Optimization of Missions to Mars for Robotic and Manned Spacecraft

A. Miele, T. Wang, S. Mancuso

Rice University, Houston, Texas, 77005-1892, USA

This paper presents an assessment of some fundamental issues of orbital transfers for round-trip Mars missions, in particular, the interplay between flight time, characteristic velocity, and mass ratio for both robotic and manned missions. For robotic missions, the best trajectory is the minimum energy trajectory computed without constraining the stay time on Mars and the total mission time; this is nearly a Hohmann transfer trajectory. For manned missions, a substantial shortening of the stay time on Mars and the total missions time is needed so as to generate a fast transfer trajectory; while this might be technically feasible, it translates into stiff penalties in characteristic velocity (it might double) and mass ratio (it might have a more than tenfold increase). At this time, the best that can be done is to continue the exploration of Mars via robotic spacecraft. We are simply not ready for the exploration of Mars via manned spacecraft. This requires advances yet to be achieved in the areas of spacecraft structural factors, engine specific impulses, and life support systems.

The objective of this paper is to assess the fundamental issues of orbital transfer for round-trip Mars missions, in particular, the interplay between flight time, characteristic velocity, and mass ratio. The point of view taken is that of feasibility, rather than that of precision. In this spirit, the assumed physical model is the restricted four-body model, the four bodies being the Sun, Earth, Mars, and spacecraft. A voyage from Earth to Mars and back includes interplanetary branches and planetary branches. Here, we analyze the interplanetary portion of the voyage; specifically, we study the round-trip LEO-LMO-LEO, where LEO denotes a low Earth orbit and LMO denotes a low Mars orbit.

First, with reference to interplanetary flight, we look at the interplay between flight time and characteristic velocity. Then, we convert the values of the characteristic velocity into mass ratios under certain assumptions for the spacecraft structural factor and engine specific impulse. Later on in the paper, we supply data relative to the mass ratios for planetary flight in either the Earth atmosphere or Mars atmosphere. In turn, this leads to an estimate of the overall mass ratios for both a minimum energy trajectory and a fast-transfer trajectory, and hence to conclusions on the relative merits of these trajectories.