# Venus Magnetotail Long-Term Sensing Using Solar Sails
## 1 Introduction
In fact, propellantless thrusters, such as the well-known photonic solar sails [11; 12; 13], the exotic Magnetic Sails proposed by [PERSON] and [PERSON] in the early 1990s [14; 15], the more recent Electric Solar Wind Sails invented by [PERSON] in 2004 [16; 17; 18], or the advanced Solar Wind Ion Focusing Thrusters (SWIFTs) recently proposed by [PERSON] et al. [19; 20; 21], theoretically allow for extremely complex space missions that require a very high value in terms of the velocity change [22; 23]. In this specific case, keeping in mind that the spacecraft flies (and, therefore, the propellantless thruster operates) inside the planet's magnetosphere, the only possible option among those just indicated consists of the use of a photonic solar sail [24]. For this reason, the use of a small (reflective) solar sail as the primary propulsion system was proposed roughly 20 years ago during the preliminary design of the GeoSail mission concept [25], the aim of which was to create a geocentric orbit capable of enabling the in situ sensing of the Earth's upper magnetosphere. To this end, it was proposed to use the propulsive acceleration induced by the reflective solar sail to achieve an artificial precession of the apsidal line of an assigned (elliptic) geocentric orbit, so as to maintain the apogee within the Earth's magnetotail over time. In this context, thanks to the pioneering studies of [PERSON] et al. [26], it was demonstrated that such artificial precession could be achieved using a low-performance reflective solar sail with a substantially constant attitude with respect to the direction of propagation of the solar rays [27; 28]. This interesting result was then subsequently refined by the author using an optimized guidance law [29; 30] or a suitable on/off control strategy [31], which allows the mission concept to be extended to smart dust with an electrochromic control device. More recently, as a valid alternative to the classical reflective sails, solar sails with advanced metamaterial films have been proposed. This is, for example, the case of the so-called refractive sails [32; 33; 34], or the interesting diffractive sails whose performance has been thoroughly investigated by [PERSON] et al. [35; 36; 37; 38]. The interesting aspect of a diffractive or a refractive sail is the possibility of generating a transverse component of the propulsive acceleration even with a Sun-facing orientation, that is, an orientation in which the direction of the solar rays is perpendicular to the nominal plane of the thin sail membrane [39; 40]. Such a Sun-facing attitude can be maintained passively through a suitable design of the sail's external structure, i.e., by providing a slightly conical form with the apex directed to the Sun [41]. Taking inspiration from the GeoSail concept, and leaving the study of the performance of diffractive or refractive sails to future works, this study analyzes the performance of a reflective solar sail-based spacecraft in (artificially) precessing the apse line of a high-elliptic orbit around Venus with the aim of exploring the planet's induced magnetotail. In particular, the solar sail-induced propulsive acceleration vector allows a scientific probe to obtain long-term residence within the induced magnetotail of the planet. The mission concept is schematized in Figure 3, which shows how the apse line of the spacecraft's planetocentric orbit remains substantially aligned with the Sun-Venus direction during the flight. In particular, Figure 3 indicates that the plane \(\mathcal{P}\) of the spacecraft's science (planetocentric) orbit coincides with the plane of the heliocentric orbit of Venus. Bearing in mind that the axial tilt of Venus is roughly \(177\deg\), one can infer that \(\mathcal{P}\) substantially coincides with the equatorial plane of the planet.