## 1 Introduction  
### Electric Solar Wind Sail and Plasma Brake in the Context of Propellantless Propulsion  
The electric solar wind sail, or E-sail, was envisioned by [PERSON] during the period 2004-2006 [1; 2]. It has been thoroughly studied and developed since then and analysed in dozens of scientific articles in an E-sail publication repository curated by [PERSON]: [[https://www.electric-sailing.fi/publications.html](https://www.electric-sailing.fi/publications.html)]([https://www.electric-sailing.fi/publications.html](https://www.electric-sailing.fi/publications.html)) (accessed on 10 March 2024), including comprehensive reviews of the E-sail concept and its applications [3; 4]. The technology consists of one or multiple thin wires, so-called tethers, which are deployed centrifugally. The tether deployment requires angular momentum and spin-up that can be provided by either a _remote unit_ with a miniature propulsion system, or by attaching an _endmass_ at the tip of a tether while hosting the propulsion unit on the main spacecraft body. The E-sail tether can be as thin as possible as long as it can carry the charge, but it should also have a redundant multi-wire structure for micrometeoroid resilience [5]. Deployed tethers are charged to a high positive voltage in the order of tens of kV, and electrons that are gathered by the field are removed by electron emitters located on the spacecraft. An electrostatic sheath is created around the positively charged tether, which works as an _E-sail_, deflecting solar wind particles and generating _interplanetary spacecraft propulsion_ as a result. The most effective direction to apply orbital thrust is along the orbital tangent. Chemical propulsion uses short impulses, or kicks, which change the orbital parameters nearly immediately. Electric propulsion applies low continuous thrust, which creates an orbital spiral as a result. The E-sail works in a similar way; by inclining the sail with respect to the orbital tangent and the incoming solar wind direction and by modulating the tether charge such that the _thrust-vector projection onto the orbital tangent_ is maximal, the technology can create a small but continuous thrust, which allows for spiralling, travelling away from the Sun and even leaving the Solar System [6]. The E-sail thrust efficiency reduces following \(1/d\), with \(d\) being the solar distance [7]. The reflective solar light sail and electric propulsion efficiency drops following the inverse square law of \(1/d^{2}\) due to the reduction in solar photon flux. While the solar wind dynamic pressure also follows the inverse square law, the E-sail efficiency is partly compensated by the electrostatic sheath's expansion in the lower plasma density further away from the Sun. The E-sail's spin is maintained in order to keep the tethers stretched and to modulate the charge depending on the spin sector. The E-sail's angle with respect to the solar direction can be controlled by modulating the voltage of each tether separately to produce the net torque for attitude control and thrust vectoring [8; 9]. The attitude control modes of continuous and on-off modulation have been analysed in terms of the power efficiency [10]. The research of E-sail control is actively evolving with recent results in single-tether E-sail modelling and guidance in heliocentric orbit [11]. Just as the reflective solar light sail can be used as an atmospheric drag sail for low Earth orbit (LEO) deorbiting, the E-sail can be used as an _ionospheric plasma brake_ for LEO deorbiting [12; 13]. The plasma brake always works against the orbital trajectory vector, and, therefore, creates a spiral with decreasing altitude. The plasma brake is charged negatively in the order of kV and does not require electron emitters. A common term for E-sail and plasma brake applications is _Coulomb drag propulsion_ (CDP) [14]. The ESTCube-LuNa consortium, as described in Section 1.4, represents the majority of E-sail developers in Europe.