## 1 Introduction
### E-Sail and Plasma Brake Demonstration Missions  
The primary scientific and technological advancement of the ESTCube-LuNa concept, as presented by this article, is to characterise the Coulomb drag effect in solar wind environment, which would also serve as a novel demonstration of the fundamental E-sail principles in its operational environment. Several missions have attempted to demonstrate the plasma brake and E-sail components in _LEO environment_. ESTCube-1 [24; 25] and Aalto-1 [26; 27] were launched in 2013 and 2017, respectively, with the first generation of CDP experiments. However, neither the ESTCube-1 nor Aalto-1 CDP tether deployed due to engineering difficulties in accommodating a vacuum-qualified piezoelectric motor, which is sensitive to launch vibrations [28; 29]. Foresail-1 [30] and ESTCube-2 were launched in 2022 and 2023, respectively, with the second generation of CDP experiments--this time, the two-phase bipolar stepper motor, _phy_SPACE 19 (from _Phytron_), is space-qualified [14]. However, the Foresail-1 communication link was lost, and ESTCube-2 did not deploy from the launch vehicle. While an updated Foresail-1p is already in development, the official investigation of ESTCube-2 deployment from the rocket remains unresolved at the time of article submission. Following the Foresail-1 objectives, the Foresail-1p mission focuses on plasma brake demonstration. The ESTCube-2 mission included both the negative-mode plasma brake and the positive-mode E-sail with electron emitters, which are still of high importance to be demonstrated in LEO environment before the solar wind.  
The Foresail-2 mission to reach _highly elliptical orbit_, such as geostationary transfer orbit (GTO), has been proposed to include a tether payload [31]. However, the Foresail-2 team reported in the Winter Satellite Workshop 2024 that the planned CDP experiment is being descoped [32]. The primary reason for taking the CDP experiment off the Foresail-2 mission is the difficulty of accommodating spinning tether requirements with other instruments and the GTO's geometry for communications, state estimation and satellite control. The Foresail-2 descope provides us with an additional motivation to develop the ESTCube (Electric Sail Test Cube) roadmap from LEO to Moon's orbit with LuNa (lunar nanospaccraft), as discussed in Section 1.3.  
As compared with the previous E-sail demonstration mission concepts in Earth's orbit, the case of an interaction between the lunar orbit and direct solar wind necessitates,as presented by this article, new mission analyses in terms of fundamental E-sail forces, experiment geometry and E-sail characterisation methods. Lunar orbit provides pristine solar wind conditions for about two thirds of the time: this is a major benefit over the GTO option proposed for the Foresail-2 mission. We propose a _spin-stabilised_ mission geometry where the E-sail spin plane remains inertially fixed due to the large angular momentum a deployed tether provides. With a fixed spin plane, we will have favourable experiment conditions four times a year: when the spin plane is perpendicular or parallel to the solar wind direction. Our proposed E-sail experiments take advantage of both orientations by estimating the orbital displacement due to the thrust and the spin rate changes due to the torque.  
The current paper also presents a case for an E-sail-propelled lunar escape manoeuvre. We are fully aware of the fact that the inertially fixed spin axis cannot be used for optimal trajectory control. As a \"stretch goal\", the escape trajectory from lunar orbit provides us with a clear ambition for future development. By placing E-sail spin plane control thrusters on a remote unit at the tether's tip, we would provide an optimal solution for spin plane control and, in turn, enable the trajectory manoeuvre. Most other ESTCube-LuNa technologies could be reused in a future mission concept with a remote unit.