Lunar Polar Orbiter Research Articles

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.

Thermal Design of the High-Resolution Volatiles and Minerals Moon Mapper (HVM3) Instrument on the Lunar Trailblazer Mission (LTB)

The Lunar Trailblazer (LTB) Mission consists of a spacecraft that hosts two science instruments, the High-resolution Volatiles and Minerals Moon Mapper (HVM3) and the Lunar Thermal Mapper. Flight system delivery is scheduled for the Fall of 2022. The purpose of the mission is to understand the form, abundance and distribution of water on the Moon as well as the lunar water cycle. HVM3 is a pushbroom shortwave infrared (SWIR) Offner imaging spectrometer that is optimized for the detection of volatiles to map OH, bound H2O and water ice. It has a spatial resolution of 70 m/pixel over a 20 km swath width and a spectral resolution of 10 nm over a spectral range of 0.6 µm to 3.6 µm. The spacecraft will deploy from an ESPA Grande and enter a 100±30 km lunar polar orbit where it may operate over all beta angles. The HVM3 thermal control architecture consists of active and passive elements. It leverages the passive cryogenic cooler design developed for the Moon Minerology Mapper (M3) that underwent a similar orbit. The first stage of the passive cooler is used to reject the heat of a Lockheed Martin Micro1-2 cryocooler that is used to actively cool the focal plane array (FPA). The second stage of the passive cooler is used to passively cool the optics. An overview of the overall thermal control design approach is presented.

Korea Pathfinder Lunar Orbiter Magnetometer Instrument and Initial Data Processing

The Korea Pathfinder Lunar Orbiter (KPLO), the first South Korea lunar exploration probe, successfully arrived at the Moon on December, 2022 (UTC), following a 4.5-month ballistic lunar transfer (BLT) trajectory. Since the launch (4 August, 2022), the KPLO magnetometer (KMAG) has carried out various observations during the trans-lunar cruise phase and a 100 km altitude lunar polar orbit. KMAG consists of three fluxgate magnetometers capable of measuring magnetic fields within a ± 1,000 nT range with a resolution of 0.2 nT. The sampling rate is 10 Hz. During the originally planned lifetime of one year, KMAG has been operating successfully while performing observations of lunar crustal magnetic fields, magnetic fields induced in the lunar interior, and various solar wind events. The calibration and offset processes were performed during the TLC phase. In addition, reliabilities of the KMAG lunar magnetic field observations have been verified by comparing them with the surface vector mapping (SVM) data. If the KPLO’s mission orbit during the extended mission phase is close enough to the lunar surface, KMAG will contribute to updating the lunar surface magnetic field map and will provide insights into the lunar interior structure and lunar space environment.

Science Missions of the Korean Lunar Exploration Program

The Korea Pathfinder Lunar Orbiter (KPLO), which is the Korean first lunar and space exploration spacecraft, will be launched in August 2022 and arrive in a lunar orbit in December 2022. The KPLO will carry out nominal missions while in a lunar polar orbit an ~100-km altitude for one year. The KPLO has five lunar science mission payloads and one technology demonstration payload in order to achieve their own science and technology goals. The science payloads consist of four Korean domestic instruments and one internationally collaborated science instrument for scientific investigations on the lunar surface and in a space environment. The Korean dometstic science instruments are the gamma-ray spectrometer named KGRS, the wide-angle polarimetric camera named PolCam, the fluxgate magnetometer named KMAG, and the high resolution camera named LUTI. The name of the internationally collaborated science instrument is ShandowCam, which was developed by Arizona State University, U.S., and funded and managed by NASA. The science data acquired by the science payloads will be released to the public in order to enhance scientific and educational achievements. The science data acquired by each science instrument will be archived and released through the web sites of the KPDS (KARI Planetary Data System) for the Korean science instruments and the NASA PDS (Planetary Data System) for the internationally collaborated science instrument.

Earth as an exoplanet mission concept for a lunar orbiting cubesat

There is a growing interest in Lunar exploration fed by the perception that the Moon can be made accessible to low-cost missions in the next decade. The on-going projects to set a communications relay in Lunar orbit and a deep space Gateway, as well as the spreading of commercial-of-the shelf (COTS) technology for small space platforms such as the cubesats contribute to this perception. Small, cubesat size satellites orbiting the Moon offer ample opportunities to study the Moon and enjoy an advantage point to monitor the Solar System and the large scale interaction between the Earth and the solar wind. In this article, we describe the technical characteristics of a 12U cubesat to be set in polar Lunar orbit for this purpose and the science behind it. The mission is named EarthASAP (Earth AS An exoPlanet) and was submitted to the Lunar Cubesats for Exploration (LUCE) call in 2016. EarthASAP was designed to monitor hydrated rock reservoirs in the Lunar poles and to study the interaction between the large Earth's exosphere and the solar wind in preparation for future exoplanetary missions.

Observational Arc-Length Effect on Orbit Determination for KPLO Using a Sequential Estimation Technique

In this study, orbit determination (OD) simulation for the Korea Pathfinder Lunar Orbiter (KPLO) was accomplished for investigation of the observational arc-length effect using a sequential estimation algorithm. A lunar polar orbit located at 100 km altitude and 90° inclination was mainly considered for the KPLO mission operation phase. For measurement simulation and OD for KPLO, the Analytical Graphics Inc. Systems Tool Kit 11 and Orbit Determination Tool Kit 6 software were utilized. Three deep-space ground stations, including two deep space network (DSN) antennas and the Korea Deep Space Antenna, were configured for the OD simulation. To investigate the arc-length effect on OD, 60-hr, 48-hr, 24-hr, and 12-hr tracking data were prepared. Position uncertainty by error covariance and orbit overlap precision were used for OD performance evaluation. Additionally, orbit prediction (OP) accuracy was also assessed by the position difference between the estimated and true orbits. Finally, we concluded that the 48-hr-based OD strategy is suitable for effective flight dynamics operation of KPLO. This work suggests a useful guideline for the OD strategy of KPLO mission planning and operation during the nominal lunar orbits phase.

Influence of the Choice of Lunar Gravity Model on Orbit Determination for Lunar Orbiters

We examine the influence of the lunar gravity model on the orbit determination (OD) of a lunar orbiter operating in a 100 km high, lunar polar orbit. Doppler and sequential range measurements by three Deep Space Network antennas and one Korea Deep Space Antenna were used. For measurement simulation and OD analysis, STK11 and ODTK6 were utilized. GLGM2, LP100K, LP150Q, GRAIL420A, and GRAIL660B were used for investigation of lunar gravity model selection effect. OD results were assessed by position and velocity uncertainties with error covariance and an external orbit comparison using simulated true orbit. The effect of the lunar gravity models on the long-term OD, degree and order level, measurement-acquisition condition, and lunar altitude was investigated. For efficiency verification, computational times for the five lunar gravity models were compared. Results showed that significant improvements to OD accuracy are observed by applying a GRAIL-based model; however, applying a full order and degree gravity modeling is not always the best strategy, owing to the computational burden. Consequently, we consider that OD using GRAIL660B with 70 × 70 degree and order is the most efficient strategy for mission preanalysis. This study provides useful guideline for KPLO OD analysis during nominal mission operation.

Comparative analysis of one- and two-way planetary laser ranging concepts

Laser ranging is an emerging technology for tracking interplanetary missions, offering both range accuracy and precision at the level of several millimeters. The ground segment uses existing Satellite Laser Ranging (SLR) technology, whereas the space segment requires an active system for either one- or two-way ranging. We numerically investigate the performance of one- and two-way active planetary laser ranging systems to quantify the difference in science return from missions employing this technology. In doing so, we assess the added value of the more complicated two-way system compared to its one-way counterpart. We simulate range measurement errors for both types of systems, using clock error time histories generated from Allan variance profiles. We use two test cases: a lunar polar orbiter and a Phobos lander. In the Phobos lander simulations, we include the estimation of Phobos librations and C¯2,2 gravity field coefficient. For the lunar orbiter, we include an empirical force-error model in our truth model. We include the estimation of clock parameters over a variety of arc lengths for one-way range data analysis and use a variety of state arc durations for the lunar orbiter simulations. For the lunar orbiter, performance of the one- and two-way system is similar for sufficiently short clock arcs. This indicates that dynamical-model error, not clock noise, is the dominant source of estimation uncertainty. However, correlations between the clock and state parameters cause an exchange between clock and state signal for the one-way system, making these results less robust. The results for the Phobos lander show superior estimation accuracy of the two-way system. However, knowledge of Phobos' interior mass distribution from both the one- or two-way system would currently be limited to the same level by inaccuracies in our knowledge of Phobos' volume. Both the lunar orbiter and Phobos lander simulations show that the use of two-way planetary laser ranging should be accompanied by improvements in associated measurements and models to allow this data type to be exploited to its full potential.

The Chang’e 3 Mission Overview

The Chang’e 3 (CE-3) mission was implemented as the first lander/rover mission of the Chinese Lunar Exploration Program (CLEP). After its successful launch at 01:30 local time on December 2, 2013, CE-3 was inserted into an eccentric polar lunar orbit on December 6, and landed to the east of a 430 m crater in northwestern Mare Imbrium (19.51°W, 44.12°N) at 21:11 on December 14, 2013. The Yutu rover separated from the lander at 04:35, December 15, and traversed for a total of 0.114 km. Acquisition of science data began during the descent of the lander and will continue for 12 months during the nominal mission. The CE-3 lander and rover each carry four science instruments. Instruments on the lander are: Landing Camera (LCAM), Terrain Camera (TCAM), Extreme Ultraviolet Camera (EUVC), and Moon-based Ultraviolet Telescope (MUVT). The four instruments on the rover are: Panoramic Camera (PCAM), VIS-NIR Imaging Spectrometer (VNIS), Active Particle induced X-ray Spectrometer (APXS), and Lunar Penetrating Radar (LPR). The science objectives of the CE-3 mission include: (1) investigation of the morphological features and geological structures of and near the landing area; (2) integrated in-situ analysis of mineral and chemical composition of and near the landing area; and (3) exploration of the terrestrial-lunar space environment and lunar-based astronomical observations. This paper describes the CE-3 objectives and measurements that address the science objectives outlined by the Comprehensive Demonstration Report of Phase II of CLEP. The CE-3 team has archived the initial science data, and we describe data accessibility by the science community.

External Heat Flux of the Solar Array on the Polar Lunar Orbit

准确的月球表面温度分布模型对于开展月球探测具有重要意义. 目前有关月球表面温 度模型还缺乏对完整月球表面温度分布的计算方法研究. 本文建立了一套计算完整月球表面温度的方法, 其中月球阳面温度采用Racca模型直接计算得到; 对于月球阴面, 将其沿纬度方向划分为若干区域, 每个区域的地表土壤采用一维非稳态热传导模型, 根据嫦娥三号着陆器太阳电池阵在轨环月阶段的温度数据, 修正得到月球表面土壤导热系数、密度及比热容, 通过数值计算求解一维非稳态热传导方程, 得出任意时刻月球阴面表面温度随时间的变化. 嫦娥三号着陆器太阳电池阵环月阶段热分析结果与在轨温度符合较好, 初步说明本文建立的完整月球表面温度计算方法正确可行. 基于本文方法计算得到整个月球表面温度分布, 进一步研究了极月轨道太阳电池阵外热流变化规律.

Detection of the lunar body tide by the Lunar Orbiter Laser Altimeter.

The Lunar Orbiter Laser Altimeter instrument onboard the Lunar Reconnaissance Orbiter spacecraft collected more than 5 billion measurements in the nominal 50 km orbit over ∼10,000 orbits. The data precision, geodetic accuracy, and spatial distribution enable two-dimensional crossovers to be used to infer relative radial position corrections between tracks to better than ∼1 m. We use nearly 500,000 altimetric crossovers to separate remaining high-frequency spacecraft trajectory errors from the periodic radial surface tidal deformation. The unusual sampling of the lunar body tide from polar lunar orbit limits the size of the typical differential signal expected at ground track intersections to ∼10 cm. Nevertheless, we reliably detect the topographic tidal signal and estimate the associated Love number h2 to be 0.0371 ± 0.0033, which is consistent with but lower than recent results from lunar laser ranging.Key PointsAltimetric data are used to create radial constraints on the tidal deformationThe body tide amplitude is estimated from the crossover dataThe estimated Love number is consistent with previous estimates but more precise

Uplink Radio Frequency Signal Fluctuations

KAGUYA was a Japanese large lunar explorer that orbited and observed the moon from 2007 to 2009. During in-orbit operation, unexpected fluctuations and lock-offs of KAGUYA’s uplink radio frequency telecommand signal were sometimes observed. To investigate the cause of the fluctuations, archived telemetry data were surveyed, and the telemetry data that showed the fluctuation were extracted. According to fault tree analysis of the telemetry data, multipath interference from the lunar surface and the spacecraft structure was deemed the most probable cause. This finding was verified by geometrical analysis of multipath interference. On the basis of the analysis results, the fluctuation map for a lunar polar orbiter was presented for the first time. This map was used to avoid the telecommand operation failure of KAGUYA. This lesson can be generally applied to operations of planet and lunar orbiters.

Bursts of energetic electron induced large surface charging observed by Chang'E-1

A relationship between surface charging and bursts of energetic electron (BEE) event is presented in this paper. In a 200km lunar polar orbit, during quiet time, 0.1–2.0MeV BEE events were observed by High Energetic Particles Detectors (HPD) on board Chang'E-1, on December 22, 2007, when the spacecraft was within the inner terrestrial magnetosheath. At the same time, a large surface charging of ∼−5.4kV was observed by Chang'E-1, which was evidenced by increasing the ions energy observed by Solar Wind Ion Detectors (SWIDs). We found that the surface charging is strongly correlated with BEE events, and the potentials of spacecraft surface charging was experientially expressed as U≈3.6×10−5·fT (kV). The BEE events did occur in the solar wind, geomagnetic tail and magnetosheath alternately, whereas the surface charging during the BEE events is in the magnetosheath or transition region of boundaries. Though the observed surface charging was fewer than the BEE events, it is expected that the occurrence of the charging events caused by the bursts of energetic electrons should be more frequent than the Chang'E-1 observations. Meanwhile, the spacecraft charging indicates the lunar surface can be charged to negative kilovolt-scale by the BEE events even in quiet times.

Lunar radiation environment and space weathering from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER)

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) measures linear energy transfer by Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs) on the Lunar Reconnaissance Orbiter (LRO) Mission in a circular, polar lunar orbit. GCR fluxes remain at the highest levels ever observed during the space age. One of the largest SEP events observed by CRaTER during the LRO mission occurred on June 7, 2011. We compare model predictions by the Earth‐Moon‐Mars Radiation Environment Module (EMMREM) for both dose rates from GCRs and SEPs during this event with results from CRaTER. We find agreement between these models and the CRaTER dose rates, which together demonstrate the accuracy of EMMREM, and its suitability for a real‐time space weather system. We utilize CRaTER to test forecasts made by the Relativistic Electron Alert System for Exploration (REleASE), which successfully predicts the June 7th event. At the maximum CRaTER‐observed GCR dose rate (∼11.7 cGy/yr where Gy is a unit indicating energy deposition per unit mass, 1 Gy = 1 J/kg), GCRs deposit ∼88 eV/molecule in water over 4 billion years, causing significant change in molecular composition and physical structure (e.g., density, color, crystallinity) of water ice, loss of molecular hydrogen, and production of more complex molecules linking carbon and other elements in the irradiated ice. This shows that space weathering by GCRs may be extremely important for chemical evolution of ice on the Moon. Thus, we show comprehensive observations from the CRaTER instrument on the Lunar Reconnaissance Orbiter that characterizes the radiation environment and space weathering on the Moon.

Lifetime test and heritage on orbit of coolers for space use

This report describes the results and operating status of ground lifetime testing and achievements on orbit of coolers for space use. Ground lifetime tests of coolers of three types were conducted to demonstrate their long life and reliability. Three single-stage Stirling coolers were tested for 89,016, 71,871 and 68,273h from 1998, a two-stage Stirling cooler was tested for 72,906h, and a 4-K class cooler with a two-stage Stirling cooler and a Joule–Thomson cooler was tested for over 2.5years. After lifetime tests were completed, a few coolers were investigated to determine the cause of the cooling performance degradation. Additionally, the filled gas of the coolers was analyzed. These coolers have shown good results on orbit. Three single-stage Stirling coolers were carried on the X-ray astronomical satellite “SUZAKU” (launched in July 2005), Japanese lunar polar orbiter “KAGUYA” (launched in September 2007), and the Japanese Venus Climate Orbiter “AKATSUKI” (launched in June 2010). Two units of a two-stage Stirling cooler were carried on the infrared astronomical satellite “AKARI” launched in February 2006. A 4-K class cooler was carried on the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) aboard the Japanese Experiment Module (JEM) of the International Space Station (ISS). SMILES was launched in September 2009.

Development of Precise Lunar Orbit Propagator and Lunar Polar Orbiter's Lifetime Analysis

Korea Aerospace Research Institute, Deajeon 305-600, KoreaTo prepare for a Korean lunar orbiter mission, a precise lunar orbit propagator; Yonsei precise lunar orbit propagator (YSPLOP) is developed. In the propagator, accelerations due to the Moon’s non-spherical gravity, the point masses of the Earth, Moon, Sun, Mars, Jupiter and also, solar radiation pressures can be included. The developed propagator’s performance is validated and propagation errors between YSPOLP and STK/Astrogator are found to have about maxi-mum 4-m, in along-track direction during 30 days (Earth’s time) of propagation. Also, it is found that the lifetime of a lunar polar orbiter is strongly affected by the different degrees and orders of the lunar gravity model, by a third body’s gravitational attractions (especially the Earth), and by the different orbital inclinations. The reliable lifetime of circular lunar polar orbiter at about 100 km altitude is estimated to have about 160 days (Earth’s time). However, to estimate the reasonable lifetime of circular lunar polar orbiter at about 100 km altitude, it is strongly recommended to consider at least 50 × 50 degrees and orders of the lunar gravity field. The results provided in this paper are expected to make further progress in the design fields of Korea’s lunar orbiter missions.

Foreword: An Exploration-Driven Renaissance in Lunar Science

In the 31⁄2 years between the first human lunar landing and the final launch, the Apollo engineering team enormously expanded the range and performance of surface systems and added orbital mapping capabilities. High-resolution cameras and a laser altimeter in the Service Module Bay photographed and profiled the lunar surface, including areas that might be future Apollo landing sites. A gamma ray spectrometer and an X-ray spectrometer provided data for construction of maps indicating the concentration of certain elements. In today’s mode of doing business, a similar systems evolution would take a decade or more. While the gains in surface mobility, astronaut EVA time, and precision landing were impressive and productive, the first geochemical (elemental) maps from the orbital spectrometers startled many planetary scientists. Those maps could be interpreted in terms of the rock types identified in the returned samples. In principle, scientists could begin to infer the geological evolution of a planet had the mapping been global. To that end, leaders in the lunar science community approached NASA about the addition of a small, unmanned polar orbiter mission. When the Administrator suggested that Apollo 17 could be an orbit-only mission, the conversation ceased. In 1973, the Manned Spacecraft Center (now the Johnson Space Center) submitted an unsolicited proposal to NASA Headquarters for an inexpensive lunar polar orbiter with only geochemical instruments, i.e., no cameras. The Office of Space Science was taken by surprise and eventually decided to compete the mission. Of the three NASA Centers that proposed, MSC had the most robust lunar science capability; but the evaluation board doubted that a human spaceflight center would be capable of cost containment on a small spacecraft project. The mission was first assigned to the Goddard Space Flight Center and, after a year of little progress, was transferred to the Jet Propulsion Laboratory. There, the mission acquired the designation Terrestrial Bodies Orbiter—Lunar (TBOL) to be consistent with the recently issued report of the Terrestrial Bodies Science Working Group. That advisory com-

Lunar Reconnaissance Orbiter (LRO): Observations for Lunar Exploration and Science

The Lunar Reconnaissance Orbiter (LRO) was implemented to facilitate scientific and engineering-driven mapping of the lunar surface at new spatial scales and with new remote sensing methods, identify safe landing sites, search for in situ resources, and measure the space radiation environment. After its successful launch on June 18, 2009, the LRO spacecraft and instruments were activated and calibrated in an eccentric polar lunar orbit until September 15, when LRO was moved to a circular polar orbit with a mean altitude of 50 km. LRO will operate for at least one year to support the goals of NASA’s Exploration Systems Mission Directorate (ESMD), and for at least two years of extended operations for additional lunar science measurements supported by NASA’s Science Mission Directorate (SMD). LRO carries six instruments with associated science and exploration investigations, and a telecommunications/radar technology demonstration. The LRO instruments are: Cosmic Ray Telescope for the Effects of Radiation (CRaTER), Diviner Lunar Radiometer Experiment (DLRE), Lyman-Alpha Mapping Project (LAMP), Lunar Exploration Neutron Detector (LEND), Lunar Orbiter Laser Altimeter (LOLA), and Lunar Reconnaissance Orbiter Camera (LROC). The technology demonstration is a compact, dual-frequency, hybrid polarity synthetic aperture radar instrument (Mini-RF). LRO observations also support the Lunar Crater Observation and Sensing Satellite (LCROSS), the lunar impact mission that was co-manifested with LRO on the Atlas V (401) launch vehicle. This paper describes the LRO objectives and measurements that support exploration of the Moon and that address the science objectives outlined by the National Academy of Science’s report on the Scientific Context for Exploration of the Moon (SCEM). We also describe data accessibility by the science and exploration community.

Nonsphericity of the Moon and Near Sun‐Synchronous Polar Lunar Orbits

Herein, we consider the problem of a lunar artificial satellite perturbed by the nonuniform distribution of mass of the Moon taking into account the oblateness (J2) and the equatorial ellipticity (sectorial term C22). Using Lie‐Hori method up to the second order short‐period terms of the Hamiltonian are eliminated. A study is done for the critical inclination in first and second order of the disturbing potential. Coupling terms due to the nonuniform distribution of mass of the Moon are analyzed. Numerical simulations are presented with the disturbing potential of first and second order is. It an approach for the behavior of the longitude of the ascending node of a near Sun‐synchronous polar lunar orbit is presented.

Lunar Surface Roughness Estimation Using Stereoscopic Data

On September 14, 2007, Selenological and Engineering Explorer (SELENE), which is a Japanese lunar polar orbiter, was launched. SELENE carries the optical instrument, which is referred to as the Lunar Imager / SpectroMeter (LISM). The LISM is composed of three sensors. One of sensors of the LISM is the Terrain Camera (TC). TC is used to produce Digital Terrain Model (DTM) from stereo observation of lunar surface. In this work, the lunar surface bidirectional reflectance is investigated from images acquired by the TC. The Radiance Ratio (RR) of surface-reflected solar radiance measured from two view angles is obtained. The estimation of lunar surface roughness using images of the TC is discussed. As a result, it is found that the moon surface is not based on Lambert's law.