Asteroid deflection and exploitation : possible synergies

The asteroid and cometary impact hazard has long been recognised as an important issue requiring risk assessment and contingency planning. At the same time asteroids have also been acknowledged as possible sources of raw materials for future large-scale space engineering ventures. This paper explores possible synergies between these two apparently opposed views; planetary protection and space resource exploitation. In particular, the paper assumes a 5 tonne low-thrust spacecraft as a baseline spacecraft for asteroid deflection and capture (or resource transport) missions. The system is assumed to land on the asteroid and provide a continuous thrust able to modify the orbit of the asteroid according to the mission objective. The paper analyses the capability of such a near-term system to provide both planetary protection and asteroid resources to Earth. Results show that a 5 tonne spacecraft could provide a high level of protection for modest impact hazards: airburst and local damage events (caused by 15 to 170 meters diameter objects). At the same time, the same spacecraft could also be used to transport to bound Earth orbits significant quantities of material through judicious use of orbital dynamics and passively safe aero-capture manoeuvres or low energy ballistic capture. As will be shown, a 5 tonne low-thrust spacecraft could potentially transport between 12 and 335 times its own mass of asteroid resources by means of ballistic capture or aero-capture trajectories that pose very low dynamical pressures on the object

Asteroid resource map for near-Earth space

Most future concepts for the exploration and exploitation of space require a large initial mass in low Earth orbit. Delivering this required mass from the Earth’s surface increases cost due to the large energy input necessary to move mass out of the Earth’s gravity well. An alternative is to search for resources in-situ among the near Earth asteroid population. The near Earth asteroid resources that could be transferred to a bound Earth orbit are determined by integrating the probability of finding asteroids inside the Keplerian orbital element space of the set of transfers with an specific energy smaller than a given threshold. Transfers are defined by a series of impulsive maneuvers and computed using the patched-conic approximation. The results show that even moderately low energy transfers enable access to a large mass of resources

Gauss’ variational equations for low-thrust optimal control problems in low-energy regimes

With the pursuit of increasingly innovative and complex space missions, the focus of the space industry has been turning towards electric propulsion systems. Due to their high specific impulse - about ten times that of a chemical engine - they provide large savings in propellant mass, decreasing the overall cost of the mission. This proves to be essential for small low cost missions, such as interplanetary CubeSats, and more ambitious endeavours such as asteroid retrieval or crewed missions to Mars. Designing a low-thrust trajectory is a more complex task than doing so for a high-thrust one, since computing the thrust sequence that minimizes the fuel spent requires a search over a huge and complex design space. Setting up the optimal control problem generally requires a good first-guess solution, a fine tuning of the parameters involved, and the definition of feasible bounds for the trajectory. In order to converge to a solution, the problem settings are simplified as much as possible. This includes the dynamical framework used, which often may not be sensitive enough to describe the low-energy trajectory regime necessary for some of the mission examples mentioned above. This abstract proposes a new set of equations of motion to solve the optimal control problem. These are derived from the disturbing function of the previously studied Keplerian Map, formulated from the Hamiltonian of the CR3BP. Its motion corresponds to the propagation of Gauss’s planetary equations with both the disturbing potential of the CR3BP, and the accelerations of the electric engine. The novelty of this formulation is that it describes a third-body motion in terms of the orbital elements that define the osculating orbit of the spacecraft, in a barycentric coordinate system. This is advantageous in several respects: first, low-thrust sub-optimal control laws can be easily generated and explored to find a first guess solution near global optima. Second, bounds for the optimal control problem, as well as the boundary values, can be easily defined, which allows for a much faster convergence. This dynamical framework is accurate until very close to the sphere of influence of the perturbing body, and thus can be efficiently used to target low-energy hyperbolic invariant manifold structures associated with periodic orbits near it. The paper presents the methodology as well as a full retrieval trajectory for asteroid 2018 AV2, a small co-orbital asteroid that could be retrieved during its next Earth encounter in 2037

The feasibility of using an L1 positioned dust cloud as a method of space-based geoengineering

In this paper a method of geoengineering is proposed involving clouds of dust placed in the vicinity of the L1 point as an alternative to the use of thin film reflectors. The aim of this scheme is to reduce the manufacturing requirement for space-based geoengineering. It has been concluded that the mass requirement for a cloud placed at the classical L1 point, to create an average solar insolation reduction of 1.7%, is 7.60x1010 kg yr−1 whilst a cloud placed at a displaced equilibrium point created by the inclusion of the effect of solar radiation pressure is 1.87x1010 kg yr−1. These mass ejection rates are considerably less than the mass required in other unprocessed dust cloud methods proposed and are comparable to thin film reflector geoengineering requirements. Importantly, unprocessed dust sourced in-situ is seen as an attractive scheme compared to highly engineered thin film reflectors. It is envisaged that the required mass of dust can be extracted from captured near Earth asteroids, whilst stabilised in the required position using the impulse provided by solar collectors or mass drivers used to eject material from the asteroid surface

Planetary protection efficiency by a small kinetic impactor

This paper re-examines the deflection concept with, arguably, the highest technological readiness level: the kinetic impactor. A baseline design for the concept with a 1,000 kg spacecraft launched from Earth is defined. The paper then analyses the capability of the kinetic spacecraft to offer planetary protection, thus, deflecting asteroids on a collision trajectory with Earth. In order to give a realistic estimate, the paper uses a set of more than 17 thousand Earth-impacting trajectories and has computed the largest asteroid mass that could be deflected to a sufficiently safe distance from Earth. By using the relative impact frequency of the different impact orbits, which can be estimated by modeling the asteroid population and the collision probability of the different impact geometries, a figure on the level of planetary protection that such a system could offer can be estimated. The results show that such a system could offer very high levels of protection, around 97% deflection reliability, against objects between 15 to 75 meters, while decreases for larger sizes

Use of orbiting reflectors to decrease the technological challenges of surviving the lunar night

In this paper the feasibility of using lunar reflectors to decrease the technological challenges of surviving the lunar night is investigated. This is achieved by attempting to find orbits in the two-body problem where the argument of periapsis is constantly Sun-pointing to maximise the time spent by the reflectors over the night-side of the Moon. Using these orbits the ability of reflectors of varying sizes to provide sufficient illumination to a target point on the surface is determined for scenarios where a latitude band is constantly illuminated and a scenario where a specific point is tracked. The optimum masses required for these far-term scenarios are large. However, a nearer-term scenario using low altitude orbits suggest that the effective duration of the lunar night can be reduced by up to 50% using a set of 300 parabolic reflectors of 100m radius with a total system mass of 370 tonnes. A system is also demonstrated that will allow a partial illumination of the craters in the Moon’s polar region for a mass up to 700kg

Autonomous navigation and guidance for CubeSats to flyby near-Earth asteroids

Recent advancements in CubeSat technology unfold new mission ideas and the possibility to lower the cost of space exploration. Exploiting the natural dynamics around the Sun-Earth barycentric Lagrange points, minimal-ΔV trajectories to flyby asteroids appear which are compatible with current CubeSat propulsive capabilities. Ground operations costs for an interplanetary CubeSat, however, still represent a major challenge towards low-cost missions; hence certain levels of autonomy are desirable. Considering the limited allocation of sensors and actuators in CubeSats, and their limited performance, Monte Carlo simulations are implemented to understand the flyby accuracies that can be achieved through autonomous navigation and guidance. Primary sources of error analyzed in this study include: (1) uncertainties in the departure conditions, (2) errors in the propulsive maneuvers, (3) errors in the observations, and (4) uncertainties in the ephemeris of the target asteroid. An autonomous navigation and guidance strategy is proposed and evaluated, employing observations of the Sun, visible planets and of the target asteroid, and two trajectory correction maneuvers along the trajectory. Flyby accuracies below 100 km are found possible if the mission characteristics are suitable in terms of available ΔV, on-board asteroid visibility time, mission duration, and asteroid ephemeris uncertainty before the mission. Ultimately, this study assesses the readiness level of current CubeSat technology to autonomously flyby near-Earth asteroids, with realistic component specifications and modeling of relevant errors and uncertainties. The effect of the different mission factors on the final flyby accuracies is evaluated, and a feasible autonomous navigation and guidance strategy is proposed in the effort to reduce ground operations and overall mission costs

Opportunities for asteroid retrieval missions

Asteroids and comets are of strategic importance for science in an effort to uncover the formation, evolution and composition of the Solar System. Near-Earth Objects (NEOs) are of particular interest because of their accessibility from Earth, but also because of their speculated wealth of material resources. The exploitation of these resources has long been discussed as a means to lower the cost of future space endeavours. In this chapter, we analyze the possibility of retrieving entire objects from accessible heliocentric orbits and moving them into the Earth’s neighbourhood. The asteroid retrieval transfers are sought from the continuum of low energy transfers enabled by the dynamics of invariant manifolds; specifically, the retrieval transfers target planar, vertical Lyapunov and halo orbit families associated with the collinear equilibrium points of the Sun-Earth Circular Restricted Three Body problem. The judicious use of these dynamical features provides the best opportunity to find extremely low energy transfers for asteroidal material. With the objective to minimise transfer costs, a global search of impulsive transfers connecting the unperturbed asteroid’s orbit with the stable manifold phase of the transfer is performed. A catalogue of asteroid retrieval opportunities of currently known NEOs is presented here. Despite the highly incomplete census of very small asteroids, the catalogue can already be populated with 12 different objects retrievable with less than 500 m/s of Δv. All, but one, of these objects have an expected size in the range that can be met by current propulsion technologies. Moreover, the methodology proposed represents a robust search for future retrieval candidates that can be automatically applied to a growing survey of NEOs

Near-Earth asteroid resource accessibility and future capture mission opportunities

In-Situ Resource Utilization (ISRU) has always been suggested for ambitious space endeavours; and asteroids and comets in particular are generally agreed to be ideal sources, both in terms of its accessibility and wealth. The future utilisation of asteroid resources is here revisited by, firstly, providing an estimate of the total amount of accessible resources in the Earth’s neighbourhood and, secondly, by envisaging a series of missions in order to retrieve resources from the most accessible objects known today. An analytical multi-impulsive transfer model is proposed in order to define the region in Keplerian space from which resources are accessible, and mapped subsequently into a near-Earth asteroid model, to understand the availability of material. This estimate shows a substantial amount of resources can be accessible at relatively low energy-cost; on the order of 1014 kg of material could potentially be accessed at an energy cost lower than that required to access the resources in the Moon. Most of this material is currently undiscovered, but the current surveyed population of near-Earth asteroid provides a good starting point for a search for future capture opportunities. The possibility of capturing, i.e., placing the asteroid into an orbit in permanent close proximity to Earth, a small-size NEO or a segment from a larger object would be of great scientific and technological interest in the coming decades. A systematic search of capture candidates among catalogued NEOs is presented, which targets the L2 region as the destination for the captured material. A robust methodology for systematic pruning of candidates and optimisation of capture trajectories through the stable manifold of planar Lyapunov orbits around L2 has been implemented and tested. Five possible candidates for affordable asteroid retrieval missions have been identified among known NEOs, and the transfers to the L2 region calculated. These transfers enable the capture of bodies with 2-8 meters diameter with modest propellant requirements. Because of the optimal departure dates, two of them have been identified as attractive targets for capture missions in the 2020-2030 time frame

Comparison of single and multi-spacecraft configurations for NEA deflection by solar sublimation

Since the first Near Earth Object (NEO) defence system, Project Icarus, was published in 1967, where the driving factor was a very short anticipating time (i.e. time available to act on the asteroid), the Near Earth Asteroid (NEA) hazard outlook has changed drastically. In the current state of NEO research, long-term missions are becoming more realistic and as such, the options for low thrust systems are being investigated as a viable option for deviating the asteroid path. Surface ablation approaches have been previously proposed using several techniques such as lasers and nuclear explosives. The method presented here hinges on directing solar energy using mirrors onto a small area on the surface of the asteroid. This concentrated heat then sublimates the surface matter creating narrow but expanding jets of gas and dust that produce a low continuous thrust. This low thrust would then alter the orbit of the NEA by producing a change in velocity, similar to the effect of the ‘tail’ on a comet. This paper adds a new important trade-off to the problem: a comparison between a single structure and a multi-mirror system. The systems analysed include both single and dual mirror configurations, for both a single spacecraft and multiple spacecraft in formation. The criteria include ease of launch, reliability, flexibility in achieving the mission objective, in-space mass and a basic cost analysis. The concept, and potential benefits, of formation flying have long been known. The barrier however, has been the high level of complexity involved in the control of the individual spacecraft within the formation. Advances in control algorithms and associated technologies have opened the door to using spacecraft formations for specialized missions, such as small-body missions which operate in highly perturbed environments. This paper compares the complexities of deploying and operating a large rigid structure around asteroids, with the control of a swarm of smaller structures. Configurations are presented for different NEAs, time-in-advance, and achieved deflections