On orbit inspection with CubeSats: State of the art and future perspective

The current Space Agencies exploration programme encompasses the analysis and development of gap-filling activities towards future reliable missions. Within this context, examples of these activities could be found in orbit debris management, reusable Space Transportation Systems and orbital stations based on the heritage of the ISS. For all of these, a system performing on orbit inspection could represent an opportunity to increase both mission reliability and safety, while reducing the need of Astronauts Extra Vehicular Activity. Indeed, a constant monitoring of the external conditions of a target could: 1) track down damages and geometry of components 2) support maintenance activities 3) identify and characterize dynamics and materials. Since the introduction of the CubeSat standard in the early 2000s, there has been a proliferation of nanosatellites in Low Earth Orbit with more than 100-200 launched annually, pushing towards interplanetary applications. It has been verified that CubeSats can efficiently support a wide range of space missions: several studies have been already conducted and are ongoing. For example, the CubISSat mission to inspect the International Space Station has been developed within SysNova ESA framework with interesting results. Other examples are the e-Inspector CDF study, a mission targeted at gathering data about Envisat to prepare the removal mission and the ESA SROC study, a mission targeted at the observation of the new-born Space Rider. The paper presents design solutions for CubeSats equipped with stereoscopic vision system and hyperspectral camera payloads, involved in the visual inspection of specific target. Different mission concepts and CubeSat configurations are evaluated and compared. For each solution, advantages and challenges that impact both on payload performances and system requirements are identified and discussed. A preliminary study is presented on the quality of the multispectral information in relation to the changes in distance from the target, spectral bands involved, relative velocity and spectral resolution. Similarly, stereoscopic system performances are analysed with respect to spatial resolution of the camera, field of view, image size and complexity of the algorithms for data processing. All these parameters impose constraints on the CubeSat design and assembly in terms of features such as pointing accuracy and stability, precise attitude control and navigation, data storage capability, processor throughput and data rate. Thanks to the analysis of general case studies, a baseline feasibility analysis of an inspection mission is then traced, taking into account the most promising COTS CubeSat-based technology but facing the state of the art towards more effective future missions. As a result, a technology development roadmap is suggested, highlighting critical accomplishments to fulfil in order to improve reliability and performances of inspection missions. Giving an overview of current and future capabilities of CubeSats for inspection missions, this paper aims to put the basics for future CubeSat applications development, assisting the human vision of space exploration.