The Fourth International Symposium on Solar Sailing 2017, Kyoto, Japan. Copyright c 2017 by A. Farrés, S. Soldini and J. Tsuda. Published by the Secretariat of the ISSS 2017, with permission and released to the Secretariat of the ISSS 2017 to publish in all forms. JAXA’s Trojan Asteroids Mission: Trajectory Design of the Solar Power Sail and its Lander By Ariadna Farrés Basiana 1) , Stefania Soldini 2) and Yuichi Tsuda 2) 1) Institut de Mathemátiques, Universitat de Barcelona, Barcelona, Spain 2) Institute of Space and Astronautical Science/JAXA, Sagamihara, Japan (Received 1st Dec, 2016) In this paper we use dynamical system tools to design trajectories in the vicinity of Trojan asteroids for both: JAXA’s solar power sail and its lander. The current JAXA baseline considers a solar power sail hovering a Trojan asteroid at 40 km from its surface. Which will then descend to 1 km of the surface to release the lander. First we will exploit solar radiation pressure to place a solar power sail in orbit around the asteroid and illustrate how the effects of changing the sail orientation can enhance the hovering opportunities. Second we will focus on the lander release, performing a sensitivity analysis on its deployment velocity together with possible bouncing trajectory. To model the dynamics of the solar power sail and the lander we use the augmented Hill three body problem for the far gravity field dynamics and a perturbed two-body problem approximating the asteroid’s as triaxial ellipsoid for the close gravity field dynamics. Key Words: Solar sail, Far vs near field dynamics, Augmented Hill problem, Triaxial ellipsoid, Lander. 1. Introduction Jovian Trojans are asteroid located in the L 4 /L 5 equilibrium points of the Sun-Jupiter system. Little is known about their origin and their evolution as the observations provide limited clues about the source of these asteroids. Currently, these aster- oids are completely unexplored and the future JAXA’s sample and return mission to the Trojan asteroids will revolutionise our understanding of these bodies. Trojan asteroids are believed to either, being captured during the migration mechanism of Gi- ant Planets, or formed during the planetary formation. 19 Their orbits are also destabilised by collision ejection. Thus, Trojans could also be the remnants of a much more substantial initial population of trapped bodies, or these objects could continually be replenished from an unidentified external source. 3 A mission to the Trojan asteroids represents the new frontier in mission ex- ploration, and their study would also explain what happened to the primordial Trojans of Uranus and Neptune. 11 With respect to NEOs exploration, a space mission to Trojan asteroids faces the challenge of reaching the unexplored far side of our solar system where large amount of fuel is required and an efficient power supply is difficult. 14 JAXA’s Trojans assessment study concluded that the solar power sail is the best way to perform such a challenging mission. This paper covers two different aspects of a solar sail mission to the Trojan asteroids, analysing possible trajectories to hover the asteroid and the landing of a small probe on the surface of the asteroid. Hence, we address both: far and near operations at the asteroid, dealing with different dynamical models that include the relevant perturbations in each scenario. The current JAXA’s baseline proposes hovering the asteroid with an Earth-pointing solar sail. 14 However, exploring the ef- fect of Solar Radiation Pressure (SRP) can enhance interesting orbit solution for a solar sail. For this purpose we study the dy- namics of the sail in the far gravity field, where the asteroid is approximated as a point mass and the effects of SRP and Sun’s gravity are also taken into account, considering the augmented Hill 3 body problem as a model. In previous works 2, 7, 8 termi- nator orbits and other quasi-periodic orbits that appear in the system are proposed for hovering and monitoring an asteroid. In Section 3. we will review some of these orbits, moreover, by changing the sail orientation (keeping it fixed with respect to the Sun-sail line) we can artificially displace these orbits changing the visibility conditions, enlarging the monitoring sites. The solar power sail will be equipped with a lander that will be released at 1 km from the surface of the asteroid. In this case, the dynamics of the lander is mainly influenced by the irregular shape of the asteroid. 1 Hence, for the near field dy- namics we consider the perturbed two-body problem where we include spherical harmonics for the irregular shape of the as- teroid and the SRP effect is neglected. In Section 4. we discus how to select the initial velocity at which the lander is released to guarantee a bounded motion of the lander around the aster- oid. 16 This will prevent bouncing trajectories of the lander after touchdown, as in the Rosetta mission, with the potential risk of escaping from the asteroid surface. 2. Asteroid Parameters Little is known on physical properties of Trojans asteroids due to the challenges one faces when observing and taking mea- surements of these objects from Earth. In this paper, we con- sider as an example the Trojan asteroid 2001 DY103 to set the orbital period and mean Sun distance (although most asteroids in the Trojan region present similar values). However, to de- termine the spin rate, shape and density we have used some statistics on the Trojan population. 14 These physical parame- ters are summarised in Table 1. Moreover, we approximate the shape of the asteroid by a triaxial ellipsoid where a, b and c (satisfying a>b>c) are the semi-major axis of the ellipsoid along the x, y and z axis respectively using an Asteroid-Centred Asteroid-Fixed (ACAF) reference frame. Table 2 contains the corresponding Stockes coefficients for this triaxial ellipsoid.