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The autonomous approach of spacecraft to a small body (comet or asteroid) relies on using all available information at each phase of the approach. This letter presents new algorithms for global shape reconstructions from sparse tracked surface points. These methods leverage estimates from earlier phases, such as rotation pole, as well as a priori knowledge, such as a genus-0 body (i.e. without boundaries or topological holes). A mapping algorithm is proposed, which performs faithful reconstructions while enforcing genus-0 output through spherical parameterization. To estimate the shape of permanently shadowed regions of the body, a symmetry reconstruction method is added to the reconstruction algorithms. This method is shown to substantially increase the reconstruction accuracy but is subject to the symmetry of the body perpendicular to the rotation pole. The proposed mapping algorithm is compared to state-of-the-practice surface reconstruction algorithms, assessing their accuracy and ability to correctly generate genus-0 shape models for 2400 datasets and three small bodies. The proposed spherical parameterization algorithm performed consistently with the state-of-the-practice while being the only algorithm to always produce genus-0 shape models.

Purpose of Review The purpose of this review is to highlight space autonomy advances across mission phases, capture the anticipated need for autonomy and associated rationale, assess state of the practice, and share thoughts for future advancements that could lead to a new frontier in space exploration. Recent Findings Over the past two decades, several autonomous functions and system-level capabilities have been demonstrated and used in spacecraft operations. In spite of that, spacecraft today remain largely reliant on ground in the loop to assess situations and plan next actions, using pre-scripted command sequences. Advances have been made across mission phases including spacecraft navigation; proximity operations; entry, descent, and landing; surface mobility and manipulation; and data handling. But past successful practices may not be sustainable for future exploration. The ability of ground operators to predict the outcome of their plans seriously diminishes when platforms physically interact with planetary bodies, as has been experienced in two decades of Mars surface operations. This results from uncertainties that arise due to limited knowledge, complex physical interaction with the environment, and limitations of associated models. Summary Robotics and autonomy are synergistic, wherein robotics provides flexibility, autonomy exercises it to more effectively and robustly explore unknown worlds. Such capabilities can be substantially advanced by leveraging the rapid growth in SmallSats, the relative accessibility of near-Earth objects, and the recent increase in launch opportunities.

FARSIDE (Farside Array for Radio Science Investigations of the Dark ages and Exoplanets) is a Probe-class concept to place a low radio frequency interferometric array on the farside of the Moon. A NASA-funded design study, focused on the instrument, a deployment rover, the lander and base station, delivered an architecture broadly consistent with the requirements for a Probe mission. This notional architecture consists of 128 dual polarization antennas deployed across a 10 km area by a rover, and tethered to a base station for central processing, power and data transmission to the Lunar Gateway. FARSIDE would provide the capability to image the entire sky each minute in 1400 channels spanning frequencies from 100 kHz to 40 MHz, extending down two orders of magnitude below bands accessible to ground-based radio astronomy. The lunar farside can simultaneously provide isolation from terrestrial radio frequency interference, auroral kilometric radiation, and plasma noise from the solar wind. This would enable near-continuous monitoring of the nearest stellar systems in the search for the radio signatures of coronal mass ejections and energetic particle events, and would also detect the magnetospheres for the nearest candidate habitable exoplanets. Simultaneously, FARSIDE would be used to characterize similar activity in our own solar system, from the Sun to the outer planets, including the hypothetical Planet Nine. Through precision calibration via an orbiting beacon, and exquisite foreground characterization, FARSIDE would also measure the Dark Ages global 21-cm signal at redshifts z=50-100. The unique observational window offered by FARSIDE would enable an abundance of additional science ranging from sounding of the lunar subsurface to characterization of the interstellar medium in the solar system neighborhood.

A close partnership between people and partially autonomous machines has enabled decades of space exploration. But to further expand our horizons, our systems must become more capable. Increasing the nature and degree of autonomy - allowing our systems to make and act on their own decisions as directed by mission teams - enables new science capabilities and enhances science return. The 2011 Planetary Science Decadal Survey (PSDS) and on-going pre-Decadal mission studies have identified increased autonomy as a core technology required for future missions. However, even as scientific discovery has necessitated the development of autonomous systems and past flight demonstrations have been successful, institutional barriers have limited its maturation and infusion on existing planetary missions. Consequently, the authors and endorsers of this paper recommend that new programmatic pathways be developed to infuse autonomy, infrastructure for support autonomous systems be invested in, new practices be adopted, and the cost-saving value of autonomy for operations be studied., 10 pages (compared to 8 submitted to PSADS), 2 figures, submitted to National Academy of Sciences Planetary Science and Astrobiology Decadal Survey 2023-2032

An ultra-long-wavelength radio telescope on the far side of the Moon has significant advantages compared to Earth-based and Earth-orbiting telescopes, including: 1. Enabling observations of the Universe at wavelengths longer than 10 meters (i.e., frequencies below 30 MHz), wavelengths at which critical cosmological or extrasolar planetary signatures are predicted to appear, yet cannot be observed from the ground due to absorption from the Earth's ionosphere; and 2. The Moon acts as a physical shield that isolates a far-side lunar-surface telescope from radio interference from sources on the Earth's surface, the ionosphere, Earth-orbiting satellites, and the Sun's radio emission during the lunar night. In this paper, we present the conceptual design of the Lunar Crater Radio Telescope (LCRT) on the far side of the Moon. We propose to deploy a wire mesh using wall-climbing DuAxel robots in a 3–5 km diameter crater, with a suitable depth-to-diameter ratio, to form a parabolic reflector with a 1 km diameter. LCRT will be the largest filled-aperture radio telescope in the Solar System; larger than the former Arecibo telescope (305 m diameter, 3 cm - 1 m wavelength band, 0.3-10 GHz frequency band) and the Five-hundred-meter Aperture Spherical radio Telescope (FAST) (500 m diameter, 0.1-4.3 m wavelength band, 60–3000 MHz frequency band). LCRT's science objective is to track the evolution of the neutral intergalactic medium before and during the formation of the first stars in the 10–100 m wavelength band (3–30 MHz frequency band), which is consistent with priorities identified in the Astrophysics decadal survey. We describe LCRT's science objectives and the key technology challenges that need to be overcome to make this concept a reality. We envisage that LCRT will open a new window for humanity's exploration of the Universe.

Many areas of scientific interest in planetary exploration, such as lunar pits, icy-moon crevasses, and Martian craters, are inaccessible to current wheeled rovers. Rappelling rovers can safely traverse these steep surfaces, but require techniques to navigate their complex terrain. This dynamic navigation is inherently time-critical and communication constraints (e.g. delays and small communication windows) will require planetary systems to have some autonomy.Autonomous navigation for Martian rovers is well studied on moderately sloped and locally planar surfaces, but these methods do not readily transfer to tethered systems in non-planar 3D environments. Rappelling rovers in these situations have additional challenges, including terrain-tether interaction and its effects on rover stability, path planning and control.This paper presents novel traversability analysis and path planning algorithms for rappelling rovers operating on steep terrains that account for terrain-tether interaction and the unique stability and reachability constraints of a rapelling system. The system is evaluated with a series of simulations and an analogue mission. In simulation, the planner was shown to reliably find safe paths down a 55 degree slope when a stable tether-terrain configuration exists and never recommended an unsafe path when one did not. In a planetary analogue mission, elements of the system were used to autonomously navigate Axel, a JPL rappelling rover, down a 30 degree slope with 95% autonomy by distance travelled over 46 meters.

Characterizing the composition, properties, and environments of Small Bodies is key to understanding the origins and processes of the Solar System. Traditionally, our knowledge has been limited to ground observations and selected few missions which cannot fully characterize the diversity of Small Bodies. Advances in miniaturized spacecraft technologies have recently enabled small spacecraft to perform missions in deep space, as demonstrated by Mars Cube One in 2018. Additional missions are being developed to further mature these technologies and expand their capabilities. We investigate a new approach to exploring Small Bodies, where standalone small spacecraft can be used as a more affordable approach to autonomously navigate, rendezvous, and characterize them. We review relevant mission concepts, available targets, architecture trade-offs, and required technologies for baseline mission design options. Using near-term technologies that will be available in less than 3 years, Our results indicate that it is possible for standalone small spacecraft to rendezvous one of several Small Bodies. It was found that a 24 kg and 180 kg spacecraft would be capable of delivering payloads of 1.5 kg and 10 kg, respectively, to several near-Earth asteroids candidates. With a departure window of 2022 to 2030, the number of available targets for the 24 kg and 180 kg architectures are 9 to 177 and 104 to 1132 respectively. The number of targets depends on additional requirements, such as orbit uncertainty, composition, and diameter. Enabling technologies include high-delta-V (>3 km/s) miniature electric propulsion system, high-efficiency (>100 W/kg) deployable solar arrays, and improved computational hardware and software for autonomy. Advances in miniaturized instruments, high-performance radiation-tolerant avionics, and interplanetary communications systems can also be leveraged. In the long term, a standardized autonomous small spacecraft architecture could enable a fleet of spacecraft to perform a cursory exploration of a representative sample of Small Bodies.

Sites of increasing interest for planetary science, such as craters, cold traps, and vents lie in terrains that are inaccessible to state-of-the-art rovers. The Jet Propulsion Laboratory, in collaboration with Caltech, is actively developing a tethered mobile robot, Axel, for traversing and exploring extremely steep terrain, such as Recurring Slope Lineae on Mars and vertical pits on the Moon. However, on Mars, where landing-site ellipses are large due to the presence of an atmosphere, Axel may need to traverse several kilometers from its lander untethered given its finite tether carrying capacity (∼300 m). This paper proposes a novel design for a hybrid mobility system that allows a pair of Axel rovers to dock, lock, and drive long distances as a four-wheeled, articulated steering vehicle. The design improves upon prior efforts to achieve DuAxel mobility by leveraging two actuated docking mechanisms attached on opposite ends of a central module to enable ‘sit/stand’ functionality; the prior DuAxel system was limited to skid steering, which was inefficient due to Axel's grouser-style, high-friction wheels. In the proposed system, the ‘sit’ configuration is achieved by aligning each dock parallel to the surface, allowing one Axel to detach and explore while the other remains docked and serves as a backup. While ‘sitting’, the central module rests on the ground and is outfitted with shovel-style wedges for passive anchoring to sandy terrain (an optional drill can be integrated for anchoring to rock). In order to ‘stand’, the exploring Axel reattaches, locks, and both docks are rotated until Axel's tether caster arm is upright and the central module is lifted off the ground. Once upright, each Axel rotates about a pivot point for articulated, all-wheel steering, which is accomplished by applying differential wheel torques. The main contributions of this paper are i) a detailed systems design of the docking mechanism and central module, ii) kinematic modeling of articulated mobility and ‘sit/stand’ docking functionality, and iii) initial testing in a relevant environment to characterize the mobility of the proposed system.

When the Apollo astronauts collected samples from Tranquility Base in 1969, they provided an unprecedented window into the processes that shaped the Moon. Ever since, the Moon has served as a “keystone” for understanding planetary geological processes throughout the Solar System. Like all samples that have been returned from the Moon, the Apollo 11 samples were collected from the lunar regolith, a layer of jumbled and pulverized rocks that blankets and obscures the Moon's bedrock geology. When geologists reconstruct the history of the Moon, these samples are like scattered puzzle pieces, each representing important information, but removed from the context of their formation and isolated from the bigger picture of how the Moon's crust was formed. The goal of Moon Diver is to return to Mare Tranquillitatis, taking advantage of the discovery of a natural pit cave entrance exposing a deep cross-section through both the lunar regolith as well as tens of meters of bedrock lava layers. Collecting information on the chemistry, mineralogy, and morphology of these intact bedrock layers would allow us to investigate where rocky crusts come from, how they are emplaced, and the process by which they are transformed into the regolith layer that we see from space. In doing so, the mission would combine the deep knowledge gained by Apollo with the unprecedented in situ access to secondary crust granted by the lunar mare pit to understand these fundamental processes on the Moon, and to use this knowledge as a keystone for understanding the same processes across the Solar System. The success of the Moon Diver concept hinges on accessing the subsurface. The existence of the mare pit provides a cross-section through the lunar maria. Access to the record exposed in the wall of this pit is provided by two critical space technologies: pinpoint landing (allowing the delivery of the payload close to the pit) and extreme terrain mobility (allowing the delivery of capable instruments to the cliff wall). Pinpoint landing is a closed-loop guidance and navigation capability that repeatedly matches visual features from a downward-facing camera to a priori acquired terrain maps. This body-relative navigation is then used with closed-loop control to guide the spacecraft toward its landing target, yielding a tight landing ellipse. Once on the surface, an extreme-terrain robotic explorer, called Axel, would egress from the lander and traverse tens of meters to the pit. The lander provides mechanical support, power and communication to the rover through its umbilical tether. Anchored to the lander, the two-wheeled, tail-dragger rover would pay out its tether as it traverses toward the pit. With the aid of its 300-meter tether, the rover can traverse the steep slopes of the pit funnel and rappel its vertical walls. The rover carries a surface preparation tool together with a suite of three instrument types: (a) a trio of high-resolution cameras (Mars 2020's EECAMs) for acquiring context images of the near and far walls with the near-wall pair in a stereoscopic configuration, (b) an alpha-particle-X-ray Spectrometer (MSL's APXS) for elemental composition, and (c) a multi-spectral microscopic imager (MMI) that uses controlled lighting for minerology. The surface-preparation tool removes dust and patina that may be present on the rock wall by grinding a small area. The surface-preparation tool, the MMI and the APXS would be deployed from their instrument bays in one of the wheel wells. The rover would independently point each of its instruments at the same target of interest on the wall with millimeter-level repeatability. Confidence in the technologies of pinpoint landing and extreme-terrain access is based on helicopter testing of terrain-relative navigation and field testing of extreme terrain mobility respectively. The latter was tested using Axel rover prototypes with integrated science instruments at multiple terrestrial analog sites including a basaltic pit in Arizona. Landing shortly after sunrise, the surface mission timeline is just shy of a lunar day (14 Earth days). Upon landing, the rover would egress from the lander, traverse toward the pit, descend along the pit funnel and rappel down its wall. Throughout its traverse, the rover would acquire multiple measurements of both regolith and mare layers. After descending to the bottom of the layers, the rover will reach a significant overhang. This void space may open into a large cave or lava tube, which could someday provide a protected location for a lunar base. For these reasons, lunar pits provide an exciting new target for lunar exploration.

In this paper, we present a novel Shape from Silhouette (SfS) algorithm to estimate the physical and dynamical properties of a small body-such as an asteroid or comet-from periodic images taken from a distant approaching spacecraft. Standard mapping techniques such as Stereo-Photo-Clinometry (SPC) and Stereo-Photo-Grammetry (SPG) are designed for close-proximity observations in which the body is 1000s of pixels in area, and there are enough surface features on the object. In contrast, our algorithms are suited for distant observations (i.e. during first approach) in which the body is only 10s-100s of pixels in area and does not present any useful visual features. First, using the Fast-Fourier-Transform of the light curve of the small body, we estimate its rotation rate. Then, using our novel silhouette-based 3D shape reconstruction technique, we estimate the shape and size of the small body and its pole of rotation. In this paper, we assume that the small body is performing pure rotation (no tumbling) about its principal axis, that the Sun is directly behind the spacecraft, and that the distance from the spacecraft to the small body is known. These algorithms have been tested using both simulated data from Comet 67P, Asteroids Eros and Itokawa; and real data from the Rosetta mission.

In the work, we present dielectric spectroscopy (DS) is an analytical method capable of characterizing and quantifying electrochemically active materials (water/ice, ions, etc.) in planetary regoliths. Detection of these materials occurs when a small amount of energy is applied to an in situ sample, causing perturbation in the form of potential kinetic energy and inducing polarization and/or relaxation processes in materials of interest. The frequency dispersion on the electrical properties of impendences, relative permittivity, and conductivity spectra represents chemical and physical properties in association with inherent material properties. Therefore, measuring these energetic states can provide a unique fingerprint of the materials, depending on the frequencies that are scanned. In the case of water content in soils, the Debye equivalent circuit model can accurately predict its electrical properties and provide excellent quantification of the water content in soils. Use of DS is enabled by miniaturization of the key components and the low power requirements. DS is therefore capable of offering substantial scientific returns as an in situ sampling and non-destructive measurement technique, making it ideally suitable for deployment onboard many smaller space exploratory platforms. This work reports the first deployment use of DS from a field test for geochemical characterization of in situ soils, specifically on the quantification of water onboard the AXEL mobility system in the Mojave Desert. Laboratory testing has confirmed the sensitivity of DS with respect to particle size (fine vs coarse) determination as well as water content (down to 0.01 ​wt%) under a wide range of environmental conditions. Water content associated with bound and bulk water was uniquely identified in the soil mixture. We also describe a field demonstration using DS using two distinct insertion dielectric probes specifically design for sub-surface measurements, verifying their mechanical robustness and chemical stability. The field measurements are in good agreement with laboratory data and effectively quantified water content at about 3.0 ​wt% and 8.0 ​wt% for field test sites #1 and #2, respectively.

The Axel rappelling rover was designed to enable access to intriguing and important science sites that lie in difficult terrains that are inaccessible to conventional rovers. Extended autonomous rappelling calls for careful control of tether tension, precise management of tether spooling, and some measure of shock tolerance. This paper covers the design and testing of a first-generation tether management system (TMS) for Axel. The system uses a double bull-wheel capstan driven by a low-stiffness series elastic actuator (SEA) to provide tension control and decouple internal spooling tension from external tether tension. A series elastic actuator was chosen for this application to permit closed-loop tether tension control and to provide shock/drop tolerance of the rappelling system both while moving and when the system is inactive with the motors locked. Experiments on the new TMS show that this design performs well in keeping nearly constant spooling tension while rejecting large dynamic disturbances at the output. While the SEA is very effective at maintaining a given tension contribution, the additional effects of friction and the unique mechanical properties of the tether result in substantial errors in the measured output tension. Upcoming field trials will be used to evaluate the effectiveness and sufficiency of this system when integrated in Axel.

Recurring Slope Lineae, RSL, are visible steaks on the sides of Martian craters and other steep landforms with three distinct characteristics: i) they occur seasonally, ii) they lengthen with time, and iii) they fade. Recently, 474 RSL-like (>= 1 characteristic) and 69 RSL (all 3 characteristics) sites have been identified and are located in four primary regions of Mars: Chryse and Acidalia Planitiae, Valles Marineras, the Equatorial highlands, and the Southern Mid-Latitudes. There are several hypotheses that explain the origin of RSL, ranging from the motion of dry, granular material downhill, to the presence of underground aquifers. These hypotheses can be partially resolved by proximal ( = 1km) observations. However, complete disambiguation can only be achieved through surface contact science. The Jet Propulsion Laboratory recently conducted a study into the methods by which a landed asset could access RSL sites to assist in the separation of RSL hypotheses. During this study, 8 of the 69 RSL sites were considered to be landable based on their latitude, elevation, topography, and ability to support an 8 km landing ellipse. In this paper, the accessibility of these sites is detailed through a discussion of their geometry, terrain types, slope and surface roughness. Particular attention is paid to three craters: Selevac, Andapa, and Garni, which were assessed to be strong candidates for scientific investigation. The challenges of accessibility provide the basis for which an assessment of the concepts of operation of 22 architectures is given. Though individually assessed, these concepts can be broadly compiled into balloon, helicopter, missile, and ground ascent/descent groups. First order sizing, power, and mass calculations for each group are detailed along with their advantages and concept specific challenges. The merits of each concept is assessed as a function of the science they enable, technology readiness and feasibility, cost, and risk. A ranking matrix is presented along with selected weightings. Based on the results of the trade study, the highest rated concepts are identified and their next stages of analysis and development are proposed.

The localization of landers on the surface of small bodies has traditionally relied on observations from a mothership (e.g. Rosetta's Philae lander and Hayabusa 2's MASCOT and MINERVA landers). However, when line-of-sight with the mothership is not always available, or for surface rovers that travel large distances, alternative mothership-independent localization techniques may be required. On-board vision-based techniques have demonstrated effective localization in terrestrial applications as well as for Mars rovers, but may be unreliable on small bodies where rovers must contend with fast-moving shadows and issues acquiring images such as dusty lenses, sun blinding, and a rotating field of view. We investigate the feasibility of an entirely new localization approach based on surface gravimetry, where a rover can constrain its location on the surface by precisely measuring the local gravity vector. This mothership-independent localization technique is well-suited to hopping rovers that can bounce and tumble over the surface of small bodies; it is insensitive to surface illumination, and even works at night. We develop a Bayesian framework for computing localization “likelihood maps” from gravimetry data, accounting for all sensor and model uncertainties. We then propose a method for deriving landing distributions of a bouncing rover from simulation data to serve as a prior for the localization estimate. Finally, we conduct a case study on the Philae lander, where we show how this approach could have helped reject localization hypotheses and significantly narrow the search area for the lost Philae lander. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.