Special Issue: Advances in CubeSat Sails and Tethers (1st Edition)

Currently, most space missions rely on chemical and electric propulsion systems, both of which are limited by the propellent tank volume whose “wet mass” is launched from the Earth. In the field of propellantless propulsion with space sails and tethers, we are exploring and developing propulsion methods which employ the natural forces originating in the Sun or in or around planets—the photon pressure, the atmospheric drag, the Coulomb drag and the Lorentz force. Reflective light sails use the photon pressure for interplanetary travel or atmospheric drag for deorbiting in low Earth orbit (LEO). Electric solar wind sails (E-sails) use the solar wind’s Coulomb drag for interplanetary propulsion and the plasma brake for ionospheric LEO deorbiting with the same Coulomb drag interaction. Electrodynamic tethers interact with planet’s magnetic field for exerting the Lorentz force.

While the size of CubeSats has grown from one kilogram to tens of kilograms, they are still far from hosting conventional propulsion systems, such as carried by tonne-class Cassini and Rosetta spacecraft (see Figure 1 in [1]). Therefore, for CubeSats and other small missions to gain significant (e.g., interplanetary) orbital maneuverability functionality, propellantless propulsion systems have the potential to overcome physical limitations of chemical and electric propulsion. The challenges in developing novel miniature propellantless propulsion systems are at least threefold. First, the new fundamental and mission analysis frameworks must be developed from scratch. Second, in-orbit experiments must be developed and fitted on small demonstration platforms, as they provide affordable flight opportunities. Third, in-orbit experiments of novel propulsion systems must be deployed and operated in space. Each propellantless system is at different levels of progress right now, as discussed in recent review papers—“Space sails for achieving major space exploration goals: Historical review and future outlook” [2] and “A comprehensive review of Electric Solar Wind Sail concept and its applications” [3]. Here, we provide highlights of technological achievements and setbacks leading up the the application of CubeSats for space sailing.

The first photon sailing demonstration was performed by the JAXA’s IKAROS spacecraft in 2010 [4]. NanoSAIL-D2 was deployed in 2011 but the mission was too low in the atmosphere to determine the atmospheric drag effect [5]. The LightSail 1 mission in 2015 was also too low [6]; however, in 2022, the LightSail 2 mission [7] that was launched in 2019 performed deorbiting, as reported in this Special Issue [8]. LightSail 2 achieved three-axis solar sail control, which in turn was used for introducing orbit changes. This Special Issue also reports a simulation study on sunlight reflectors using a formation visible from the Earth [9].

In the field of electrodynamic tethers, the E.T.PACK-Fly project is preparing a LEO demonstration mission of an electrodynamic tether [10]. This Special Issue includes new laboratory results of a bare-photovoltaic-tether feasibility study [11].

In the field of electric sails, the NASA Innovative Advanced Concept (NIAC) “Heliopause Electrostatic Rapid Transit System” (HERTS) project has worked on particle-in-cell (PIC) simulations and a mission concept for sailing to 100 au in 10 years [12,13]. PIC analysis has also been performed for ionospheric LEO conditions [14]. Aurora Propulsion Technologies develop plasma brakes in European Innovation Council (EIC) and European Space Agency (ESA) projects [15,16].

The E-sail is also prepared for in-orbit demonstration missions. The first unsuccessful attempts were performed in 2010s by ESTCube-1 and Aalto-1 missions [17,18,19,20]. Both missions were able to perform high-level CubeSat attitude determination and control [21,22,23], including high-rate spin up to 2.4 revolutions per second required for the planned E-sail tether deployment [24,25].

This Special Issue provides a thorough material on modeling the E-sail, on in-orbit demonstration plans and on future concepts. For ionospheric deorbiting with the plasma brake, two approximation methods for decay time and for trajectory are proposed [26,27]. For E-sail modeling, a generic mission design tool Electric Sail Mission Expeditor (ESME) is in early development [1], and multiple specific mission scenarios are studied–optimal trajectory for E-sail with Sun-facing attitude [28], optimal Earth gravity-assist maneuvers [29], optimal trajectories to heliostationary points [30], and an option for plunging a spacecraft into the Sun’s atmosphere [31].

The first edition of this Special Issue includes an article which describes the E-sail tether manufacturing technology [32]. The manufactured E-sail tether was carried onboard ESTCube-2 [33], published in the second edition; unfortunately, the ESTCube-2 satellite did not deploy from the Vega VV23 launcher. The team has also proposed the ESTCube-LuNa mission concept for demonstrating the E-sail in the solar wind of the Moon’s orbit [34].

The idea of solar wind propulsion is developing further in recent years. The dipole drive is constructed from two parallel screens, one charged positive, the other negative, creating an electric field between them with no significant field outside [35]. Ambient solar wind protons entering the dipole drive field from the negative screen side are reflected out, with the angle of incidence equaling the angle of reflection, thereby providing lift if the screen is placed at an angle to the plasma wind. The solar wind ion focusing thruster (SWIFT) utilizes a net of positively charged wires arranged as a cone and kept facing towards the Sun while the cone funnels the solar wind into the spacecraft, where it is manipulated into a particle beam before being ejected at high velocities to accelerate the spacecraft [36]. The application of SWIFT has been analyzed for optimal interplanetary transfer, including simplified Earth–Venus and Earth–Mars transfers [37].

As compared with the E-sail, whose thrust control is in a close loop with the spin plane control [38], the dipole drive and SWIFT technology concepts are proposing methods with more authority over thrust vector control, either by two parallel screens or wires arranged as a cone. Future research and development includes the following topics: What is the performance of the E-sail in the ionosphere and in solar wind? How E-sail performance, size and complexity compares with the dipole drive and with SWIFT? Can dipole drive and SWIFT be hosted by a CubeSat-class spacecraft?