Mission Analysis of Robotic Low Thrust Missions to the Martian Moons Deimos And Phobos

Kurzfassung

The Martian moons Deimos and Phobos are interesting targets for exploration missions, especially within the frame of a crewed Mars orbit mission. To minimize the risk to a crew and also to support EVA site selection, a robotic precursor mission should investigate both moons in advance. The focus of this study is on mission analysis of such a precursor mission that utilizes low-thrust propulsion, in particular Electric Propulsion, for the transfer to the Martian system. We assumed a launch by a Soyuz Fregat in 2018 and a direct injection into an escape trajectory with a hyperbolic excess velocity v∞ = 0 km/s. The spacecraft uses electric propulsion for the interplanetary transfer to Mars and also for spiraling down from an elliptic capture orbit to its destination. The mission analysis comprises dedicated missions to either Deimos or Phobos, and combined missions with Deimos as the primary target and, during a possible mission extension, Phobos as the secondary target. We used two different electric engine types for this study, which represent a wide range of specific impulse Isp. The employed thruster types were the Snecma PPS®1350-G with Isp = 1,650 s and the Astrium RIT-22 in two configurations having Isp = 3,704 s and Isp = 4,763 s. Within the analysis, we varied the number of engines and the available electrical power, followed by a down selection of a system design. In the second part of this study we investigated the implications of transfer time and thruster count on the mission itself caused by permanent degradation of the power and propulsion subsystems. Therefore we selected 15 state vectors of the nominal transfer trajectory and, starting from each of these 15 new initial states, optimized new minimum-duration transfers under the assumption of permanent engine failures or degraded solar cells.