# Interplanetary Student Nanospacecraft: Development of the LEO Demonstrator ESTCube-2
## 2 ESTCube-2 Mission  
### Coulomb Drag Propulsion Mission Summary  
ESTCube-2 is a three-unit (3U) CubeSat form-factor low-Earth orbit technology demonstration platform for future deep-space nanospacecraft missions. The primary objective of ESTCube-2 is to demonstrate the electric solar wind sail (E-sail) in the LEO ionosphere conditions where the electric sail can be used as an ionospheric plasma brake (IPB) for deorbiting. ESTCube-2 includes several secondary experiments: a deep-space platform prototype, Earth observation cameras, and a materials corrosion experiment. The primary driver of the ESTCube-2 mission design has been the ESTCube-3 concept of bringing the E-sail to the solar wind, which fits in the broader vision of the Estonian Student Satellite Program.  
The E-sail is a propellantless propulsion concept that uses the natural space plasma flux of the solar wind to accelerate a spacecraft. The E-sail consists of one or multiple tethers deployed from the spacecraft centrifugally. A static sheath is formed around the tethers by applying a high voltage to the sail, creating a synthetic sail. A single tether is a multi-wire micrometeoroid-resistant structure consisting of two 50-\(\upmu\)m parallel wires connected with zig-zag wires (Figure 1). The synthetic sail deflects charged particles of the solar wind, or ionosphere, in the case of LEO. For operational missions, the length of E-sail tethers can reach tens of kilometers. With only tens of microns thin, such tethers can be accommodated by CubeSats [31].  
Developed for the LEO environment, ESTCube-2 will demonstrate tether deployment and ionospheric plasma brake deorbiting in the ionosphere [7]. In solar wind and ionospheric conditions, the Coulomb drag between the charged tether and the plasma flow creates a propulsive effect. While the solar wind allows the spacecraft to accelerate outwards and tack inwards, the ionosphere is relatively stationary, allowing it to drag the satellite downwards or deorbit. An ionospheric plasma brake of 300 m could shorten the deorbiting time by an order of magnitude [7]. The main difference between the E-sail and the ionospheric plasma brake is the high voltage polarity--the E-sail is more optimal in the positive polarity mode, which requires an electron emitter, and the ionospheric plasma brake is more optimal in negative polarity mode [32].  
The ESTCube-2 will carry 30 m of tether to \(\sim\)565 km orbit and demonstrate the proof of concept for (a) centrifugal tether deployment, (b) charging with respect to the satellite spin (Figure 2), (c) testing the positive E-sail mode with the electron emitter, and (d) demonstrating the ionospheric plasma brake for deorbiting (Figure 3) [7].  
Figure 1: The Coulomb drag propulsion tether has a redundant structure, making it resistant to micrometeoroid and orbital debris impacts. The wire thickness is 50 \(\upmu\)m. A human hair is used for scale below.  
Figure 2: Satellite spin rate change with the E-sail tether in the Earth’s ionosphere [33]. The tether is charged while moving downstream or upstream of ionospheric plasma to increase or decrease the satellite spin rate, respectively. The change in spin rate would be noticeable in one or a few orbits (depending on the deployed tether length). Artwork credit: [PERSON], [PERSON], and [PERSON].