|Fig. 1: Required solar array size under ideal conditions at Earth and Mars orbital distances. Efficiency values are from reference .|
Since the conclusion of the Apollo program in 1972, human spaceflight has been limited to a region of space called Low Earth Orbit (LEO). In that time, crews have demonstrated the ability to launch, recover, and repair satellites, have studied the effects of long exposure to microgravity, and ushered in a new era of international collaboration in space activities with the construction of the International Space Station (ISS).
Within the last decade, there has been a resurgence of U.S. political interest in a return to human spaceflight beyond LEO. Since 2004, space policy has been driven by the Vision for Space Exploration, and the Review of Human Spaceflight Plans Committee, directing the development of the next generation of NASA spacecraft and exploration technology. [1,2] The long-term goals, stated by NASA, focus on technology development to support a manned landing on the surface of Mars. To accomplish this, one of three technology development paths can be taken. These paths are described below:
Moon First Path: This approach dictates a return to lunar surface exploration as a means to test the technologies necessary for a Martian landing. This includes the establishment of a permanent lunar habitat.
Flexible Path: This approach would demand technology and spacecraft capable of exploring deep space, with possible destinations including asteroids, quasi-stable gravitational locations, called lagrange points, lunar orbit, and Martian moons.
Mars First Path: This approach would set a Martian landing as an immediate priority, with a brief stop on the moon to test technology only if needed.
The selection of the path forward has great significance on the mission architecture and the necessary hardware. This report will discuss the implications of that choice on the power subsystem and the role that nuclear-based energy sources may play in the future of manned spaceflight.
Space systems require power to maintain orientation, communicate, operate scientific instruments, and to run life-support systems. In the last half-century, four technologies have primarily been used for spacecraft and satellites: batteries, fuel cells, photovoltaic cells, and nuclear systems. A brief description of each of these technologies follows.
Batteries and Fuel Cells: These expendable and rechargable energy sources are used for primary power on short-duration missions only. Most modern satellites and spacecraft use batteries and fuel cells as backup power sources, or to bridge portions of the duty cycle where the primary power system is ineffective (e.g., a system primarily powered by solar cells, that is temporarily out of direct sunlight).
Photovoltaic Cells: Solar arrays are able to convert incident solar radiation into electic power via the photoelectric effect. For this reason, these systems are popular for long-duration missions, since the sun is able to supply power indefinitely, provided that the array remains in direct sunlight. The amount of power generated is proportional to the surface area of the array, and as a consequence, power generation on large scales is not always possible, as array sizes can become prohibitively large. Additionally, the intensity of incident solar radiation decreases proportionally to the square of the distance from the sun, making solar-based power generation less attractive as the array moves further from the center of the solar system.
Nuclear Power: Nuclear-based systems can be further subdivided into two technologies, Radioisotope Thermoelectric Generators (RTGs) and fission reactors.
RTGs: Power is generated using heat created by the decay of a radioactive isotope. These systems have no moving parts, and, depending on the isotope, can supply power over the course of several decades. Unfortunately, RTGs are highly inefficient, typically converting only 7% of the available energy to electricity.  Partially due to these inefficiencies, modern RTG power sources, like those used on the Cassini (1997) and the New Horizons (2006) spacecraft are only able to supply about 300 W of electricity, necessitating multiple coupled RTG systems to supply the required power. RTGs have been used extensively for deep space exploration missions including Galileo, Voyager, Cassini, New Horizons, and the Mars Science Laboratory. 
Fission Reactors: These systems Generate power via heat released during nuclear fission. These systems are similar to ground-based nuclear power sources, but, in general, are far smaller. Space-based fission reactors are able to supply large quantities of electrical power, and have been flown by both the U.S. and Russian space programs, demonstrating robustness to the vibrational and mechanical stresses imposed on the reactor during launch.  These reactors require the same control mechanisms as ground-based systems, necessitating care in the design of the control algorithms to be robust to faults caused by incident high-energy cosmic rays in the space environment.
Generally, nuclear systems have shown attractive mass-scaling with power, but require additional radiation shielding to prevent intereference with onboard electronics and crew compartments. These power systems have been used sparingly for non-military missions, due to public sensitivity to nuclear energy and the risk of dispersing radioactive material high in the atmosphere in the event of a catastrophic failure during launch.
Each of the missions previously highlighted impose specific requirements and challenges on the power subsystem that must be matched with the advantages and disadvantages inherent in the power generating technologies currently available. In particular, three items are of primary interest: mission duration, space environmental conditions, and desired electrical power. Fig. 1 relates solar array size to desired power under ideal conditions at Earth and Mars. This figure represents a "best case" scenario and forms a foundation upon which we can evaluate the suitability for solar-based power generation for missions of interest. 
Lunar Surface Exploration/Habitation: Lunar surface exploration systems are exposed to demanding environmental conditions not encountered on the other two exploration paths. Incident solar radiation intensity is roughly the same as that in LEO, making solar-based power systems attractive. However, the lunar night lasts for fourteen days and during that time, an alternate power source must be utilized, energy must be stored, or exploration must be situated at one of the lunar poles, where near-permanent sunlight is available.
Estimates for power requirements depend largely on the nature of the scientific mission. Surface exploration estimates have placed required power levels in the 10s of kW, while a permanent habitat has electrical power projections as high as 1 MW. [7,8] Assuming that permanent sunlight is available at a polar location, the amount of power required may exceed feasible solar array sizes. Even for more modest electrical budgets, the array must be carefully designed so that panels do not shade one another as the moon rotates about its axis.
Based on the duration of the lunar night, the challenges of the array design, and the potential for very large electrical power requirements necessary for a permanent facility, a nuclear-based surface energy source will likely be the most suitable to support lunar surface exploration and habitation. A surface-based fission reactor allows for flexibility in the selection of the habitat location, supplies power at levels in accordance with the highest projected power requirements, supports future expansion, and eliminates the need to store very large amounts of energy for the lunar night.
Near-Earth Objects, Lagrange Points, and Martian Moons: The flexible path to inner planet destinations has the most benign power requirements of the three possible paths. Mission duration will be the shortest of the three options, on the order of weeks to months, making it possible to utilize advanced fuel cell technologies. Longer missions, including those to Martian moons will exceed the capabilities of expendable power sources, necessitating the use of solar or nuclear technologies. For inner planet destinations, spacecraft will have near-constant solar exposure, at solar radiation intensities comparable to those in LEO.
Power requirements for flexible path destinations will be much lower than those necessary for a lunar habitat. Considering the favorable sun exposure and incident solar radiation intensity, solar-based power generation systems seem the most appropriate choice for flexible path destinations.
Martian Landing: This path has the most complex mission architecture of the three. A successful landing on the martian surface will require interplanetary cruise stages, crew and cargo landers, surface exploration equipment, Mars ascent vehicles, and Earth entry systems, each subject to unique environmental conditions and mission requirements. A discussion of the major phases of the mission follows.
Interplanetary Cruise: This mission phase has similar space environmental conditions, mission, and power requirements as those for the flexible path. Despite a reduction in incident solar radiation to 43% of that in LEO, solar-based power generation is capable of providing the required amount of power at reasonable array sizes and is still the logical choice.
Surface Exploration: The duration of the surface exploration mission phase depends on the overall mission strategy. A minimal time approach (conjunction-class) leaves merely 30-60 days on the surface, while a minimum required propellant mass (opposition-class) approach leads to well over 500 days for surface exploration. Regardless of whether a conjunction or opposition-class mission is selected, it seems logical to assume that the large monetary investment and high-risk involved in landing large payloads on the martian surface would lead to an overall exploration strategy based on the re-use of existing surface equipment. Based on this assumption, surface equipment will suffer long exposure to martian surface conditions, namely intense dust storms, winds and cold temperatures. These conditions work against the strengths of solar-based surface power generation. Nuclear-based energy sources are subject to none of these adverse environmental conditions, with the capability of providing power to a fixed surface research station for many years. This approach permits rotating astronaut deployments to the Martian surface and provides ample electrical power to support the expansion of surface facilities to support more sophisticated scientific research during subsequent missions. This follows the same strategy used to create the International Space Station. Furthermore, heat generated by a surface based fission reactor or network of RTGs can simulataneously be used to heat surface habitats and vital electrical systems.
We have discussed three potential futures for human spaceflight in the 21st century. The environment, power, and mission requirements have great influence on the selection of the best electrical power source to accomplish the mission. Undoubtedly, a more detailed analysis is necessary to make the final decisions moving forward, but it is clear at a high level that nuclear-based power sources offer some clear advantages for lunar and martian surface exploration missions. It is likely that nuclear technologies will be heavily involved in mankind's next giant leap.
© Sean R. Copeland. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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