1 Extraterrestrial EnvironmentsDifferences in the Martian and lunar environments present a challenge to designing a robust system that is optimized for both. In the sections ahead, the reactor core, power conversion unit, shielding and radiator, as well as their respective relationships with both environments, will be described in great detail. The information presented here will serve both as a reference and as a guide to the numerous design decisions concerning these various systems.ReferencesLunar Surface ReactorGeneral1 Extraterrestrial EnvironmentsThe Martian and lunar environments are quite different from the Earth and from each other inmany respects, such as: reduced atmospheric pressure, weaker gravitational pull, surfaceelemental composition, and the wide and often unpredictable temperature shifts. In order toallow for appropriate and effective design adaptations and innovations, these differences mustbe taken into account. This section provides a brief description and listing of some importantphysical properties of the Martian and lunar environments. The properties described andtabulated below will serve as a standard reference, and will be useful for determining theappropriate materials and designs for the core, power conversion unit, shielding and radiator. 1.1 The MoonSince the actual geological and weather conditions vary across the lunar surface, accuratevalues can only be determined once a landing site is determined. However, until that locationis known, it is instructive to consider the average values for these factors. It is also importantto note that while the lunar poles are the most likely future mission sites, many of the belowvalues were obtained from the early lunar missions that landed predominately in theequatorial regions. In the polar region, the sun continuously occupies a low position on the horizon and surfacetemperatures are more moderate and less variable, generally working to our advantage. In thisdesign, while we anticipate going to the poles, we have designed the reactor to be robust andwork anywhere on the lunar surface.1.1.1 Lunar Atmosphere and GravityThe moon has almost no atmosphere. With an atmospheric pressure of ~10-8 Pa very littleconvection can occur on the lunar surface, which has direct implications for material options,radiator, and shielding systems. Yet with an atmosphere composed almost entirely of inertnoble gases such as helium, neon and argon, minimal corrosion is expected. Hydrogenrepresents a major exception, but at a concentration of roughly 35,000 particles per cubiccentimeter, hydrogen can be disregarded as a threat to material stability. [1] Gravity on theMoon is roughly one ninth of that on the Earth. See tables 1 and 2 below for data oncomposition and properties of the lunar atmosphere.Gas Particles/m3H235,000He-4 40,0001Radiator Ne-20 40,000Ne-22 5,000Ar-36 2,000Ar-40 30,000CO21,000H3N 1,000CH41,000Table 1: Composition of Lunar AtmosphereTable 2: Average Properties of Lunar Atmosphere1.1.2 Lunar GeologyMuch of the moon’s cratered surface is composed of igneous rock, not unlike that found nearvolcanoes on Earth. [2] See tables 3 and 4 below for data on composition and properties ofthe lunar soil.Compound Weight PercentSiO247.3TiO21.6Al2O317.8FeO 10.5MgO 9.6CaO 11.4Na2O 0.7 Page 2Property (surface) Value UnitsThermal ConductivityEffectively zero W/m*KPressure 3 x 10-10PaDensity 2 x 1011particles/m3Gravity 1.62 m/s2Radiator K2O 0.6MnO 0.1Cr2O30.2Table 3: Composition of Lunar SoilProperty Value UnitsThermal Conductivity10-3W/m*KAlbedo 0.11Table 4: Properties of Lunar Soil 1.1.3 Lunar Meteorological PropertiesTemperature variations on the moon are extreme when compared with the Earth. The lunarday is equivalent to 28.5 Earth days, and all parts of the lunar surface, excepting the polarregions, spend half this time in the sun. With virtually nonexistent atmospheric convectionand limited surface conductivity, the side of the moon exposed to the sun heats to over 373K .When the sun sets, however, temperatures drop below 160K. Owing to the low surfaceconductivity, shadowed regions on the sunny side will also fall to the night temperature,resulting in large temperature gradients. Table 5 displays some of the lunar meteorologicalproperties.Property Value UnitsTemperature100 to 400KBlackbody Temperature 274.5 KSolar Radiation 1367.6 W/m2Table 5: Lunar Meteorological Properties1.2 Mars Page 3Radiator The Martian environment differs greatly from the moon, since Mars has a more significantatmosphere, different elemental composition, and is farther from the sun. Again, because theactual properties that will be encountered on Mars are highly variable based on location andseason, average values are given below. Values obtained from the Viking I mission are alsogiven for comparison. Once likely landing sites are identified more specific values can beobtained.1.2.1 Martian Atmosphere and GravityThe Martian atmosphere consists of 95% carbon dioxide, with the other 5% consisting ofnitrogen, argon, and trace amounts of oxygen and carbon monoxide. Unlike the inert lunarenvironment, the Martian atmosphere is potentially corrosive, especially to potential reactorcore materials. Atmospheric pressure ranges from 600-800 Pa on Mars, considerably thinnerthan the Earth’s by about 1/150 [3] though many orders of magnitude thicker than theMoon’s. The surface gravity of Mars is about one-third of that of Earth’s. See tables 6–9 belowfor data on composition and properties of the Martian atmosphere.Gas Volume PercentCO295.32N22.7Ar 1.6O2.13CO .08H2O 210 ppmNO 100 ppmNe 2.5 ppmHDO0.85 ppmKr 0.3 ppmXe 0.08 ppmTable 6: Composition of Martian Atmosphere [2]Property (surface) Value UnitsThermal Conductivity [3]0.012 W/(m K)Pressure [2]600 to 800Pa Page 4Radiator Density [2] 0.020 kg/m3Gravity 3.69 m/s2Table 7: Properties of Martian Atmosphere Page 5Radiator Gas Volume PercentCO295.32N22.7Ar 1.6O20.13CO 0.07H2O0.03Ne 2.5 ppmKr 0.3 ppmXe 0.08 ppmTable 8 Composition of Martian Atmosphere as measured during the Viking I mission [1]Property (surface)Value UnitsPressure 800 PaDensity 0.0166 kg/m3Table 9: Properties of MartianAtmosphere as measured during theViking I mission [1]1.2.2 Martian GeologyThe Martian crust and soil is rich in metal oxides such as silicon dioxide, ferrous oxide andmagnesium oxide. The terrain is much less severe on Mars. Flat plains