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We obtained New Horizons LORRI images to measure the cosmic optical background (COB) intensity integrated over 0.4 μm ≲ λ ≲ 0.9 μm. The survey comprises 16 high-Galactic-latitude fields selected to minimize scattered diffuse Galactic light (DGL) from the Milky Way, as well as scattered light from bright stars. This work supersedes an earlier analysis based on observations of one of the present fields. Isolating the COB contribution to the raw total sky levels measured in the fields requires subtracting the remaining scattered light from bright stars and galaxies, intensity from faint stars within the fields fainter than the photometric detection limit, and the DGL foreground. DGL is estimated from 350 μm and 550 μm intensities measured by the Planck High Frequency Instrument, using a new self-calibrated indicator based on the 16 fields augmented with eight additional DGL calibration fields obtained as part of the survey. The survey yields a highly significant detection (6.8σ) of the COB at 11.16 ± 1.65 (1.47 sys, 0.75 ran) nW m−2 sr−1 at the LORRI pivot wavelength of 0.608 μm. The estimated integrated intensity from background galaxies, 8.17 ± 1.18 nW m−2 sr−1, can account for the great majority of this signal. The rest of the COB signal, 2.99 ± 2.03 (1.75 sys, 1.03 ran) nW m−2 sr−1, is formally classified as anomalous intensity but is not significantly different from zero. The simplest interpretation is that the COB is completely due to galaxies.

The NASA New Horizons Venetia Burney Student Dust Counter (SDC) measures dust particle impacts along the spacecraft’s flight path for grains with mass ≥10−12 g, mapping out their spatial density distribution. We present the latest SDC dust density, size distribution, and flux measurements through 55 au and compare them to numerical model predictions. Kuiper Belt objects (KBOs) are thought to be the dominant source of interplanetary dust particles in the outer solar system due to both collisions between KBOs and their continual bombardment by interstellar dust particles. Continued measurements through 55 au show higher than model-predicted dust fluxes as New Horizons approaches the putative outer edge of the Kuiper Belt (KB). We discuss potential explanations for the growing deviation: radiation pressure stretches the dust distribution to further heliocentric distances than its parent body distribution; icy dust grains undergo photosputtering that rapidly increases their response to radiation pressure forces and pushes them further away from the Sun; and the distribution of KBOs may extend much further than existing observations suggest. Ongoing SDC measurements at even larger heliocentric distances will continue to constrain the contributions of dust production in the KB. Continued SDC measurements remain crucial for understanding the Kuiper Belt and the interpretation of dust disks around other stars.

The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>10 4 km 3 ) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto’s interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto’s history. Giant icy volcanos (cryovolcanos) on Pluto are unique in the imaged solar system and provide evidence for unexpected, active geology late in Pluto’s history.

The large-scale dynamics of the heliosphere is driven by solar activity and variable solar wind (SW) conditions. In situ SW observations inform time-dependent heliosphere modeling efforts but are only available up until the present day, thus restricting informed predictions to the very near future. We developed and implemented a method to provide long-term forecasting of SW parameters at 1 au. Such forecasting supports modeling efforts of the time-dependent global heliosphere by providing realistic boundary conditions for heliospheric models at the upwind model-domain boundary ahead of time in order to understand the global dynamic processes in the heliosphere such as the evolution of the termination shock (TS), heliosheath, and heliopause. Such forecasting capabilities are needed to better constrain the timing of New Horizons’ TS encounter and predict energetic neutral atom flux variation for IMAP. We analyzed SW direct measurement time series for periodicities at various timescales using Lomb–Scargle periodogram analysis. Based on identified prominent periodicities, we construct quasiperiodic functions for the SW parameters. By extrapolating these functions forward in time, we obtain a prediction of the SW evolution over the next two solar cycles. Our results indicate that the next pulse in the SW dynamic pressure, which controls the global heliosphere, will occur around 2035.

This study provides a detailed analysis of five distant interplanetary shocks observed by the Solar Wind Around Pluto instrument on board New Horizons, which exhibit the signature of a suprathermal H+ pickup ion (PUI) tail in the downstream distribution. These shocks were observed with a PUI data cadence of approximately 24 hr, covering a heliocentric distance range of 23.71–36.75 au. The shock compression ratio varies between approximately 1.4 and 3.2. The H+ PUI density and temperature show a gradual increase across the shock, while the H+ solar wind density shows erratic behavior without a distinct downstream compression. The H+ PUI cooling index variation across the shock displays different characteristics in each shock. This study demonstrates, for the first time, the variation of the number density of downstream H+ PUI tails with the shock compression ratio, revealing an increase in tail density with stronger shocks. Additionally, theoretical estimates of reflected PUI number densities derived from the electrostatic cross-shock potential agree very well with the observed H+ PUI tail densities for stronger shocks.

We present new observations of the cosmic ultraviolet background (CUVB) at high Galactic latitudes (∣b∣ > 40∘), made using the Alice UV spectrograph on board the New Horizons spacecraft. These observations were taken at about 57 au from the Sun, outside much of the foreground emission affecting previous missions, and allowed a new determination of the spectrum of the CUVB between 912–1100 Å and 1400–1800 Å. We found a linear correlation between the CUVB and the Planck E(B − V) with offsets at zero-reddening of 221 ± 11 photon units at 1000 Å and 264 ± 24 photon units at 1500 Å (4.4 ± 0.2 nW m−2 sr−1 at 1000 Å and 5.3 ± 0.5 nW m−2 sr−1 at 1500 Å). The former is the first firm detection of the offset in the range 912–1100 Å while the latter result confirms previous results from the Galaxy Evolution Explorer, showing that there is little emission from the solar system from 1400 to 1800 Å. About half of the offset may be explained by known sources (the integrated light of unresolved galaxies, unresolved stars, emission from ionized gas, and two-photon emission from warm hydrogen in the halo) with the source of the remaining emission as yet unidentified. There is no detectable emission below the Lyman limit with an upper limit of 3.2 ± 3.0 photon units.

Enceladus' cratered terrains contain large numbers of unusually shallow craters consistent with deformation by viscous relaxation of water ice under conditions of elevated heat flow. Here we use high‐resolution topography to measure the relaxation fraction of craters on Enceladus far from the active South Pole. We find that many craters are shallower than expected, with craters as small as 2 km in diameter having relaxation fractions in excess of 90%. These measurements are compared with numerical simulations of crater relaxation to constrain the minimum heat flux required to reproduce these observations. We find that Enceladus' nominal cold surface temperature (70 K) and low surface gravity strongly inhibit viscous relaxation. Under such conditions less than 3% relaxation occurs over 2 Ga even for relatively large craters (diameter 24 km) and high, constant heat fluxes (150 mW m−2). Greater viscous relaxation occurs if the effective temperature at the top of the lithosphere is greater than the surface temperature due to insulating regolith and/or plume material. Even for an effective temperature of 120 K, however, heat fluxes in excess of 150 mW m−2are required to produce the degree of relaxation observed. Simulations of viscous relaxation of Enceladus' largest craters suggest that relaxation is best explained by a relatively short‐lived period of intense heating that decayed quickly. We show that infilling of craters by plume material cannot explain the extremely shallow craters at equatorial and higher northern latitudes. Thus, like Enceladus' tectonic terrains, the cratered regions of Enceladus have experienced periods of extreme heat flux. Key Points Many craters (some ~2‐km in diameter) on Enceladus are highly viscously relaxed The degree of relaxation observed requires extremely high heat fluxes Infilling by plume material cannot account for Enceladus' shallow craters

The outer heliosphere is profoundly influenced by nonthermal energetic pickup ions (PUIs), which dominate the internal pressure of the solar wind beyond ~10 au, surpassing both solar wind and magnetic pressures. PUIs are formed mostly through charge exchange between interstellar neutral atoms and solar wind ions. This study examines the apparent heating of PUIs in the distant supersonic solar wind before reaching the heliospheric termination shock. New Horizons’ SWAP observations reveal an unexpected PUI temperature change between 2015 and 2020, with a notable bump in PUI temperature. Concurrent observations from the ACE and Wind spacecraft at 1 au indicate a ~50% increase in solar wind dynamic pressure at the end of 2014. Our simulation suggests that the bump observed in the PUI temperature by New Horizons is largely associated with the enhanced solar wind dynamic pressure observed at 1 au. Additional PUI temperature enhancements imply the involvement of other heating mechanisms. Analysis of New Horizons data reveals a correlation between shocks and PUI heating during the declining phase of the solar cycle. Using a PUI-mediated plasma model, we explore shock structures and PUI heating, finding that shocks preferentially heat PUIs over the thermal solar wind in the outer heliosphere. We also show that the broad shock thickness observed by New Horizons is due to the large diffusion coefficient associated with PUIs. Shocks and compression regions in the distant supersonic solar wind lead to elevated PUI temperatures and thus they can increase the production of energetic neutral atoms with large energy.

We analyze ∼40–200 keV energetic particle observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on board the New Horizons (NH) spacecraft from 2007 to 2024 and compare them with similar measurements from the Low Energy Charged Particle experiments on the Voyager 1 and Voyager 2 spacecraft when they were at comparable heliocentric distances (5–60 au). In both Voyager data sets, particle intensities generally decrease with increasing radial distance from the Sun, reaching a minimum in the outer heliosphere before rising again prior to their respective encounters with the heliospheric termination shock (TS). This radial behavior in the intensity-time profiles is described as a heliospheric valley. The NH/PEPSSI time series from 5 au (2007) to 60 au (2024) exhibits a similar decrease in particle fluxes with distance. Analysis of the radial dependence of energetic particle intensities from all three spacecraft normalized by observations at 1 au to account for the solar cycle effects reveals an approximate piece-wise composite power-law relationship, with a slope break (steeper decline) beyond ∼33 au. This break may reflect differences in the dominant transport and acceleration mechanisms operating in the two regions demarcated by this radial break. In addition, a radial scaling method is applied to Voyager observations to best match the NH data. This comparison provides an estimate for the NH TS crossing between 2027 (68 au) and 2034 (83 au).

We present New Horizons Alice spectral observations of interstellar hydrogen Lyα emissions at a vantage point of 57 au from the Sun. The observations were conducted as a pair of orthogonal scans to produce high spatial angular resolution measurements of a dark patch of sky located at high Galactic latitudes. We find that the brightness of the UV background remains relatively constant over the selected sky region, with the exception of areas near a few bright stars and galaxies. Most stars do not affect the brightness of Lyα in this region; however, we detect a 25 Rayleigh Lyα enhancement near the location of hot subdwarf CD-38 222. This enhancement is consistent with nebular emission produced by ionizing photons from a bow shock as the object interacts with a high-latitude interstellar gas cloud. We do not find correlations of Lyα or background UV brightness with dust structures seen by the Planck space observatory. Our analysis at higher angular resolution and with the full Alice bandpass of 520–1870 Å confirms the results of the global Lyα maps produced by G. R. Gladstone et al. at lower spatial resolution. The near-constant Lyα is consistent with their interpretation that scattering of H within our Local Bubble produces a nearly isotropic Lyα distribution. Future Alice global scans in photometer mode can be used to assess changes in the Lyα sky as a function of solar distance. High-resolution FUV Lyα scans with the full spatial and spectral information should be repeated over the entire sky to resolve additional small-scale structures.