Joel William Parker

The PEPSSI instrument onboard New Horizons is sampling the energy range that bridges pickup and suprathermal ions for the first time in the outer heliosphere. New Horizons repeatedly encountered structures beyond 30 au that we interpret as the spiral of a corotating interaction region (CIR) for an extended period from 2015 to 2016. Ion intensities abruptly increase at each CIR crossing, followed by a gradual, energy‐dependent decrease until the next passage. PEPSSI also has been able to constrain the instantaneous pitch angle distribution of pickup ions, suggesting the intensities in the hemispheres parallel and anti‐parallel to the assumed field to differ by an order of magnitude. Such observations are traditionally explained through transport equations involving diffusion coefficients, which are difficult to measure directly. Here, we are using an entirely different approach and make the Ansatz that after acceleration at the CIR ions are propagating deterministically without critical reliance on diffusive processes. We develop a comprehensive theoretical framework and find that it yields promising results. The new framework is indeed able to fit examples of the observed spectral evolution, while also naturally maintaining the observed hemispheric asymmetry.

The main belt asteroid (52246) Donaldjohanson (DJ) is a likely member of the Erigone asteroid family. This implies that DJ is a fragment of a larger parent body that was destroyed in a collision about 155 million years ago. We report observations taken during a flyby of DJ by the Lucy spacecraft. We found that DJ is composed of two heavily cratered lobes, connected by a smoother neck, with overall dimensions 8.8 kilometers (km) by 4.4 km by 3.1 km. The crater density is consistent with the Erigone family’s age, except for craters <0.4 km, which have been preferentially erased. DJ rotates slowly in a tumbling state, likely owing to spin-down by radiative forces. Surface spectra show iron-bearing phyllosilicates, indicating moderate aqueous evolution on the parent body.

The solar wind gradually slows with increasing distance from the Sun as interstellar material becomes ionized and picked up by the solar wind. When observing the solar wind in the outer heliosphere, it is difficult to differentiate temporal decreases in the speed originating at the Sun from the slowing of the solar wind with increase distance as interstellar material is picked up. In order to resolve this issue, we examine the solar wind speed in the outer heliosphere relative to the corresponding solar wind speed observed at 1 au. Additionally, we use a simulation to help discern the temporal and spatial changes in speed at New Horizons (NH) by driving the inner boundary of a heliospheric simulation with 1 au speed measurements. We conclude that the long interval of slow solar wind from 2018–2023 found at NH was caused by a combination of slow wind being directed toward NH owing to a low current sheet tilt angle keeping the streamers in the ecliptic, and the gradual slowing with increasing distance caused by additional pickup of interstellar material as the solar wind travels farther way from the Sun. When NH was between 50 and 58.5 au, the measured solar wind speed was reduced by ∼13%–15% relative to 1 au owing to pickup of interstellar material. This reduction is a further decrease from the previously reported slowing of 5%–7% in the 30–43 au region.

We present New Horizons Alice spectral observations of interstellar hydrogen Ly α 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 Ly α in this region; however, we detect a 25 Rayleigh Ly α 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 Ly α 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 Ly α maps produced by G. R. Gladstone et al. at lower spatial resolution. The near-constant Ly α is consistent with their interpretation that scattering of H within our Local Bubble produces a nearly isotropic Ly α distribution. Future Alice global scans in photometer mode can be used to assess changes in the Ly α sky as a function of solar distance. High-resolution FUV Ly α scans with the full spatial and spectral information should be repeated over the entire sky to resolve additional small-scale structures.

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).

Currently ∼62 au from the Sun, the New Horizons spacecraft is en route to the outer heliosphere boundaries. The first boundary it will encounter is the heliospheric termination shock (HTS), where the solar wind ion (SWI) and interstellar pickup ion (PUI) plasma mixture is slowed down to subsonic speeds, compressed, and heated. Some particles, mostly PUIs, undergo preferential acceleration at the HTS due to their higher energies and thus gain the capability to reflect from the shock and undergo, e.g., shock drift acceleration. This produces a tail in the downstream PUI energy distribution, with the potential for multiple power-law breaks. In anticipation of crossing the HTS, we have constructed a test particle model with synthetic turbulence to simulate New Horizons’ Solar Wind Around Pluto (SWAP) observations downstream of the HTS. SWAP can measure the energy/charge of multiple particle populations (thermal solar wind (SW) protons, alphas, and PUIs). Here, we calculate what SWAP might observe after it crosses the HTS. Our model shows that the count rate distribution will be very different from what is observed in the supersonic SW, with a hotter SW+PUI distribution and no sharp PUI cutoff. This will require a different method to quantify the moments of the SWIs and PUIs in the heliosheath. SWAP may be able to observe part of the PUI tail downstream of the HTS, if the upstream SW speed is ≲ 300 km s⁻¹. Moreover, if the shock compression ratio is also ≳ 2, SWAP may observe multiple populations of the accelerated PUI distribution.

As NASA’s New Horizons spacecraft exits the solar system bound for interstellar space, it has traveled so far that the nearest stars have shifted markedly from their positions seen from Earth. We demonstrated this by imaging the Proxima Centauri and Wolf 359 fields from Earth and New Horizons on 2020 April 23, when the spacecraft was 47.1 au distant. The observed parallaxes for Proxima Centauri and Wolf 359 are 32 . ″ 4 and 15 . ″ 7 , respectively. These measurements are not of research grade, but directly seeing large stellar parallaxes between two widely separated simultaneous observers is vividly educational. Using the New Horizons positions of the two stars alone, referenced to the three-dimensional model (3D) of the solar neighborhood constructed from Gaia DR3 astrometry, further provides the spacecraft spatial position relative to nearby stars with 0.44 au accuracy. The range to New Horizons from the solar system barycenter is recovered to 0.27 au accuracy, and its angular direction to 0 . ° 4 accuracy, when compared to the precise values from NASA Deep Space Network tracking. This is the first time optical stellar astrometry has been used to determine the 3D location of a spacecraft with respect to nearby stars and the first time any method of interstellar navigation has been demonstrated for a spacecraft on an interstellar trajectory. We conclude that the best astrometric approach to navigating spacecraft on their departures to interstellar space is to use a single pair of the closest stars as references, rather than a large sample of more distant stars.

New Horizons’ Solar Wind Around Pluto (SWAP) instrument has observed interplanetary shocks from ∼20 to 60 au from the Sun. Different studies have analyzed the effects of these shocks on the solar wind ion (SWI) and interstellar pickup ion (PUI) distributions. A key finding includes the observation of PUI tails downstream of some shocks. However, PUI tails have only been observed when SWAP was measuring 24 hr averages of the count rate histograms. After a software change in 2021 February, SWAP’s measurement resolution was improved to ∼30 minutes averages, but no PUI tails have been observed since this change. In this study, we use a test particle code to simulate the downstream SWI and PUI distributions to study the possible existence and formation of PUI tails, using the shock compression ratio, shock thickness, and turbulence as free parameters. We find that in most cases, a suprathermal tail does not develop downstream of interplanetary shocks, but a persistent hump and PUI cutoff exists at energies above the PUI injection energy. The extent of the hump largely depends on the shock compression ratio. We find PUI tails do form when turbulence is included in the model. After we convert our distributions to count rates in the spacecraft frame, the results show similar structure to the data. We conclude that significant H⁺ PUI tails can form downstream of distant interplanetary shocks if there is a relatively large compression ratio (>2) and significant turbulence present.