Interplanetary CubeSats system for space weather evaluations and technology demonstration

The paper deals with the mission analysis and conceptual design of an interplanetary 6U CubeSats system to be implemented in the L1 Earth-Sun Lagrangian Point mission for solar observation and in-situ space weather measurements. Interplanetary CubeSats could be an interesting alternative to big missions, to fulfill both scientific and technological tasks in deep space, as proved by the growing interest in this kind of application in the scientific community and most of all at NASA. Such systems allow less costly missions, due to their reduced sizes and volumes, and consequently less demanding launches requirements. The CubeSats mission presented in this paper is aimed at supporting measurements of space weather. The mission envisages the deployment of a 6U CubeSats system in the L1 Earth-Sun Lagrangian Point, where solar observations for in situ measurements of space weather to provide additional warning time to Earth can be carried out. The proposed mission is also intended as a technology validation mission, giving the chance to test advanced technologies, such as telecommunications and solar sails, envisaged as propulsion system. Furthermore, traveling outside the Van Allen belts, the 6U CubeSats system gives the opportunity to further investigate the space radiation environment: radiation dosimeters and advanced materials are envisaged to be imple- mented, in order to test their response to the harsh space environment, even in view of future implementation on other spacecrafts (e.g. manned spacecrafts). The main issue related to CubeSats is how to fit big science within a small package - namely power, mass, volume, and data limitations. One of the objectives of the work is therefore to identify and size the required subsystems and equipment, needed to accomplish specific mission objectives, and to investigate the most suitable configuration, in order to be compatible with the typical CubeSats (multi units) standards

Artificial Potential Fields for Autonomous Cluster Keeping

In recent years the concepts of distributed systems and fractionation spread considerably, as they allow the replacement of a single monolithic spacecraft with multiple smaller ones. The desire for such a design transformation stems from the many limitations that are associated with the traditional monolithic option and can be instead overcome through the use of a fractionated architecture. A cluster of spacecraft working together could enhance the mission performances in many ways, e.g., by augmenting flexibility and redundancy, by reducing costs and risks, by overcoming physical limitations. On the other hand these benefits come at a price, since the cluster brings a new series of challenges concerning, for example, the sharing of data, the communication, and the relative motion between the objects. In the field of formation flight, much has been already done in these areas but not everything is directly applicable in a cluster scenario. For what concerns the relative motion, for instance, it is clear that this should always be safe to ensure the success of the mission, but while a formation requires the modules to remain in a precise relative configuration, a cluster only requires satisfaction of minimum and maximum distance constraints to ensure neither that modules drift away, nor that they collide. This peculiar type of requirements on the one side kept boosting the research in the field of the relative motion models, with a particular push in the direction of long-term passive distance-bounded relative orbits, on the other side promoted the development of cluster keeping algorithms, which are desired to be scalable, autonomous and responsive. In this thesis, at first several types of relative configurations are investigated and compared to observe how they influence the evolution of the relative motion and which advantages they bring into a cluster flight scenario. By considering the number of deployable objects and the v budget required for station keeping as main performance indexes, one specific configuration is selected and further explored with the application of the artificial potential field method. The approach of artificial potentials represents a simple and effective path planner, that can influence the motion of the considered objects, for example to steer them towards goal positions while ensuring obstacle avoidance, by using proper attractive and repulsive behaviours. On top of that the method deals well with the desired requirements of autonomy, responsiveness and scalability, and that is why it has been selected to be applied in the cluster keeping problem. The artificial potentials are widely studied and applied in the field of robotics, but the complexity of the equations of motion in the orbital environment significantly limited their spread in the space domain. Space research involving this method is nowadays restricted to small-sized relative motion problems, in which the simplified Hill-Clohessy-Wiltshire model could be used. In this thesis, on the other hand, both small- and large-sized clusters are considered, leading to the need for dropping distance-related simplified assumptions and developing a general cluster keeping approach. Two different artificial-potentials-based architectures are presented, one exploiting the use of virtual reference states that the spacecraft of the cluster track to (indirectly) satisfy minimum and maximum distance constraints, and one dealing directly with the relative distance between the spacecraft to alter their motion and prevent violations of the distance boundaries. By discussing the results of the extensive simulations that have been performed to study the two architectures, the strengths and limitations of these can be highlighted, and eventually a framework employing them both is proposed, showing under which conditions they can be successfully coupled. Conclusions and recommendations for future work finally wrap up the conducted research and close the thesis

Multiple spacecraft configuration designs for coordinated flight missions

Coordinated flight allows the replacement of a single monolithic spacecraft with multiple smaller ones, based on the principle of distributed systems. According to the mission objectives and in order to ensure a safe relative motion, constraints on the relative distances need to be satisfied. Initially, differential perturbations are limited by proper orbit design. Then, the induced differential drifts can be properly handled through corrective maneuvers. In this work several designs are surveyed, defining the initial configuration of a group of spacecraft while counteracting the differential perturbations. For each of the investigated designs, focus is placed upon the number of deployable spacecraft and on the possibility to ensure safe relative motion through station keeping of the initial configuration, with particular attention to the required DV budget and the constraints violations

Application of Mean-Motion-Based Artificial Potentials for a Cluster Flight Mission Scenario

Cluster flight is one of the key technologies that are required to enable the deployment of distributed space systems. Through the concept of cluster flight, a large monolithic structure can be replaced with multiple smaller spacecraft, permitting to overcome physical limitations and improve mission performance. To ensure a safe relative motion between several objects that fly in proximity, the guidance and control algorithm must be designed in order to be scalable, autonomous, and responsive. A technique to meet these requirements by employing the method of the artificial potentials is presented in this paper. For a cluster of spacecraft that are distributed in the along-track direction in a leader-follower manner, the relative distances can be altered by focusing and adjusting the mean motion of the spacecraft. An artificial-potential-based approach can be used to evaluate corrections of the semi-major axes by only reacting on the current configuration, with no need to perform trajectory predictions

Quasi-impulsive maneuvers to correct mean orbital elements in LEO

An approach is developed to compute quasi-impulsive maneuvers to steer the orbital elements of a spacecraft to a desired value. Using Gauss Variational Equations it is possible to define the location along the orbit as well as the magnitude of the maneuver(s) so that specific orbital elements can be changed with little influence on the others. The possibility to include the effect of the perturbations allows an accurate evaluation of the time required to reach the maneuvering location. Including a model of the propulsion system makes the simulation more realistic, if compared with an impulsive maneuver implementation, since a burning arc can replace the instantaneous change of the orbital elements, which is instead associated with the impulsive approach. Simulations have been performed to compare perturbed and unperturbed cases and the results from the comparisons are presented