津田 雄一

© 2019, Tsinghua University Press. This paper describes attitude dynamics properties of spinning, momentum-biased and zero-momentum solar sail spacecraft. The model called “Generalized Sail Dynamics Model” (GSDM) is introduced, which can deal with general and practical sail configurations, such as arbitrary optical property distribution, shape and surface wrinkles. Attitude stability criteria and other key dynamical characteristics are derived and compared by compact analytical equations induced from the GSDM. The newly derived zero-momentum sail dynamics is compared with that of spinning and momentum-biased sails. It is shown that the spinning and momentum sails have an advantage in terms of dynamical stability whereas zero-momentum sails are only statically stable. With this special property, angular momentum-stabilized sails can realize a sun-pointing stable attitude with almost zero-fuel, which are discussed with actual space flight experience of the JAXA’s two interplanetary missions, IKAROS and Hayabusa2.

© 2017 The Authors. The near-Earth asteroid (162173) Ryugu is a 900-m-diameter dark object expected to contain primordial material from the solar nebula. The Mobile Asteroid Surface Scout (MASCOT) landed on Ryugu’s surface on 3 October 2018. We present images from the MASCOT camera (MASCam) taken during the descent and while on the surface. The surface is covered by decimeter- to meter-sized rocks, with no deposits of fine-grained material. Rocks appear either bright, with smooth faces and sharp edges, or dark, with a cauliflower-like, crumbly surface. Close-up images of a rock of the latter type reveal a dark matrix with small, bright, spectrally different inclusions, implying that it did not experience extensive aqueous alteration. The inclusions appear similar to those in carbonaceous chondrite meteorites.

162173 Ryugu, the target of Hayabusa2, has a round shape with an equatorial ridge, which is known as a spinning top-shape. A strong centrifugal force is a likely contributor to Ryugu's top-shaped features. Observations by Optical Navigation Camera onboard Hayabusa2 show a unique longitudinal variation in geomorphology; the western side of this asteroid, later called the western bulge, has a smooth surface and a sharp equatorial ridge, compared to the other side. Here, we propose a structural deformation process that generated the western bulge. Applying the mission-derived shape model, we employ a finite element model technique to analyze the locations that experience structural failure within the present shape. Assuming that materials are uniformly distributed, our model shows the longitudinal variation in structurally failed regions when the spin period is shorter than ~3.75 h. Ryugu is structurally intact in the subsurface region of the western bulge while other regions are sensitive to structural failure. We infer that this variation is indicative of the deformation process that occurred in the past, and the western bulge is more relaxed structurally than the other region. Our analysis also shows that this deformation process might occur at a spin period between ~3.5 h and ~3.0 h, providing the cohesive strength ranging between ~4 Pa and ~10 Pa.

© 2018 IAA In the exploration of small bodies, the orbiting operation is very significant in terms of fuel consumption and scientific observation of small bodies. Although the force field around a small body is strongly perturbed, there is a family of stable orbits, called terminator orbits, that exist. However, although a terminator orbit is stable, it also has disadvantages. The orbital plane of a terminator orbit must always face the Sun and lacks flexibility in orbit design. Moreover, since the orbital plane lies in the night side because of the solar radiation pressure (SRP) shifting the equilibrium point, optical observation of the small body is extremely restricted. Therefore, the present study focuses on the perturbed terminator orbit, i.e., the quasi-terminator orbit (QTO), which is a stable orbit that does not suffer from impact with the surface or escape. The present study attempts to reveal the solution space and the usability of the QTO. We succeeded in analytically deriving the existence range of the long-term QTO and verified that the numerical and analytical solutions coincide well. As a result, the calculation time required for solving the existence range of the long-term stable QTO can be greatly shortened, and it becomes possible to quickly apply our findings to other missions by accounting for the gravity field and SRP of the mission-specific small body.

© 2018 IAA The Japan Aerospace Exploration Agency launched the asteroid sample return spacecraft “Hayabusa2” on December 3, 2014. Hayabusa2 will reach the C-type asteroid 162173 Ryugu in 2018, and return back to the Earth in 2020. Sample collections from three sites, four surface rovers deployment and a 4 MJ-class kinetic impact crater forming are planned in the 1.5 years of the asteroid-proximity operation. The mission objective of Hayabusa2 has three aspects, science, engineering and exploration, all of which would be expanded by the successful round-trip journey. The objectives and technologies used in this mission is not a direct solution for the future planetary defense, but should contribute to this field by increasing general asteroid knowledge and enhancing human capabilities of small body-surface access/roving/sampling/impacting. This paper describes the outline of the Hayabusa2 mission, overviews the kinetic impact technology as an example of planetary defense-related technologies and the current flight status after the two and a half years of the interplanetary cruise.

© 2018 IEICE This paper describes the pulse operation characteristics of GaN radar amplifiers for space and civil use. A single-ended GaN amplifier was fabricated using a single die and a printed circuit board and operated with maximum output power of 70.8 W, gain of 8.5 dB, and DC-RF efficiency of 38.5% at 9.5 GHz. The amplifier operated under a pulse width of 10 (is and duty ratio of 10%. A parallel-combined amplifier generated 125.9 W output and 31.5% efficiency under the same conditions as a single-ended amplifier. The developed GaN amplifiers are applicable to the tracking radar for the Hayabusa 2 re-entry capsule and search radars for the marine use.

Hayabusa2 mission is the ongoing JAXA's sample and return mission to Ryugu asteroid. In late 2018, Ryugu was in superior solar conjunction. Therefore, the Hayabusa2 spacecraft experienced communication blackouts while leaving its hovering position of 20 km from Ryugu. In this article, the design of a safe conjunction trajectory is given in the Hill frame and then verified in the full-body. Two Trajectory Correction Manoeuvres (TCMs) are scheduled before and after the deep conjunction. A linear covariance analysis is shown together with the results of the Monte Carlo analysis to compute the stochastic Delta V at TCMs. Pre/Post-flight operation data are also compared.

The Japanese Hayabusa2 asteroid sample return mission deployed the two small MINERVA-II-1A/B rovers to the surface of asteroid Ryugu in September 2018. An overview of the rover deployment analysis and landing site selection, which was subject to various mission constraints, is provided in this paper. During the deployment, the Hayabusa2 spacecraft obtained images using its three navigation cameras. These images are analyzed using control point matching to identify moving bright and dark spots. By combining the rectified positions of these spots, three object tracks can be observed. A basic understanding of the rover release mechanism enabled classifying the two rovers and their cover to the respective tracks. Initial results on the trajectories reconstruction of these tracks suggest that one rover flew a nominal trajectory while the other was ejected in a somewhat perturbed direction. The cover is seen impacting the surface at roughly 30 cm/s and rebounds with a relatively high energetic restitution of 0.6 to 0.8. Further analysis will narrow down the descent trajectory of the two rovers and provide a more precise estimate of the cover's rebound on the asteroid surface.

<p>The Martian Moons eXploration (MMX) mission is now under study by the Japan Aerospace Exploration Agency (JAXA). Its scope includes the world's first landing on one of the Martian moons, collecting samples from the surface, and returning to Earth. This paper describes the orbit design for MMX. Nominal and backup trajectories for launch in 2024 and 2026 are discussed. The Mars orbit insertion (MOI) sequence using 3-impulse maneuvers is introduced. A new scheme, the robust MOI, is also proposed as a contingency to enhance the robustness of the mission sequence. A method to design a robust MOI trajectory and examples are presented.</p>

An optical navigation method for autonomous landing on asteroids using asteroid shape model is presented. Vertices of the shape model are tracked in the sequential images obtained by a monocular camera. The proposed method does not need the process of landmark detection or mapping. The pose of the spacecraft is estimated using particle filter, considering the dynamics around the asteroid. The performance of the developed navigation method is evaluated via numerical simulation; it is based on the touchdown rehearsal operation in Hayabusa2 Mission to show the effectiveness of the proposed method against actual asteroid exploration missions.

© 2019 American Association for the Advancement of Science. All rights reserved. The near-Earth carbonaceous asteroid 162173 Ryugu is thought to have been produced from a parent body that contained water ice and organic molecules. The Hayabusa2 spacecraft has obtained global multicolor images of Ryugu. Geomorphological features present include a circum-equatorial ridge, east-west dichotomy, high boulder abundances across the entire surface, and impact craters. Age estimates from the craters indicate a resurfacing age of ≤ 106 years for the top 1-meter layer. Ryugu is among the darkest known bodies in the Solar System. The high abundance and spectral properties of boulders are consistent with moderately dehydrated materials, analogous to thermally metamorphosed meteorites found on Earth. The general uniformity in color across Ryugu's surface supports partial dehydration due to internal heating of the asteroid's parent body.

© 2019 American Association for the Advancement of Science. All rights reserved. The Hayabusa2 spacecraft arrived at the near-Earth carbonaceous asteroid 162173 Ryugu in 2018.We present Hayabusa2 observations of Ryugu's shape, mass, and geomorphology. Ryugu has an oblate "spinning top" shape, with a prominent circular equatorial ridge. Its bulk density, 1.19 ± 0.02 grams per cubic centimeter, indicates a high-porosity (>50%) interior. Large surface boulders suggest a rubble-pile structure. Surface slope analysis shows Ryugu's shape may have been produced from having once spun at twice the current rate. Coupled with the observed global material homogeneity, this suggests that Ryugu was reshaped by centrifugally induced deformation during a period of rapid rotation. From these remote-sensing investigations, we identified a suitable sample collection site on the equatorial ridge.

© 2019 American Association for the Advancement of Science. All rights reserved. The near-Earth asteroid 162173 Ryugu, the target of the Hayabusa2 sample-return mission, is thought to be a primitive carbonaceous object. We report reflectance spectra of Ryugu's surface acquired with the Near-Infrared Spectrometer (NIRS3) on Hayabusa2, to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 micrometers was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, which is consistent with Ryugu being a compositionally homogeneous rubble-pile object generated from impact fragments of an undifferentiated aqueously altered parent body.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. On October 3rd, 2018, the MASCOT lander has been deployed successfully by the Hayabusa2 spacecraft from an altitude of 41m onto the C-type near-Earth asteroid (162173) Ryugu. After a free-fall of approx. 6 minutes MASCOT has experienced its first contact with the asteroid. The lander underwent a bouncing phase of ~ 11 minutes before it finally came to rest at its first settlement point where it entered into its on-surface operational mode. The lander was able to perform science measurements with its payload suite at 3 locations on Ruygu. After about 17 hrs of operations, the MASCOT mission terminated with the last communication contact. The lander was jointly developed by the German Aerospace Centre (DLR) and the Centre National d'Etudes Spatiales (CNES). The payload suite of four scientific instruments is provided by DLR Berlin (MASCam wide-angle camera with colour illumination and MARA thermal IR radiometer), IAS Paris (MicrOmega hyperspectral IR soil microscope) and TU Braunschweig (MASMag magnetometer). The paper will outline the path of the lander on the asteroid and present a summary of the scientific observations. These first results are related to the progress of the Hayabusa2 mission and its MINERVA-II rovers on the background of a recap of the landing site selection process.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. One of the primary operations in the Hayabusa2 mission is touchdown on the asteroid Ryugu, which was successfully performed on February 21st, 2019. Because of the abundance of boulders on the asteroid surface, it was challenging to guarantee a safe and secure landing. To identify a promising landing site and design a feasible landing trajectory even under such a situation, this research develops detailed site selection and dispersion analysis strategies. The dispersion of the landing points is computed by a Monte Carlo simulation. Moreover, the distributions of landing conditions, such as a surface contact angle and a clearance distance, are analyzed, validating the feasibility of the touchdown operation. Consequently, a circular area with a radius of 3 m was selected as a safe landing site, leading to the successful landing.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. The asteroid explorer Hayabusa2 was launched by Japan Aerospace Exploration Agency (JAXA) on December 3rd, 2014. The main mission of the probe is to sample pieces of asteroid and bring it back to the Earth in order to do more advanced scientific analysis on the ground. After three years' cruising phase, Hayabusa2 finally arrived at the asteroid Ryugu on June 28th, 2018, and a mission operation has been started. Hayabusa2 carries several rovers and separates them to land on the asteroid surface. One of these rovers is called MASCOT which was developed under the collaboration between Deutsches Zentrum für Luft- und Raumfahrt (DLR) and Centre national d'études spatiales (CNES). This rover was planned to be separated to the asteroid surface and executes several missions on the asteroid surface. In order to support this mission, the mother ship Hayabusa2 is requested to separate this rover at very low altitude around 50m and after separation to hover around 3 km altitude for realizing the assured communication link with MASCOT. On October 2nd - 5th, 2018, we performed the operation for MASCOT release. In the decent operation, Hayabusa2 was successfully guided to the target point which is specified by the MASCOT team, and the MASCOT was released at 51m altitude which satisfies the altitude criteria defined between MASCOT team and Harabusa2 operation team. After ascent to altitude 3km, Hayabusa2 started the hovering operation in order to ensure the communication between Hayabusa2 and MASCOT. The spacecraft position was adequately controlled to hover around the target position, and the MASCOT operation after landing was successfully executed. In this paper, we introduce the GNC operation scheme and show the flight results of entire operation for MASCOT release.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. Hayabusa2 is a Japanese sample return mission from the asteroid Ryugu. The Hayabusa2 spacecraft was launched on 3 December 2014 and arrived at Ryugu on 27 June 2018. It will stay there until December 2019 for in situ science observation and surface sample collection and will return to the Earth with the collected sample in December 2020. During the stay, the spacecraft is planned to carry out several numbers of descent operation to deploy and land rovers, and to touchdown and collect surface sample. On 22nd February 2019, the spacecraft successfully touched down on Ryugu. Since the surface of Ryugu is extremely rough and full of boulders, and the number of areas with small-enough and low-enough boulders is limited. The target point named “L08-B” has “safety area” with radius of only 3m and the accuracy required to the GNC (Guidance, Navigation and Control) of the spacecraft was challenging. For the “pinpoint touchdown” it was necessary to follow step-by-step approach including touchdown rehearsal descent operations in order to check GNC sequence, performance of laser sensors such as LIDAR (long range laser altimeter) and LRF (short range four beam laser distance sensor), to collect closer images of the surface of Ryugu and to drop TM (Target Maker) that is retro-reflected ball used for image-based navigation reference. There are three key GNC features for “pinpoint touchdown”, that is (1) position control of the spacecraft from the altitude of 20km to 45m toward the area that TM is in the field of view of navigation camera, (2) feed-forward attitude control of the spacecraft in order to align to the unlevel touch down area using noisy LRF output, (3) high precision six-degrees-of-freedom feed-back control using TM as a lateral position navigation reference in order to descend and touch down to the small safety area. This paper introduces strategy, sequence and algorithms for the last two GNC features for touch down of Hayabusa2 stated above with numerical simulation results and actual flight data.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. A Japanese interplanetary prove “Hayabusa2" was launched on December 3rd, 2014. After long transfer period including the ion engine powered cruising, the prove arrived at the vicinity of C-type asteroid 162173 Ryugu on June 27th, 2018 and started its 1.5 years asteroid proximity phase aiming "touchdown" for surface sample collection to carry back to the Earth by end of 2020. Overcoming the asteroid's unexpected exceptionally rough surface covered by substantial numbers of harmful boulders, first touchdown was accomplished on February 21st, 2019 to confirm nominal progress of entire touchdown sequence which is expected to have secured surface sample. This paper focuses on planning methodology and operation of entire touchdown sequence. First, boundaries and restriction for operation planning including representative features of landing target are described. Then, operation sequence introducing “pinpoint touchdown method” newly developed for Hayabusa2 together with schematics of onboard function as well as circumstances and results of rehearsals. Finally, achievements of Hayabusa2's first touchdown operation targeting L08-E1 area are reported based on actual event timeline and flight data.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. Hayabusa2 is a Japanese interplanetary probe launched on December 3, 2014. It arrived at asteroid Ryugu on June 27, 2018. During stay around Ryugu, it has succeeded in several challenging operations, including two rovers/a lander landings, two sample collections, and a kinetic impact. The kinetic impact is one of the biggest challenges of the Hayabusa2 mission. Investigating the physical and chemical properties of the internal materials and structures is an important scientific objective. Small Carry-on Impactor (SCI) was developed to achieve the aim. The SCI is a compact kinetic impactor designed to remove the asteroid surface regolith locally and create an artificial crater. The spacecraft deployed the SCI on April 5, 2019, and the SCI successfully created an artificial crater with a diameter of 10 m. This paper describes the operation planning of the kinetic impact and summarizes the operation results.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. Hayabusa2 is a Japanese interplanetary spacecraft to explore the near-Earth asteroid Ryugu. The spacecraft touched down to Ryugu for soil sample collection on 21 February 2019 for the first time. This paper presents an overview and flight results of a guidance, navigation and control method used in the touchdown operation. The method consists of on-board and on-ground guidance systems including an image-based navigation technique using a shape model and ground control points of the asteroid. The flight results show that the performance of the systems satisfied accuracy requirements and contributed to the success of the operation.

© 2019, Univelt Inc. All rights reserved. This research investigates retrograde teardrop orbits (RTOs) about asteroids subject to strong solar radiation pressure. RTOs are closed orbits that are made periodic by introducing a deterministic impulsive delta-V within each period. This type of artificial periodic orbit provides high flexibility in orbit design compared with natural periodic orbits. RTOs are promising options for asteroid missions because of their stability and small delta-V values (on the order of 10 cm/s or less). This paper presents the dynamical theories of RTOs and possible applications for the Hayabusa2 mission.

Copyright © 2019 SPIE. Digital Electronics and Optical Navigation Camera (DE-ONC) is an edge computing node of the asteroid probe HAYABUSA2. DE-ONC was developed to provide real-time image recognition performance for optical navigation. Lightweight, low power consumption and miniaturization are realized to overcome resource restrictions. It also satisfies high reliability and safety requirements of HAYABUSA2 missions. There are static and dynamic requirements for reliability and safety. The former increases reliability by adding redundancy combining the concept of functional distribution and time-division redundancy to meet resource constraints. Functional distribution mode, standby redundancy mode and hot redundancy mode were realized with the same device configuration. The real-time performance of optical navigation exploiting image recognition functions of the unit was demonstrated through the interplanetary cruising phase, as well as touch down to and taking off from the asteroid Ryugu. DE-ONC is always required to operate in the critical operation phase. In addition to that, it must always satisfy latency requirements to complete processing within a predetermined duration and to guarantee hard real-time performance. In order to satisfy these requirements, the image processing unit of DE-ONC adopts a unified language processing system and a distributed memory model with reference to a parallel inference machine, which is a so-called the second generation artificial intelligence technology. Its image processing module integrates a radiation hardened micro-controller unit (MCU) and field programmable gate arrays (FPGAs) with the language processing system and the distributed object model. We report the evaluation result of reliability and safety with real-time performance of the unit's architecture.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. Japan Aerospace Exploration Agency (JAXA) launched the 1st C-type asteroid exploration and sample return probe "Hayabusa2" in December 3rd, 2014, and it arrived at C-type asteroid \Ryugu (1999ju3)" in the end of June, 2018. Hayabusa2 conduced science observation and several rehearsals of touch-down (TD) operation. Then, Hayabus2 has succeeded 1st TD and sampling operation on the surface of Ryugu on February 22nd, 2019. TD operation for sampling is conducted at the surface area “L08-E1” where is without 50-cm sized or larger boulders and with the local surface angle of <30 deg for safety. Hayabusa2 controls 6-DOF (position, velocity) by autonomous on-board function and attitude / angular rate by feed-forward command at the altitude of 8.5 m before the final descent phase. Then, Hayabusa2 descents by a free fall in initial velocity -7.4cm/s from 8.5m altitude. After approximately 100 seconds, Hayabusa2 touches the surface of Ryugu and collects sample using the sampling system that applies the same projectile method as that of 1st Hayabusa. The projectile is shot for sampling, which is triggered by the detection of the bending of the sampler horn by a short-range laser range finder (LRF-S2). For planning the TD operation, the important thing is to set sequence of event from final descent phase and detection parameters. We simulated applying dynamics model of Hayabusa2 including a sampler horn model and a surface reaction model to verify the sequence, detection method and parameters. Multibody dynamics and ground contact model are implemented in this simulator. Hayabusa2 has several detection methods to detect the touch-down and trigger shot of projectile for sampling, that is, fluctuation of signal of distance and intensity measured by LRF-S2, attitude /angular rate fluctuation measured by the inertia reference unit of AOCS subsystem, acceleration integral value. We discussed appropriate threshold values for each detection method based on the result of the dynamics simulation. The dynamics simulation is also used to study spacecraft dynamics and safety of both spacecraft and the sampler horn during spacecraft contacts on the asteroid which surface properties are unknown. In this paper, the design and parameter decision process which includes TD simulation results will be described. In addition, we will report the result of actual 1st TD dynamics and sampling operation, which is the world's first achievement in the deep space exploration.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. Hayabusa2 arrived at the C-type asteroid Ryugu in June 2018. During one and a half year of the Ryugu-proximity operation, we succeeded in two rovers landing, one lander landing, two spacecraft touchdown/sample collection, one kinetic impact operation and two tiny reflective balls and one rover orbiting. Among the two successful touchdowns, the second one succeeded in collecting subsurface material exposed by the kinetic impact operation. This paper describes the asteroid proximity operation activity of the Hayabuas2 mission, and gives an overview of the achievements done so far. Some important engineering and scientific activities, which have been done in synchronous with the spacecraft operations to tackle with unexpected Ryugu environment, are also described.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. This paper describes an autonomous optical navigation to estimate the relative position of a spacecraft with a target body for deep space explorations. The asteroid exploration spacecraft Hayabusa2 touched down on the asteroid Ryugu with pin-point accuracy in February 2019. In the case of Hayabusa2, the asteroid-relative position is estimated by ground operators. On the other hand, in the case of explorations to small bodies farther than Main-belt, the communication delay is unacceptably large for the asteroid-relative feedback guidance. This situation becomes worse for larger asteroids because the time constant of the dynamics becomes faster. Therefore, importance of high-seed on-board optical navigation is highlighted for explorations to far distant bodies. To accomplish the high-precision and high-speed on-board optical navigation that can be applied to various bodies, the Vector Code Correlation (VCC) algorithm suitable for field-programmable gate array (FPGA) is focused on. This method is a type of correlation calculation for the template matching that finds the most similar part of 2 images by comparing them. In the case of the VCC algorithm, by discretization of luminance gradient into 3 patterns, data size of each pixel can be reduced from 8 to 4 bit without losing feature amount. Accordingly, their correlation can be calculated at high speed via XOR operations on FPGA. In this study, the VCC-based position estimation method was developed and implemented on an FPGA board. The proposed method mainly consists of 2 steps: generation of a reference image from nominal position by rendering; estimation of the deviation between a captured image and a reference image by the VCC algorithm. In addition, the VCC algorithm on multiple planes in images was implemented in order to improve the estimation accuracy at low altitudes, where the outlines of the target bodies cannot be seen in the images. The estimation accuracy and computational time werw evaluated by comparing the proposed method with other methods. As a result, the position estimation accuracy of approximately image resolution size (1pixel size) in real space is achieved. Finally, by demonstration of the proposed method to the flight data of Hayabusa2 in landing phase, it was found that it can be applied to a real mission environment from the aspect of estimation accuracy and computational time.

© 2018 by the American Institute of Aeronautics and Astronautics, Inc. This paper investigates coupled orbit-attitude dynamics around asteroids subject to solar radiation pressure and gravity irregularities. The solutions of sun-synchronous orbits with sun-tracking attitude motion are analytically derived, and their stability is evaluated by applying linearization and averaging. To validate the analytical solutions, numerical simulations are performed based on nonlinear coupled orbit-attitude equations of motion. In addition, the nonlinear stability of such coupled motion is analyzed using finite-time Lyapunov exponents. It is demonstrated that the sun-synchronous orbit-attitude coupled motions exhibit long-term stability under certain conditions, and thus, these motions are promising options for asteroid missions.

© 2019, Univelt Inc. All rights reserved. An optical navigation method for autonomous landing on asteroids using asteroid shape model is presented. Vertices of the shape model are tracked in the sequential images obtained by a monocular camera. The proposed method does not need the process of landmark detection or mapping. The pose of the spacecraft is estimated using particle filter, considering the dynamics around the asteroid. The performance of the developed navigation method is evaluated via numerical simulation; it is based on the touchdown rehearsal operation in Hayabusa2 Mission to show the effectiveness of the proposed method against actual asteroid exploration missions.

© 2019, Univelt Inc. All rights reserved. Hayabusa2 mission is the ongoing JAXA’s sample and return mission to Ryugu asteroid. In late 2018, Ryugu was in superior solar conjunction. Therefore, the Hayabusa2 spacecraft experienced communication blackouts while leaving its hovering position of 20 km from Ryugu. In this article, the design of a safe conjunction trajectory is given in the Hill frame and then verified in the full-body. Two Trajectory Correction Manoeuvres (TCMs) are scheduled before and after the deep conjunction. A linear covariance analysis is shown together with the results of the Monte Carlo analysis to compute the stochastic ΔV at TCMs. Pre-/Post-flight operation data are also compared.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. The scientific interest towards asteroids has increased in the last few years, leading to several successful missions such as past Hayabusa (JAXA), Rosetta (ESA), and currently Hayabusa2 (JAXA) and OSIRIS-REx (NASA). Several efforts have been made to study the environmental condition around asteroids, to characterise the dynamical behaviour of orbits about small bodies. One of the most challenging aspects of such missions is to collect and sample asteroids material by means of an on-ground collection, involving landing (or touchdown) and mining. This paper analyses the evolution of the dust dynamics around asteroids in the context of the circular restricted three-body problem, perturbed by the solar radiation pressure and the aspherical potential of the asteroid (J2 effect). The aim is to carry out an analysis showing if particles ejected by means of a kinetic impactor can be temporary captured around the asteroid, leading to a potential threat for the spacecraft's sampling operations. The main goal of this work is to study the dynamics arising from the re-impact and bouncing of particles ejected from the asteroid surface, analysed by considering non-elastic collisions. These collisions can potentially cause a trail of particles, captured for several months; this mechanism could explain the recent discovery of trails, observed for asteroid P/2010 A2. In case these mechanisms are well understood, the asteroid's impact location can be selected, as a function of the high survival on-orbit probability. Since the dynamics involved is different depending on the particles size, as already showed by past works, the solar radiation pressure acceleration acts like a passive in-situ mass spectrometer. Therefore, future missions could consider on-orbit collection as an alternative to landing or touchdown operations. The artificial impact performed during the Hayabusa2 mission makes the asteroid Ryugu the ideal case study.

© 2019, Univelt Inc. All rights reserved. The Japanese Hayabusa2 asteroid sample return mission deployed the two small MINERVA-II-1A/B rovers to the surface of asteroid Ryugu in September 2018. An overview of the rover deployment analysis and landing site selection, which was subject to various mission constraints, is provided in this paper. During the deployment, the Hayabusa2 spacecraft obtained images using its three navigation cameras. These images are analyzed using control point matching to identify moving bright and dark spots. By combining the rectified positions of these spots, three object tracks can be observed. A basic understanding of the rover release mechanism enabled classifying the two rovers and their cover to the respective tracks. Initial results on the trajectories reconstruction of these tracks suggest that one rover flew a nominal trajectory while the other was ejected in a somewhat perturbed direction. The cover is seen impacting the surface at roughly 30 cm/s and rebounds with a relatively high energetic restitution of 0.6 to 0.8. Further analysis will narrow down the descent trajectory of the two rovers and provide a more precise estimate of the cover’s rebound on the asteroid surface.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. As for missions that explore small bodies such as asteroids or comets, landmark-based optical navigation is widely used in such operations as descent or landing. The Japanese asteroid explorer Hayabusa2 successfully performed two touchdowns on the asteroid Ryugu in 2019, using one of the landmark-based optical navigation. Hayabusa2 realized the guidance, navigation, and control with accuracy of less than 5 m at the touchdowns. On the other hand, this navigation method strongly depends on the terrain surface of the target celestial bodies, and also requires laborious work to detect sufficient number of landmarks in images. This paper presents an optical navigation method that is independent of landmarks as an advanced study for future missions. The movement of a global surface, rather than a local point, is focused to enable visual tracking without relying on landmarks. The result of the visual tracking yields the pose of the probe via perspective projection equation. The function of the developed method is simulated using the flight data of Hayabusa2.

© 2018 IAA This paper presents the robust planning of the Hayabusa2-Ryugu proximity operation and landing site selection process considering unknown asteroid environment and the spacecraft constraints. The proximity operation scenario is described together with the relationship between the selection process and the in-situ observation. The mission constraints are summarized for the possible asteroid environment, including the rotation state, thermal condition and gravity.

© 2018 Mars Moon eXplorer (MMX) is JAXA's next candidate flagship mission to be launched in the early 2020s. MMX will explore the Martian moons and return a sample from Phobos. This paper presents the mission analysis work, focusing on the transfer legs and comparing several architectures, such as hybrid options with chemical and electric propulsion modules. The selected baseline is a chemical-propulsion Phobos sample return, which is discussed in detail with the launch- and return-window analysis. The trajectories are optimized with the jTOP software, using planetary ephemerides for Mars and the Earth; Earth re-entry constraints are modeled with simple analytical equations. Finally, we introduce an analytical approximation of the three-burn capture strategy used in the Mars system. The approximation can be used together with a Lambert solver to quickly determine the transfer Δv costs.

© 2018 The Society of Instrument and Control Engineers - SICE. We proposed a localization method for spacecraft navigation in the proximity of asteroids. This method uses particle filtering with LIDAR measurement and the terrain data of an asteroid. This type of localization is called terrain-aided localization (TAL), and has been studied intensively in the fields of aeronautics and underwater robotics. However, there exist characteristic difficulties caused by the terrain uncertainty in the application of the TAL to asteroid proximity operations. To overcome these difficulties, we developed a robust terrain-aided localization method, which is effective for general TAL problems unlimited to spacecraft applications. The strength of this method lies in the capability to probabilistically address the collision of a LIDAR ray with an asteroid. The core idea of this method is to perform Monte Carlo sampling at the point at which a LIDAR ray collides with different independently simulated terrains in every trial via stochastic process. This sampling is enabled by utilizing the signed distance function. To confirm the effectiveness of the proposed method, we conducted numerical simulations. The results show the effectiveness of the proposed method.

This study investigates coupled orbit-attitude dynamics around asteroids subject to solar radiation pressure and gravity irregularities. The solutions of Sun synchronous orbits with Sun-tracking attitude motion are analytically derived by applying linearization and averaging. To verify the validity of the analytical solutions, numerical simulations are performed based on non-linear coupled orbit attitude equations of motion. In addition, the stability of such coupled motion is analyzed using finite-time Lyapunov exponents. It is demonstrated that the Sun synchronous orbit-attitude coupled motions exhibit long-term stability under certain conditions, and thus, these motions are useful and feasible options for asteroid missions.

This paper addresses a stability analysis for an attitude model called the Generalized Sail Dynamics Model, which describes attitude dynamics of a momentum-biased spacecraft with arbitrary shape and optical reflectance properties. In the model, there is a coupling between internal angular momentum of a spacecraft and solar radiation pressure (SRP) torque. This results in passive sun tracking attitude motion, and therefore, stability is particularly important. In this paper, general stability conditions are derived analytically by eigenvalue and phase plane analyses, and are verified with a numerical analysis by computing SRP torque acting on a spacecraft. The results provide an insight into spacecraft design for stable attitude motion.

This paper describes a methodology to find almost-optimum trajectories which are robust against inflight stochastic events, such as navigation/guidance error and unexpected missed thrust due to temporal spacecraft malfunctions. A Monte-Carlo based solution search technique was developed which can generate robustness-increased trajectories by deoptimizing the original solution. Arbitrary practical control constraints can be imposed, and one can obtain a solution range in the neighborhood of the original solution which improves the stochastic events-robustness. The technique was applied to an asteroid sample-return mission Hayabusa2 to improve the missed-thrust recoverability, which are presented in detail in this paper.

Copyright © 2018 by the International Astronautical Federation (IAF). All rights reserve It will be nearly a four year long journey from launch with the Hayabusa2 spacecraft on Dec 3 rd , 2014, until the beginning of October 2018 when MASCOT ('Mobile Asteroid surface SCOuT') should land on the Near-Earth Asteroid (162173) Ryugu. During cruise phase, MASCOT underwent several inflight and ground based health checks, instrument calibration and subsystem tests to prepare the lander for its biggest challenge: operating autonomously on the asteroid's surface for up to 16 hours. Within this time, MASCOT should provide scientific data of the surface and physical properties of asteroid Ryugu with its 4 scientific instruments: a wide angle camera with night-time colour illumination (MASCAM), an imaging IR spectrometer microscope (MicrOmega), a multichannel radiometer (MARA), and a magnetometer (MasMAG). MASCOT has an internal mobility unit which enables it to self-right, to place itself into the desired science operation orientation and to relocate on the surface in order to explore more than one site. The lander was jointly developed by the German Aerospace Centre (DLR) and the Centre National d'Etudes Spatiales (CNES). The four payloads are provided by DLR Berlin (MASCAM and MARA), IAS Paris (MicrOmega) and TU Braunschweig (MASMAG). The landing of MASCOT is currently scheduled between 2018, October 1 st - 4 th .

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Hayabusa2 is an asteroid sample return mission operated by the Japanese space agency, JAXA. It was launched in December 2014. In July 2018 the spacecraft will reach the mission target after a 4-year-long cruise. The objective is a C-type primordial asteroid called Ryugu, in search of organic and hydrated minerals that might give essential clues for the solar system formation. The small lander MASCOT (Mobile Asteroid surface SCOuT) carried aboard Hayabusa2 intends to land on the surface for in-situ investigations while the probe is aiming to study Ryugu on a global scale and to return samples to Earth. MASCOT was jointly developed by the German Aerospace Centre (DLR) and the Centre National d'Etudes Spatiales (CNES). It is equipped with a sensor suite consisting of four fully-fledged instruments. DLR was responsible for developing the MASCOT lander and ground segment, and is in charge of planning and conducting lander operations. CNES supplied the hyperspectral IR spectrometer (MicrOmega, IAS Paris), antennas and electrical power system that would be essential contributors to the on-asteroid operation success. These subsystems are partly inherited from Philae lander onboard Rosetta mission. CNES is responsible for MASCOT flight dynamics and is also providing an operational support for RF and power system, based on the expertise gained on the past science missions. The lander mission is similar to the previous Philae mission nevertheless the timeline and the resources are more constrained than it was for Philae. Indeed the characteristics of the Ryugu asteroid such as the shape and the gravity will be known only after arrival of Hayabusa2 in July 2018. In addition to this, MASCOT’s battery allows to operate only for a few hours. So the time available to prepare those 2 asteroid days of science activities on the surface will be very short and dedicated processes and tools were developped to cope with these constraints. Thus a training phase is mandatory to deal with the different processes and all the teams involved. An expertise and science mission support was developped at CNES Toulouse to prepare the operations and provide a support to the operational team at DLR. Its functionnalities and interfaces will be detailed as well as the specific training put in place. This paper is a complement to the overall MASCOT operations paper submitted by Christian Krause (DLR). It will summarize the performed and planned activities to prepare the CNES team and means for MASCOT operations while focusing on the improvements/adaptations made on the subsystems inherited from Philae. Some specific topics as flight dynamics team’s involvement in landing site selection, visualisation tool, specific RF link tests and power system tests and developped models are also detailed in specific papers submitted for this spaceops in the dedicated sessions.

Copyright © 2018 by the International Astronautical Federation (IAF). All rights reserved. The solar power sail mission OKEANOS, planned to be launched in 2026, aims for sample return from a Jovian Trojan asteroid. In this mission, propagation delay (100min) is longer than that of Hayabusa2 (60min). Moreover, it is difficult to get sufficient information to make a success of the image-based navigation used in the previous mission. Another GNC method is required under the condition. This paper proposes an image-based on-board GNC method.

Copyright © 2018 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. This paper documents the prearrival planning strategy and analysis for the deployment of rovers on board the Japanese Hayabusa2 asteroid sample return mission. This will allow for rigorous comparison with postarrival results obtained at a future date. Hayabusa2 will arrive at asteroid Ryugu sometime between June and July of 2018. During its stay, the spacecraft will deploy three MINERVA-II rovers and obtain several surface samples. The autonomous Hayabusa2 descent guidance requires the rovers to avoid some exclusion zone around the target sample site(s). Although the limited current knowledge of the Ryugu system prohibits accurate deployment predictions, a mock shape model allows for a qualitative prearrival deployment analysis. Using a signed distance field shape, constant-density polyhedron gravity, and an impulsive contact model, high-fidelity simulations of the MINERVA-II deployment are performed. Simulations to the Ryugu reference sphere identify general trends in the rover motion and suggest the inclusion of a horizontal prerelease maneuver that minimizes the rover tangential velocity at first impact. These insights are applied to the Ryugu training model, in which appreciably small surface dispersions are observed when including the prerelease maneuver and when deploying to a large crater. These results will adjust the nominal mission operation plan at the asteroid.

© 2018 Univelt Inc. All rights reserved. This paper addresses a stability analysis for an attitude model called the Generalized Sail Dynamics Model, which describes attitude dynamics of a momentum-biased spacecraft with arbitrary shape and optical reflectance properties. In the model, there is a coupling between internal angular momentum of a spacecraft and solar radiation pressure (SRP) torque. This results in passive sun-tracking attitude motion, and therefore, stability is particularly important. In this paper, general stability conditions are derived analytically by eigenvalue and phase plane analyses, and are verified with a numerical analysis by computing SRP torque acting on a spacecraft. The results provide an insight into spacecraft design for stable attitude motion.

© 2018 Univelt Inc. All rights reserved. This study investigates coupled orbit-attitude dynamics around asteroids subject to solar radiation pressure and gravity irregularities. The solutions of Sun-synchronous orbits with Sun-tracking attitude motion are analytically derived by applying linearization and averaging. To verify the validity of the analytical solutions, numerical simulations are performed based on non-linear coupled orbit-attitude equations of motion. In addition, the stability of such coupled motion is analyzed using finite-time Lyapunov exponents. It is demonstrated that the Sun-synchronous orbit-attitude coupled motions exhibit long-term stability under certain conditions, and thus, these motions are useful and feasible options for asteroid missions.

Copyright © 2018 by the authors. All rights reserved. This paper describes a modelling technique of the solar radiation pressure (SRP) exerted on spacecraft which can incorporate general surface with anisotropic reflectance property. The SRP torque is a major disturbance for the attitude of spacecraft especially in deep space missions. It depends not only on the shape and attitude of spacecraft, but also on their surface optical reflectance property. Some missions even attempted to exploit the SRP to stabilize and reorient spacecraft attitude rather than compensating it. Two such typical missions are the JAXA's solar sail technology demonstration spacecraft IKAROS and the asteroid explorer Hayabusa2. They modelled very precise SRP torque exerted on the spacecraft bodies (GSDM: Generalized Solar-sail Dynamics Model) and successfully extracted a useful behavior to control the spacecraft attitude. On the other hand, owing to precise SRP modeling used in the operation of IKAROS and Hayabusa2, at least 3% of directional inconsistency has found between the SRP torque estimated from the conventional SRP model used in the GSDM and the actual flight data. Our detailed sensitivity analysis implies the error should be due to unmodelled anisotropic optical reflectance of a large area on the spacecraft, such as solar array panels (SAPs). Therefore, the aim of this paper is to create a precise reflectance distribution model by measurement of a bidirectional reflection distribution functions (BRDF). A BRDF, a technique originally developed in the field of Computer Graphics, describes a reflectance map between arbitrary incident and emitted rays in a general mathematical form. The BRDF measurement was performed for test cells of the Hayabusa2 SAPs. As a result, a distinctive anisotropic reflectance property was found which hadn't been modelled in the conventional model. It was also suggested that this reflection should be due to the microscopic structure of the solar cell. Then a compact mathematical model of the reflection which reproduces the anisotropic property observed in the BRDF measurement has been developed. This model successfully describes the inconsistency between the conventional SRP torque and the flight data and relates material's microscopic structure to the optical property.

© 2018 Univelt Inc. All rights reserved. This paper describes a methodology to find almost-optimum trajectories which are robust against inflight stochastic events, such as navigation/guidance error and unexpected missed thrust due to temporal spacecraft malfunctions. A Monte-Carlo based solution search technique was developed which can generate robustness-increased trajectories by deoptimizing the original solution. Arbitrary practical control constraints can be imposed, and one can obtain a solution range in the neighborhood of the original solution which improves the stochastic events-robustness. The technique was applied to an asteroid sample-return mission Hayabusa2 to improve the missed-thrust recoverability, which are presented in detail in this paper.

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Hayabusa2 is a sample return mission to the C-type asteroid 162173 Ryugu. Hayabusa2 was launched by the Japan Aerospace Exploration Agency in December 2014 and will arrive at the asteroid vicinity in the summer of 2018. Hayabusa2 will explore Ryugu for 1.5 years and return to the Earth in winter 2020. The entire flight period of Hayabusa2 is divided into 4 phases; (i)EDVEGA (launch to Earth gravity assist), (ii) Transfer (Earth gravity assist to asteroid arrival), (iii) Asteroid Proximity, (iv) Return (asteroid to Earth reentry). Different operations are required in each phase. Especially in the asteroid proximity phase, there are many critical events such as low-altitude observation, touching down, rover deployment and crater forming. There are some difficult characteristics in performing these critical operations from ground; navigation and guidance in the submeter accuracy against the microgravity environment of Ryugu, 40 minutes round-trip light time due to the 3.6 billion kilometers distance between Hayabusa2 and the Earth. We have developed a ground operation system to clear the characteristics. This paper presents the ground operation systems overview for Asteroid Proximity Operation to realize high quality operation for mission success.

© 2017 The Author(s). We report results of a laser link experiment between a laser altimeter called light detection and ranging (LIDAR) aboard Hayabusa2 and ground-based satellite laser ranging stations conducted when the spacecraft was near the Earth before and after the gravity assist operation. Uplink laser pulses from a ground station were successfully detected at a distance of 6.6 million km, and the field of view direction of the receiving telescope of the LIDAR was determined in the spacecraft frame. The intensities of the received signals were measured, and the link budget from the ground to the LIDAR was confirmed. By detecting two successive pulses, the pulse intervals from the ground-based station were transferred to the LIDAR, and the clock frequency offset was thus successfully calibrated based on the pulse intervals. The laser link experiment, which includes alignment measurement of the telescopes, has proven to be an excellent method to confirm the performance of laser altimeters before they arrive at their target bodies, especially for deep space missions.[Figure not available: see fulltext.]

© 2017 IAA The motion of a spacecraft in proximity to a small body is significantly perturbed due to its irregular gravity field and solar radiation pressure. In such a strongly perturbed environment, the coupling effect of the orbital and attitude motions exerts a large influence that cannot be neglected. However, natural orbit-attitude coupled dynamics around small bodies that are stationary in both orbital and attitude motions have yet to be observed. The present study therefore investigates natural coupled motion that involves both a Sun-synchronous orbit and Sun-tracking attitude motion. This orbit-attitude coupled motion enables a spacecraft to maintain its orbital geometry and attitude state with respect to the Sun without requiring active control. Therefore, the proposed method can reduce the use of an orbit and attitude control system. This paper first presents analytical conditions to achieve Sun-synchronous orbits and Sun-tracking attitude motion. These analytical solutions are then numerically propagated based on non-linear coupled orbit-attitude equations of motion. Consequently, the possibility of implementing Sun-synchronous orbits with Sun-tracking attitude motion is demonstrated.