Hayabusa2 pinpoint touchdown near the artificial crater on Ryugu: Trajectory design and guidance performance
Keywords:Asteroid landing;Orbital motion;Gravity field;Payload deployment;Autonomous navigation;Trajectory reconstruction
Abstracts:One of the major challenges in the Hayabusa2 sample-return mission was the second touchdown on the asteroid Ryugu, which was designed to collect subsurface materials near the artificial crater formed by a small carry-on impactor. Due to engineering and scientific requirements, a narrow area with a radius as small as 3.5 m was selected as the target landing site. To achieve pinpoint touchdown at the selected site, an artificial landmark called a target marker (TM) was used for optical navigation. The key to a successful touchdown was precise deployment of the TM in the microgravity environment of the asteroid. This study therefore investigates a viable trajectory for TM deployment, incorporating the uncertainty in the impact and rebound motions of the TM. Following the TM deployment operation, a detailed survey of the landing site around the TM settlement point is performed to assess the terrain conditions. To guarantee the observation quality and spacecraft safety, multi-impulse low-altitude trajectories are introduced in this paper, along with covariance analyses based on the high-fidelity polyhedral gravity model of Ryugu. Subsequently, a pinpoint touchdown trajectory that satisfies various engineering requirements, such as landing accuracy and velocity, is designed, taking advantage of optical TM tracking. The feasibility of the touchdown sequence is further validated by a Monte Carlo dispersion analysis. Consequently, Hayabusa2 successfully touched down within the target site on 11 July 2019. The current research also conducts a post-operation trajectory reconstruction based on the flight data to demonstrate the actual guidance performance in the TM deployment, landing site observations, and pinpoint touchdown.
Developing novel multi-plane satellite constellation deployment methods using the concept of nodal precession
Keywords:Satellite constellation deployment;Nodal precession;Multi-objective optimization;Total deployment time;Deployment deltaV
Abstracts:Deployment of multi-plane satellite constellations has become a fresh challenge in the space program missions since all the direct solutions including plane changing maneuvers and separate launches are energy and time-consuming. A proposed solution to this challenge is launching several satellites simultaneously and deploying them using gravitational perturbations of the Earth. Utilizing this solutions, only in-plane satellite maneuvers are sufficient to deploy a constellation of satellites in several orbital planes. The aim of this study is to extend and develop the present methods that use the concept of nodal precession to deploy a multi-plane satellite constellation with only one launch in order to cover a wider range of mission specifications, provide mathematical expressions for the total deployment time and the fuel expenditure of these methods, optimize the methods multi-objectively and provide a thorough analysis and comparison between them. Two general methods are proposed in this study; one that places all the satellites in a parking orbit and injects them into their final orbits sequentially, and one that first places them in different drifting orbits and after a certain amount of time, maneuvers them to their final orbits. The findings of the optimization process suggest that the method which involves all the satellites in the process of deployment simultaneously and by this means exploits the full nodal precessing potential of all of them provides better results in comparison with the method with a more passive approach, which considers the deployment a sequential process. Finally, since this mathematical modeling is performed for the first time in the literature, the mathematical expressions for both methods are analyzed to provide beneficial insights for the designers.
Robust finite-time control design for attitude stabilization of spacecraft under measurement uncertainties
Keywords:Attitude stabilization;Measurement uncertainties;Robust control;Finite-time control;Finite-time disturbance observer
Abstracts:This paper investigates the finite-time attitude stabilization of spacecraft under multiple uncertainties. Besides inertia uncertainties, external disturbances, and actuator faults, the measurement uncertainties are particularly considered. When including the measurement uncertainties, the spacecraft attitude control system becomes a mismatching system, which brings a great challenge to the control design. To resolve this problem, a novel robust finite-time attitude control approach is proposed by using a dual-disturbance-observer-based structure. First, two finite-time disturbance observers are developed to estimate the mismatched and matched lumped disturbances in the spacecraft attitude kinematics and dynamics, respectively. Then, the robust finite-time attitude controller is synthesized by incorporating the two finite-time disturbance observers into the adding a power integrator technique. It is strictly proved that the proposed robust finite-time attitude controller can ensure the actual attitude and angular velocity stabilize to the small neighborhoods around the origin in finite time even subject to multiple uncertainties. Benefiting from the feedforward disturbance compensation, the proposed controller is not only robust against inertia uncertainties, external disturbances, and actuator faults, but also insensitive to measurement uncertainties. Lastly, the effectiveness and advantages of the proposed control approach are validated through simulations and comparisons.
Homotopy-SQP coupled method for optimal control of far-distance nonplanar rapid cooperative rendezvous with multiple specific-direction thrusts
Keywords:Cooperative rendezvous;Multiple specific-direction thrusts;Indirect method;Intelligent algorithm;Optimal control
Abstracts:In this paper, the far-distance nonplanar rapid cooperative rendezvous with multiple specific-direction thrusts is investigated. In the optimization process, the indirect method is adopted. The homotopy method is used to smooth the dynamical equations. The solutions of the initial co-state variables of the energy-optimal problem are obtained by the quantum-behaved particle swarm optimization (QPSO) algorithm. Sequential quadratic programming (SQP) algorithm is coupled into the homotopy method to overcome the difficulty in nonsmooth integration when the thrust switches on & off frequently. The solutions of the fuel-optimal problem are obtained by the homotopy-SQP coupled method. The simulation results show the feasibility of the cooperative rendezvous with multiple specific-direction thrusts. The “suitable point” of normal thrust is investigated.
Trajectory optimization and maintenance for ascending from the surface of Phobos
Keywords:Mars-Phobos system;Ascend;Trajectory optimization;Particle swarm optimization;Trajectory maintenance
Abstracts:The ascending trajectory from the surface of Phobos to a resonant quasi-satellite orbit around Phobos is investigated with a newly proposed dynamical model based on the Elliptic Restricted Three-Body Problem. The proposed model incorporates the non-spherical gravity field and physical libration of Phobos as well. The trajectories from the surface of Phobos are classified into three types according to their z-componentsas short-, middle-, and long-term ascending trajectories. The total ΔV of the two-impulse ascending trajectories is optimized with the particle swarm optimization method. The total ΔV and time of flight of the optimized ascending trajectories are analyzed, and the Pareto Front is refined from the optimized solutions. A multi-impulse maintenance strategy based on the target point method is constructed to ensure that the ascender can insert into the target orbit accurately along the nominal trajectory in the real environment. The robustness of the maintenance strategy is validated by Monte-Carlo simulations in a high-fidelity model, i.e., the N-body problem with the ephemeris, Phobos’ physical libration, non-spherical gravity field of Phobos, and a Gaussian uncertain perturbation. The number of the correction impulses needed is highly positively correlated with the time of flight of the trajectory. Based on the results, middle-term ascending trajectories with less time of flight on the Pareto Front are recommended.
Future Venus missions and flybys: A collection of possible measurements with mass spectrometers and plasma instruments
Keywords:Venus;Mass spectrometry;BepiColombo;Solar Orbiter;Phosphine
Abstracts:This study contains predictions for mass spectrometry and plasma instrument measurements during upcoming Venus flybys of BepiColombo and Solar Orbiter and discusses the possibility of a phosphine detection with mass spectrometry in Venus’ upper atmosphere. The results extend the ones published previously in Gruchola et al. (2019), where predictions for the proposed ESA mission EnVision and the Venus flyby of JUICE were included. Both the Venus flyby of BepiColombo and the one of Solar Orbiter will take place in August 2021, only 1 day apart. BepiColombo, carrying the neutral mass gas spectrometer STROFIO, could probe the atmosphere around closest approach and obtain data on the thermal and hot neutral particle populations in the upper atmosphere. According to this study, the thermal hydrogen population as well as the hot species H, C, N and O, including isotopes, should be visible to STROFIO. Especially data on the abundance of the hot species would yield important insight into the planetary escape processes. The Solar Orbiter on the other hand carries the plasma instrument SWA-HIS, designed to measure the energetic solar wind ions. During its second Venus gravity assist it will traverse the magnetosheath, the region between bow shock and ion composition boundary, where in addition to the solar wind ions energetic planetary ions are present. The planetary pickup ions can be measured by SWA-HIS, providing important information on the outflow of planetary ions and the ionization processes in the upper atmosphere itself. The recent reports of phosphine in Venus’ cloud decks with an abundance of 20 ppb at 80 km probably overestimate the actual PH3 abundance, as the data are currently being reanalyzed (Greaves et al., 2021). However, even with this upper limit of 20 ppb a phosphine detection with a mass spectrometer e.g. on-board ESA’s proposed EnVision mission, seems unlikely. To resolve the PH3 peak and the 16O18O peak a mass resolution of almost 10’000 is required, and the PH2D peak is masked by the Cl fragment peak of HCl. Furthermore, NGMS on-board Pioneer Venus with a mass resolution of around 440 did most likely not detect phosphine, as it scanned only a few masses in the mass range of interest where more abundant species than phosphine are present.
On the horizontal currents over the Martian magnetic cusp
Keywords:Horizontal currents;Martian Magnetic cusps;Ion-neutral frequency;Electron-neutral frequency;Ion and electron gyro-frequencies
Abstracts:Localized crustal magnetization over heavily cratered southern hemisphere at Mars gives rise to open magnetic field configurations which interact with the solar wind magnetic field to form magnetic cusp. The downward acceleration of energetic electrons in these cusps can produce aurora and an extended topside ionospheric structure over regions of magnetic anomalies. We report plasma collisions with the neutral atmosphere at one of the Martian cusps located at 82oS and 180oE, where the crustal field is strong with a radial component ~35° from the local zenith. We find that the dynamo region in the upper ionosphere of Mars is located between altitudes of 102 km and 210 km. The electrons in this region are constrained to gyrate along magnetic field lines while ions are dragged by neutrals and move along the direction of applied force. In the absence of the electric field, the horizontal current in the Martian dynamo is generated by the differential motion of ions and electrons. We find that the bulk of the current density is equatorward and confined within the Martian dynamo near the ionospheric peak with a magnitude of ~3.5 µA/m2. We also find that the eastward current density of magnitude ~0.4 µA/m2 peaking near the upper boundary of the Martian dynamo is generated by magnetized ions in the F × B direction.
Scaling uncertainties on asteroid characteristics to prepare datasets for machine learning
Keywords:Asteroids;Solar system;Space resources;Planetary defence;Machine learning
Abstracts:Physical and dynamical characterisations of asteroids are used in different fields, such as Solar System formation modelling, Planetary Defence and Resources Prospecting. The vast majority of asteroids are not known in detail - have at best their orbit well defined - and the knowledge on the composition or internal structure is derived by models of reflectivity curves, with limited certainty. Machine learning methods have begun to be used on asteroid datasets, but the major uncertainties about their characteristics are slowing down the applicability. This paper reviews some stakes and challenges of asteroid exploration, and why the introduction of common characterisation factors would be beneficial for the asteroid science community, especially with the application of machine learning methodologies. A preliminary scale to quantify the characterisation of asteroids is proposed, and finally discusses its interests and limitations for machine learning applications. The investigated characteristics of asteroids are: size/shape, orbital dynamics, mass/density, spin, internal structure and composition. This paper reviews the current methods used to determine these parameters, and provides a preliminary scale based on the certainty associated with the different measurements. Characterisation factors are useful to build datasets that will be used in machine learning algorithms applied to asteroid science. The ratio of currently known asteroids in each defined bin of characterisation factor is estimated. Moreover, a total characterisation factor that yield a preliminary quantification of our knowledge about a specific asteroid (i.e. the sum of all the certainties about its different characteristics) is defined. Finally, characterisation factors for specific applications can be introduced using an adapted weighting system. This preliminary work provides a baseline for scaling uncertainties of asteroids properties. The next step is to create viable datasets using application specific characterisation factors that would allow the use of advanced machine learning algorithms already available.
Progressive aqueous alteration and iron oxidation record in the matrix of Mukundpura CM2 chondrite, a new fall
Keywords:Mukundpura;Carbonaceous chondrite CM2;Aqueous alteration;Fe oxidation state;EXAFS
Abstracts:The matrices of CM chondrites are fine-grained and have undergone aqueous alteration. Based on mineral chemical composition of matrix phyllosilicate of Mukundpura CM2, the evidence of progressive aqueous alteration is understood and also reflected in gradual changes in S/SiO2 and FeO/SiO2 ratios. Further, we performed X-ray absorption near-edge spectroscopy at the Fe K-edge to explain the redox state of the phyllosilicate. The Iron Oxidation Index of Mukundpura yields 2.423 and the Fe+3/ Fe+2 ratio is 0.733. The Fe speciation in phyllosilicate is also sensitive to the degree of aqueous activity. Our data are consistent with the presence of Cronstedtite and Tochilinite-Cronstedtite intergrowths in the matrix suggested for different extent of alteration and recorded history of pervasive aqueous alteration in the asteroidal parent body.
Performance assessment of real-time precise point positioning using BDS PPP-B2b service signal
Keywords:BDS;PPP-B2b;GPS;Precise point positioning;Real-time PPP
Abstracts:China’s BeiDou Navigation Satellite System (BDS) can provide real-time precise point positioning (PPP) service for users in China and surrounding areas with PPP-B2b signal. In this paper, real-time PPP is assessed and studied based on 8 days of PPP-B2b and BDS/GPS broadcast messages. Real-time precise ephemerides are compared against Multi-GNSS Experiment (MGEX) final products and the positioning performance of real-time PPP is evaluated with MGEX/iGMAS stations. Results show that the real-time orbit accuracy is about 0.0725, 0.2479 and 0.2587 m in directions of radial (R), along (A) and cross (C) for BDS-3 satellites, and 0.0729, 0.3098 and 0.2193 m for GPS satellites. The standard deviation values of the clock offset errors are about 0.05–0.18 ns for BDS-3 satellites, and 0.05–0.15 ns for GPS satellites. In addition, the differential code bias (DCB) parameters of PPP-B2b agree well with parameters of MGEX DCB and time group delay (TGD), with the differences less than 2 and 0.15 ns, respectively. Furthermore, for the positioning experiments, the average positioning accuracies of BDS and GPS integrated real-time PPP are about 1.07, 2.69 and 2.25 cm in north (N), east (E), and up (U) directions for static mode; and about 3.6, 5.9 and 9.4 cm for kinematic mode. Moreover, the convergence times for an accuracy level of 20 cm in N, E and U components are about 8–22 min, and 10–8 min, for static mode and kinematic mode, respectively.