# Venus Magnetotail Long-Term Sensing Using Solar Sails
## 1 Introduction  
Since the pioneering Soviet missions of the Venera family, which were the first to successfully enter the atmosphere of Venus and make a soft landing on the planet, or NASA's missions such as the Mariner 2 launched in the early 1960s and the Pioneer Venus Orbiter launched in the late 1970s [1], and more recently, thanks to the ESA's Venus Express mission and the ESA/JAXA's BepiColombo, which is currently on an interplanetary flight to its target planet, the exploration of Venus's induced magnetosphere and the related magnetotail has provided useful data to better understand the complex Sun-Venus connection, and how the solar wind interacts with the upper atmosphere of the second Solar System's planet [2; 3]. In fact, the particular physical origin of Venus's magnetic field [4], which originates from the interaction of the solar wind stream of charged particles with the ionized atoms that populate the planet's upper atmosphere, differentiates Venus's magnetotail from that of magnetized planets as, for example, the Earth [5]. In this context, the artistic impression in Figure 1 schematically describes the differences between the induced magnetosphere of Venus (and Mars) and that of the Earth. Over the decades, Venus has been the study object (or a flyby planet on its way to the final target) of several interplanetary spacecraft. For example, NASA's Pioneer Venus Orbiter observed the central region of Venus's magnetotail at a distance between8 \(R_{V}\) and 12 \(R_{V}\)[6], where \(R_{V}\simeq 6052\) km is Venus's mean radius, while the ESA's Venus Express observed a downstream region of the magnetosphere at a distance of roughly 4 \(R_{V}\), due to the high orbital inclination of that space probe with respect to the equatorial plane of the planet [7; 8]. Figure 2 shows an artistic impression of the Venus Express spacecraft approaching the planet on its successful science mission, which ended in early 2015. More recently (i.e., in October 2020 and August 2021), the two Venus flybys of the ESA-JAXA's BepiColombo spacecraft provided new and interesting data regarding both the physical composition of the planet's upper atmosphere and the structure of its induced magnetosphere [9; 10]. To date, however, a systematic and in situ exploration of Venus's comet-like induced magnetotail has not been performed. In fact, a continued and long-term probing of the physical characteristics of the trailing part of a magnetosphere usually requires a continuous-thrust propulsion system, which is able to change the orientation of a suitable elliptic  
Figure 1: Artistic impression of the magnetosphere of Venus (top part of the figure), Earth (middle part), and Mars (bottom part). The topology of the induced magnetosphere of Venus and Mars is substantially different from that of the Earth, whose internal magnetic field interacts with the solar wind charged particles. Image: European Space Agency (ESA). Figure 2: Artistic impression of the ESA’s Venus Express orbiting around the second planet of the Solar System. Using the installed onboard magnetometer and low-energy particle detector, the spacecraft observed Venus’s magnetotail on 15 May 2006 at a distance of about 1.5 planet’s radii downstream of Venus. Image: ESA—D. [PERSON]. (planetocentric) orbit whose apocytherion lies inside the planet's magnetotail. Consequently, the continuous and prolonged application of propulsive acceleration indicates that this type of mission requires a considerable velocity change, which, in turn, makes them difficult to achieve using a conventional thruster whose capabilities are constrained by the mass of storable propellant on board. A possible solution, in this respect, is offered by a propellantless propulsion system.