Sazuna/UAT_keywords huggingface dataset:
The Use of Digital Terrain Models for Natural Feature Tracking at Asteroid Bennu. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission rendezvoused with asteroid (101955) Bennu in 2018 with the primary objective of collecting a sample of regolith from the surface. As the first NASA asteroid sample return mission, OSIRIS-REx deployed several new technologies to achieve program objectives. Here we present an overview of Natural Feature Tracking (NFT), a system developed to autonomously guide the spacecraft to the desired sampling site using optical navigation and the natural terrain on the surface of Bennu. NFT utilized a series of image-based digital terrain models (DTMs) constructed by means of stereophotoclinometry to represent patches on the surface of the asteroid. These DTMs were used to generate synthetic renderings of the terrain and identify features for use in navigating to the sampling location. In addition, high-resolution models of the sampling site constructed from scanning lidar data were used for predicting the time and location of contact with the surface. These models went through a series of validation tests to ensure the performance of the NFT system. When the spacecraft executed the sampling trajectory in 2020 October, NFT enabled real-time guidance updates that delivered it safely to the desired sampling location while also providing critical hazard avoidance capabilities in the rocky Bennu environment.

Keywords: Deep space probes, Space probes, Asteroids, Asteroid rotation, Asteroid surfaces
Autonomous Navigation Performance Using Natural Feature Tracking during the OSIRIS-REx Touch-and-Go Sample Collection Event. When the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft collected a sample of surface material from asteroid Bennu in 2020 October, it was the first time that an autonomous optical navigation system relying on natural terrain features had been used to guide a spacecraft to a planetary surface. This system, called Natural Feature Tracking (NFT), works by rendering features from digital terrain models and then correlating them with the terrain in real-time navigation images to estimate the spacecraft's position and velocity with respect to the asteroid. Here we describe how the OSIRIS-REx mission built the catalog of features for NFT and how those features performed during rehearsals for and execution of the Touch-and-Go (TAG) sample collection event. Feature performance (quality and accuracy of match) in the rendering and correlation process is the basis of the NFT measurement. All features scored well above the minimum correlation threshold thanks to the effort invested in selecting and modeling them. Residuals across the TAG trajectory were small, indicating that features in the catalog were defined consistently relative to each other. NFT delivered the spacecraft to within 1 m of the targeted location, with a difference of only 3.5 cm and 1.4 s from the predicted location and time of touch. This exceptional performance was crucial for spacecraft safety given Bennu's rough and hazardous terrain.

Keywords: Asteroid surfaces
Practical Stereophotoclinometry for Modeling Shape and Topography on Planetary Missions. Stereophotoclinometry (SPC) is a technique to extract topographic information from images acquired by spacecraft. It combines stereophotogrammetry and photoclinometry to produce a product that has the accuracy of stereo with the resolution of photoclinometry without the restrictions common to both. We describe the implementation of this technique in the context of digital terrain model (DTM) generation for a small-body mission. We detail the process and the data used to generate SPC-derived DTMs at progressively increasing resolutions. The highest-quality DTMs are generated using four images optimized for topography, a 30° emission angle with the emission azimuth (spacecraft position) to the north, east, south, and west of the target, and one image optimized for albedo (a low incidence angle such that most of the image pixels' digital numbers are based upon albedo rather than topography). We discuss implications for mission planning and how SPC-based DTM generation can support spacecraft navigation. As a case study, we share outcomes from the modeling performed for the OSIRIS-REx mission to asteroid Bennu.

Keywords: Asteroids
Quality Assessment of Stereophotoclinometry as a Shape Modeling Method Using a Synthetic Asteroid. The stereophotoclinometry (SPC) software suite has been used to generate global digital terrain models (DTMs) of many asteroids and moons, and was the primary tool used by the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission to model the shape of asteroid Bennu. We describe the dedicated preflight testing of SPC for the OSIRIS-REx mission using a synthetic "truth" asteroid model. SPC has metrics that determine the internal consistency of a DTM, but it was not known how these metrics are related to the absolute accuracy of a DTM, which was important for the operational needs of the mission. The absolute accuracy of an SPC-generated DTM cannot be determined without knowing the truth topography. Consequently, we developed a realistic, but synthetic, computer-generated representation of asteroid Bennu, photographed this synthetic truth model in an imaging campaign similar to that planned for the OSIRIS-REx mission, and then generated a global SPC DTM from these images. We compared the SPC DTM, which was represented by a radius every 70 cm across the asteroid surface, to the synthetic truth model to assess the absolute accuracy. We found that the internal consistency can be used to determine the 3D root-mean-square accuracy of the model to within a factor of two of the absolute accuracy.

Keywords: Near-Earth objects, Planetary science
Ground Testing of Digital Terrain Models to Prepare for OSIRIS-REx Autonomous Vision Navigation Using Natural Feature Tracking. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft collected a sample from the asteroid Bennu in 2020. This achievement leveraged an autonomous optical navigation approach called Natural Feature Tracking (NFT). NFT provided spacecraft state updates by correlating asteroid surface features rendered from previously acquired terrain data with images taken by the onboard navigation camera. The success of NFT was the culmination of years of preparation and collaboration to ensure that feature data would meet navigation requirements. This paper presents the findings from ground testing performed prior to the spacecraft's arrival at Bennu, in which synthetic data were used to develop and validate the technical approach for building NFT features. Correlation sensitivity testing using synthetic models of Bennu enabled the team to characterize the terrain properties that worked well for feature correlation, the challenges posed by smoother terrain, and the impact of imaging conditions on correlation performance. The team found that models constructed from image data by means of stereophotoclinometry (SPC) worked better than those constructed from laser altimetry data, except when test image pixel sizes were more than a factor of 2 smaller than those of the images used for SPC, and when topography was underrepresented and resulted in incorrect shadows in rendered features. Degradation of laser altimetry data related to noise and spatial sampling also led to poor correlation performance. Albedo variation was found to be a key contributor to correlation performance; topographic data alone were insufficient for NFT.

Keywords: Asteroids, Asteroid surfaces
Near-infrared Spectroscopy of the Nucleus of Low-activity Comet P/2016 BA<SUB>14</SUB> during Its 2016 Close Approach. The near-Earth comet P/2016 BA14 (PanSTARRS) is a slow-rotating, nearly dormant object, a likely dynamical twin of 252P/LINEAR, and was recently shown to have a mid-infrared spectrum very dissimilar to other comets. Comet BA14 was also recently selected as one of the backup targets for the ESA's Comet Interceptor, so a clearer understanding of BA14's modern properties would not just improve our understanding of how comets go dormant but could also aid in planning for a potential spacecraft visit. We present observations of BA14 during its 2016 Earth close approach taken with the NASA Infrared Telescope Facility on two dates, both of which are consistent with direct observations of its nucleus. The reflectance spectrum of BA14 is similar to 67P/Churyumov-Gerasimenko, albeit highly phase-reddened. Thermal emission contaminates the reflectance spectrum at longer wavelengths, which we correct with a new Markov Chain Monte Carlo thermal modeling code. The models suggest that BA14's visible geometric albedo is p  V  = 0.01-0.03, consistent with radar observations; its beaming parameter is typical for NEOs observed in its geometry; and its reflectance spectrum is red and linear throughout the H and K bands. It appears very much like a "normal" comet nucleus despite its mid-infrared oddities. A slow loss of fine grains as the object's activity diminished might help to reconcile some of the lines of evidence, and we discuss other possibilities. A spacecraft flyby past BA14 could get closer to the nucleus than with a more active target, and we highlight some science questions that could be addressed with a visit to a (nearly) dormant comet.

Keywords: Comets, Short period comets, Spectroscopy
Ariel's Elastic Thicknesses and Heat Fluxes. The surface of Ariel displays regions that were resurfaced in the geologically recent past. Some of these regions include large chasmata that exhibit evidence for flexure. To estimate Ariel's heat fluxes, we analyzed flexure associated with the Pixie Group of chasmata, including Pixie, Kewpie, Brownie, Kra, Sylph, and an unnamed chasma, and the Kachina Group of chasmata, which includes Kachina Chasmata. We analyzed topography of these chasmata using digital elevation models developed for this work. Our results indicate that Ariel's elastic thicknesses range between 4.4 ± 0.7 km and 11.4 ± 1.4 km across the imaged surface. The younger Kachina Group has a relatively low elastic thickness of 4.4 ± 0.7 km compared to most chasmata in the older Pixie Group (4.1 ± 0.3 km to 11.4 ± 1.4 km). A pure H2O ice lithosphere would correspond to heat fluxes ranging from 17 to 46 mW m-2 for the Kachina Group and from 6 to 40 mW m-2 for the Pixie Group. Alternatively, if NH3 hydrates are present in Ariel's lithosphere, then the estimated heat fluxes are lower, ranging from 3 to 18 mW m-2 for the Kachina Group and from 1 to 16 mW m-2 for the Pixie Group. These results indicate that accounting for NH3 hydrates in the lithosphere substantially alters the resulting heat flux estimates, which could have important implications for understanding the lithospheric properties of other icy bodies where NH3-bearing species are expected to be present in their lithospheres. Our results are consistent with Ariel experiencing tidal heating generated from mean motion resonances with neighboring satellites in the past, in particular Titania and Miranda.

Keywords: Uranian satellites, Natural satellite surfaces, Natural satellites (Solar system), Planetary science, Tectonics
Discovery of the Closest Saturnian Irregular Moon, S/2019 S 1, and Implications for the Direct/Retrograde Satellite Ratio. We present a tracked orbit for a recently discovered 25th magnitude irregular moon of Saturn, using Canada-France-Hawaii Telescope imaging. Our 2 yr of observational arc on the moon leads to an orbit with a semimajor axis of 11.2 million kilometers and an inclination of 44 deg. This makes it one of the smallest Saturnian irregular moon orbits known and puts the moon in the Inuit group. This moon is also a magnitude brighter than the faintest known Saturnian irregulars. We show that the moon's small semimajor axis results in it spending most of the time lost in the glare of the often-nearby planet, thus explaining how it escaped detection in previous surveys. We postulate that the disparity in the known inventory with more retrograde than direct irregular moons is partly due to the selection bias against finding the direct moons (whose groupings have smaller semimajor axis).

Keywords: Irregular satellites, Saturn, Saturnian satellites, Natural satellites (Solar system)
Resurfacing: An Approach to Planetary Protection for Geologically Active Ocean Worlds. The putative and potential ocean worlds of our solar system span the asteroid belt to the Kuiper Belt, containing within their icy shells past or present global saltwater oceans. Among these worlds, those bearing signs of present-day geologic activity are key targets in the search for past or extant life in the solar system. As the icy surfaces of these bodies are modified by geologic processes, landforms are erased and replaced through what is called "resurfacing." To avoid contaminating sites for robotic spacecraft exploration, planetary protection requirements obligate missions to these ocean worlds to demonstrate a less than 10-4 probability of introducing a viable terrestrial microorganism into a liquid water body. To constrain the probability of subsurface contamination, we investigate the interaction with geologic resurfacing on an active ocean world. Through the example of Europa, we show how the surface age can be used to constrain the resurfacing rate, a critical parameter to estimate the probability that nonsterile spacecraft material present on the surface is geologically incorporated into the subsurface, and extend this example to mission scenarios at Ganymede and Enceladus. This approach was critical to demonstrating compliance with planetary protection requirements for the Europa Clipper mission, reducing its probability of contamination by two to five orders of magnitude. We also show how a Europa lander mission might be brought close to complying with planetary protection requirements, that a Ganymede impactor could easily comply, and that the situation of Enceladus, while more complex, can greatly benefit from this approach.

Keywords: Planetary surfaces, Ocean planets, Ganymede, Titan, Europa, Astrobiology, Galilean satellites, Saturnian satellites, Tectonics
Detection of Radio Thermal Emission from the Kuiper Belt Object (486958) Arrokoth during the New Horizons Encounter. The New Horizons spacecraft encountered the Kuiper Belt object (KBO) Arrokoth (486958), originally designated as 2014 MU69 and formerly called "Ultima Thule," on 2019 January 01. At 43.3 au from the Sun and 44.4 au from Earth, this was the most distant spacecraft reconnaissance of a solar system body to date. The Radio Science Experiment (REX) on New Horizons performed radiometry measurements of the KBO's thermal emission at λ = 4.2 cm in two observation slots, one before (dayside) and one after (nightside) the point of closest approach. Owing to the small size of the target, the intensity of the thermal emission was expected to be only marginally detectable. The KBO was not detected on approach because of unexpectedly large variations in the REX system temperature. A brightness temperature T  b  = 29 ± 5 K was derived for the nightside observation, considerably less than the predicted equilibrium temperature of ~50 K derived for Arrokoth on the dayside. A model explaining this day-night contrast is used to constrain the global values of emissivity, thermal inertia, and electrical skin depth of the KBO. In particular, models with small values of thermal inertia and small values of electrical skin depth are excluded. Relatively high values of the effective radio emissivity (E eff &gt; 0.8) provide better agreement with the REX observation.

Keywords: Radio continuum emission, Planetary surfaces, Small Solar System bodies, Classical Kuiper belt objects
A Near-surface Temperature Model of Arrokoth. A near-surface thermal model for Arrokoth is developed based on the recently released 105 facet model of the body. This thermal solution takes into account Arrokoth's surface reradiation back onto itself. The solution method exploits Arrokoth's periodic orbital character to develop a thermal response using a time-asymptotic solution method, which involves a Fourier transform solution of the heat equation, an approach recently used by others. We display detailed thermal solutions assuming that Arrokoth's near-surface material's thermal inertia ${ \mathcal I }\,=\,$ 2.5 W/m-2 K-1 s1/2. We predict that at New Horizons' encounter with Arrokoth, its encounter hemisphere surface temperatures were ~57-59 K in its polar regions, 30-40 K in its equatorial zones, and 11-13 K for its winter hemisphere. Arrokoth's orbitally averaged temperatures are around 30-35 K in its polar regions and closer to 40 K near its equatorial zones. Thermal reradiation from the surrounding surface amounts to less than 5% of the total energy budget, while the total energy ensconced into and exhumed out of Arrokoth's interior via thermal conduction over one orbit is about 0.5% of the total energy budget. As a generalized application of this thermal modeling together with other Kuiper Belt object origins considerations, we favor the interpretation that New Horizons' REX instrument's 29 ± 5 K brightness temperature measurement is consistent with Arrokoth's near-surface material being made of sub-to-few-millimeter-size tholin-coated amorphous H2O ice grains with 1 W/m-2 K-1 s1/2 $\lt \,{ \mathcal I }\,\lt $ 10-20 W/m-2 K-1 s1/2 and which are characterized by an X-band emissivity in the range 0.9 and 1.

Keywords: Natural satellite surfaces, Computational methods, Small Solar System bodies, Classical Kuiper belt objects
Upper Limits on the Escape of Volatiles from (486958) Arrokoth Using New Horizons Alice Ultraviolet Spectrograph Observations. During the New Horizons flyby of Kuiper Belt object (486958) Arrokoth on 2019 January 1, a search was made for escaping volatiles. Using the Alice spectrograph, appulse observations of both the Sun and the star HD42954 were performed to look for signatures of far-ultraviolet absorption by various species. No escaping volatiles were detected, and in this article we present upper limits to the escape rates for several species using a Haser model for the maximum line-of-sight column densities that would be undetected at 3σ. These results are useful for models for the formation and evolution of Kuiper Belt objects.

Keywords: Neutral coma gases, Classical Kuiper belt objects
A Predicted Dearth of Majority Hypervolatile Ices in Oort Cloud Comets. We present new, ice species-specific New Horizons/Alice upper gas coma production limits from the 2019 January 1 MU69/Arrokoth flyby of Gladstone et al. and use them to make predictions about the rarity of majority hypervolatile (CO, N2, CH4) ices in Kuiper Belt objects and Oort Cloud comets. These predictions have a number of important implications for the study of the Oort Cloud, including the determination of hypervolatile-rich comets as the first objects emplaced into the Oort Cloud, the measurement of CO/N2/CH4 abundance ratios in the protoplanetary disk from hypervolatile-rich comets, and population statistical constraints on early (&lt;20 Myr) planetary aggregation driven versus later (&gt;50 Myr) planetary migration driven emplacement of objects into the Oort Cloud. They imply that the phenomenon of ultradistant active comets like C/2017K2 should be rare, and thus not a general characteristic of all comets. They also suggest that interstellar object 2I/Borisov may not have originated in a planetary system that was inordinately CO rich, but rather could have been ejected onto an interstellar trajectory very early in its natal system's history.

Keywords: Ice destruction, Ice formation, Comae, Pluto, Long period comets, Neutral coma gases, Trans-Neptunian objects, Comets, Oort cloud objects
OSSOS XXV: Large Populations and Scattering-Sticking in the Distant Trans-Neptunian Resonances. There have been 77 TNOs discovered to be librating in the distant trans-Neptunian resonances (beyond the 2:1 resonance, at semimajor axes greater than 47.7 au) in four well-characterized surveys: the Outer Solar System Origins Survey (OSSOS) and three similar prior surveys. Here, we use the OSSOS Survey Simulator to measure their intrinsic orbital distributions using an empirical parameterized model. Because many of the resonances had only one or very few detections, j:k resonant objects were grouped by k in order to have a better basis for comparison between models and reality. We also use the Survey Simulator to constrain their absolute populations, finding that they are much larger than predicted by any published Neptune migration model to date; we also find population ratios that are inconsistent with published models, presenting a challenge for future Kuiper Belt emplacement models. The estimated population ratios between these resonances are largely consistent with scattering-sticking predictions, though further discoveries of resonant TNOs with high-precision orbits will be needed to determine whether scattering-sticking can explain the entire distant resonant population or not.

Keywords: Resonant Kuiper belt objects
Titan Stratospheric Haze Bands Observed in Cassini VIMS as Tracers of Meridional Circulation. We analyzed Cassini data to derive the nature and evolution of circumglobal annuli observed in the stratosphere of Titan, Saturn's largest moon. The annuli were observed between 2004 and 2017 in data acquired by the Visual and Infrared Mapping Spectrometer on board the Cassini spacecraft. We observed a north polar annulus, an equatorial annulus, and several secondary annuli. Pre-Cassini telescopic observations by the Hubble Space Telescope and Keck reported an atmospheric feature consistent with the presence of a south polar annulus between 1999 and 2001, although this feature was not observed by Cassini. Relative to the atmosphere near the annuli, they appear dark at 300-500 nm and bright in methane absorption channels such as the ones at 900 and 1150 nm. The stratosphere seems to rotate around the north pole. Alternatively, it seems to rotate about a point offset from solid-body rotation axis by a few degrees; this point in turn rotates around the solid-body rotation axis.

Keywords: Atmospheric circulation, Titan, Seasonal phenomena, Stratosphere, Spectrophotometry