A Precursor for a Crewed Mission to Near-Earth Asteroids: The SESAME Concept

Kurzfassung

In the last decade of the 20th century, a new era in asteroid exploration began mainly driven by scientific interest regarding the origin of our solar system, technological interest in paving the way for human missions to Mars, resource utilization of minerals present on these bodies and mitigation of potentially hazardous asteroids. A crewed mission would maximize the quality and the amount of returned information. To effectively prepare such a mission, a robotic precursor should visit the asteroid target(s) to ensure its relevance and safety. This thesis describes the architecture, trajectory and system design of a precursor mission. Using low-thrust trajectories for multi-rendezvous approaches to Near Earth Asteroids (NEAs), this mission named SESAME (Spacecraft for Evaluation of Suitability of Asteroids for Manned Exploration) is the first multi-targets precursor mission towards NEAs. First a comparison of the lists of accessible targets for human spaceflight with the trajectories from the 5th Global Trajectory Optimization Competition (GTOC5) is presented. GTOC5 trajectories are highly optimized, low thrust trajectories reaching multiple (up to 18) NEAs. However, the low correlation between the lists led to a mission analysis meant to define appropriate drivers for a new trajectory computation. The outcome was a trajectory with a departure in 2023 reaching seven asteroids in less than 10 years (all in the top 25 of the Near-Earth Object Human Space Flight Accessible Targets Study (NHATS)). To comply with this trajectory, the final design, which focuses on payload, propulsion, communication and power subsystems, resulted in a precursor spacecraft reaching the first five targets of the trajectory: 2001 QJ142, 2000 SG344, 2009 OS5, 2007 YF and 1999 AO10. SESAME carries five 27.6 kg landers each having a short term observation function and a long term beacon function to guide the future crew. The spacecraft is powered by two 17 m² solar arrays and its propulsion system consists of three RIT-22 ion engines (one redundant). The total fuel mass of 582 kg consists of 457.5 kg of xenon for the ion propulsion and 124.5 kg of bi-propellant for the attitude control thrusters. The total launch mass is 1570.3 kg, well within the capabilities of current launchers. To evaluate the impact of changes in the ∆V required for the maneuvers at each target, in the power supply and in the dry mass, a sensitivity analysis of the design has been conducted. Results show that the margins taken for the design were sufficient for the spacecraft to fly the trajectory even with a 20 % power loss. Also, 118 kg of additional payload could be carried if only the first four targets were visited and 135.8 kg of the dry mass should be dropped in order to fly to six asteroids in the trajectory considered. SESAME represents a feasible spacecraft capable of reaching five of the most promising NEAs in less than seven years, with an average of five months of operation at each target.