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This paper investigates the effects of model fidelity and parameter uncertainty on the performance of a hybrid model-based feedback-feedforward control scheme for attitude tracking of a satellite with flexible appendages. The feedforward component is an inverse model-based term produced through a computational approach known as inverse simulation (InvSim), which works by iteratively solving a discretised reference trajectory. The hybrid controller's feedback is proportional-derivative (PD) based, using body attitude and rate feedback to provide stability and robustness. Furthermore, to ensure that the flexible modes do not trigger instability, the PD control gains are tuned to give a closed-loop response that is significantly slower than the flexible modes. Additionally, excitation of the flexible modes is reduced by minimising jerk through polynomial rest-to-rest manoeuvres, following the shortest quaternion path using spherical–linear-interpolation (SLERP). The effects of the appendage flexing on attitude tracking are then compensated through the feedforward element of the hybrid controller, with performance being compared to a traditional PD tracking law. The effect of the model fidelity on the performance of the hybrid controller is investigated through the use of both rigid body and multiple-fidelity finite-element mathematical models. Additionally, the effect of uncertainties in the model parameters is investigated to determine the accuracy of the model required to obtain significant improvement in attitude tracking. It is found that in the absence of any model parameter uncertainty, the hybrid controller outperforms the PD tracking control law by at least one order of magnitude when the finite-element model is used. Increasing the number of finite elements was found to provide no significant improvement in performance, with one element being sufficient and favourable with its lower computational overhead. It was also found that to ensure good performance compared to the PD tracking controller, the uncertainty in the inertia tensor should be < 1%. Similarly, uncertainty in the first flexible modal frequency should be < 0.5 rad/s. • Inverse simulation (InvSim) used to produce model-based feedforward control. • Combined with PD feedback to improve attitude tracking. • Effects of model uncertainty and fidelity on model-based control investigated. [ABSTRACT FROM AUTHOR]

This paper investigates the dynamics of an orbital siphon anchored to a rotating ellipsoidal asteroid. The siphon is a chain of tether-connected payload masses arranged vertically from the asteroid surface, envisaged for propellantless delivery of payloads (e.g., mined material) from the asteroid surface to a collecting spacecraft. If the structure is long enough, the centrifugal-induced force can overcome the gravitational force on the payloads, eventually allowing resource payloads to escape. By connecting new payloads at the bottom of this chain while removing upper payloads a net orbital siphon effect is established, which provides a net continuous flow of resources from the asteroid surface to a collecting spacecraft, attached at the top of the siphon. The dynamics of the siphon is investigated in detail by varying a set of relevant parameters, in particular, chain length, anchor location and asteroid shape. It is shown that the system exhibits oscillatory behaviour in the equatorial plane, with decreasing oscillation amplitude over time and that the longest equatorial end is the best anchor location to guarantee proper siphon operation while minimizing the chain length. Eventually, a method is proposed to exploit the equatorial Coriolis-induced oscillations of the siphon to transfer payload masses from the collecting spacecraft to the stable equilibrium points associated with the effective potential of the ellipsoidal asteroid, where a catcher would collect the material. • Dynamics of orbital siphon anchored at ellipsoidal asteroid is studied. • Anchoring at longest equatorial end minimizes siphon length. • Siphon exhibits damped oscillatory behaviour. • Stable equilibrium points can be used as gravitational depots. [ABSTRACT FROM AUTHOR]

A new type of space debris in near geosynchronous orbit (GEO) was recently discovered and later identified as exhibiting unique characteristics associated with high area-to-mass ratio (HAMR) objects, such as high rotation rates and high reflection properties. Observations have shown that this debris type is very sensitive to environmental disturbances, particularly solar radiation pressure, due to the fact that its motion depends on the actual effective area, orientation of that effective area, reflection properties and the area-to-mass ratio of the object is not stable over time. Previous investigations have modelled this type of debris as rigid bodies (constant area-to-mass ratios) or discrete deformed body; however, these simplifications will lead to inaccurate long term orbital predictions. This paper proposes a simple yet reliable model of a thin, deformable membrane based on multibody dynamics. The membrane is modelled as a series of flat plates, connected through joints, representing the flexibility of the membrane itself. The mass of the membrane, albeit low, is taken into account through lump masses at the joints. The attitude and orbital motion of this flexible membrane model is then propagated near GEO to predict its orbital evolution under the perturbations of solar radiation pressure, Earth’s gravity field (J 2 ), third body gravitational fields (the Sun and Moon) and self-shadowing. These results are then compared to those obtained for two rigid body models (cannonball and flat rigid plate). In addition, Monte Carlo simulations of the flexible model by varying initial attitude and deformation angle (different shape) are investigated and compared with the two rigid models (cannonball and flat rigid plate) over a period of 100 days. The numerical results demonstrate that cannonball and rigid flat plate are not appropriate to capture the true dynamical evolution of these objects, at the cost of increased computational time. [ABSTRACT FROM AUTHOR]

Hill's approximation models the motion of two small masses gravitationally interacting with each other and perturbed by a large central body. An application of the model is employed in this paper to manoeuvre the relative motion of two asteroids by a small impulse to capture them in bound binary motion in Earth's orbit. The initial conditions prior to the capture manoeuvre are restricted in a parameter space termed the gateway region. The gateway region is produced by applying the constraint that the capture impulse is a real-valued function, and the zero-velocity curve closes, enveloping both asteroids. A full mission scenario is designed with three impulses. The first impulse transfers one asteroid from a far field region to the gateway region, where there is mutual interaction with the second asteroid, which is assumed to be the origin of the relative motion. The second impulse changes the trajectory of the asteroid in the gateway region to a linear drift approaching the second asteroid. The third impulse is calculated by means of increasing the Jacobi integral to reach the critical value required to close the zero-velocity curve. It is demonstrated in principle that the triple impulses enable the candidate asteroids to be captured in a bound pair in Earth's orbit. It is also shown that the thruster on-time for the capture impulse as applied for the different small asteroids considered is short relative to the natural time-scale of the orbital dynamics of the problem, which shows the feasibility of the impulse approximation with the typical thruster forces applied. This strategy could provide a basis for parking small captured near-Earth asteroids in Earth's orbit. • A three-impulse scenario is proposed to form an asteroid pair in bound motion. • Hill's equations are modified with capture impulse to the transverse relative velocity. • An analytical investigation for position and velocity constraints is applied to the capture impulse. • A gateway map is generated to provide possible initial conditions for the capture impulse scenario. • The scenario is illustrated by forming an artificial binary pair in high Earth orbit. [ABSTRACT FROM AUTHOR]

Multilayer insulation (MLI) is a recently-discovered type of debris originating from delamination of aging spacecraft; it is mostly detected near the geosynchronous orbit (GEO). Observation data indicates that these objects are characterised by high reflectivity, high area-to-mass ratio (HAMR), fast rotation, high sensitivity to perturbations (especially solar radiation pressure) and change of area-to-mass ratio (AMR) over time. As a result, traditional models (e.g. cannonball) are unsuitable to represent and predict this debris' orbital evolution. Previous work by the authors effectively modelled the flexible debris by means of multibody dynamics to improve the prediction accuracy. The orbit evolution with the flexible model resulted significantly different from using the rigid model. This paper aims to present a methodology to determine the dynamic properties of thin membranes with the purpose to validate the deformation characteristics of the flexible model. A high-vacuum chamber (10 −4 mbar) to significantly decrease air friction, inside which a thin membrane is hinged at one end but free at the other provides the experimental setup. A free motion test is used to determine the damping characteristics and natural frequency of the thin membrane via logarithmic decrement and frequency response. The membrane can swing freely in the chamber and the motion is tracked by a static, optical camera, and a Kalman filter technique is implemented in the tracking algorithm to reduce noise and increase the tracking accuracy of the oscillating motion. Then, the effect of solar radiation pressure on the thin membrane is investigated: a high power spotlight (500–2000 W) is used to illuminate the sample and any displacement of the membrane is measured by means of a high-resolution laser sensor. Analytic methods from the natural frequency response and Finite Element Analysis (FEA) including multibody simulations of both experimental setups are used for the validation of the flexible model by comparing the experimental results of amplitude decay, natural frequencies and deformation. The experimental results show good agreement with both analytical results and finite element methods. [ABSTRACT FROM AUTHOR]

The fascinating idea of shepherding asteroids for science and resource utilization is being considered as a credible concept in a not too distant future. Past studies identified asteroids which could be efficiently injected into manifolds which wind onto periodic orbits around collinear Lagrangian points of the Sun-Earth system. However, the trajectories are unstable, and errors in the capture maneuver would lead to complete mission failure, with potential danger of collision with the Earth, if uncontrolled. This paper investigates the controllability of some asteroids along the transfers and the periodic orbits, assuming the use of a solar-electric low-thrust system shepherding the asteroid. Firstly, an analytical approach is introduced to estimate the stability of the trajectories from a dynamical point of view; then, a numerical control scheme based on a linear quadratic regulator is proposed, where the gains are optimized for each trajectory through a genetic algorithm. A stochastic simulation with a Monte Carlo approach is used to account for different perturbed initial conditions and the epistemic uncertainty on the asteroid mass. Results show that only a small subset of the considered combinations of trajectories/asteroids are reliably controllable, and therefore controllability must be taken into account in the selection of potential targets. [ABSTRACT FROM AUTHOR]

A Riccati-based tracking controller with collision avoidance capabilities is presented for proximity operations of spacecraft formation flying near elliptic reference orbits. The proposed dynamical model incorporates nonlinear accelerations from an artificial potential field, in order to perform evasive maneuvers during proximity operations. In order to validate the design of the controller, test cases based on the physical and orbital features of the Prototype Research Instruments and Space Mission Technology Advancement (PRISMA) will be implemented, extending it to scenarios with multiple spacecraft performing reconfigurations and on-orbit position switching. The results show that the tracking controller is effective, even when nonlinear repelling accelerations are present in the dynamics to avoid collisions, and that the potential-based collision avoidance scheme is convenient for reducing collision threat. [ABSTRACT FROM AUTHOR]

This paper investigates orbits and transfer trajectories for continuous polar Earth observation in the Earth-Moon system. The motivation behind this work is to complement the services offered by polar-orbiting spacecraft, which offer high resolution imaging but poor temporal resolution, due to the fact that they can only capture one narrow swath at each polar passage. Conversely, a platform for high-temporal resolution imaging can enable a number of applications, from accurate polar weather forecasting to Aurora study, as well as direct-link telecommunications with high-latitude regions. Such a platform would complement polar orbiters. In this work, we make use of resonant gravity swing-by manoeuvres at the Moon in order to design trajectories that are suitable for quasi-continuous polar observation. In particular, it is shown that the Moon can flip the line of apsides of a highly eccentric, highly inclined orbit from north to south, without the need for thrust. In this way, a spacecraft can alternatively loiter for an extended period of time above the two poles. In addition, at the lunar encounter it is possible to change the period of time spent on each pole. In addition, we also show that the lunar swing-by can be exploited for transfer to a so-called pole-sitter orbit, i.e. a spacecraft that constantly hovers above one of the Earth's poles using continuous thrust. It is shown that, by using the Moon's gravity to change the inclination of the transfer trajectory, the total Δv is less than using a trajectory solely relying on high-thrust or low-thrust, therefore enabling the launchers to inject more mass into the target pole-sitter position. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides a detailed mission analysis and systems design of a near-term and far-term pole-sitter mission. The pole-sitter concept was previously introduced as a solution to the poor temporal resolution of polar observations from highly inclined, low Earth orbits and the poor high-latitude coverage from geostationary orbit. It considers a spacecraft that is continuously above either the north or south pole and, as such, can provide real-time, continuous and hemispherical coverage of the polar regions. Being on a non-Keplerian orbit, a continuous thrust is required to maintain the pole-sitter position. For this, two different propulsion strategies are proposed, which result in a near-term pole-sitter mission using solar electric propulsion (SEP) and a far-term pole-sitter mission where the SEP thruster is hybridized with a solar sail. For both propulsion strategies, minimum propellant pole-sitter orbits are designed. In order to maximize the spacecraft mass at the start of the operations phase of the mission, the transfer from Earth to the pole-sitter orbit is designed and optimized assuming either a Soyuz or an Ariane 5 launch. The maximized mass upon injection into the pole-sitter orbit is subsequently used in a detailed mass budget analysis that will allow for a trade-off between mission lifetime and payload mass capacity. Also, candidate payloads for a range of applications are investigated. Finally, transfers between north and south pole-sitter orbits are considered to overcome the limitations in observations due to the tilt of the Earth's rotational axis that causes the poles to be alternately situated in darkness. It will be shown that in some cases these transfers allow for propellant savings, enabling a further extension of the pole-sitter mission. [Copyright &y& Elsevier]

Abstract: The paper provides a survey of novel mission concepts for continuous, hemispheric polar observation and direct-link polar telecommunications. It is well known that these services cannot be provided by traditional platforms: geostationary satellites do not cover high-latitude regions, while low- and medium-orbit Sun-synchronous spacecraft only cover a narrow swath of the Earth at each passage. Concepts that are proposed in the literature are described, including the pole-sitter concept (in which a spacecraft is stationary above the pole), spacecraft in artificial equilibrium points in the Sun–Earth system and non-Keplerian polar Molniya orbits. Additionally, novel displaced eight-shaped orbits at Lagrangian points are presented. For many of these concepts, a continuous acceleration is required and propulsion systems include solar electric propulsion, solar sail and a hybridisation of the two. Advantages and drawbacks of each mission concept are assessed, and a comparison in terms of high-latitude coverage and distance, spacecraft mass, payload and lifetime is presented. Finally, the paper will describe a number of potential applications enabled by these concepts, focusing on polar Earth observation and telecommunications. [Copyright &y& Elsevier]

Abstract: Recent studies have shown the feasibility of an Earth pole-sitter mission using low-thrust propulsion. This mission concept involves a spacecraft following the Earth''s polar axis to have a continuous, hemispherical view of one of the Earth''s poles. Such a view will enhance future Earth observation and telecommunications for high latitude and polar regions. To assess the accessibility of the pole-sitter orbit, this paper investigates optimum Earth pole-sitter transfers employing low-thrust propulsion. A launch from low Earth orbit (LEO) by a Soyuz Fregat upper stage is assumed after which solar electric propulsion is used to transfer the spacecraft to the pole-sitter orbit. The objective is to minimize the mass in LEO for a given spacecraft mass to be inserted into the pole-sitter orbit. The results are compared with a ballistic transfer that exploits manifold-like trajectories that wind onto the pole-sitter orbit. It is shown that, with respect to the ballistic case, low-thrust propulsion can achieve significant mass savings in excess of 200kg for a pole-sitter spacecraft of 1000kg upon insertion. To finally obtain a full low-thrust transfer from LEO up to the pole-sitter orbit, the Fregat launch is replaced by a low-thrust, minimum time spiral, which provides further mass savings, but at the cost of an increased time of flight. [Copyright &y& Elsevier]

Abstract: This paper presents the preliminary systems design of a pole-sitter. This is a spacecraft that hovers over an Earth pole, creating a platform for full hemispheric observation of the polar regions, as well as direct-link telecommunications. To provide the necessary thrust, a hybrid propulsion system combines a solar sail with a more mature solar electric propulsion (SEP) thruster. Previous work by the authors showed that the combination of the two allows lower propellant mass fractions, at the cost of increased system complexity. This paper compares the pure SEP spacecraft with the hybrid spacecraft in terms of the launch mass necessary to deliver a certain payload for a given mission duration. A mass budget is proposed, and the conditions investigated under which the hybrid sail saves on the initial spacecraft initial mass. It is found that the hybrid spacecraft with near- to mid-term sail technology has a lower initial mass than the SEP case if the mission duration is 7years or more, with greater benefits for longer duration missions. The hybrid spacecraft with far-term sail technology outperforms the pure SEP case even for short missions. [Copyright &y& Elsevier]

Abstract: In this paper we propose a pole-sitter spacecraft hybridising solar electric propulsion (SEP) and solar sailing. The intriguing concept of a hybrid propulsion spacecraft is attractive: by combining the two forms of propulsion, the drawbacks of the two systems cancel each other, potentially enabling propellant mass saving, increased reliability, versatility and lifetime over the two independent systems. This almost completely unexplored concept will be applied to the continuous monitoring of the Earth''s polar regions through a pole-sitter, i.e. a spacecraft that is stationary above one pole of the Earth. The continuous, hemispherical, real-time view of the pole will enable a wide range of new applications for Earth observation and telecommunications. In this paper, families of 1-year-periodic, minimum-propellant orbits are found, for different values of the sail lightness number and distance from the pole. The optimal control problem is solved using a pseudo-spectral method. The process gives a reference control to maintain these orbits. In addition, for stability issues, a feedback control is designed to guarantee station-keeping in the presence of injection errors, sail degradation and temporary SEP failure. Results show that propellant mass can be saved using a medium-sized solar sail. Finally, it is shown that the feedback control is able to maintain the spacecraft on-track with only minimal additional effort from the SEP thruster. [Copyright &y& Elsevier]

Abstract: Given a set of celestial bodies, the problem of finding an optimal sequence of swing-bys, deep space manoeuvres (DSM) and transfer arcs connecting the elements of the set is combinatorial in nature. The number of possible paths grows exponentially with the number of celestial bodies. Therefore, the design of an optimal multiple gravity assist (MGA) trajectory is a NP-hard mixed combinatorial–continuous problem. Its automated solution would greatly improve the design of future space missions, allowing the assessment of a large number of alternative mission options in a short time. This work proposes to formulate the complete automated design of a multiple gravity assist trajectory as an autonomous planning and scheduling problem. The resulting scheduled plan will provide the optimal planetary sequence and a good estimation of the set of associated optimal trajectories. The trajectory model consists of a sequence of celestial bodies connected by two-dimensional transfer arcs containing one DSM. For each transfer arc, the position of the planet and the spacecraft, at the time of arrival, are matched by varying the pericentre of the preceding swing-by, or the magnitude of the launch excess velocity, for the first arc. For each departure date, this model generates a full tree of possible transfers from the departure to the destination planet. Each leaf of the tree represents a planetary encounter and a possible way to reach that planet. An algorithm inspired by ant colony optimization (ACO) is devised to explore the space of possible plans. The ants explore the tree from departure to destination adding one node at the time: every time an ant is at a node, a probability function is used to select a feasible direction. This approach to automatic trajectory planning is applied to the design of optimal transfers to Saturn and among the Galilean moons of Jupiter. Solutions are compared to those found through more traditional genetic-algorithm techniques. [ABSTRACT FROM AUTHOR]

An optimal steering law for sails that exploit both solar and infrared planetary radiation pressure is presented in this paper. The optimal steering law maximises the orbit raise over one revolution of the sail around the planet. An indirect analytical approach, that uses Pontryagin Minimum Principle, is used to develop specialised steering laws for the sunlit and eclipse cases in a planar motion scenario. The law for the sunlit case uses both the solar and infrared radiation emitted from the planet, while the law for the eclipse case finds the optimal sail attitude that maximises the raise of the orbit using only the planetary radiation. Numerical results show that these laws lead to better performance in terms of orbit raising against other sub-optimal and optimal strategies exploiting the solar radiation pressure only. A numerical study is also carried out to show the effects of the reflectivity coefficient in the infrared band on the orbital motion of the sail. [ABSTRACT FROM AUTHOR]

Space resource utilization (SRU) can be a catalyst for a growing space economy. Significant reductions in cost, launch mass and risk for human and robotic activities beyond Earth can be achieved. Commercial entities will have the opportunity to generate revenue through economic development, allowing for a decreased dependency on governmental agencies for the exploration of space. A wide variety of materials can be found on planetary and small solar system bodies, which are potentially of great use to customers in space. This paper conducts a comparison of the economic and commercial potential of SRU on the surface of the Moon, Mars, a representative near-Earth asteroid and a representative main-belt asteroid. In particular, mining of water will be considered which will be electrolysed into hydrogen and oxygen using solar power. Coupled economic and trajectory optimization then results in the minimum specific cost to deliver propellant (LOX/LH 2) from these bodies to a number of customer locations. Customers on the surface of or in orbit around the Earth, Moon, and Mars are investigated, along with Psyche, a large metallic main-belt asteroid. Using a cost model that considers technology maturation to allow current aviation-like costs to be used for development and manufacturing, missions are optimized. Results show that for customers beyond low-Earth orbit, SRU should be used instead of launching water-derived resource directly from the Earth. In particular, near-Earth asteroids are shown to be in an attractive location for mining, processing and delivering a large quantity of LOX/LH 2 at a low specific cost, for customers in the vicinity of the Earth and the Moon, including nearby collinear Lagrange points. Exceptions are customers on the surface of the Earth and the Moon, who, together with customers on the surface of Mars, are better off mining and processing on that same surface. A sensitivity analysis shows that, for all investigated customers except Psyche, there are near-Earth asteroids available which can be used to deliver water-derived resources for a lower specific cost than the most ideal main-belt asteroid. Only once these near-Earth asteroids have been depleted, mining main-belt asteroids and transporting resources back to a customer in the vicinity of the Earth is worth considering. • Economic modelling and trajectory optimization are coupled to minimize specific cost. • A parametric approach allows for unbiased comparison of source/customer-pair results. • The cheapest sources of LOX/LH2 for customers on/near the Earth/Moon/Mars are given. • Using near-Earth asteroids is often more cost-effective than using main-belt asteroids. • It is cost-prohibitive to use Mars-derived propellant elsewhere. [ABSTRACT FROM AUTHOR]

A low-jerk attitude guidance method is developed, based on an analytical smoothing of a bang-off-bang maneuver. A set of closed-form equations are derived and used to plan constrained low-jerk maneuvers, with prescribed boundary conditions, inertia, time and maximum control torque. The guidance law is first developed for one-dimensional and then three-dimensional rotations using two different approaches: (i) by designing a rotation about the Euler axis and (ii) by using the inverse kinematics equations. A generic model of the torque induced by a multi-body appendage is derived using the lumped-parameter method. This method can also be used to approximate the dynamic behavior of flexible appendages. The simulations results show that the smoothing techniques reduce the excitation of multi-body and flexible structures during a slew maneuver. • Lumped-parameter method is used to model multi-body and flexible appendages dynamics. • Trigonometric functions can be used to plan smooth slew maneuvers. • Smoothing can be applied to both eigenaxis rotations and generic 3-D maneuvers. • Smoothed maneuvers lead to lower appendage oscillations than classical control. [ABSTRACT FROM AUTHOR]

Abstract In 2013, the well-known Chelyabinsk meteor entered the Earth's atmosphere over Chelyabinsk, Russia. It is estimated that the meteor exploded at altitude near 30 km, which damaged thousands of buildings and injured a thousand of residents. The estimated size of the meteor is approximately 20 m. Because the meteor approached to Earth from Sun direction, no ground-based observatories could not detect until the impact. Considering such situations, the paper proposes a concept to detect Chelyabinsk-class small Near-Earth Objects. The concept addresses a "last-minute" warning system of NEO impact, in the same manner of "Tsunami" warning. To achieve the mission objective, two locations are assumed for the space telescope installation point i.e., Sun-Earth Lagrange point 1, SEL1 and Artificial Equilibrium Point, AEP. SEL1 is one of the natural equilibrium points, on the other hand, AEP is artificially equilibrated point by Sun and Earth gravity, centrifugal force and low-thrust acceleration. The magnitude of the acceleration to keep AEP is sufficiently small near 1 au radius orbit around the Sun i.e., the order of μm/s2 which can be achieved by solar sail. Through some cases of numerical simulations considering the size of NEOs and detector capability, this paper will show the feasibility of the proposed concept. Highlights • This is a concept study of a hazardous NEO detection and impact warning system. • It is assumed to use space-based telescopes to detect virtual impactors before the impact. • Sun-Earth L1 point and Artificial Equilibrium Point are used for space-telescope. • SEL1 case shows the deficiency to detect NEOs in-coming from Sun-direction. • On the other hand, AEP case shows almost 100% detectability of NEOs.5th IAA Planetary Defense Conference – PDC 2017. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the concept of capturing near-Earth asteroids into bound orbits around the Earth by using aerobraking. To guarantee that the candidate asteroids cannot present an impact risk during aerobraking, an initial aerobraking hazard analysis is undertaken and accordingly only asteroids with a diameter less than 30 m are considered as candidates in this paper. Then, two asteroid capture strategies utilizing aerobraking are defined. These are termed single-impulse capture and bi-impulse capture, corresponding to two approaches to raising the perigee height of the captured asteroid's orbit after the aerobraking manoeuvre. A Lambert arc in the Sun-asteroid two-body problem is used as an initial estimate for the transfer trajectory to the Earth and then a global optimisation is undertaken, using the total transfer energy cost and the retrieved asteroid mass ratio (due to ablation) as objective functions. It is shown that the aerobraking can in principle enable candidate asteroids to be captured around the Earth with, in some cases, extremely low energy requirements. Highlights • An aerobraking hazard analysis is undertaken to select candidate asteroids. • Two asteroid capture strategies using aerobraking are proposed. • Maximum mass ratio of captured asteroid and spacecraft is obtained. • Best candidate asteroids which can be captured using aerobraking are found. [ABSTRACT FROM AUTHOR]

This paper investigates two strategies to engineer the capture of one member of a binary asteroid pair by a planetary body after close encounter with that planetary body. It is assumed that the binary pair consists of a smaller minor asteroid in orbit about a larger main asteroid, which encounters a planetary body. In order to develop an engineering model of the problem, first we neglect the mass of the smaller minor asteroid in the binary pair and approximate the model as planar parabolic restricted three-body problem (PPRTBP). Second, the related regularised dynamical equations for the problem are developed. An approximate analytical solution to the problem is then obtained for motion in the vicinity of the main asteroid using the regularised coordinates through a linearized model. This provides insight into the motion of the minor asteroid about the main asteroid, allowing strategies to engineer the capture process to be developed. Based on the topology of the zero velocity curves (ZVCs) for the PPRTBP, we determine the capture region for the problem by developing initial condition maps (ICMs) and investigate the details of the dynamical process for capture. Two capture strategies are then proposed to engineer and extend the possibility for capture of the minor asteroid in binary pair. One is a re-phasing manoeuvre before encounter, which guarantees that the particle is within the capture region of the ICMs. The other is an optimal, single-impulse transfer during encounter to ensure transfer through the ZVC bottleneck and capture of the minor asteroid by the planetary body. The purpose of the paper is to explore such engineering strategies, rather than to provide new insights into natural capture dynamics. [ABSTRACT FROM AUTHOR]

Abstract The scientific interest in near-Earth asteroids (NEAs) and the classification of some of those as potentially hazardous for the Earth stimulated the interest in their exploration. Close-up observations of these objects will drastically increase our knowledge about the overall NEA population. For this reason, a multiple NEA rendezvous mission through solar sailing is investigated, taking advantage of the propellantless nature of this propulsion technology. Considering a spacecraft based on the DLR/ESA Gossamer technology, this work focuses on a method for searching possible sequences of NEA encounters. The effectiveness of the approach is demonstrated through a number of fully-optimised trajectories. The results show that it is possible to visit five NEAs within 10 years with near-term solar-sail technology. Moreover, a study on a reduced NEA database demonstrates the reliability of the approach used, showing that 58% of the sequences found with an approximated trajectory model can be converted into real feasible solar-sail trajectories. Overall, the study shows the effectiveness of the proposed automatic optimisation algorithm, which is able to find solutions for a large number of mission scenarios without any input required from the user. [ABSTRACT FROM AUTHOR]

To overcome the congestion of geostationary orbit slots, previous work proposed to use vertically-displaced, non-Keplerian geostationary orbits by means of continuous low-thrust propulsion in the form of hybrid solar sail and solar electric propulsion (hybrid sail). This work extends and generalizes that concept by loosening the position constraint and introducing a station-keeping box. Sub-optimal orbits are first found with an inverse method that still satisfy the geostationary position constraint (i.e., no station-keeping box), which will be referred to as ideal displaced geostationary orbits. For these sub-optimal orbits, it is found that the hybrid sail saves propellant mass compared to the pure solar electric propulsion case: for solar sail lightness numbers of up to a value of 0.2 and the most favorable time during the year (i.e., at summer solstice), the hybrid sail saves up to 71.6% propellant mass during a single day compared to the use of pure solar electric propulsion. Subsequently, the sub-optimal orbits are used as a first-guess for a direct optimization algorithm based on Gauss pseudospectral transcription, which loosens the position constraint. This enables a more flexible trajectory around the ideal displaced geostationary orbit and lets the solar sail contribute more efficiently to the required acceleration. It therefore leads to a further propellant savings of up to 73.8%. Finally, the mass budget shows that by using by using far-term solar sail technology, the hybrid propulsion system enables an evident reduction in the required initial mass of the spacecraft for a given payload mass with a relatively long mission duration. [ABSTRACT FROM AUTHOR]

Since the Sun-Earth libration points L 1 and L 2 are regarded as ideal locations for space science missions and candidate gateways for future crewed interplanetary missions, capturing near-Earth asteroids (NEAs) around the Sun-Earth L 1 /L 2 points has generated significant interest. Therefore, this paper proposes the concept of coupling together a flyby of the Earth and then capturing small NEAs onto Sun–Earth L 1 /L 2 periodic orbits. In this capture strategy, the Sun-Earth circular restricted three-body problem (CRTBP) is used to calculate target Lypaunov orbits and their invariant manifolds. A periapsis map is then employed to determine the required perigee of the Earth flyby. Moreover, depending on the perigee distance of the flyby, Earth flybys with and without aerobraking are investigated to design a transfer trajectory capturing a small NEA from its initial orbit to the stable manifolds associated with Sun-Earth L 1 /L 2 periodic orbits. Finally, a global optimization is carried out, based on a detailed design procedure for NEA capture using an Earth flyby. Results show that the NEA capture strategies using an Earth flyby with and without aerobraking both have the potential to be of lower cost in terms of energy requirements than a direct NEA capture strategy without the Earth flyby. Moreover, NEA capture with an Earth flyby also has the potential for a shorter flight time compared to the NEA capture strategy without the Earth flyby. [ABSTRACT FROM AUTHOR]

The problem of finding an optimal solar-sail trajectory must be solved by means of numerical methods, since no analytical, closed-form solutions exist. A new tool named ATOSS (Automated Trajectory Optimizer for Solar Sailing) has been developed for optimizing multi-phase solar-sail trajectories. A shape-based method for solar sailing and a two-stage approach for the optimization are the keys to the success of ATOSS, which operates with minimum inputs required to the user. Once the initial guess is generated by means of the shape-based method, the above mentioned two-stage approach works as follows. First, a solution to the optimal control problem at hand is sought; subsequently, the boundaries on the times are modified so that a better solution, in terms of total mission duration, is searched. Several numerical test cases are presented to demonstrate ATOSS' ability to automatically find optimal solar-sail trajectories for single- and multi-phase optimization problems. Moreover, the shape-based method for solar sailing has been validated as a viable method to produce initial guess solutions for a direct optimization algorithm. [ABSTRACT FROM AUTHOR]

Active debris removal (ADR) allows for the disposal of inactive satellites and larger objects, preventing the build-up of space junk and allowing to replace aging agents in a constellation. To make ADR missions more commercially viable, the removal and disposal of multiple debris objects using a single spacecraft are investigated. This paper proposes the use of artificial neural networks (ANNs) to quickly estimate the cost and duration of the transfers to de-orbit a range of debris objects, so that it is possible to identify the optimal sequence of objects which minimizes the cost and/or the duration of the mission, for the maximum number of de-orbited objects. To this end, the ANN is integrated within a sequence search algorithm based on a tree search. The performance of the proposed methodology is assessed by analyzing three distinctive sequences of multiple space debris removals. A near-term low-thrust propulsion technology enables to dispose of up to 13 debris objects within 10 years, when the optimal design parameters are chosen. The use of ANN allows for this solution to be found 26 times faster than current methods, while enabling the selection of faster and less expensive (being the propellant mass required lower) options. • Artificial Neural Networks (ANN) can be trained to estimate complex transfer models. • ANN can quickly estimate transfers for the disposal of multiple space debris. • ANN is used within a sequence search algorithm to identify optimal missions. • Missions where up to 13 debris can be disposed of within 10 years are obtained. • The use of machine learning greatly reduces the computational time by 26 times. [ABSTRACT FROM AUTHOR]

Abstract Any effort which intends to physically interact with specific asteroids requires understanding at least of the composition and multi-scale structure of the surface layers, sometimes also of the interior. Therefore, it is necessary first to characterize each target object sufficiently by a precursor mission to design the mission which then interacts with the object. In small solar system body (SSSB) science missions, this trend towards landing and sample-return missions is most apparent. It also has led to much interest in Mascot -like landing modules and instrument carriers. They integrate at the instrument level to their mothership and by their size are compatible even with small interplanetary missions. The DLR-ESTEC Gossamer Roadmap NEA Science Working Groups' studies identified Multiple NEA Rendezvous (MNR) as one of the space science missions only feasible with solar sail propulsion. Parallel studies of Solar Polar Orbiter (SPO) and Displaced L 1 (DL1) space weather early warning missions studies outlined very lightweight sailcraft and the use of separable payload modules for operations close to Earth as well as the ability to access any inclination and a wide range of heliocentric distances. These and many other studies outline the unique capability of solar sails to provide access to all SSSB, at least within the orbit of Jupiter. Since the original MNR study, significant progress has been made to explore the performance envelope of near-term solar sails for multiple NEA rendezvous. However, although it is comparatively easy for solar sails to reach and rendezvous with objects in any inclination and in the complete range of semi-major axis and eccentricity relevant to NEOs and PHOs, it remains notoriously difficult for sailcraft to interact physically with a SSSB target object as e.g. the Hayabusa missions do. The German Aerospace Center, DLR, recently brought the Gossamer solar sail deployment technology to qualification status in the Gossamer -1 project. Development of closely related technologies is continued for very large deployable membrane-based photovoltaic arrays in the GoSolAr project. We expand the philosophy of the Gossamer solar sail concept of efficient multiple sub-spacecraft integration to also include landers for one-way in-situ investigations and sample-return missions. These are equally useful for planetary defence scenarios, SSSB science and NEO utilization. We outline the technological concept used to complete such missions and the synergetic integration and operation of sail and lander. We similarly extend the philosophy of Mascot and use its characteristic features as well as the concept of Constraints-Driven Engineering for a wider range of operations. Highlights • Asteroid exploration is key to planetary science, new resources & planetary defense. • Multiple near-Earth asteroid rendezvous (MNR) is uniquely possible by solar sail. • Sail-carried asteroid landers enable direct surface access for science operations. • Small spacecraft solar sails and landers are cost-efficient to build and launch. • Mature technologies were created in the projects Gossamer-1 and MASCOT. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, displaced geostationary orbits using hybrid low-thrust propulsion, a complementary combination of Solar Electric Propulsion (SEP) and solar sailing, are investigated to increase the capacity of the geostationary ring that is starting to become congested. The SEP propellant consumption is minimized in order to maximize the mission lifetime by deriving semi-analytical formulae for the optimal steering laws for the SEP and solar sail accelerations. By considering the spacecraft mass budget, the performance is also expressed in terms of payload mass capacity. The analyses are performed both for the use of pure SEP and hybrid low-thrust propulsion to allow for a comparison. It is found that hybrid low-thrust control outperforms the pure SEP case both in terms of payload mass capacity and mission lifetime for all displacements considered. Hybrid low-thrust propulsion enables payloads of 255–487kg to be maintained in a 35km displaced orbit for 10–15 years. Adding the influence of the and terms of the Earth''s gravity field has a small effect on this lifetime, which becomes almost negligible for small values of the sail lightness number. Finally, two SEP transfers that allow for an improvement in the performance of hybrid low-thrust control are optimized for the propellant consumption by solving the accompanying optimal control problem using a direct pseudospectral method. The first type of transfer enables a transit between orbits displaced above and below the equatorial plane, while the second type of transfer enables customized service for which a spacecraft is transferred to a Keplerian parking orbit when geostationary coverage is not needed. While the latter requires a modest propellant budget, the first type of transfer comes at the cost of an almost negligible SEP propellant consumption. [Copyright &y& Elsevier]

The analysis of the optimal control law that steers a solar sail-based spacecraft from a given initial condition toward a final target state is typically carried out using either indirect or direct approaches. Both these methods are usually time-consuming and require good initial guesses of costates or state vector. This paper presents a procedure requiring minimum user-computer interaction to compute an approximate three-dimensional optimal trajectory using a shape-based approach. To that end, novel shaping functions are introduced to describe the time evolution of the spacecraft state vector. The optimization problem is solved using a genetic algorithm, in which a set of shape coefficients and the initial and final spacecraft position are computed while enforcing suitable constraints on the magnitude and direction of the propulsive acceleration vector. Numerical simulations of transfers from Earth to potentially hazardous asteroids show that this method provides good estimates of solar sail trajectories, which can be used as guesses for more refined direct optimization approaches. [ABSTRACT FROM AUTHOR]