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Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft that finally defines the missions lifetime unless way to compensate for it is provided. This environment is named Very Low Earth Orbit (VLEO) and is defined for $h, arXiv admin note: substantial text overlap with arXiv:2106.15912

Operating satellites in Very Low Earth Orbit (VLEO) benefit the already expanding New Space industry in applications including Earth Observation and beyond. However, long-term operations at such low altitudes require propulsion systems to compensate for the large aerodynamic drag forces. When using conventional propulsion systems, the amount of storable propellant limits the maximum mission lifetime. The latter can be avoided by employing Atmosphere-Breathing Electric Propulsion (ABEP) system, which collects the residual atmospheric particles and uses them as propellant for an electric thruster. Thus, the requirement of on-board propellant storage can ideally be nullified. At the Institute of Space Systems (IRS) of the University of Stuttgart, an intake, and a RF Helicon-based Plasma Thruster (IPT) for ABEP system are developed within the Horizons 2020 funded DISCOVERER project. To assess possible future use cases, this paper proposes and analyzes several novel ABEP-based mission scenarios. Beginning with technology demonstration mission in VLEO, more complex mission scenarios are derived and discussed in detail. These include, amongst others, orbit maintenance around Mars as well as refuelling and space tug missions. The results show that the ABEP system is not only able to compensate drag for orbit maintenance but also capable of performing orbital maneuvers and collect propellant for applications such as Space Tug and Refuelling. Thus, showing a multitude of different future mission applications. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 737183. This reflects only the author’s view, and the European Commission is not responsible for any use that may be made of the information it contains

Very Low Earth Orbits (VLEOs), those below 450 km, present a number of benefits and challenges for the development and operation of Earth observation spacecraft at both the system and mission level. This paper examines the design of constellations of satellites for operation in VLEO for Earth observation considering both system and mission level trade-offs. The resulting analysis identifies general design trends and proposes suitable mission architectures for Earth observation from VLEO. The principal benefit for satellites operating in VLEO is that the reduction in the distance to the Earth’s surface allows better imaging resolution to be achieved using smaller and less powerful payloads. This has corresponding benefits for the system mass and cost. However, the sustained and controlled operation of spacecraft in VLEO is challenging due to the increased atmospheric density at these altitudes, which increases propulsive and attitude control requirements. Technologies to facilitate the commercially viable operation of spacecraft in VLEO are currently being developed, for example materials to facilitate drag-reduction and aerodynamic control and atmosphere-breathing electric propulsion systems (ABEP), each of which influence the design of other sub-systems, requiring, for example, varying levels of power or new geometric considerations. At the mission level, the reduction in altitude has a generally negative influence on the coverage and revisit characteristics of a given satellite. However, deployment of these satellites in constellations can provide improvements in the overall system metrics. Systems operating in VLEO may also benefit from improved launch vehicle capability and assured end-of-life deorbit. It is clear, therefore, that important and non-intuitive trade-offs between the satellite platform design, constellation configuration, and total cost arise in the design of these systems. This paper uses combined platform-level system modelling and mission analysis to explore the design of constellations of satellites in VLEO for Earth observation and demonstrates the necessity of a holistic approach to mission and system design when considering operations in VLEO.

The DISCOVERER project commenced in 2017 with the aim to advance the development of key technologies to enable the commercially viable, sustained operation of satellites in very low Earth orbits (VLEO). Funded by the European Commission through Horizon 2020, the project ends this month. This paper presents an overview of the key achievements and current status of the project.The project set out to advance the development of, and demonstrate, several technologies with the long-term aim of enabling the commercial use of VLEO. These technologies include:1. aerodynamic materials which encourage specular scattering of the incoming flow to minimise drag andincrease the performance of aerodynamic surfaces in the highly rarefied flows experienced in VLEO2. aerodynamic attitude control methods to compensate for the dynamic flow environment, especiallylower in the VLEO altitude range3. atmosphere breathing electric propulsion (ABEP), combining an optimised atmospheric intake withadvanced RF Helicon-based plasma thruster, for drag compensationDISCOVERER’s test satellite, the Satellite for Orbital Aerodynamics Research or SOAR, was deployed from the International Space Station in June 2021 and re-entered the atmosphere in March 2022. The primary aim was to measure the induced drag and lift on different aerodynamic materials candidates in VLEO by exposing panels, coated in various novel and control materials, to the flow at different orientations whilst observing the induced attitude and orbit perturbations produced. Early analysis of the results from the mission shows promising results for the novel materials developed as part of the project. Parallel studies on the long-term survivability of these materials to the space environment have been on-going through exposure tests on the exterior of the International Space Station through the MISSE programme.The project has also been developing a ground-based facility, the Rarefied Orbital Aerodynamics Research facility, to characterise the gas surface interaction properties of materials to atomic oxygen at orbital velocities. Characterisation of the facility itself is on-going.In support of ABEP technology, the experimental development and characterisation of an RF Helicon-based plasma thruster has been on-going, along with detailed computational modelling of aerodynamic intakes. Whilst the thruster has already been operated, current work focusses on the characterisation of its performance.Finally, work to place these technological developments into context has also been progressed. On overview of the overall achievements in this area is provided, including business modelling of the VLEO market ecosystem, which identifies the enormous market potential for VLEO missions.

The Satellite for Orbital Aerodynamics Research (SOAR) is a 3U CubeSat that has been designed to investigate the aerodynamic performance of different materials and perform demonstrations of aerodynamic attitude control manoeuvres in very low Earth orbit (VLEO). SOAR was deployed from the ISS on 14th June 2021 into a naturally decaying orbit and deorbited on 14th March 2022. This paper provides an overview of the operations performed during the mission and presents preliminary experimental results obtained from this spacecraft.SOAR was designed and launched within the frame of DISCOVERER, a Horizon 2020 project that aimed to support the development of a new class of commercially viable spacecraft operating in VLEO, i.e., orbits below 450 km in altitude. Operating in these lower altitude orbits has several benefits to the design of spacecraft, particularly for Earth observation and communications applications. However, development of spacecraft that can operate sustainably at these altitudes requires advancement in foundational technologies, for example atmosphere-breathing electric propulsion (ABEP) and novel aerodynamic materials.The primary aim of SOAR was to characterise the aerodynamic performance of different materials at very low altitudes and accomplished this task using a set of steerable fins that exposed different materials to the oncoming flow and an ion and neutral mass spectrometer (INMS) to provide in-situ measurements of atmospheric properties. SOAR was also designed to perform novel aerodynamic attitude control manoeuvres and measurements of thermospheric winds.Two of the materials carried to orbit were selected for their atomic oxygen erosion resistance and potential improvement in aerodynamic performance. The identification of such materials would allow for a reduction in the drag experienced in VLEO, the design of atmospheric intakes with greater efficiency used for ABEP, and implementation of enhanced aerodynamic attitude and orbit control. Ongoing ground-based experimentation seeks to further characterise the properties of such materials and to deepen our understanding of the physical interaction mechanisms that occur in the rarefied flow environment of VLEO.

The DISCOVERER project commenced in 2017 with the aim to advance the development of key technologies to enable the commercially viable, sustained operation of satellites in very low Earth orbits (VLEO). Funded by the European Commission through Horizon 2020, the project ends this month. This paper presents an overview of the key achievements and current status of the project. The project set out to advance the development of, and demonstrate, several technologies with the long-term aim of enabling the commercial use of VLEO. These technologies include: 1. aerodynamic materials which encourage specular scattering of the incoming flow to minimise drag and increase the performance of aerodynamic surfaces in the highly rarefied flows experienced in VLEO 2. aerodynamic attitude control methods to compensate for the dynamic flow environment, especially lower in the VLEO altitude range 3. atmosphere breathing electric propulsion (ABEP), combining an optimised atmospheric intake with advanced RF Helicon-based plasma thruster, for drag compensation DISCOVERER’s test satellite, the Satellite for Orbital Aerodynamics Research or SOAR, was deployed from the International Space Station in June 2021 and re-entered the atmosphere in March 2022. The primary aim was to measure the induced drag and lift on different aerodynamic materials candidates in VLEO by exposing panels, coated in various novel and control materials, to the flow at different orientations whilst observing the induced attitude and orbit perturbations produced. Early analysis of the results from the mission shows promising results for the novel materials developed as part of the project. Parallel studies on the long-term survivability of these materials to the space environment have been on-going through exposure tests on the exterior of the International Space Station through the MISSE programme. The project has also been developing a ground-based facility, the Rarefied Orbital Aerodynamics Research facility, to characterise the gas surface interaction properties of materials to atomic oxygen at orbital velocities. Characterisation of the facility itself is on-going. In support of ABEP technology, the experimental development and characterisation of an RF Helicon-based plasma thruster has been on-going, along with detailed computational modelling of aerodynamic intakes. Whilst the thruster has already been operated, current work focusses on the characterisation of its performance. Finally, work to place these technological developments into context has also been progressed. On overview of the overall achievements in this area is provided, including business modelling of the VLEO market ecosystem, which identifies the enormous market potential for VLEO missions. Peer Reviewed Article signat per 30 autors: Peter C.E. Roberts, Nicholas H. Crisp, Steve Edmondson, Antonio Arcos, Georg H. Herdrich, Jonathan Skalden, Silvia Rodriguez-Donaire, Daniel Garcia-Almiñana, Alejandro Macario-Rojas, Katharine L. Smith, Ciara Mcgrath, Sarah J. Haigh, Vitor T.A. Oiko, Brandon E.A. Holmes, Luciana A. Sinpetru, Virginia Hanessian, Simon Christensen, Thomas Kauffman Jensen, Jens Nielsen, Morten Bisgaard, Francesco Romano, Stefanos Fasoulas, Constantin Traub, Konstantinos Papvramidis, Miquel Sureda, Dhiren Kataria, Badia Belkouchi, Alexis Conte, Simon Seminari, Rachel Villain

The Satellite for Orbital Aerodynamics Research (SOAR) is a 3U CubeSat that has been designed to investigate the aerodynamic performance of different materials at low orbital altitudes. The spacecraft has been developed within the scope of DISCOVERER, a Horizon 2020 project that aims to develop foundational technologies to enable sustainable operations of Earth observation spacecraft in very low Earth orbits (VLEO) i.e., those below 450 km. SOAR features two payloads: i) a set of steerable fins that can expose different materials to the oncoming atmospheric flow developed by The University of Manchester, and ii) a forward-facing ion and neutral mass spectrometer (INMS) that provides in-situ measurements of the atmospheric density, flow composition, and velocity from the Mullard Space Science Laboratory (MSSL) of University College London. These payloads enable characterisation of the aerodynamic performance of different materials at very low altitudes with the aim to advance understanding of the underlying gassurface interactions in rarefied flow environments. The satellite will also be used to test novel aerodynamic attitude control methods and perform atmospheric characterisation in the VLEO altitude range. SOAR will perform the first in-orbit test of two novel materials that are expected to have atomic oxygen erosion resistance and drag-reducing properties, providing valuable in-orbit validation data for ongoing ground-based experimentation. Such materials hold the promise for extending operations at lower altitudes with benefits particularly for Earth observation and communications satellites that can correspondingly be reduced in size and cost. The platform for SOAR is largely based on GOMX-3 heritage and the spacecraft was assembled, integrated, and tested by GomSpace A/S. The satellite was launched on the SpX-22 commercial resupply service mission to the International Space Station in on 3rd June 2021 was subsequently deployed into orbit on the 14th June 2021. This paper presents the final preparations of SOAR prior to launch and provides an overview of the planned operations of the spacecraft following deployment into orbit.Keywords: Orbital Aerodynamics; Drag and Lift Coefficient; Gas-Surface Interactions; Thermospheric Wind; CubeSat.

The Satellite for Orbital Aerodynamics Research (SOAR) is a CubeSat mission, due to be launched in 2021, to investigate the interaction between different materials and the atmospheric flow regime in very low Earth orbits (VLEO). Improving knowledge of the gas-surface interactions at these altitudes and identification of novel materials that can minimise drag or improve aerodynamic control are important for the design of future spacecraft that can operate in lower altitude orbits. Such satellites may be smaller and cheaper to develop or can provide improved Earth observation data or communications link-budgets and latency. Using precise orbit and attitude determination information and the measured atmospheric flow characteristics the forces and torques experienced by the satellite in orbit can be studied and estimates of the aerodynamic coefficients calculated. This paper presents the scientific concept and design of the SOAR mission. The methodology for recovery of the aerodynamic coefficients from the measured orbit, attitude, and in-situ atmospheric data using a least-squares orbit determination and free-parameter fitting process is described and the experimental uncertainty of the resolved aerodynamic coefficients is estimated. The presented results indicate that the combination of the satellite design and experimental methodology are capable of clearly illustrating the variation of drag and lift coefficient for differing surface incidence angle. The lowest uncertainties for the drag coefficient measurement are found at approximately 300 km, whilst the measurement of lift coefficient improves for reducing orbital altitude to 200 km., Comment: 19 pages, 11 figures. Accepted for publication in Acta Astronautica (2020-12-14)

Interest in operating spacecraft in very low Earth orbits (VLEO), those below approximately 450 km, is growing due to the numerous benefits offered by reducing altitude. For remote sensing and Earth observation applications, improvements in resolution can be achieved or smaller instruments used with associated benefits in cost or mission value. Similarly, for communications applications, link-budgets and data latency can be improved by reducing the operational altitude. However, a key challenge to sustainedoperations in lower altitude orbits is to minimise and compensate for the aerodynamic drag that is produced by the interaction with the residual atmosphere. A principal aim of the DISCOVERER project is to identify, develop, and characterise materials thatcan promote specular reflections of the residual atmosphere in VLEO whilst also remaining resistant to the erosive atomic oxygen that is predominant at these altitudes. In combination with geometric design, such materials would be able to reduce the aerodynamic drag experienced by satellites in orbit and would also be able to generate usable aerodynamic lift enabling novel aerodynamic attitude and orbit control. SOAR (Satellite for Orbital Aerodynamics Research) is a 3U CubeSat that has been designed to investigate the aerodynamic performance of different materials in the VLEO environment and provide validation data for further ground-based experiments. To achieve this, the spacecraft features a set of steerable fins that can expose different materials to the oncoming atmospheric flow. A forward-facing ion and neutral mass spectrometer (INMS) provides in-situ measurements of the atmospheric density and flow composition. SOAR is scheduled for launch to the ISS in March 2021. This paper will present the design of the spacecraft, the experimental method that will be used to investigate the aerodynamic properties of materials in orbit, and will provide an update on the status of the spacecraft as it prepares for launch. The DISCOVERER project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 737183. This publication reflects only the view of the authors. The European Commission is not responsible for any use that may be made of the information it contains

Nanosatellites have gained considerable presence in low Earth orbits wherein the atmospheric interaction with exposed surfaces plays a fundamental role in the evolution of motion. These aspects become relevant with the increasing applicability of nanosatellites to a broader range of missions objectives. This investigation sets out to determine distinctive drag coefficient development and attributes of atmospheric gas-surface interactions in nanosatellites in the common form of standard 3U CubeSats from observed orbital decay. As orbital decay can be measured with relative accuracy, and its mechanism broken down into its constituent sources, the value of drag-related coefficients can be inferred by fitting modelled orbit predictions to observed data wherein the coefficient of interest is the adjusted parameter. The analysis uses the data of ten historical missions with documented passive attitude stabilisation strategies to reduce uncertainties. Findings indicate that it is possible to estimate fitted drag coefficients in CubeSats with physical representativeness. Assessment of atomic oxygen surface coverage derived from the fitted drag coefficients is broadly consistent with theoretical trends. The proposed methodology opens the possibility to assess atmospheric interaction characteristics by using the unprecedented opportunity arising from the numerous observed orbital decay of nanosatellites.

The ability to accurately measure the level of thrust during in-orbit operations is fundamental to the characterisation of emerging propulsion systems for nanosatellites. Many new CubeSat missions use propulsion systems with thrust levels in the order of few micro-Newtons. Whilst laboratory sensing resources are able to resolve such low thrust values, in complementary in-orbitcharacterisation are limited and in the main not compatible with the standard CubeSat mission. Additionally, typical in-orbit assessment of micro-thrust is generally carried out through body angular speed changes, the effectiveness of which is drastically reduced when external perturbations and sensor noise approach or exceed the thruster action on the CubeSat. This investigationsets out to improve in-orbit micro-thrust characterisation via changes in body angular velocity periodicity due to off-centred thrust action in nearly axisymmetric CubeSats. Unlike traditional methods that rely on determining angular acceleration this method employs a frequency analysis of the transversal component of the angular velocity signal with the aim of reducing measurementerror. Numerical simulations support the feasibility and adequacy of the proposed low-thrust gauging method, particularly for weak and noisy sensor signals. The robustness of the method allows for interchangeable analysed signal and enables the use of simple commercial-off-the-shelf rate sensors in fine micro-thrust characterisation.

Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft. To extend the lifetime of such missions, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP) that collects atmospheric particles to be used as propellant for an electric thruster. The system would minimize the requirement of limited propellant availability and can also be applied to any planetary body with atmosphere, enabling new missions at low altitude ranges for longer times. IRS is developing, within the H2020 DISCOVERER project, an intake and a thruster for an ABEP system. The article describes the design and simulation of the intake, optimized to feed the radio frequency (RF) Helicon-based plasma thruster developed at IRS. The article deals in particular with the design of intakes based on diffuse and specular reflecting materials, which are analysed by the PICLas DSMC-PIC tool. Orbital altitudes $h=150-250$ km and the respective species based on the NRLMSISE-00 model (O, $N_2$, $O_2$, He, Ar, H, N) are investigated for several concepts based on fully diffuse and specular scattering, including hybrid designs. The major focus has been on the intake efficiency defined as $\eta_c=\dot{N}_{out}/\dot{N}_{in}$, with $\dot{N}_{in}$ the incoming particle flux, and $\dot{N}_{out}$ the one collected by the intake. Finally, two concepts are selected and presented providing the best expected performance for the operation with the selected thruster. The first one is based on fully diffuse accommodation yielding to $\eta_c, Comment: Accepted Version

To extend missions lifetime at very low altitudes, an efficient propulsion system is required to compensate for aerodynamic drag. One solution is Atmosphere-Breathing Electric Propulsion (ABEP). It collects atmospheric particles to be used as propellant for an electric thruster. The system ideally nullifies the requirement of onboard propellant storage. An ABEP system can be applied to any celestial body with atmosphere (Mars, Venus, Titan, etc.), enabling new mission at low altitude ranges for longer times. Challenging is operation of the thruster on reactive chemical species, such as atomic oxygen (AO), that is highly present in low Earth orbit, as they cause erosion of (not only) propulsion system components, i.e. acceleration grids, electrodes, neutralizers, and discharge channels of conventional EP systems. For this reason, a contactless plasma thruster is developed: the RF helicon-based plasma thruster (IPT). The paper describes the thruster design, implementation, and first ignition tests. The thruster presents a novel antenna called the birdcage antenna that is implemented for decades in magnetic resonance imaging (MRI) machines. The design is supported by the simulation tool XFdtd®. The IPT is aided by an externally applied static magnetic field that provides the boundary condition for the helicon wave formation within the plasma discharge. The antenna working principle allows to minimize losses in the electric circuit and provides, together with the applied magnetic field, acceleration of a quasi-neutral plasma plume.

The DISCOVERER project is developing technologies to enable commercially-viable sustained-operation of satellites in very low Earth orbits for communications and remote sensing applications. Operating closer to the surface of the Earth significantly reduces latency for communications applications and improves link budgets, whilst remote sensing also benefits from improved link budgets, the ability to have higher resolution or smaller instruments, all of which provide cost benefits. In addition, all applications benefit from increased launch mass to lower altitudes, whilst end-of-life removal is ensured due to the increased atmospheric drag. However, this drag must also be minimised and compensated for. DISCOVERER is developing several critical technologies to enable commercially viable operations in at these lower altitudes including aerodynamic materials, aerodynamic attitude and orbit control methods, atmosphere breathing electric propulsion and an in-situ environment monitoring payload. The current status of these developments are summarised, along with the plans for the coming year.

In very low Earth orbits (VLEO), below 450 km altitude, the aerodynamic properties of satellites are primarily determined by the flow regime, free molecular flow, and the interaction of atomic oxygen with the surfaces of the spacecraft. The Rarefied Orbital Aerodynamics Research (ROAR) facility is a novel experimental facility designed to simulate these conditions in a controlled environment to characterise the aerodynamic properties of materials. It is built as part of DISCOVERER, a Horizon 2020 project developing the different technologies required to enable the sustainable operation of satellites in VLEO. Because ROAR isn’t intended to perform erosion studies, it differs quite significantly from other atomic oxygen exposure experiments and its characteristics are discussed in this work. ROAR consists of an ultrahigh vacuum system, responsible for generating the free molecular flow conditions, a source of hyperthermal oxygen atoms at orbital velocities, and mass spectrometers; the latter used to characterise the gas-surface interactions, and therefore the material’s aerodynamic performance. This paper includes a description of ROAR’s main components, together with the experimental methodology for materials testing and early results. Among the main parameters to be considered are atomic oxygen flux, beam shape and energy spread, mass resolution, and signal-tonoise ratio.

The use of very low Earth orbits (VLEO), for communications and remote sensing satellites, offers a number of significant payload and platform benefits. Imaging from these altitudes allows higher resolution or smaller optical payloads, whilst radar also benefits from improved link budgets leading to smaller antennas and lower transmission power. Communications payloa ds also have improved link budgets, reduced latency, and improved frequency reuse factors. Platform benefits include a more benign radiation environment, lower cost per kilogram to launch satellites, and atmospheric drag makes the environment inherently sustainable, simultaneously removing debris objects and ensuring satellites are quickly removed from orbit at the end of their operational lives. However, the impact of drag on satellite and mission operations must also be addressed.The DISCOVERER project, which commenced in 2017, is addressing the following key questions about technologies that would enable the commercially viable and sustained operation of satellites in VLEO:1. Are there materials or processes which reduce the induced drag on spacecraft surfaces?2. Are there propulsion methods which use the residual atmospheric gas as a propellant, providing drag compensation whilst removing the lifetime limits caused by carrying a limited amount of propellant?3. How can we improve our understanding of, and make best use of, the orbital aerodynamics of a space platform and its ability to perform attitude control manoeuvres?4. And what are the new opportunities that these technologies may bring to the market?This paper provides highlights from the developments made during the DISCOVERER project to date, demonstrating the potential for a new, commercially attractive, class of aerodynamic satellites operating in VLEO.Keywords: Orbital Aerodynamics; Very Low Earth Orbit; Remote Sensing; Earth Observation; SatelliteCommunications