Abundant power is needed to significantly explore and expand into space. A robust power source is needed that can provide high energy and power density while being independent of location, environment and application. The primary energy alternatives that are available in the near-term are discussed below.
Solar Power
Using the sun as an energy source in space is often the preferred option, but is limited in power density and is location specific; if you’re ever in the sun’s shadow or in a dusty/cloudy environment, or as you move deeper into the solar system the sun becomes a poor energy source. Even on Earth, the sun is not practical as a source of baseload electricity. Solar power on the Moon is significantly hampered by the 28 day cycle (i.e. 14 days of darkness). The technology to efficiently and reliably store energy in the deep cold on the lunar night presents substantial challenges, and would likely be more complex and heavier than the solar power system itself. On Mars, the diminished sunlight decreases the value of solar power, but it is the dust, and more importantly dust storms that make solar power unattractive. The potential of a protracted dust storm will require more baseload power and a longer term storage system, presenting the same challenges as a lunar system (in addition to the decreased solar insolation during the day). Finally, the lower power density of sunlight beyond the asteroid belt would make the required size of a solar powered system excessive for a high power mission.
Chemical Power
Since the dawn of the human race, our favorite energy alternative to the sun has been to burn things (primarily wood until the industrial revolution). On Earth we are surrounded by materials laden in hydrogen and carbon, fortuitously surrounded by a gas that contains oxygen (air). When we move about, we generally worry about our fuel supply but take the oxidizer for granted. In space, we have to bring both fuel and oxidizer with us, or have a means of generating either in-situ (which requires power). Space does not change fundamental chemistry (except for gravitational effects on combustion), but using chemical power in space is extremely inefficient because of all the power needed to transport or create constituents in-situ. In addition, storing these constituents in the extreme temperatures of space presents problems in logistics and reliability.
Energy Storage
Energy-storage systems (e.g. batteries) have a role for almost any space mission, but generally only in the very early stages. The advantage of batteries is that we can use low-cost Earth-based technologies to generate the energy, and then extract it from the storage technology to utilize it in space. There is considerable ongoing research in fuel-cells and alternative means of storing energy, which could be much more mass efficient than traditional batteries; however, there is no chance that a chemical energy storage system would ever be practical to power ambitious space exploration. Some nuclear sources, such as anti-matter and even 238Pu might fit within the definition of an energy storage system, but they are better classified as nuclear power sources.
Nuclear Power
Nuclear power can be used to provide electricity, propulsion and/or heat in space. The major benefit of nuclear technology is that power can be provided in a robust form that is independent of location or application. There is a great deal of experience in space radioisotope power systems, and limited experience in space fission power systems. On Earth, the physics of fission are well understood, and engineered systems are well established. Other nuclear power types like fusion, anti-matter, or options like triggered isotopes are not well developed for either terrestrial of space application.
“Nuclear power in space” encompasses a wide range of sources, technologies and applications. The physical sources of nuclear energy that can be utilized are radioactive decay, fission, fusion, and antimatter. The technologies to harness these forms of energy are almost limitless, but the list of practical near-term technologies rather small. The applications of nuclear technology include electricity, propulsion, and/or heat (for warmth or to drive chemical processes). There are numerous combinations of nuclear power sources, technologies, and applications that can be used to enable our exploration and expansion into space. In the end, the attractiveness of nuclear energy in space is the ability to provide robust, long-lived, high power density (kW/kg) systems at any location.
The current forms of nuclear power that we know of and can reproduce on some level are radioactive decay, fission, fusion and anti-matter.
Energy Source | Energy Density | Max Power Density |
LH2-LOx Combustion | 13 MJ/kg | limited only by engineering |
Pu-238 Decay | 2,100,000 MJ/kg | 0.54 kW/kg |
U-235 fission | 82,000,000 MJ/kg | limited only by engineering |
D-He3 fusion | 354,000,000 MJ/kg | limited only by engineering |
Antimatter | 90,000,000,000 MJ/kg | limited only by engineering |
The 2 key points of the above table is that 1) nuclear power sources offer enormously higher energy densities than chemical systems, and 2) that power density is limited for Pu-238. Fission, fusion and antimatter can all provide energy and power densities beyond what we could feasibly utilize in the foreseeable future. However, discussions of fusion and anti-matter are essentially moot for decades or perhaps centuries to come, because even if we engineer the technologies required to make these power sources practical, we will not have the technologies to engineer the high power density systems needed for space application. Restated, fission has the ability to provide energy densities so high that it will take many generations of technology advancement (materials and fabrication) until we could take advantage of a higher energy density power source (even if that source itself was off the shelf). Therefore, since fission technology is established and well-understood, it is the obvious technology to focus on for space nuclear technology development.
It is possible that once we establish large scale settlements on Mars or elsewhere, that we could shift away from nuclear power to an in-situ source (geothermal power, solar power with an in-situ energy storage mechanism, wind power where there is an atmosphere, or perhaps nuclear if we could mine thorium, uranium, D, He-3, etc.). Until then, we need to have a technology that can enable the energy intensive missions and operations that could get us to that end goal, and that power source is fission power.
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