# Interplanetary Student Nanospacecraft: Development of the LEO Demonstrator ESTCube-2
## 1 Introduction
### Interplanetary Nanospacecraft Ambition  
With the nanosatellite success in LEO, the interplanetary community is considering the nanospacecraft utility--the use of nanosatellites in deep space. The value of deep-space satellites has already been demonstrated; MarCO CubeSats could relay InSight mission data from Mars [11], and LICIACube returned high-resolution images after DART impact into Dimorphos [12].  
The CAPSTONE mission has returned the first results, establishing the pathway for cislunar operations and navigation for the upcoming Lunar Gateway [13]. On 16 November 2022, NASA's Artemis 1 launched ten deep-space CubeSats [14], from which only three missions were not announced as operational as of April 2023 [15; 16; 17].  
The ESTCube-2 mission is an integral part of the Estonian Student Satellite Program--ESTCube-2 builds upon the ESTCube-1 lessons learned [8]. The ESTCube-2 will test technologies necessary to bring CubeSats and nanospacecraft into deep interplanetary space [18; 19; 20]. The ESTCube-1 was the first mission of the program [21] as well as the first mission and space technology experience of Estonia [22], from which a range of space and terrestrial spin-offs have emerged in the industry and academia. The satellite was launched in May 2013 and operated for two years. Although students built ESTCube-1, its mission was a significant step in enabling nanospacecraft propulsion--ESTCube-1 carried the first E-sail experiment, which included a 10-m-long tether to be charged at a high voltage [23]. The team operated the satellite to (a) take images of the Earth, including Estonia, which was the secondary mission objective, (b) spin the satellite to 2.5 revolutions per second [24], which was required for tether deployment [25; 26], (c) take images for attitude determination characterization [27], and d) make tens of firmware updates [28]. However, the ESTCube-1 tether deployment was not successful [8]. A similar tether payload was carried on board Aalto-1, which also did not deploy [9; 29].  
In the wake of the ESTCube-1 tether deployment results, the Estonian Student Satellite Program faced a major challenge--how to square the failed E-sail experiment with the future vision of sailing in deep space where the solar wind blows? In the early days of the ESTCube-2 project, the feasibility of demonstrating the E-sail seemed further than ever because everything had to be performed all over again. The program accepted the challenge to continue working on the E-sail demonstration, and this time with a clear vision of where to go with it.  
One of the largest stepping stones in bringing the E-sail into operations is demonstrating it in the solar wind. We consider the lunar orbit perfect from an experimental and operational point of view. ESTCube-2 includes several experimental systems necessary for operating in lunar orbit and deep space, in addition to an entirely new design of the E-sail experiment by the Finnish Meteorological Institute [7]. If ESTCube-2 successfully demonstrates the E-sail ionospheric plasma brake mode in LEO, then the next step will be to develop ESTCube-3 for lunar orbit. To prepare for future ESTCube-3 challenges, ESTCube-2 will demonstrate a deep-space attitude solution consisting of cold-gas propulsion, reaction wheels, and a star tracker (ST). A dedicated article will present the ESTCube-3 mission and the 'LEO to lunar orbit' technology development roadmap.