Horizons. Neither Pluto nor Charon show any evidence for tidal/rotational distortions; upper bounds on the oblateness are <0.6% and <0.5%, respectively., Comment: revised version submitted to Icarus (Pluto special issue)