Solar Panels and Nuclear Spacecraft
Austin Morris, Director of Engineering
5-minute read
In a previous column, I discussed various methods of propulsion and the respective “oomph” that each one has. Included in that was a mention that KMI plans to use a Hall-Effect thruster on our Laelaps spacecraft, which is a form of electric propulsion. One of the notable characteristics of electric propulsion is that it draws a significant amount of power, especially compared to other spacecraft components. Due to this need, an important consideration for a spacecraft with onboard propulsion is a power generation system, because unfortunately extension cord technology hasn’t yet come far enough (get it?) to plug them into the wall. As such, spacecraft need to generate their own power in order to keep running and there’s a few different ways to do that. The two methods that I am going to discuss in this column are the well-known solar panels and the lesser-known radioisotope thermoelectric generator, or RTG.
Solar panels, which are sometimes referred to by their scientific name photovoltaic or PV panels, are a technology that are familiar to most as they have become so widespread in the last few decades. When most people think of renewable, clean energy sources, solar and wind power are usually at the top of their mental list. Historically, the concepts that led to solar panels have been around since 1839, with the first commercial solar panel created in 1881. There have been many iterations and improvements to the function and efficiency of solar panels since then but the overall concepts still remain. Essentially, light emitted from the Sun in the form of photons hits the solar panel, which excites valence electrons within the solar cells and causes them to jump and become free electrons. This in turn excites me and other nerds like me because it means we can harness the energy from this as electrical power.
Electrical power generated from the Sun is effectively free, once you have the solar panels themselves, as you don’t need to keep feeding the Sun with fuel to maintain the output of photons. Arguably, we will eventually lose this capability to freeload when the Sun eventually dies, but seeing as we’ve got a few billion years until then, we might as well make use of the energy getting expended now. One of the main drawbacks of solar panels is that, predictably, they really work best when in sunlight. Which means on Earth, they don’t typically generate much power in the nighttime. In space however, this means that they don’t generate power when in an eclipse. Depending on your path, you could be in eclipse always, never, or alternating back and forth. In LEO, this alternating period is approximately 60 minutes of sunlight followed by 30 minutes of darkness. On the other hand, there are particular Sun-Synchronous Orbits in which you are always in sunshine, which makes solar panels an excellent method of power generation. If, for instance, you have an orbital regime that spends a lot of time in darkness, or if you are running a mission like the Voyager 1 spacecraft that, at the time of this writing, is about 14.4 billion miles away from the Sun, you may not be able to rely on generating a lot of power from the Sun. Solar panels continue to increase in efficiency and decrease in size and mass, making them the go-to solution for anything orbiting close to the Earth’s neighborhood or closer to the sun. So for missions like Voyager 1 and 2 that go farther away, you can instead turn to nuclear power.
Radioisotope thermoelectric generators, or RTGs, utilize the heat generated by the steady decay of a radioactive material to produce electricity by converting that heat energy into electrical power, similar to the purpose of a nuclear power plant as a constant and steady source of power. RTGs have worked sufficiently on a multitude of missions, including the latest Mars rover, Perseverance, and the above-mentioned Voyager probes. In the case of Voyager 1, power is generated by three RTGs, which have been supplying power for the last 40+ years since it launched. This type of system allows you to operate on a self-contained power source and can eliminate the need for solar panels. This is especially useful for missions like the Voyager missions, which have been flung so far out from the Sun that if they stopped to ask for directions home, the locals would likely not even know where our Sun is (or speak the language, but let’s not think too hard about the logistics of this scenario). There is also the possible use-case of utilizing an RTG not to entirely remove the need for solar panels but to supplement the power generated from them to power more equipment than would otherwise be possible, or to reduce the amount of solar panel area needed in order to power the equipment aboard the spacecraft. An example of potential need for increased power in space is on missions that plan to use electric propulsion for interstellar travel. As the size of spacecraft increases to accommodate crew and colony supplies, chemical rockets could be replaced with electric propulsion for their higher efficiency and lower propensity to spontaneously explode. Regardless, RTGs and solar panels both come with their own benefits and drawbacks when it comes to efficiency, cost, usage, and even lifespan. At the end of the day, these trade-offs will likely be the factors that determine whether it is more appropriate for your particular mission and spacecraft to be solar-powered or nuclear-powered.
On the note of nuclear-powered spacecraft, there’s one final concept that I’d like to introduce you to today. I would like to add a disclaimer first, in that this idea may not throw sanity completely out the window, but at the very least it certainly leans out the window and dangles sanity from a great height by its ankles. Nuclear pulse propulsion is an idea that sprouted up in the late 1940’s, which entails using nuclear explosions as a method of propulsion by detonating below a rocket or behind a spacecraft and allowing the explosion force to serve as a mechanism of thrust for the craft. Fortunately for those of us who enjoy living in the world as it is today, this project was reportedly shut down in the 1960’s after nearly two decades of work on the idea. As with any other propulsion method, this too had its benefits and drawbacks. The main benefit was the speed, which could have led to missions that could fly to Mars and back in four weeks (as compared to the current rate of roughly nine months) or out to Saturn’s moons in seven months (instead of nine years). The drawbacks to such a system are...mostly everything else as it pertains to the health, safety, and well-being of humans who live on this planet.
Space is already a dangerous place. Detonating nuclear bombs to reach space and leaving behind a trail of radioactive fallout, in addition to carrying numerous additional bombs as continuing propellant with you, is not a way to make it safer. Instead, there is a lot of modern work ongoing toward the endeavor of making space a safer place to operate, from radiation shielding and cleaner, safer propulsion methods, to collision avoidance and debris mitigation. It is important that we all continue working toward these goals that will further humanity’s future in space and is why KMI will continue work on the mission of #KeepingSpaceClearForAll.
Recommended column to read next: The Oomph of Different Engines