FAST Spacecraft Research Articles

Fast Spacecraft Charging Mitigation With Plasma Contactors During High‐Power Electron Beam Emission in the GEO Environment

AbstractThe emission of artificial electron beams causes an active spacecraft charging effect that poses challenges for electron beam sounding experiments. Plasma contactors have been suggested as a potential solution to this issue based on theoretical and simulation studies, but the impact of these contactors on spacecraft potentials in complex space environments remains unexplored. This paper investigates the fast active charging effect of spacecraft under the action of plasma contactors in the geostationary Earth orbit environment by using a two‐dimensional Particle‐in‐Cell model. We analyze and compare the effects of the “ion emission” process of the plasma contactor and the “electron collection” process of the background plasma on the spacecraft potential at different beam and environment parameters, as well as the coupling effects when both processes occur simultaneously. Our findings indicate that the “ion emission” and “electron collection” processes are the primary factors in determining the spacecraft potential in relatively thin and dense plasma environments, respectively. When the two processes occur together, the contactor plasma cloud increases the “electron collection” return current by expanding the range of the positive potential area around the spacecraft, while the dense background plasma mediates and provides the return current to ensure that the plasma contactor can effectively suppress the active charging effect even after the ion sheath is present. These results suggest that a reasonable adjustment of the contactor parameters based on the relationship between the electron beam and the background environment can effectively extend the normal emission time of the electron beam.

Fast spacecraft charging induced by a high-power electron beam emission and its mitigation through a plasma contactor

Active sounding experiments, including tracing magnetic field lines and magnetospheric sounding, may be accomplished in the GEO environment by the active emission of high-power electron beams. The continual emission of electron beam pulses during such experiments may result in the accumulation of positive charges on the spacecraft surface and a rise in the spacecraft potential, which might prohibit the normal emission of succeeding pulses. The plasma contactor has been shown to be a reliable method for neutralizing the spacecraft potential. But as the active charging effect brought on by the high-current electron beam manifests itself more quickly, it is still unknown whether the plasma contactor can reliably control the spacecraft potential in this circumstance. In this study, a two-dimensional PIC model is used to examine the active charging effect brought on by high-power electron beam emission. Findings indicate that the potential neutralization process may be affected by the ion sheath that forms close to the emitting surface of the plasma contactor, which cuts the electrical connection between the spacecraft and plasma in space. By evaluating the quantities and growth speeds of different particles during active charging, we discover that lowering the particle density or pre-emission time of the plasma contactor may bring the spacecraft potential to the equilibrium state. Additionally, the high-current electron beam raises the peak potential, making it more difficult to launch the electron beam properly before the spacecraft potential reaches equilibrium. In contrast, the high-energy electron beam is less susceptible to the active charging effect.

Radial distribution of plasma at comet 67P

Context. The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko (67P) for more than two years at a slow walking pace (~1 m s−1) within 1500 km from the nucleus. During one of the radial movements of the spacecraft in the early phase of the mission, the radial distribution of the plasma density could be estimated, and the ionospheric density was found to be inversely proportional to the cometocentric distance r from the nucleus (a 1/r distribution). Aims. This study aims to further characterise the radial distribution of plasma around 67P throughout the mission and to expand on the initial results. We also aim to investigate how a 1/r distribution would be observed during aflyby with a fast (~10’s km s−1) spacecraft, such as the upcoming Comet Interceptor mission, when there is also an asymmetry introduced to the outgassing over the comet surface. Methods. To determine the radial distribution of the plasma, we used data from the Langmuir probe and Mutual Impedance instruments from the Rosetta Plasma Consortium during six intervals throughout the mission, for which the motion of Rosetta was approximately radial with respect to the comet. We then simulated what distribution a fast flyby mission would actually observe during its passage through a coma when there is a 1/r plasma density distribution as well as a sinusoidal variation with a phase angle (and then a sawtooth variation) multiplied to the outgassing rate. Results. The plasma density around comet 67P is found to roughly follow a 1/r dependence, although significant deviations occur in some intervals. If we normalise all data to a common outgassing rate (or heliocentric distance) and combine the intervals to a radial range of 10–1500 km, we find a 1/r1.19 average distribution. The simulated observed density from a fast spacecraft flying through a coma with a 1/r distribution and an asymmetric outgassing can, in fact, appear anywhere in the range from a 1/r distribution to a 1/r2 distribution, or even slightly outside of this range. Conclusions. The plasma density is distributed in such a way that it approximately decreases in a manner that is inversely proportional to the cometocentric distance. This is to be expected from the photoionisation of a collision-less, expanding neutral gas at a constant ionisation rate and expansion speed. The deviation from a pure 1/r distribution is in many cases caused by asymmetric outgassing over the surface, additional ionisation sources being present, electric fields accelerating plasma, and changing upstream solar wind conditions. A fast flyby mission can observe a radial distribution that deviates significantly from a 1/r trend if the outgassing is not symmetric over the surface. The altitude profile that will be observed depends very much on the level of outgassing asymmetry, the flyby velocity, the comet rotation rate, and the rotation phase. It is therefore essential to include data from both the inbound and outbound legs, as well as to compare plasma density to neutral density to get a more complete understanding of the radial distribution of the plasma.

Kelvin-Helmholtz Vortices as an Interplay of Magnetosphere-Ionosphere Coupling

The solar wind-magnetosphere interaction drives diverse physical processes on the flanks of Earth’s magnetopause, and in turn these processes couple to the ionosphere. We investigate simultaneous multipoint in-situ spacecraft and ground-based measurements to determine the role of Kelvin-Helmholtz waves at the Earth’s magnetopause and the low-latitude boundary layer in the magnetosphere-ionosphere coupling process. Nonlinear Kelvin-Helmholtz waves develop into flow vortices that twist and/or shear flux tube magnetic fields, thereby generating localized field-aligned currents. Kelvin-Helmholtz vortices on the dusk (dawn) flanks of the magnetosphere generate clockwise (counter-clockwise) rotations and upward (downward) field-aligned currents inside the flux tubes, consistent with the region-1 field-aligned current. We present in-situ MMS and Cluster spacecraft observations of Kelvin-Helmholtz vortices at the magnetopause that map to the poleward edge of the auroral regions. The FAST spacecraft and the ground-based magnetometers from which spherical elementary currents (acting as a proxy for vertical currents) can be calculated observe corresponding field-aligned current signatures. This study demonstrates the role played by the Kelvin-Helmholtz waves in linking magnetopause boundary fluctuations to ionospheric phenomena.

Heuristic decomposition planning for fast spacecraft reorientation under multiaxis constraints

Spacecraft are required to achieve fast attitude reorientation in many space missions. However, the attitude maneuver will be limited by the bounded and pointing constraints. The presence of these complex multiaxis constraints will largely reduce the feasible attitude space and greatly degrade the solving efficiency of the planning algorithm. For the time-optimal spacecraft reorientation problem under multiaxis constraints, a new heuristic decomposition planning on the virtual domain (HDPV) method is proposed in this paper. The parameterized attitude path is first determined on the virtual domain. Then the virtual-domain-based variable-step approach is presented to rapidly check the pointing constraints and generate the sets of candidate rotational-path decomposition nodes. The composite-position heuristics is developed to select the best node during the decomposition optimization, which considers the maneuver time and connection smoothness of paths. The total continuous parameterized path under keep-out and keep-in constraints is yielded through recursive planning. Finally, the time-optimal path parameterization approach is adopted to minimize the maneuver time along the total path and generate a fast attitude trajectory under bounded constraints on the time domain. Simulation results demonstrate the high computational efficiency and good optimization effect of the proposed method.

Field Localization and Density Cavitation in Low-Beta Plasmas

In the present paper, filamentous structure formation, associated turbulent spectrum, and density cavity formation phenomena have been investigated for low-β plasma β≪me/mi applicable to the auroral region. A set of dimensionless equations governing the dynamics of three dimensionally propagating inertial Alfvén wave (3D-IAW) and perpendicularly propagating magnetosonic wave (PMSW) has been developed. Ponderomotive force due to 3D-IAW has been included in the dynamics of the PMSW. Numerical simulation has been performed to study the nonlinear coupling of these two waves. From the obtained results, we found that the field intensity localization takes place which may further lead to the additional dissipation/turbulence process for particle heating and acceleration in space plasma. The associated turbulent spectrum is obtained with scaling nearly k−4.28 at smaller scales (in the dissipation range). Relevance of the obtained results with the observations reported by various spacecrafts such as Hawkeye and Heos 2 has been discussed. Also, density fluctuations (depletion) of ∼0.10 n0 are calculated, which are consistent with the FAST spacecraft observation reported.

Observations of Electron Precipitation During Pulsating Aurora and Its Chemical Impact

AbstractPulsating auroras (PsAs) are low‐intensity diffuse aurora, which switch on and off with a quasiperiodic oscillation period from a few seconds to ∼10 s. They are predominantly observed after magnetic midnight, during the recovery phase of substorms and at the equatorward boundary of the auroral oval. PsAs are caused by precipitating energetic electrons, which span a wide range of energies between tens and hundreds of keV. Such energetic PsA electrons will deposit their energy at mesospheric altitudes and induce atmospheric chemical changes. To examine the effects of energetic PsA electrons on the atmosphere, we first collect electron flux and energy measurements from low‐latitude spacecraft to construct a typical energy spectrum of precipitating electrons during PsA. Among the 840 PsA events identified using ground‐based auroral all‐sky camera (ASC) network over the Fennoscandian region, 253 events were observed by DMSP, POES, and FAST spacecraft over the common field of view of five ASCs. The combined measurements from these spacecraft enable us to obtain an energy spectrum consisting of nonrelativistic and relativistic (30 eV to 1,000 keV) electrons during PsA. The median spectrum was found to be in good agreement with earlier estimates of the PsA spectra. We then use the Sodankylä Ion‐neutral Chemistry (SIC) model to assess the chemical effect of PsA electrons. The observed extreme and median spectra of PsA produce a significant depletion in the mesospheric odd oxygen concentration up to 78%.

3DI: A novel ion composition and three-dimensional velocity analyzer for the topside ionosphere

A new ion composition and three-dimensional velocity analyzer, 3-Dimensional ion velocity and mass Imager (3DI), measures 3D velocity distribution functions (VDFs) for each major ion species in Earth’s topside ionosphere. The 3DI instrument is composed of a miniaturized electrostatic analyzer (ESA) and a deflector, backed by a static, magnet-based, mass spectrometer. We have developed a micro-pixel read-out anode technique that significantly saves power in the particle detection system, and integrated it into an imaging microchannel plate (MCP). We tested the ESA and deflector, magnet-based mass spectrometer, and anode in the laboratory to demonstrate the 3DI prototype’s performance. We have applied numerical calculations to evaluate and discuss 3DI’s performance and dynamic range. Due to complexities associated with imaging 3D distribution functions during fast spacecraft motion, we also discuss the operation strategy for 3DI to capture and resolve the VDF within the field of view. Once applied to flight investigations, the 3DI observations will be extremely useful in identifying ionosphere composition, mass-dependent ion transport such as upflows, and mass-dependent ion heating. Furthermore, the precise measurement of non-thermal plasma VDFs provides information to improve ionospheric environment modeling and ground-based radar observations.

Fast spacecraft solar radiation pressure modeling by ray tracing on graphics processing unit

A novel method is presented to evaluate on the graphics processing unit (GPU) the force and torque on a spacecraft due to solar radiation pressure. The method employs efficient ray tracing techniques, developed in the graphics rendering discipline, to resolve spacecraft self-shadowing and reflections at faster than real-time computation speed. The primary algorithmic components of the ray tracing process which contribute to the method’s computational efficiency are described. These components include two-level bounding volume hierarchy acceleration data structures, fast ray to bounding box intersection testing using the slab intersection algorithm and fast triangle intersection testing using the Möller-Trumbore algorithm. Spacecraft material optical properties are represented as a combination of Lambertian diffuse and ideal specular reflections. Both diffuse and specular ray-surface interactions are modeled. The approach is implemented using C++ and OpenCL and executed on a consumer grade GPU. Model validation is presented comparing ray traced force and torque values to the same quantities produce by a faceted analytic model. Numerical results illustrate the impact of self-shadowing on the force and torque calculation, and demonstrate the fast computational speed that is enabled with this implementation.

Fast and Accurate Spacecraft Pose Estimation From Single Shot Space Imagery Using Box Reliability and Keypoints Existence Judgments

Real-time 6DOF (6 Degree of Freedom) pose estimation of an uncooperative spacecraft is an important part of proximity operations, e.g., space debris removal, spacecraft rendezvous and docking, on-orbit servicing, etc. In this article, a novel efficient deep learning based approach is proposed to estimate the 6DOF pose of uncooperative spacecraft using monocular-vision measurement. Firstly, we introduce a new lightweight YOLO-liked CNN to detect spacecraft and predict 2D locations of the projected keypoints of a prior reconstructed 3D model in real-time. Then, we design two novel models for predicting the bounding box (bbox) reliability scores and the probability of keypoints existence. The two models not only significantly reduce the false positive, but also speed up convergence. Finally, the 6DOF pose is estimated and refined using Perspective-n-Point and geometric optimizer. Results demonstrate that the proposed approach achieves 73.2% average precision and 77.6% average recall for spacecraft detection on the SPEED dataset after only 200 training epochs. For the pose estimation task, the mean rotational error is 0.6812°, and the mean translation error is 0.0320m. The proposed approach achieves competitive pose estimation performance and extreme lightweight ( $\sim ~0.89$ million learnable weights in total) on the SPEED dataset while being efficient for real-time applications.

A projection-based controller for fast spacecraft detumbling using magnetic actuation

Magnetic control has been used for decades for spacecraft detumbling, i.e., to bring a spacecraft to a final condition with a sufficiently small angular momentum after separation from the launcher. This task is typically achieved by controllers based on the so-called b-dot principle, which stands out thanks to its simplicity, reliability and ease of on-board implementation. In this paper, we first review existing control methods and study their convergence properties with tools borrowed from general averaging theory, which allows addressing in an accurate manner the time-varying nature of magnetic actuation. Then, some effort is devoted to showing the performance limitations of existing controllers for which increasing the gain too much deteriorates the convergence rate. To overcome this issue, a novel projection-based control law with a state-dependent time-varying gain is presented. By means of Lyapunov arguments for non-autonomous systems, we prove that the proposed controller guarantees that the spacecraft angular momentum converges exponentially fast to zero for all initial conditions, robustly with respect to sufficiently small uncertainties in the inertia matrix, and that the closed-loop solutions are globally uniformly ultimately bounded in the presence of exogenous bounded disturbances. Several numerical simulations have been carried out by referring to realistic detumbling scenarios to show the performance improvement with respect to existing controllers.

Solar electric propulsion for human mars missions

While solar electric propulsion (SEP) is being widely considered for cargo transport to Mars, its value for propelling fast human missions is often viewed as marginal. This conclusion is driven by the high electric power requirement (multi megawatts) of a fast human spacecraft, coupled to the low power density of traditional solar arrays. For these applications, nuclear electric propulsion (NEP) appears to provide a better electric alternative. However, recent progress in the field of thin film photovoltaic cells and large deployable structures may, at least in the short-term, challenge this conclusion. Although ultimately NEP systems might very well become the mainstay of fast human deep space transport, we examine the human SEP option as an attractive intermediate path on this journey, one that capitalizes on the rapid evolution of the solar array technology being experienced today. We investigated the challenges of building suitably large, lightweight, solar arrays to produce the required electric power for both cargo and human interplanetary spacecraft and examined the advantages of such SEP architectures in the context of a long-stay human Mars mission. Within this framework, we present some conclusions regarding the prospects for this technology.

Envelope oriented singularity robust steering law of control moment gyros for spacecraft attitude maneuver

The problem of steering pyramid control moment gyro (CMG) cluster for fast spacecraft attitude maneuver along eigenaxis is investigated. A novel steering law is proposed to continuously attempt to reduce the difference between the current gimbal angle and the desired one corresponding to the angular momentum envelop of the CMG cluster. The proposed steering law can be decomposed into two parts: the first one is a singularity robust term to keep maneuverability and produce control torque, and the other is a null motion term to rearrange the gimbal angles toward momentum envelope. By involving this steering law, it is expected to possess both rapid angular momentum exchange and singularity avoidance ability. In addition, by introducing a new limit vector on attitude error, classical cascade-saturation control algorithm is revised to guarantee spacecraft eigenaxis rotation. Both open-loop steering law test and closed-loop attitude maneuver simulations are performed to evaluate the efficacy of the proposed methods.

Fast spacecraft adaptive attitude tracking control through immersion and invariance design

This paper presents a novel non-certainty-equivalence adaptive control method for the attitude tracking control problem of spacecraft with inertia uncertainties. The proposed immersion and invariance (I&I) based adaptation law provides a more direct and flexible approach to circumvent the limitations of the basic I&I method without employing any filter signal. By virtue of the adaptation high-gain equivalence property derived from the proposed adaptive method, the closed-loop adaptive system with a low adaptation gain could recover the high adaptation gain performance of the filter-based I&I method, and the resulting control torque demands during the initial transient has been significantly reduced. A special feature of this method is that the convergence of the parameter estimation error has been observably improved by utilizing an adaptation gain matrix instead of a single adaptation gain value. Numerical simulations are presented to highlight the various benefits of the proposed method compared with the certainty-equivalence-based control method and filter-based I&I control schemes.

The range of Alfvénic turbulence scales in the topside auroral ionosphere

The minimal scale of the Alfvenic turbulence transverse to the external magnetic field in the topside auroral ionosphere is investigated using electric field observations of the FAST spacecraft (the resolution 512 s–1). The events in which the power law form of the electric fluctuation spectra with a 2.0–2.5 slope (typical of Alfvenic turbulence) remains unchanged down to acoustic gyroradius ρs or ion gyroradius ρi local values are illustrated for the first time. In this case, the character of spectrum variation does not change at the electron inertial length λe, which is much larger than ρs and ρi for FAST altitudes (apogee ~4000 km). We have tried to explain this experimental fact by consideration of the known scenarios of the appearance of a small transverse scale in an Alfvenic perturbation. It has been noted that the effects of front steepening in an inertial Alfven wave with a finite amplitude, which propagates at an angle smaller than (me/mi)1/2 with respect to the transverse direction, can result in small transverse scales comparable with acoustic gyroradius appearing in a perturbation.

Ionospheric flow shear associated with the preexisting auroral arc: A statistical study from the FAST spacecraft data

AbstractAn auroral substorm is a disturbance in the magnetosphere that releases energy stored in the magnetotail into the high‐latitude ionosphere. By definition, an auroral substorm commences when a discrete auroral arc brightens and subsequently expands poleward and azimuthally. The arc that brightens is usually the most equatorward of several auroral arcs that remain quiescent for ~5 to ~60 min before the breakup commences. This arc is often referred to as the “preexisting auroral arc (PAA)” or the “growth‐phase arc.” In this study, we use FAST measurements to establish the statistics of flow patterns near PAAs in the ionosphere. We find that flow shear is present in the vicinity of a preexisting arc. When a PAA appears in the evening sector, enhanced westward flow develops equatorward of the arc, whereas when a PAA appears in the morning sector, enhanced eastward flow develops poleward of the arc. We benchmark locations of the PAAs relative to large‐scale field‐aligned currents (FACs) and convective flows in the ionosphere, finding that the arc forms in the upward current region within ~1° of the Region 1/Region 2 boundary in all local time sectors from 20 MLT to 03 MLT. We also find that near midnight in the Harang region, most of the PAAs lie within 0.5° poleward of the low‐latitude Region 1/Region 2 currents boundary and sit between the westward and eastward flow peak but equatorward of the flow reversal point. Finally, we examine arc‐associated electrodynamics and find that the FAC of the PAA is mainly closed by the north‐south Pedersen current in the ionosphere.

Localization of 3D inertial Alfvén wave and generation of turbulence

The present paper deals with the nonlinear interaction of Inertial Alfven wave (IAW) and fast magnetosonic wave in the low beta plasma, where beta is the ratio of thermal pressure to the background magnetic pressure. In this paper, the localization and turbulent spectra of IAW along with the density dips correlated with the fast magnetosonic wave have been investigated. Variation of parallel electric field along and across the field lines has also been studied. Taking ponderomotive nonlinear effect in the dynamics of fast magnetosonic wave, couple of dimensionless equations has been derived. These coupled equations have been simulated numerically using the pseudo-spectral method. The obtained results reveal that the Kolmogorov scaling is followed by a steeper scaling in magnetic power spectrum, which is consistent with the observations by the FAST and Hawkeye spacecraft in auroral region. The relevance of present investigation has been discussed for auroral plasmas.

Density cavity formation through nonlinear interaction of 3‐D inertial Alfvén wave and ion acoustic wave

AbstractDensity cavities having transverse‐scale size of the order of electron inertial length have been observed in auroral ionosphere by Freja spacecraft. In the present study, density cavity formation in auroral regions has been investigated using numerical simulation techniques. Nonlinear interaction of inertial Alfvén waves and ion acoustic waves has been suggested as a possible mechanism to apprehend density cavity formation in auroral regions. In the proposed mechanism, strong modification in the density profile takes place due to the ponderomotive force of inertial Alfvén waves and accounts for these density depleted regions. In literature also, it has been demonstrated that these cavities are associated with the ponderomotive forces of inertial Alfvén wave. Our presented model also attempts to understand the salient features of auroral density cavities which are reported in literature from the analysis of data available from FAST spacecraft. Relevance of obtained results with observations in auroral ionosphere has also been discussed.

A unified model of auroral arc growth and electron acceleration in the magnetosphere‐ionosphere coupling

AbstractA unified physical model of auroral arc growth and electron acceleration is constructed from the gyrokinetic and two‐fluid equations for the magnetosphere‐ionosphere (M‐I) coupling system. The present theory describes destabilization of kinetic Alfvén waves (KAWs) in the M‐I coupling, where development of upward and downward field‐aligned currents carried by the KAWs leads to ionospheric density enhancement and depletion, respectively. The feedback M‐I coupling through KAWs elucidates growth of auroral arcs and field‐aligned acceleration of electrons self‐consistently and provides a possible explanation to the Alfvénic auroras observed by the FAST spacecraft.

Optimal Current Switching Control of Electrodynamic Tethers for Fast Deorbit

Electrodynamic tethers have the ability to remove space debris from Earth’s orbit without the use of propellant. Unfortunately, the periodic variations of electrodynamic force will lead to the tumbling of tethers, and attitude control is needed to achieve a successful deorbit of space debris at the cost of deorbit efficiency. This paper develops an optimal current switching control scheme to enable a fast and stable spacecraft deorbit simultaneously by electrodynamic tethers. In addition, the computational effort of the proposed optimal control is significantly reduced by using a piecewise treatment that discretizes the deorbit process into consecutive time intervals. Within each interval, the system dynamic model is simplified based on different timescales of state variables and the current on–off switching is optimized by solving a constrained minimization problem of a control index representing the deorbit efficiency. Direct Hermite–Simpson discretization is adopted to convert the optimal control problem into a standard nonlinear programming problem. The validity and efficiency of the proposed control strategy is shown by numerical simulations. The deorbit rate increases significantly with the proposed optimal current switching control compared with the existing simple current on–off switch, whereas the computational efforts are reduced.