Working in technology, it is easy to forget the importance of power. Electricity is like water, in that we are so awash in it we don’t realize its core value as a primary need. People from places where water is scarce often marvel at city sidewalks being washed down with a hose. Just like water, the scope of a project is also often determined by its availability.
In Star Trek, Scotty was always telling Captain Kirk that he couldn’t ignore the laws of physics (although the show’s very premise broke several). Real space travel is bounded by them, however, in this case the physical law we are referring to is the scientific one that states a specified physical quantity is inversely proportional to the square of the distance.
The inverse-square law is why we need to worry about power in space, even though the sun shines 24/7 up there. Ignoring for a moment what happens when the spacecraft is in shadow, the fact is that the farther and farther out you go, the less power you can generate per square meter of solar panel (now you can think about what happens when the spacecraft goes into shadow).
More than solar
Solar panels are very cool and useful, but just as electric cars won’t drive fossil-fueled vehicles out of every niche in the marketplace due to intrinsic utility, Now that we’ve established you need more than a bunch of solar panels, what do you do? Fuel cells are great for on-board power systems in habitable spacecraft, as the added side-benefit is water. However, they require fuel, which adds weight and bulk.
Nuclear power is the best alternative, and being used in spacecraft outside of the Earth’s ecosystem, exhibit a low risk to it (barring launch crashes, and the systems are sufficiently contained for that expediency). Putting a nuclear power source on a spacecraft also extends its lifetime significantly, as the power keeps coming as long as the nuclear isotope has neutrons to spare.
The United Nations Office for Outer Space Affairs recognizes “that for some missions in outer space nuclear power sources are particularly suited or even essential owing to their compactness, long life and other attributes” and “that the use of nuclear power sources in outer space should focus on those applications which take advantage of the particular properties of nuclear power sources.”
Nukes in space
Space-rated radioisotope power systems (RPSs) convert the heat from the decay of the plutonium-238 (Pu-238) into heat and electricity for decades. Proven safe, reliable, and maintenance-free in operational missions throughout the solar system. The RPS-powered New Horizons spacecraft transited the Pluto system on July 14, 2015, for example.
The US Department of Energy (DOE) creates RPSs for space exploration and national security missions. There are two primary types, power systems for electricity, like radioisotope thermoelectric generators (RTGs), and small heat sources called radioisotope heater units (RHUs). DOE is also responsible for nuclear safety throughout a given mission.
The Mars 2020 Perseverance Rover will be the first rover in more than 30 years to use US-produced plutonium. Perseverance will be equipped with a multi-mission Pu-238 RTG to power the rover and its tools once it lands on the red planet. To date, DOE has built nearly 50 radioisotope power system units that have powered more than two dozen U.S. space missions.
The US’ Oak Ridge National Laboratory provided the plutonium oxide fuel and fuel cladding for the Perseverance power system. Los Alamos National Laboratory manufactured the fuel by encapsulating the plutonium-238, and Idaho National Laboratory assembled and tested the unit.
Recently The US Department of Energy placed a formal request for a fission surface power system for harsh space environments. Small reactors are needed because the smaller RTGs will not have sufficient output to drive a space-pioneer village . NASA, the Idaho National Laboratory, and the Energy Department will evaluate ideas for developing reactors.
The Department of Energy, NASA, and the Battle Energy Alliance, the American contractor that manages the Idaho National Laboratory, plans to hold a government-industry webcast technical meeting in August regarding expectations for such a program. The plan is to develop a reactor design, build a test reactor, send another reactor to the moon, and develop a flight system and lander to transport it to the moon. The target of the plan is to build a reactor, flight system, and lander by the end of 2026.