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In a similarly brief event some 10½ years before Apophis' fly-by on Friday, April 13th, 2029, the Mobile Asteroid Surface Scout, MASCOT, successfully completed its 17-hours mission on the ~km-sized C-type potentially hazardous asteroid (162173) Ryugu. Investigating the surface and its thermal properties, looking for a magnetic field, and imaging the stark landscapes of this dark rubble pile, it contributed valuable close-up information before the surface sampling by its mothership, HAYABUSA2. We outline the capabilities of the asteroid nanolanders MASCOT, MASCOT2, and the options for optimized MASCOT@Apophis designs in particular for small spacecraft rendezvous missions to Apophis.

By the time of Apophis' fly-by on Friday, April 13th, 2029, more satellites than have ever been launched since the beginning of the space age to this day will reach low Earth orbit (LEO). Almost all of them will be microsatellites of less than ~250 kg equipped with solar-electric propulsion (SEP). We propose the use of already created low-thrust trajectories to Apophis to help advance design trades in the early study phases of missions to Apophis. It appears that small spacecraft missions could benefit from solar-electric or sail propulsion.

This paper aims to provide a detailed analysis of the preliminary Attitude Determination and Control System (ADCS) design of DLR’s (German Aerospace Center) demonstrator mission GoSolAr (Gossamer Solar Array). The goal of this mission is to demonstrate the two-dimensional deployment of a 25m2 flexible solar array in orbit. Understanding of the satellite configurations and control phases is critical for the design of the ADCS. The main structural configurations are stowed and deployed, in which the satellite consists of a central part to which the solar array is attached via four composite booms. The control phases are detumbling, deployment and acquiring and maintaining an orientation w.r.t. the Sun. This study focuses in developing a control approach for the attitude of the satellite able to deal with the difficulties inherent to the GoSolAr satellite.These difficulties can be divided in two groups, related with the particularities of the deployed structure and to the limitations of the attitude actuators selected. In relation with the structure, the most concerning issues are related with the considerably high area-to-mass and moment of inertia-to-mass ratios, which increase the effect of external disturbances and reduce that of the control actuators. This initial design contains only magnetorquers, generating a locally underactuated system. The analyses focuses in the pointing phase, which aims to reach and maintain a relative orientation of the main axis of inertia w.r.t. the Sun, while generating a spin around this axis to stabilize the satellite. In relation to this phase, two control approaches are explained, implemented and evaluated. The first one is based on using a linearization of the plant combined with an LQR (linear-quadratic regulator) approach. The second control approach is known as the Udwadia-Kalaba approach, and is based in the parallelism between constrained and controlled systems. This approach leads to a non-linear controller which can include complex guidance instructions. The performance of these controllers is evaluated for the nominal case, confirming that they are able to fulfill the requirements. The difference in performance between LQR and Udwadia-Kalaba control approaches is explained, focusing on convergence time and long term error. Finally, some limitations in relation to the ADCS design are pointed out, related to control actuation limitations and to assumptions made when deriving the controllers. In relation to the control actuation, the use of magnetorquers imposes a limitation in altitude and in orbital inclination. The potential consequences of neglecting the flexibility are also addressed qualitatively.

During the last decades, large deployable structures are starting to be seen as a plausible configuration to multiple space missions, such as solar sailing, LEO (Low Earth Orbit) deorbiting missions or solar power generation. The potential of this kind of structures lays in their high area-to-mass ratio and their low launch volume, which decreases the overall cost of the mission. Technological advances in key areas, such as thin film solar cells or new deployment methods, as well as the miniaturization of satellites and their components, have considerably increased the usefulness of this design. The research presented in this document aims to contribute to these technological developments, helping to unfold the whole potential of this structural solution. This research comprises the MSc thesis of the author, in partial fulfillment of the MSc in Aerospace Engineering by the Technical University of Delft. The research was conducted during an internship at the department of Guidance Navigation and Control (GNC) Systems, Institute of Space Systems, German Aerospace Center (DLR). The research focuses in the study of the behavior of large thin flexible structures (LTFSs) in space from the perspective of the attitude determination and control system (ADCS). This work was performed in close relation to the DLRs mission GoSolAR (Gossamer Solar Array), which targets the technical developments that are necessary to unfold large flexible photovoltaic arrays in space.

Deployment systems for innovative space applications such as solar sails require technology for a controlled and autonomous deployment in space. Before employing such technology for a dedicated mission, it is necessary to demonstrate its reliability with a Technology Readiness Level (TRL) of six or higher. On the example of the design implemented in the Gossamer-1 project of the German Aerospace Center (DLR), a stowing and deployment process for large deployable membranes mainly considered for solar sailing is analyzed and tested. It is based on a combination of zig-zag folding and coiling of triangular sail segments spanned between crossed booms. Possible membrane materials are evaluated and a deployment technique is explored through theoretical analysis and tests in order to verify their functionality for large membrane space systems. The requirements for membranes that are exposed to the space environment are studied and the materials are analyzed regarding their resistance against atomic oxygen, radiation and their thermal properties. The folding geometry and force progressions are described mathematically. Load introduction aspects, the stress-strain state and the billowing of the deployed membrane are analyzed with finite element models. The folding lines were examined with microscopes, and their impact on thermal behavior is shown by analytical analysis. The membrane and deployment mechanisms were manufactured and integrated in an ISO 8 clean room environment, and the deployment process was verified in an extensive test campaign. It ranged from component level to system level and included mechanical vibration, static acceleration, fast decompression, thermal vacuum and laboratory deployment tests. It is shown that state-of-the-art aluminum-coated polyimide foils are sufficient for a demonstration of deployment technology in an Low Earth Orbit (LEO) and that coating systems based on a combination of aluminum, silicon oxide and titanium oxide enhance the membrane properties for solar sails. The model of the deployment force progression under zero gravity shows a tendency that the loads are transferred along the cathetus of the sail segments. The finite element models show generally low stresses in the deployed membrane and interface forces on the order of several Newtons for a 25m2 membrane. The analysis of the folding lines reveals that coatings in this region are damaged, and that hot spots can occur due to multiple reflections. The verification testing showed the general suitability of the membrane and of the deployment strategy itself. Materials, mechanisms, and a stowing and deployment strategy are presented that enable the controlled and autonomous membrane deployment for space sails. While the analysis presented is applied on a sail with an edge length of about 5 m, it allows an analysis of other configurations as well. This is of particular interest because currently-considered solar sails are about one order of magnitude bigger. With the environmental tests conducted, the membrane-related aspects of the deployment technology are on TRL six for a 25m2 LEO deployment demonstrator. The deployment strategy is scalable and materials are available that can be used for bigger solar sails as well. With respect to membrane-related aspects there is nothing to prevent the development of full-scale solar sails.

Following the highly successful flight of the first interplanetary solar sail, JAXA's IKAROS, with missions in the pipeline such as NASA's NEASCOUT nanospacecraft solar sail and JAXA's Solar Power Sail solar-electric propelled mission to a Jupiter Trojan asteroid, and on the back-ground of the ever increasing power demand of GEO satellites now including all-electric spacecraft, there is renewed interest in large lightweight structures in space. Among these, deployable membrane or 'gossamer' structures can provide very large functional area units for innovative space applications which can be stowed into the limited volumes of launch vehicle fairings as well as secondary payload launch slots, depending on the scale of the mission. Large area structures such as solar sails or high-power photovoltaic generators require a technology that allows their controlled and thereby safe deployment. Before employing such technology for a dedicated science or commercial mission, it is necessary, to demonstrate its reliability, i.e., TRL 6 or higher. A reliable technology that enables controlled deployment was developed in the GOSSAMER-1 solar sail deployment demonstrator project of the German Aerospace Center, DLR, including verification of its functionality with various laboratory tests to qualify the hardware for a first demonstration in low Earth orbit. We provide an overview of the GOSSAMER-1 hardware development and qualification campaign. The design is based on a crossed boom configuration with triangular sail segments. Employing engineering models, all aspects of the deployment were tested under ambient environment. Several components were also subjected to environmental qualification testing. An innovative stowing and deployment strategy for a controlled deployment and the required mechanisms are described. The tests conducted provide insight into the deployment process and allow a mechanical characterization of this process, in particular the measurement of the deployment forces. The stowing and deployment strategy was verified by tests with an engineering qualification model of one out of four GOSSAMER-1 deployment units. According to a test-as-you-fly approach the tests included vibration tests, venting, thermal-vacuum tests and ambient deployment. In these tests the deployment strategy proved to be suitable for a controlled deployment of gossamer spacecraft, and deployment on system level was demonstrated to be robust and controllable. The GOSSAMER-1 solar sail membranes were also equipped with small thin-film photovoltaic arrays intended to supply the core spacecraft. In our follow-on project GOSOLAR, the focus is now entirely on deployment systems for huge thin-film photovoltaic arrays. Based on the GOSSAMER-1 experience, deployment technology and qualification strategies, new technologies for the integration of thin-film photovoltaics are being developed and qualified for a first in-orbit technology demonstration within five years. Main objective is the further development of deployment technology for a 25 m² gossamer solar power generator and a flexible photovoltaic membrane. GOSOLAR enables a wider range of deployment concepts beyond solar sail optimized methods. It uses the S²TEP bus system developed at the Institute of Space Systems as part of the DLR satellite roadmap.

In recent years the German Aerospace Center (DLR) developed gossamer deployment systems in the GOSSAMER- 1 project with a focus on solar sails also equipped with small thin-film photovoltaic arrays. With our new project GOSOLAR ahead, the focus is now entirely on gossamer deployment systems for huge thin-film photovoltaic arrays. Based on the previous achievements in the field of deployment technology and qualification strategies, new technology for the integration of thin-film photovoltaics will be developed and qualified with the goal of a first inorbit technology demonstration. The time frame for this development is about five years. The two major objectives of the project are the further development of deployment technology for a 25 m² gossamer solar power generator and the development of a flexible photovoltaic membrane. In contrast to the GOSSAMER-1 deployment approach, GOSOLAR enables a wider range of deployment concepts. The technology demonstration is supposed to employ the S²TEP bus system which is developed on-site in parallel. While the development of a bus system is in consequence not part of the GOSOLAR project, there are special challenges when it comes to the development of huge solar arrays. The level of power required in the solar array application is about two orders of magnitude higher than for a sailcraft of the same size. The currents required to carry power off the thin-film structure at commonly used bus voltages result in a substantial harness cross-section. At the same time, there is a desire for higher voltages, e.g. to power electrical propulsion directly. In consequence the first system GOSOLAR will be a low voltage system employing offthe- shelf small spacecraft power system technology. The development of high power systems will be studied in parallel and its implementation is left to future projects. Using an established test strategy, a characterization of the deployment performance and deployment forces will be made based on a test-as-you-fly approach. It includes vibration testing, fast decompression, partial deployment under thermal-vacuum and full-scale ambient deployment on a test rig previously developed for GOSSAMER-1. The data gained can be used for further development and as input for mechanism and structure sizing. Examples for the application of those testing strategies are the previous DLR GOSSAMER-1 project, the ESA drag sail projects ‘Deployable Membrane’ and ‘Architectural Design and Testing of a De-Orbiting Subsystem’ (ADEO) as well as the tether deployment of the HP³ experiment on the NASA/JPL Mars mission INSIGHT.