# Closing Force Evaluation of a Sample Return Capsule for a Phobos Sample Return Mission
## 1 Introduction  
The Phobos Sample Return Mission is an incipient phase of the Mars Robotic Exploration and Preparation 2 (MREP-2) programme [1]. Due to the impact ejecta phenomenon described in [2], the regolith from Phobos may contain up to 250 ppm in a concentration of particles coming from Mars' Surface. As the robotic exploration of Phobos is easier and safer than Mars, this makes it the perfect candidate to understand the origins of the red planet. Throughout history, there have been several attempts to explore the Martian moon Phobos, in the frame of the Phobos 1 and 2 missions organized by the former Union of Soviet Socialist Republic (URSS) [3] and the Phobos-Grunt mission launched by Russia in 2011 [4]. While Phobos 1 failed prematurely, Phobos 2 managed to observe Phobos, Deimos and Mars before suffering a fault that led to losing contact with the operation centre [5]. Phobos-Grunt mission also encountered a failure in the early stages due to a programming error, which caused it to lose contact with the operation centre while it was on the Low Earth Orbit, later being destroyed as it descended uncontrolled in the Earth's atmosphere [4]. The European Space Agency (ESA) conducted a thorough study regarding a Phobos Sample Return Mission [6], highlighting the main challenges of this type of mission and proposing a possible mission architecture that would involve also Russia, considering their previous experience with the Martian moon. However, the Phobos Sample Return Mission was put on hold, as now the main goal of the space industry is to bring samples directly from Mars in the frame of the Mars 2020 mission currentlyongoing [7]. Although the scientific community is focused right now on Mars' exploration, Japan Aerospace Exploration Agency (JAXA) prepares the Martian Moons Exploration (MMX) having among its purposes to establish the origin of Mars' natural satellites, as well as to bring to Earth regolith samples from Phobos' surface [8]. Currently, the mission is scheduled for launch in 2024 [9]. National Aeronautics and Space Administration (NASA) is currently studying Mars under the Mars Exploration Program, providing information related to planet evolution, history of geological and climate processes that have shaped Mars through time and the potential to have hosted life (its biological potential) [10]. A joint NASA-ESA mission is under development as part of NASA's Mars Exploration Program to bring samples of Martian rocks and soil back to Earth. The Sample Return Mission is designed to continue the work of NASA's Mars 2020 Rover. During the mission architecture, NASA is working on a Sample Retrieval Lander that would launch the retrieved samples into Mars orbit while ESA is working on an Earth Return Orbiter that would rendezvous with the samples in Mars orbit and bring them back to Earth. The foreseen launch date for the joint program is in 2026 and to touch Mars soil in 2028 [11]. Sample Return Missions are an extremely valuable resource, both for the scientific community and society. The analysis of samples from other bodies in the Solar System can answer fundamental questions for humankind, as well as give an insight regarding their resources (ores, rocks, etc.) which could prove useful in the future. The idea of Sample Return Missions dates back to 1969 [12], starting with the first steps made on the Moon. A year later, the former URSS performed the first sampling using remote-controlled equipment [3], thus paving the way for future robotic exploration missions on other planetary bodies. A chronology of the Sample Return Missions is shown in Figure 1.  
In recent history, the Sample Return Missions that stand out as being successful are the two Hayabusa missions conducted by JAXA [3], first to the Itokawa asteroid, and the second one to Ryugu, both celestial bodies being classified as NEO and potentially hazardous asteroids [14; 15]. A third Sample Return Mission is on its way to Earth, NASA's OSIRIS-REx mission to Bennu [16].