arXiv subjects

Harold A. Weaver

Publications and source records attributed to Harold A. Weaver.

At least 19 recordsLinked to original sources

arXiv2025

A Demonstration of Interstellar Navigation Using New Horizons

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 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 $0.4^\circ$ 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 three-dimensional 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.

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arXiv2025

Excess Ultraviolet Emission at High Galactic Latitudes: A New Horizons View

We present new observations of the cosmic ultraviolet background (CUVB) at high Galactic latitudes ($|b| > 40^{\circ}$), 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 $221 \pm 11$ photon units at 1000~Å and $264 \pm 24$ \photu\ at 1500~Å ($4.4 \pm 0.2$ nW m$^{-2}$ sr$^{-1}$ at 1000~Å and $5.3 \pm 0.5$ 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 \galex, showing that there is little emission from the Solar System from 1400 -- 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 $3.2 \pm 3.0$ photon units.

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arXiv2024

New Synoptic Observations of the Cosmic Optical Background with New Horizons

We obtained New Horizons LORRI images to measure the cosmic optical background (COB) intensity integrated over $0.4\lesssimλ\lesssim0.9{~\rmμm}.$ The survey comprises 16 high Galactic-latitude fields selected to minimize scattered diffuse Galactic light (DGL) from the Milky Way galaxy, 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 Planck HFI $350 {~\rmμm}$ and $550 {~\rmμm}$ intensities, 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 ${\rm 11.16\pm 1.65~(1.47~sys,~0.75~ran) ~nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $μ$m. The estimated integrated intensity from background galaxies, ${\rm 8.17\pm 1.18 ~nW ~m^{-2} ~sr^{-1}},$ can account for the great majority of this signal. The rest of the COB signal, ${\rm 2.99\pm2.03~ (1.75~sys,~1.03~ran) ~nW ~m^{-2} ~sr^{-1}},$ 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.

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arXiv2024

The New Horizons Extended Mission Target: Arrokoth Search and Discovery

Following the Pluto fly-by of the New Horizons spacecraft, the mission provided a unique opportunity to explore the Kuiper Belt in-situ. The possibility existed to fly-by a Kuiper Belt object (KBO) as well as to observe additional objects at distances closer than are feasible from earth-orbit facilities. However, at the time of launch no KBOs were known about that were accessible by the spacecraft. In this paper we present the results of 10 years of observations and three uniquely dedicated efforts -- two ground-based using the Subaru Suprime Camera, the Magellan MegaCam and IMACS Cameras, and one with the Hubble Space Telescope -- to find such KBOs for study. In this paper we overview the search criteria and strategies employed in our work and detail the analysis efforts to locate and track faint objects in the galactic plane. We also present a summary of all of the KBOs that were discovered as part of our efforts and how spacecraft targetability was assessed, including a detailed description of our astrometric analysis which included development of an extensive secondary calibration network. Overall, these efforts resulted in the discovery of 89 KBOs including 11 which became objects for distant observation by New Horizons and (486958) Arrokoth which became the first post-Pluto fly-by destination.

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arXiv2023

Ejecta from the DART-produced active asteroid Dimorphos

Some active asteroids have been proposed to be the result of impact events. Because active asteroids are generally discovered serendipitously only after their tail formation, the process of the impact ejecta evolving into a tail has never been directly observed. NASA's Double Asteroid Redirection Test (DART) mission, apart from having successfully changed the orbital period of Dimorphos, demonstrated the activation process of an asteroid from an impact under precisely known impact conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope (HST) from impact time T+15 minutes to T+18.5 days at spatial resolutions of ~2.1 km per pixel. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact. The ejecta evolution following DART's controlled impact experiment thus provides a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact.

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arXiv2022

Large-scale cryovolcanic resurfacing on Pluto

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 (>104 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.

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arXiv2022

Anomalous Flux in the Cosmic Optical Background Detected With New Horizons Observations

We used New Horizons LORRI images to measure the optical-band ($0.4\lesssimλ\lesssim0.9{\rmμm}$) sky brightness within a high galactic-latitude field selected to have reduced diffuse scattered light from the Milky Way galaxy (DGL), as inferred from the IRIS all-sky $100~μ$m map. We also selected the field to significantly reduce the scattered light from bright stars (SSL) outside the LORRI field. Suppression of DGL and SSL reduced the large uncertainties in the background flux levels present in our earlier New Horizons COB results. The raw total sky level, measured when New Horizons was 51.3 AU from the Sun, is $24.22\pm0.80{\rm ~nW ~m^{-2} ~sr^{-1}}.$ Isolating the COB contribution to the raw total required subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the field, and the hydrogen plus ionized-helium two-photon continua. This yielded a highly significant detection of the COB at ${\rm 16.37\pm 1.47 ~nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $μ$m. This result is in strong tension with the hypothesis that the COB only comprises the integrated light of external galaxies (IGL) presently known from deep HST counts. Subtraction of the estimated IGL flux from the total COB level leaves a flux component of unknown origin at ${\rm 8.06\pm1.92 ~nW ~m^{-2} ~sr^{-1}}.$ Its amplitude is equal to the IGL.

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arXiv2021

The Dark Side of Pluto

During its departure from Pluto, New Horizons used its LORRI camera to image a portion of Pluto's southern hemisphere that was in a decades-long seasonal winter darkness, but still very faintly illuminated by sunlight reflected by Charon. Recovery of this faint signal was technically challenging. The bright ring of sunlight forward-scattered by haze in the Plutonian atmosphere encircling the nightside hemisphere was severely overexposed, defeating the standard smeared-charge removal required for LORRI images. Reconstruction of the overexposed portions of the raw images, however, allowed adequate corrections to be accomplished. The small solar elongation of Pluto during the departure phase also generated a complex scattered-sunlight background in the images that was three orders of magnitude stronger than the estimated Charon-light flux (the Charon-light flux is similar to the flux of moonlight on Earth a few days before first quarter). A model background image was constructed for each Pluto image based on principal component analysis (PCA) applied to an ensemble of scattered-sunlight images taken at identical Sun-spacecraft geometry to the Pluto images. The recovered Charon-light image revealed a high-albedo region in the southern hemisphere. We argue that this may be a regional deposit of N_2 or CH_4 ice. The Charon-light image also shows that the south polar region currently has markedly lower albedo than the north polar region of Pluto, which may reflect the sublimation of N_2 ice or the deposition of haze particulates during the recent southern summer.

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arXiv2021

Spatial Distribution of Ultraviolet Emission from Cometary Activity at 67P/Churyumov-Gerasimenko

The Alice ultraviolet spectrograph on board the \textit{Rosetta} orbiter provided the first near-nucleus ultraviolet observations of a cometary coma from arrival at comet 67P/Churyumov-Gerasimenko in 2014 August through 2016 September. The characterization of atomic and molecular emissions in the coma revealed the unexpected contribution of dissociative electron impact emission at large heliocentric distances and during some outbursts. This mechanism also proved useful for compositional analysis, and Alice observed many cases that suggested elevated levels of the supervolatile \ce{O2}, identifiable in part to their emissions resulting from dissociative electron impact. In this paper we present the first two-dimensional UV maps constructed from Alice observations of atomic emission from 67P during an increase in cometary activity on 2015 November 7-8. Comparisons to observations of background coma and of an earlier collimated jet are used to describe possible changes to the near-nucleus coma and plasma. To verify the mapping method and place the Alice observations in context, comparisons to images derived from the MIRO and VIRTIS-H instruments are made. The spectra and maps we present show an increase in dissociative electron impact emission and an \ce{O2}/\ce{H2O} ratio of $\sim$0.3 for the activity; these characteristics have been previously identified with cometary outbursts seen in Alice data. Further, UV maps following the increases in activity show the spatial extent and emission variation experienced by the near-nucleus coma, informing future UV observations of comets that lack the same spatial resolution.

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arXiv2021

Analysis of Hybrid Gas-Dust Outbursts Observed at 67P/Churyumov-Gerasimenko

Cometary outbursts offer a valuable window into the composition of comet nuclei with their forceful ejection of dust and volatiles in explosive events, revealing the interior components of the comet. Understanding how different types of outbursts influence the dust properties and volatile abundances to better interpret what signatures can be attributed to primordial composition and what features are the result of processing is an important task best undertaken with a multi-instrument approach. The European Space Agency \textit{Rosetta} mission to 67P/Churyumov-Gerasimenko carried a suite of instruments capable of carrying out this task in the near-nucleus coma with unprecedented spatial and spectral resolution. In this work we discuss two outbursts that occurred November 7 2015 and were observed by three instruments on board: the Alice ultraviolet spectrograph, the Visual Infrared and Thermal Imaging Spectrometer (VIRTIS), and the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). Together the observations show that mixed gas and dust outbursts can have different spectral signatures representative of their initiating mechanisms, with the first outburst showing indicators of a cliff collapse origin and the second more representative of fresh volatiles being exposed via a deepening fracture. This analysis opens up the possibility of remote spectral classification of cometary outbursts with future work.

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arXiv2021

A Statistical Review of Light Curves and the Prevalence of Contact Binaries in the Kuiper Belt

We investigate what can be learned about a population of distant KBOs by studying the statistical properties of their light curves. Whereas others have successfully inferred the properties of individual, highly variable KBOs, we show that the fraction of KBOs with low amplitudes also provides fundamental information about a population. Each light curve is primarily the result of two factors: shape and orientation. We consider contact binaries and ellipsoidal shapes, with and without flattening. After developing the mathematical framework, we apply it to the existing body of KBO light curve data. Principal conclusions are as follows. (1) When using absolute magnitude H as a proxy for size, it is more accurate to use the maximum of the light curve rather than the mean. (2) Previous investigators have noted that smaller KBOs have higher-amplitude light curves, and have interpreted this as evidence that they are systematically more irregular in shape than larger KBOs; we show that a population of flattened bodies with uniform proportions could also explain this result. (3) Our analysis indicates that prior assessments of the fraction of contact binaries in the Kuiper Belt may be artificially low. (4) The pole orientations of some KBOs can be inferred from observed changes in their light curves; however, these KBOs constitute a biased sample, whose pole orientations are not representative of the population overall. (5) Although surface topography, albedo patterns, limb darkening, and other surface properties can affect individual light curves, they do not have a strong influence on the statistics overall. (6) Photometry from the OSSOS survey is incompatible with previous results and its statistical properties defy easy interpretation. We also discuss the promise of this approach for the analysis of future, much larger data sets such as the one anticipated from the Rubin Observatory.

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arXiv2021

Disintegration of Long-Period Comet C/2019 Y4 (ATLAS): I. Hubble Space Telescope Observations

Near-Sun Comet C/2019 Y4 (ATLAS) is the first member of a long-period comet group observed to disintegrate well before perihelion. Here we present our investigation into this disintegration event using images obtained in a 3-day {\it Hubble Space Telescope} (\hst) campaign. We identify two fragment clusters produced by the initial disintegration event, corresponding to fragments C/2019 Y4-A and C/2019 Y4-B identified in ground-based data. These two clusters started with similar integrated brightness, but exhibit different evolutionary behavior. C/2019 Y4-A was much shorter-lived compared to C/2019 Y4-B, and showed signs of significant mass-loss and changes in size distribution throughout the 3-day campaign. The cause of the initial fragmentation is undetermined by the limited evidence but crudely compatible with either the spin-up disruption of the nucleus or runaway sublimation of sub-surface supervolatile ices, either of which would lead to the release of a large amount of gas as inferred from the significant bluing of the comet observed shortly before the disintegration. Gas can only be produced by the sublimation of volatile ices, which must have survived at least one perihelion passage at a perihelion distance of $q=0.25$~au. We speculate that Comet ATLAS is derived from the ice-rich interior of a non-uniform, kilometer-wide progenitor that split during its previous perihelion. This suggests that comets down to a few kilometers in diameter can still possess complex, non-uniform interiors that can protect ices against intense solar heating.

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arXiv2021

Morphological Comparison of Blocks in Chaos Terrains on Pluto, Europa, and Mars

Chaos terrains are characterized by disruption of preexisting surfaces into irregularly arranged mountain blocks with a chaotic appearance. Several models for chaos formation have been proposed, but the formation and evolution of this enigmatic terrain type has not yet been fully constrained. We provide extensive mapping of the individual blocks that make up different chaos landscapes, and present a morphological comparison of chaotic terrains found on Pluto, Jupiter's moon Europa, and Mars, using measurements of diameter, height, and axial ratio of chaotic mountain blocks. Additionally, we compare mountain blocks in chaotic terrain and fretted terrain on Mars. We find a positive linear relationship between the size and height of chaos blocks on Pluto and Mars, whereas blocks on Europa exhibit a flat trend as block height does not generally increase with increasing block size. Block heights on Pluto are used to estimate the block root depths if they were floating icebergs. Block heights on Europa are used to infer the total thickness of the icy layer from which the blocks formed. Finally, block heights on Mars are compared to potential layer thicknesses of near-surface material. We propose that the heights of chaotic mountain blocks on Pluto, Europa, and Mars can be used to infer information about crustal lithology and surface layer thickness.

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arXiv2020

New Horizons Observations of the Cosmic Optical Background

We used existing data from the New Horizons LORRI camera to measure the optical-band ($0.4\lesssimλ\lesssim0.9{\rmμm}$) sky brightness within seven high galactic latitude fields. The average raw level measured while New Horizons was 42 to 45 AU from the Sun is $33.2\pm0.5{\rm ~nW ~m^{-2} ~sr^{-1}}.$ This is $\sim10\times$ darker than the darkest sky accessible to the {\it Hubble Space Telescope}, highlighting the utility of New Horizons for detecting the cosmic optical background (COB). Isolating the COB contribution to the raw total requires subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the fields, and diffuse Milky Way light scattered by infrared cirrus. We remove newly identified residual zodiacal light from the IRIS $100μ$m all sky maps to generate two different estimates for the diffuse galactic light (DGL). Using these yields a highly significant detection of the COB in the range ${\rm 15.9\pm 4.2\ (1.8~stat., 3.7~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 18.7\pm 3.8\ (1.8~stat., 3.3 ~sys.)~ nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $μ$m. Subtraction of the integrated light of galaxies (IGL) fainter than the photometric detection-limit from the total COB level leaves a diffuse flux component of unknown origin in the range ${\rm 8.8\pm4.9\ (1.8 ~stat., 4.5 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 11.9\pm4.6\ (1.8 ~stat., 4.2 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$. Explaining it with undetected galaxies requires the galaxy-count faint-end slope to steepen markedly at $V>24$ or that existing surveys are missing half the galaxies with $V< 30.$

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arXiv2020

Cryogenic Cometary Sample Return

Comets likely formed in the outer regions of the protosolar nebula where they incorporated and preserved primitive presolar materials, volatiles resident in the outer disk, and more refractory materials from throughout the disk. The return of a sample of volatiles (i.e., ices and entrained gases), along with other components of a cometary nucleus, will yield numerous major scientific opportunities. We are unaccustomed to thinking of ices through a mineralogical/petrological lens, but at cryogenic temperatures, ices can be regarded as mineral components of rocky material like any other. This is truly Terra Incognita, as a sample from a natural cryogenic (10s of K) environment is unprecedented in any setting; currently, we can only make educated guesses about the nature of these materials on a microscopic scale. Such samples will provide an unparalleled look at the primordial gases and ices present in the early solar nebula, enabling insights into the gas phase and gas-grain chemistry of the nebula. Understanding the nature of the ices in their microscopic, petrographic relationship to the refractory components of the cometary sample will allow for the study of those relationships and interactions and a study of evolutionary processes on small icy bodies. The previous 2013-2022 Planetary Decadal Survey included a study of a Flagship-class cryogenic comet nucleus sample return mission, given the scientific importance of such a mission. However, the mission was not recommended for flight in the last Decadal Survey, in part because of the immaturity of critical technologies. Now, a decade later, the scientific importance of the mission remains and relevant technological advances have been made in both cryo instrumentation for flight and laboratory applications. Such a mission should be undertaken in the next decade.

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arXiv2020

Pluto's Ultraviolet Spectrum, Surface Reflectance, and Airglow Emissions

During the New Horizons spacecraft's encounter with Pluto, the Alice ultraviolet spectrograph conducted a series of observations that detected emissions from both the interplanetary medium (IPM) and Pluto. In the direction of Pluto, the IPM was found to be 133.4$\pm$0.6R at Lyman $α$, 0.24$\pm$0.02R at Lyman $β$, and <0.10R at He I 584Å. We analyzed 3,900s of data obtained shortly before closest approach to Pluto and detect airglow emissions from H I, N I, N II, N$_2$, and CO above the disk of Pluto. We find Pluto's brightness at Lyman $α$ to be $29.3\pm1.9$R, in good agreement with pre-encounter estimates. The detection of the N II multiplet at 1085Å marks the first direct detection of ions in Pluto's atmosphere. We do not detect any emissions from noble gasses and place a 3$σ$ upper limit of 0.14 R on the brightness of the Ar I 1048Å line. We compare pre-encounter model predictions and predictions from our own airglow model, based on atmospheric profiles derived from the solar occultation observed by New Horizons, to the observed brightness of Pluto's airglow. Although completely opaque at Lyman $α$, Pluto's atmosphere is optically thin at wavelengths longer than 1425Å. Consequently, a significant amount of solar FUV light reaches the surface, where it can participate in space weathering processes. From the brightness of sunlight reflected from Pluto, we find the surface has a reflectance factor (I/F) of 17% between 1400-1850Å. We also report the first detection of an C$_3$ hydrocarbon molecule, methylacetylene, in absorption, at a column density of ~5$\times10^{15}$ cm$^{-2}$, corresponding to a column-integrated mixing ratio of $1.6\times10^{-6}$.

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arXiv2020

Photometry of Kuiper Belt Object (486958) Arrokoth from New Horizons LORRI

On January 1st 2019, the New Horizons spacecraft flew by the classical Kuiper belt object (486958) Arrokoth (provisionally designated 2014 MU69), possibly the most primitive object ever explored by a spacecraft. The I/F of Arrokoth is analyzed and fit with a photometric function that is a linear combination of the Lommel-Seeliger (lunar) and Lambert photometric functions. Arrokoth has a geometric albedo of p_V = 0.21_(-0.04)^(+0.05) at a wavelength of 550 nm and ~0.24 at 610 nm. Arrokoth's geometric albedo is greater than the median but consistent with a distribution of cold classical Kuiper belt objects whose geometric albedos were determined by fitting a thermal model to radiometric observations. Thus, Arrokoth's geometric albedo adds to the orbital and spectral evidence that it is a cold classical Kuiper belt object. Maps of the normal reflectance and hemispherical albedo of Arrokoth are presented. The normal reflectance of Arrokoth's surface varies with location, ranging from ~0.10-0.40 at 610 nm with an approximately Gaussian distribution. Both Arrokoth's extrema dark and extrema bright surfaces are correlated to topographic depressions. Arrokoth has a bilobate shape and the two lobes have similar normal reflectance distributions: both are approximately Gaussian, peak at ~0.25 at 610 nm, and range from ~0.10-0.40, which is consistent with co-formation and co-evolution of the two lobes. The hemispherical albedo of Arrokoth varies substantially with both incidence angle and location, the average hemispherical albedo at 610 nm is 0.063 +/- 0.015. The Bond albedo of Arrokoth at 610 nm is 0.062 +/- 0.015.

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arXiv2018

Great Expectations: Plans and Predictions for New Horizons Encounter with Kuiper Belt Object 2014 MU69 ('Ultima Thule')

The New Horizons encounter with the cold classical Kuiper Belt object (KBO) 2014 MU69 (informally named 'Ultima Thule,' hereafter Ultima) on 1 January 2019 will be the first time a spacecraft has ever closely observed one of the free-orbiting small denizens of the Kuiper Belt. Related to but not thought to have formed in the same region of the Solar System as the comets that been explored so far, it will also be the largest, most distant, and most primitive body yet visited by spacecraft. In this letter we begin with a brief overview of cold classical KBOs, of which Ultima is a prime example. We give a short preview of our encounter plans. We note what is currently known about Ultima from earth-based observations. We then review our expectations and capabilities to evaluate Ultima's composition, surface geology, structure, near space environment, small moons, rings, and the search for activity.

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