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| Fig. 1: A photograph taken at NASA Jet Propulsion Laboratory's "Mars Yard," which provides simulated Martian terrain for testing. Sojourner, Spirit, and Opportunity used solar panels, while Curiosity used RTGs. [5] (Courtesy of NASA) |
If you asked most people on Earth how their electricity could be produced, you would find a variety of answers. Natural gas, coal, nuclear, solar, wind these are all common sources of energy that many people are familiar with. While energy production on Earth is quite diverse, being in space imposes a number of restrictions. In-situ power infrastructure, such as asteroid mining fueling depots, are still more science fiction than fact, so spacecrafts must rely solely on what is possible to bring with them. Weight, dimension, power output, mission trajectory, and mission lifetime restrictions are of utmost importance in space exploration.
Previous posts of this class have broadly introduced what power sources have been done in spacecraft, namely solar panels, nuclear, batteries, and fuel cells. [1,2] For space exploration beyond Low Earth Orbit (LEO), spacecraft overwhelmingly use two power sources as their primary power supply: solar panels, and radioisotope thermoelectric generators (RTGs). We will investigate the design trade-offs for these two power sources, discussing engineering considerations, economic cost, and political cost.
Solar arrays, as the name implies, rely on sunlight to produce electricity. Almost all spacecraft that are in LEO use solar arrays are their primary means of power production; they are cheap and easy to manufacture, especially relative to RTGs. [3] Solar arrays have also been used for deep space exploration, but their physical design has some constraints in comparison to RTGs (see next section). Some examples of these design considerations include the distance of the spacecraft from the Sun, the power requirements of the system, the mission lifetime and the mission trajectory. [3]
Solar arrays must follow the inverse square law, which says that the intensity I at any distance d is equal to the inverse square of that distance:
k is a constant depends on the power of the source.
From this relationship we see a simple yet powerful design constraint: the further we are from the sun, the larger our solar arrays must be (if keeping factors like efficiency and position the same) in order to produce the same output. A quick back-of-the-envelope calculation as shown in Table 1 reveals that if we received 100% of possible energy in an orbit around Earth, by the time we reach Mars we would only receive only about 44% of the possible energy, and at Jupiter we would receive about 4% of the possible energy.
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| Table 1: Rough estimate of the solar energy available for a spacecraft at different destinations. [10] |
Our solar system is quite expansive! The inverse-square law means that there is a trade-off for using solar panels; in general, solar panels are cheaper and more readily available than nuclear options such as RTGs, but they come with several design disadvantages for going beyond Earth. Adding more solar panels to make up for the lack of sunlight at deep space destinations adds more mass, which raises the cost of launch because launches are priced by mass, and raises the cost of the system by requiring more powerful maneuvering capabilities. [1]
There are other factors that will affect a solar panel's electrical performance in space. Solar arrays will suffer from degradation over time due to space radiation. Engineers must design around periods of total occlusion, such as during an eclipse fly-by, or periods where they are angled sub-optimally with respect to the sun. Solar panels must be able to be stowed for launch, deployed while in space, and be stiff and stable enough to withstand on-orbit accelerations. [3]
RTGs are used in some space exploration missions, most notably Voyager 1 & 2, New Horizons, and the Mars Curiosity rover. The natural decay of radioactive material provides a high temperature source, over 600°C for a silicon germanium junction. This temperature gradient provides the electrical output, and the excess heat must be removed from the spacecraft. [4]
RTGs are advantageous because they do not require sunlight to operate. They provide longer mission lifetimes than solar power systems because of the nature of radioisotope decay; PU-238 is commonly used in RTGs and has a half-life of approximately 88 years. [4] For example, the Viking landers on Mars operated for four and six years respectively, whereas the solar-powered 1997 Mars Pathfinder operated for only three months. [5] RTGs are also relatively insensitive to the extreme cold of space, and virtually invulnerable to high radiation fields. Finally, in comparison to the surface area of a solar panel, they are lightweight and compact, and provide more power for less mass. They are "maintenance free" in that they require no moving parts or fluids to operate, and are designed to withstand launch and re-entry accidents. [6]
RTGs also have their disadvantages. Unfortunately, the nuclear decay process cannot be turned on and off. This means that an RTG is active from the moment the radioisotopes are inserted into the assembly, and the power output decreases exponentially with time. Missions will sometime take years to come to fruition, with an RTG's output ticking down with that time. Furthermore, an RTG must be cooled and shielded constantly. They are also extremely inefficient, with a conversion efficiency around 5%, with much of waste heat dumped to space. [4]
In addition to these technical challenges, RTGs also sometime receive pushback for political reasons. The U.S. produced Pu-238 at Savannah River from 1964 to 1988, and after the site was shut down, it has been procured from Russia. [7] The limited supply of Pu-238 has also limited the number of scientific endeavors into deep space that may be reliant on RTGs. NASA is incentivized to try to use solar panels for most missions unless an RTG is absolutely necessary. [8] Currently, the DOE maintains 35 kg of Pu-238, about half of which meets the power specifications for flight. The U.S. only recently began manufacturing Pu-238 again in 2013. [9]
© Julia Di. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
[1] B. Johnson, "Power Sources for Space Exploration", Physics 240, Stanford University, Fall 2012.
[2] R. Marincola, "Powering Space Vehicles," Physics 240, Stanford University, Fall 2014.
[3] P. A. Jones and B. R. Spence, "Spacecraft Solar Array Technology Trends," IEEE Aerospace and Electronic Systems Magazine 26, No. 8, 17 (August 2011).
[4] G. Bennett et al., "Mission of Daring: the General-Purpose Heat Source Radioisotope Thermoelectric Generator," American Institute of Aeronautics and Astronautics, AIAA 2006-4096, 26 Jun 06.
[5] A. C. Madrigal, "From Sojourner to Curiosity: A Mars Rover Family Portrait," The Atlantic, 6 Aug 12.
[6] M. Jiang, "An Overview of Radioisotope Thermoelectric Generators," Physics 241, Stanford University, Winter 2013.
[7] K. Shi, "The Use and Re-Supply of Plutonium-238 in the United States," Physics 241, Stanford University, Winter 2018.
[8] D. Kramer, "Shortage of Plutonium-238 Jeopardizes NASA's Planetary Science Missions," Physics Today 64, No. 1, p. 24 (2011).
[9] S. S. Oakley, "Space Exploration: Improved Planning and Communication Needed for Plutonium-238 and Radioisotope Power Systems Production," U.S. Government Accountability Office, GAO-18-161T, 4 Oct 17.
[10] A. J. Hahn, Basic Calculus of Planetary Orbits and Interplanetary Flight: The Missions of the Voyagers, Cassini, and Juno (Springer, 2020).