Lunar Orbit Research Articles

Quantifying Topographic Influence on Irradiance Transfer in Permanently Shadowed Regions

Abstract. In permanently shadowed regions (PSRs), the surface temperature is influenced by secondary illumination, which changes daily and seasonally due to the sunlight reflected by the surrounding terrain. Understanding how topography affects the transfer of radiant energy can help us quickly interpret the thermal behavior using available topographic data. The amount of radiant energy transferred from a sunlit lunar surface to a PSR depends on the distance and orientation of the sunlit surface to the PSR, and is represented by view factors. In this study, we introduce an approach to systematically represent the combined effects of multiple surfaces using statistical analysis applied to view factor maps. We demonstrate that our proposed approach can explain the contrasting temperatures of two PSRs at the lunar south pole. We verify our theoretical findings using PSR images acquired by the ShadowCam instrument aboard the Danuri lunar orbiter.

Shape-from-Shading Derived DEMs from ShadowCam Images in Permanently Shadowed Regions at the Lunar South Pole: A First Look

Abstract. The lunar south pole, particularly its permanently shadowed regions (PSRs), has been the focus of future lunar exploration. The solar altitude angle at the lunar south pole is less than ∼ 2°, resulting in the PSRs not being directly illuminated and the possible existence of water ice in the PSRs due to the low temperatures. High-resolution digital elevation models (DEMs) in PSRs are crucial for future exploration missions and scientific research. However, existing image datasets (e.g., LROC NAC images) cannot penetrate the PSRs. Laser altimetry (e.g., LOLA) can measure the PSRs, but the DEMs derived from LOLA suffer from noises and interpolation defects. ShadowCam is a newly deployed camera onboard the Korea Pathfinder Lunar Orbiter, with a high imaging sensitivity. ShadowCam can capture the faint secondary illumination within PSRs, revealing previously indistinguishable topographic features in persistent darkness. Based on our existing Shape-from-Shading (SfS) pipeline for generating high-resolution DEMs from monocular images, we introduce a novel SfS approach that utilizes secondary illumination and reflectance to reconstruct pixel-wise high-resolution DEMs from ShadowCam images in PSRs. The approach consists of three steps: (1) modeling the secondary illumination on the target surfaces in PSRs based on the view factor; (2) optimizing a low-resolution LOLA DEM using the secondary illumination model; (3) refining the DEM to pixel-wise resolution based on SfS using the ShadowCam images. A test area (approximately 1 km × 1 km) within the PSR in the Shackleton crater near the lunar south pole is used for experimental evaluation. The results show that the DEM generated by the proposed approach is highly consistent with the direct LOLA measurements, particularly in small-scale topographic features such as crater floors and rims that are absent from the LOLA DEM. A first look at the results indicates that the proposed approach has great potential for generating high-resolution DEMs from ShadowCam images in PSRs, which can aid future exploration missions targeting the PSRs at the lunar south pole and support related scientific research on the water ice in the PSRs.

Giant Kelvin‐Helmholtz (KH) Waves at the Boundary Layer of the Coronal Mass Ejections (CMEs) Responsible for the Largest Geomagnetic Storm in 20 Years

AbstractStarting in the evening of 10 May 2024 the Earth's magnetosphere was hit by the coronal mass ejections (CMEs) creating the largest geomagnetic storm in 20 years. The CME encounter was characterized by variations of plasma number density and magnetic field. Here, I present the ARTEMIS observations at the lunar orbit during this event. The IMF ranged from −60 to +40 nT both with hour to minutes periodicity with plasma jets propagating in ‐direction within multi‐scale wave structures. Similar signature has been recently reported at the magnetopause by MMS spacecraft (Li et al., 2023, https://doi.org/10.1029/2023GL105539; Nykyri, 2024, https://doi.org/10.1029/2024GL108605) during a strongly southward IMF. Here, I show that the CME boundaries were KH unstable leading to multi‐scale density and magnetic field fluctuations including reconnection jets. The wavelengths varied from 60 to 270 , suggesting that the magnetosphere was periodically exposed to successive intervals of strongly northward and southward IMF leading to enhanced mass and magnetic flux loading.

Geostratigraphic Mapping of the Intrusive Valentine Domes on the Moon

AbstractLunar intrusive igneous domes have not been the center of much research in the past due to their rare occurrence on the lunar surface, and the difficulty in locating them. Most of the known structures were discovered using images with low illumination angles, including data from the Lunar Orbiter, telescopic images, and photos taken during the Apollo Missions. These intrusive domes are characterized by an oval shape and low slopes. We analyzed one of these systems, the Valentine domes, located near the rim of the western Serenitatis basin, with modern techniques and data sets from the Lunar Reconnaissance Orbiter (LRO) and Chandrayaan‐1 missions. We created a geostratigraphic map of the area, combining geomorphological and spectral classifications. The aspect map (direction of the slope) proved to be the most suitable product to locate and delimit these structures; using it, we identified a new dome southeast of the principal body, suggesting that the intrusive system is larger than previously thought. It was found that the three domes can be classified as laccoliths, and that several derived structures such as rilles, dykes, and secondary domes represent different stages of intrusive activity in the area. Based on crater counting analysis, we determined that the intrusive activity began after 2.98 ± 0.15 Ga and lasted at least until 1.88 ± 0.5 Ga ago.

The Evolution of Rock Size‐Frequency Distribution on the Moon: Effects of Rock Strength and Fragmentation Products on Centimeter‐Scale Abundances

AbstractRock abundances on the Moon represent both an opportunity to understand the history of the surface and of the regolith and a hazard to lander missions. While rock erasure by meteoroid bombardment is known to modify rock size˗frequency distributions, the interplay between rock erasure and rock exposure by impact cratering, and the resulting net rock abundance, is not known. Leveraging a coupling between modeling and optical imagery from the lunar orbit, we calculate new rock lifetimes that consider the specific shattering energy and the fragments produced by boulder shattering. We find differences between the estimated and expected specific shattering energy (Qs*), likely suggesting incomplete understanding of the scaling of the shattering energy with velocity and size. We find that the decrease in rock abundances with time on crater ejecta occurs faster than previous estimates based on thermal infrared data.

Space tether research at the University of Stuttgart

Space tether research activities at the University date back to the 1990s. First research projects investigated tether-assisted re-entry of payload return capsules from space stations. Tether research was resumed in 2015 focusing on the application of tethered planetary exploration rovers, based on the micro-rover Nanokhod. This includes robust, highly-integrated and miniaturised tethers and tether spooling systems, which are specifically adapted to the challenging lunar environmental and dust conditions. For this, several prototypes were developed and tested and a tether-dust testing facility was setup. In addition, developments have been made in the tether detection by creating a simulation framework, tracking algorithms and low-fidelity test bench for the verification of the algorithms. Based on the experience of the planetary tether applications, the research at the University of Stuttgart was expanded to Tethered Satellite Systems in 2022, utilising the tether spooling technology. In-depth studies on tethered CubeSat missions were developed together with students as part of educational activities, including tethered rendezvous capabilities and electrodynamic tether operation. In addition, breadboard models of optical detection and tracking payloads were developed for tracking a CubeSat sized object at full tether deployment of 100m. Additional subsystem development is currently in progress, consolidating the results of the mission studies, as well as dynamic analyses and simulations of the tethered satellite system. Concurrently, a third tether application research area was established, looking into studies for creating tether-based spaceflight infrastructure for lunar exploration missions. A concept study of a Momentum Exchange Tether system to transfer large payloads from Low Earth Orbit to a lunar transfer orbit was conducted and feasible tether system configurations were drafted. Various technological and operational challenges were identified and are now the focus of ongoing Momentum Exchange Tether research at the University of Stuttgart.

The LCROSS Impact Crater as Seen by ShadowCam and Mini‐RF: Size, Context, and Excavation of Copernican Volatiles

AbstractThe Lunar CRater Observations and Sensing Satellite (LCROSS) impacted a Centaur rocket stage into a permanently shadowed region (PSR) in Cabeus crater, excavating water ice and other volatiles. We used the Miniature Radio Frequency (Mini‐RF) instrument on the Lunar Reconnaissance Orbiter and the ShadowCam instrument on the Korean Pathfinder Lunar Orbiter to detect the probable 22‐m diameter crater that resulted from the LCROSS impact. The crater formed superposed upon a dense small crater population along a crater ray from a larger pre‐existing crater. From its geologic context, the ice and regolith excavated by LCROSS were likely modified within the last 0.1–0.5 Gyr. An upper limit for the excavated volatiles is ~0.9 Gyr, as the location was not a PSR prior to that time. A young age for the LCROSS‐detected volatiles supports the idea that they were mostly emplaced by an exogenic mechanism, such as from comets or the solar wind.

Autonomous Lunar rendezvous trajectory planning and control using nonlinear MPC and Pontryagin’s principle

This paper explores the application of Nonlinear Model Predictive Control (NMPC) techniques, based on the Pontryagin Minimum Principle, for a minimum-propellant autonomous rendezvous maneuver in non-Keplerian Lunar orbits. The relative motion between the chaser and the target is described by the nonlinear dynamics of the circular restricted three body-problem, posing unique challenges due to the complex and unstable dynamics of near-rectilinear halo orbits. Key aspects of the proposed NMPC include trajectory optimization, maneuver planning, and real-time control, leveraging on its ability to satisfy complex mission requirements while ensuring safe and efficient spacecraft operations and in the presence of input and nonlinear/non-convex state constraints. The proposed formulation allows the design of a minimum-propellant controller, whose optimal control signal results to be bang–bang in time. A case study based on the Artemis III mission – where the docking of the Orion spacecraft to the Gateway station is planned – is illustrated in order to demonstrate the efficiency of the proposed approach, showcasing its potential for enhancing target tracking accuracy, while reducing propellant consumption.

Reinforced Lyapunov controllers for low-thrust lunar transfers

Future missions to the Moon and beyond are likely to involve low-thrust propulsion technologies due to their propellant efficiency. However, these still present a difficult trajectory design problem, owing to the near continuous thrust, lack of control authority and chaotic dynamics. Lyapunov control laws can generate sub-optimal trajectories for such missions with minimal computational cost and are suitable for feasibility studies and as initial guesses for optimisation methods. In this work a Reinforced Lyapunov Controller is used to design optimal low-thrust transfers from geostationary transfer orbit towards lunar polar orbit. Within the reinforcement learning (RL) framework, a dual-actor network setup is used, one in each of the Earth- and Moon-centred inertial frames respectively. A key contribution of this paper is the demonstration of a forwards propagated trajectory, removing the need to define a patch point a priori. This is enabled by an adaptive patch distance and extensive initial geometry exploration during the RL training. Results for both time- and fuel-optimal transfers are presented, along with a Monte Carlo analysis of the robustness to disturbances for such transfers. Phasing is introduced where necessary to aid rendezvous with the Moon. The results demonstrate the potential for such techniques to provide a basis for the design and guidance of low-thrust lunar transfers.

Secular dynamics and the lifetimes of lunar artificial satellites under natural force-driven orbital evolution

In this paper, we study the long-term (time scale of several years) orbital evolution of lunar satellites under the sole action of natural forces. In particular, we focus on secular resonances, caused either by the influence of the multipole moments of the lunar potential and/or by the Earth’s and Sun’s third-body effect on the satellite’s long-term orbital evolution. Our study is based on a simplified secular model obtained in ‘closed form’, i.e., without expansions in the satellite’s orbital eccentricity. Contrary to the case of artificial Earth satellites, in which many secular resonances compete in dynamical impact, we give numerical evidence that for lunar satellites only the 2g−resonance (ω̇=0) affects significantly the orbits at secular timescales. We interpret this as a consequence of the strong effect of lunar mascons. We show that the lifetime of lunar satellites is, in particular, nearly exclusively dictated by the 2g resonance. By deriving a simple analytic model, we propose a theoretical framework which allows for both qualitative and quantitative interpretation of the structures seen in numerically obtained lifetime maps. This involves explaining the main mechanisms driving eccentricity growth in the orbits of lunar satellites. In fact, we argue that the re-entry process for lunar satellites is not necessarily a chaotic process (as is the case for Earth satellites), but rather due to a sequence of bifurcations leading to a dramatic variation in the structure of the separatrices in the 2g resonance’s phase portrait, as we move from the lowest to the highest limit in inclination (at each altitude) where the 2g resonance is manifested.

Space Environments Applied to the Gateway Program

NASA’s Gateway is a crewed space station with a planned mission duration of 15 years in lunar orbit. It is an international collaboration, including partnerships with the European Space Agency, Canadian Space Agency, and Japan Aerospace Explorations Agency. Additionally, the program relies on important partnerships with industry for designing and building hardware. This complex mission poses significant challenges related to space environments. Gateway’s hardware will be directly exposed to the extremes of space weather posing challenges to avionics and materials as well as operations planning. The space environments are defined in the “Design Specification for Natural Environments” and applied through program requirements to all design partners. This paper describes the space environments applicable to the Gateway program, the flow of the environments to system requirements, and the challenges that the program is currently facing with regard to the space environments.

Efficient disposal of low lunar orbiters on the lunar surface

Realistic numerical simulations are conducted to explore safe and efficient strategies for End-of-Life lunar spacecraft disposal on the lunar surface. Disposal from three different near-polar, low-altitude, and near-circular orbit types was analyzed to determine if impact locations that minimize the risk to protected regions of the lunar surface can be achieved consistently for low fuel costs using the Moon’s non-spherical gravity field. A total of 300,000 objects were propagated following retrograde disposal burns using a high-fidelity lunar trajectory model, and the disposal burn locations and magnitudes were compared against the resulting likelihoods of decay and lunar surface impact locations. The results identified disposal strategies that could achieve impact in safe locations for 10 m/s of ΔV or less for all orbit types, much lower than the ΔV cost to lower the perilune entirely to the lunar surface. A similar simulation-based methodology could be applied to operational lunar satellites to identify disposal strategies based on specific mission conditions. The results of this study support the development of safe and efficient End-of-Life disposal strategies that minimize the accumulation of debris in lunar orbit and extend the operational lifetimes of lunar missions.

The Faustini Permanently Shadowed Region on the Moon

Faustini crater (41 km diameter) hosts a large (664 km2) permanently shadowed region (PSR) with a high potential to harbor water-ice deposits. One of the 13 candidate Artemis III landing areas contains a portion of the crater rim and proximal ejecta. The ShadowCam instrument aboard the Korea Pathfinder Lunar Orbiter provides detailed images of the PSR within Faustini. We characterize the terrain and thermal environment within the Faustini PSR from ShadowCam images, Lunar Reconnaissance Orbiter thermal measurements and laser ranging, and thermal modeling. Our mapping revealed three distinct areas of the floor of Faustini based on elevations, slopes, and surface roughness. These units broadly correlate with temperatures; thus, they may be influenced by variations in volatile sublimation. Crater retention and topographic diffusion rates appear to be asymmetric across the floor, likely due to differences in maximum and average temperatures. Several irregular depressions and a pronounced lobate-rim crater are consistent with subsurface ice. However, differences in the thicknesses of deposited materials on the floor may also explain the asymmetry. Additionally, zones of elevated surface roughness across Faustini appear to result from overprinted crater ray segments, possibly from Tycho and Jackson craters. Mass wasting deposits and pitting on opposite sides of the crater wall may have resulted from the low-angle delivery of material ejected by the Shackleton crater impact event, suggesting that the Artemis III candidate landing region named “Faustini Rim A” will contain material from Shackleton.

Unveiling Mass Transfer in Solar Flares: Insights from Elemental Abundance Evolutions Observed by Chang’E-2 Solar X-Ray Monitor

Understanding how elemental abundances evolve during solar flares helps shed light on the mass and energy transfer between different solar atmospheric layers. However, prior studies have mostly concentrated on averaged abundances or specific flare phases, leaving a gap in exploring the comprehensive observations throughout the entire flare process. Consequently, investigations into this area are relatively scarce. Exploiting the Solar X-Ray Monitor data obtained from the Chang’E-2 lunar orbiter, we present two comprehensive soft X-ray spectroscopic observations of flares in active regions, AR 11149 and 11158, demonstrating elemental abundance evolutions under different conditions. Our findings unveil the inverse first ionization potential (IFIP) effect during flares for Fe for the first time, and reaffirm its existence for Si. Additionally, we observed a rare depletion of elemental abundances, marking the second IFIP effect in flare decay phases. Our study offers a CSHKP model-based interpretation to elucidate the formation of both the FIP and IFIP effects in flare dynamics, with the inertia effect being incorporated into the ponderomotive force fractionation model.

The GRB221009A gamma-ray burst as revealed by the gamma-ray spectrometer onboard the KPLO (Danuri)

The strongest gamma-ray burst (GRB) of the century, GRB20221009A, has been detected by the Korean Pathfinder Lunar Orbiter Gamma-ray Spectrometer (KGRS) instrument onboard the Korean Pathfinder Lunar Orbiter (KPLO). KGRS uses a LaBr3 detector to measure GRB counts with five energy bins in the energy range from 30 keV to 12 MeV. KGRS detected GRB221009A at a distance of 1.508 million kilometers from the Earth. The full duration of the main burst was recorded between 13:20 and 13:26 on October 9, 2022 with peak counts of over 1000 times background. The dead time of KGRS reached as high as 50%, and the intrinsic gamma-ray spectrum of LaBr3 was significantly altered.

A Relativistic Framework to Estimate Clock Rates on the Moon

As humanity aspires to explore the solar system and investigate distant worlds such as the Moon, Mars, and beyond, there is a growing need to estimate and model the rate of clocks on these celestial bodies and compare them with the rate of standard clocks on Earth. According to Einstein’s theory of relativity, the rate of a standard clock is influenced by the gravitational potential at its location and its relative motion. A convenient choice of local reference frames allows for the comparison of local time variations of clocks due to gravitational and kinematic effects. We estimate the rate of clocks on the Moon using a locally freely falling reference frame coincident with the center of mass of the Earth–Moon system. A clock near the Moon’s selenoid ticks faster than one near the Earth’s geoid, accumulating an extra 56.02 μs day−1 over the duration of a lunar orbit. This formalism is then used to compute the clock rates at Earth–Moon Lagrange points. Accurate estimation of the rate differences of coordinate times across celestial bodies and their intercomparisons using clocks on board orbiters at Lagrange points as time transfer links is crucial for establishing reliable communications infrastructure. This understanding also underpins precise navigation in cislunar space and on celestial bodies’ surfaces, thus playing a pivotal role in ensuring the interoperability of various position, navigation, and timing systems spanning from Earth to the Moon and to the farthest regions of the inner solar system.

Extended perilune rendezvous method for low Delta-V transition to NRHO for cargo transportation mission to gateway

The construction of Gateway in lunar orbit is planned as a post-ISS mission by NASA, and JAXA is considering a cargo supply mission to provide service. The Near-Rectilinear Halo Orbit (NRHO) is a candidate for a Gateway orbit, and several methods for the transfer trajectory are being researched. The In-Direct Transfer (IDT) has a short transition period of seven days, but the ΔV application is inefficient. The Weak Stability Boundary (WSB) transfer maintains a small ΔV, but the transition takes over 100 days. However, a trajectory intermediate between IDT and WSB has not yet been actively studied. This paper proposes the Extended Perilune Rendezvous Method (EPRM) of intermediate transfer to NRHO. This method reduces ΔV by orbiting in an elliptical lunar orbit for over two weeks. EPRM has several transition methods corresponding to ΔV and transition periods. This paper systematically reveals the transition mechanisms. Moreover, an effective TCM, considering several error types, is presented for an actual mission. Finally, the rendezvous accuracy with Gateway is discussed.

Operational Tests for Delay-Tolerant Network between the Moon and Earth Using the Korea Pathfinder Lunar Orbiter in Lunar Orbit

The Korea Pathfinder Lunar Orbiter (KPLO) was launched on 5 August 2022, equipped on the SpaceX Falcon 9 launch vehicle. At present, the KPLO is effectively carrying out its scientific mission in lunar orbit. The KPLO serves as a cornerstone for the development and validation of Korean space science and deep space technology. Among its payloads is the DTNPL, enabling the first-ever test of delay-tolerant network (DTN) technology for satellites in lunar orbit. DTN technology represents a significant advancement in space communication, offering stable communication capabilities characterized by high delay tolerance, reliability, and asymmetric communication speeds—a necessity for existing satellite and space communication systems to evolve. In this paper, we briefly give an overview of the Korea Lunar Exploration Program (KLEP) and present scientific data gathered through the KPLO mission. Specifically, we focus on the operational tests for DTN-ION conducted for message and file transfer, as well as real-time video streaming, during the initial operations of the KPLO. Lastly, this study offers insights and lessons learned from KPLO DTNPL operations, with the goal of providing valuable guidance for future advancements in space communication.

Establishing requirements for lunar and cislunar orbital debris tracking

There is a need to formally coordinate lunar and cislunar space traffic management to mitigate the risk of collisions with micrometeoroids and orbital debris in this space domain. To control this hazard, it will be critical to develop a high-fidelity orbital debris catalog. This catalog will be maintained by monitoring and propagating the trajectory of objects. The need to perform a debris avoidance maneuver for collision avoidance will depend on the fidelity of the propagated debris trajectory. A larger uncertainty (magnitude as a function of time) will require a larger maneuver and a higher likelihood of the need to perform this maneuver. This study assesses debris avoidance maneuvers and corresponding corrective actions to recover the desired mission trajectory as a way to evaluate the desired capability of tracking of objects in lunar and cislunar orbit. The baseline Earth-to-Moon trajectory was that of the Artemis I mission. Typical conjunction assessments were postulated at 100,000 km from Earth's center (approximately a quarter of the way to the moon) and at lunar orbit insertion (LOI). For these two cases, the required debris avoidance maneuver (and hence impact to the mission) is tied to the uncertainty in orbital debris tracking. The study provides a methodology and baseline inputs to establish future requirements of debris tracking in lunar and cislunar orbits. It also reinforces the importance of long-term sustainability for lunar missions, specifically preventing the generation of orbital debris.

Code-Based Differential GNSS Ranging for Lunar Orbiters: Theoretical Review and Application to the NaviMoon Observables

In the near future, international space agencies have planned to achieve significant milestones in investigating the utilization of Global Navigation Satellite Systems (GNSS) within and beyond the current space service volume up to their application to lunar missions. These initiatives aim to demonstrate the feasibility of GNSS navigation at lunar altitudes. Based on the outcomes of such demonstrations, dozens of lunar missions will likely be equipped with a GNSS receiver to support autonomous navigation in the lunar proximity. Relying on non-invasive, consolidated differential techniques, GNSS will enable baseline estimation, thus supporting a number of potential applications to lunar orbiters such as collaborative navigation, formation flight, orbital manoeuvers, remote sensing, augmentation systems and beyond. Unfortunately, the large dynamics and the geometry of such differential GNSS scenarios set them apart from current terrestrial and low-earth orbit use cases. These characteristics result in an increased sensitivity to measurements time misalignment among orbiters. Hence, this paper offers a review of baseline estimation methods and characterizes the divergences and limitations w.r.t. to terrestrial applications. The study showcases the estimation of the baseline length between a lunar CubeSat mission, VMMO, and the communication relay Lunar Pathfinder mission. Notably, real GNSS measurements generated by an Engineering Model of the NaviMoon receiver in the European Space Agency (ESA/ESTEC) Radio Navigation Laboratory are utilized. A radio-frequency constellation simulator is used to generate the GNSS signals in these hardware-in-the-loop tests. The performed analyses showed the invalidity of common terrestrial differential GNSS ranging techniques for space scenarios due to the introduction of significant biases. Improved ranging algorithms were proposed and their potential to cancel ranging errors common to both receivers involved was confirmed.