monoxide, an entirely Mars derived fuel, is used. D shows how the mass is reduced when terrestrial hydrogen is imported to Mars to produce methane and oxygen for the return to Earth. E shows the mass requirements if the stoichiometric amount of hydrogen is brought to Mars, and the excess methane produced is put to waste. This represents the simplest possible methane production plant. F shows the mass requirements if terrestrial hydrogen is used for the return, and oxygen is produced on Mars. G shows the mass requirements with hydrogen and oxygen produced on Mars for the return. The above missions employ conjunction class trajectories (low energy) that require flight times of 325 days. They typically require a 400 day wait for a launch window between arrival on Mars and the next available Earth return opportunity [1]. Thus, for the sample return mission, a 5 to 15 kg/day propellant production facility would suffice. For the manned mission, a 30 to 90 kg/day facility would be adequate. Martian Resources and Environment In order to utilize local Martian resources it is necessary to characterize them and understand the environmental conditions that can affect propellant production on the Martian surface. The following information has been adapted from Meyer and McKay [7]. The Martian environment has variations in temperature of up to 60 K between day and night, and seasonal variations in atmospheric pressure of with a low of 6 mbar and a high of 10 mbar. In addition, these numbers may vary with landing site. The nominal Martian conditions are 215 K and 8 mbar [7]. However, the components are sized in the present study using worst case values of 260 K, and 6 or 10 mbar. These values are typical of a mid-latitude northern location, such as the Viking Lander sites. The most obvious Martian resource available for ISMU is the atmosphere, particularly carbon dioxide. CO2 averages 95.3% of the atmosphere, nitrogen 2.7%, and argon 1.6% (mole or volume basis). Various other gases exist in trace amounts. Those gases were not considered in the present work. In addition, dust is present in the atmosphere in amounts up to 10 ppm nominally, and potentially in much greater amounts during dust storms. Another potential Martian resource is water. Water is believed to exist in the atmosphere as vapor, and may also exist in the polar caps as ice, as a deep subsurface permafrost or aqueous layer, or in the soil. Although the presence of subsurface water (ice) has not been verified at any specific site, Martian water would be a valuable resource. Thus, the assumption is made that water might be available for propellant manufacturing at some properly selected landing sites, so that processes which use Martian water can be compared to those which do not.
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