Will NASA's Nuclear Powered Spacecraft Revolutionize Space Exploration?

Hello, it's Scott Manley here. Just over a month ago, NASA dropped a surprise on the space community when they announced Space Reactor 1 Freedom or SR1F for short. A spacecraft that's intended to be the first nuclear fishing powered interplanetary mission. Now, the name might sound a bit clunky, but it kind of matches the design of the mission hardware. It takes the existing power and propulsion element from the cancelled gateway. It pairs it with a reactor and a payload to create a minimal viable product to demonstrate space nuclear electric propulsion. Simultaneously using existing hardware that has no future, the PPE, buying down risk on space fishing reactors which are needed for the space moon base and picking a payload that can be thrown together from things that are largely already designed. Skyfall. It launches multiple Ingenuity class helicopters from an entry vehicle that's the same design as used on Curiosity and Perseverance. So launch is targeted for December 2028 and it's heading to Mars with a primary goal of demonstrating its nuclear electro propulsion in deep space. Dropping off JPL's Skyfall mission to scout potential human landing sites and find subsurface water is really just like a bonus goal, right? an extra side quest that gives the primary spacecraft something to do, a reason to exist. Skyfall could easily get to Mars on an existing launch system and cruise stage. But the real goal is to prove that after years of studies that never have turned into missions that nuclear power can open up the solar system in ways that we've only really dreamed about. If the Mars exploration mission doesn't work, it's not as big of a problem than if the space nuclear reactor power system turns out to be fatally flawed in some way. So, the PPE for the Gateway is being repurposed again. If you remember, it was originally intended to propel the asteroid redirect mission and be a technology demonstrator for high power electric thrusters before it became the heart of the lunar gateway, providing the power and the propulsion. Hence its name, power and propulsion element. Now that solar electric bus with its hall thrusters and all that power management gear already were tested on the ground. It's going to be getting paired with a brand new compact fishing reactor using higha lowenrich uranium and that will drive a closed braen cycle turbine spitting out more than 20 kowatts of electrical power. It looks at first glance like a bunch of existing space parts slapped together with a nuclear heart because in a sense it is. But if it works, it can mean that NASA can finally stop talking about nuclear fishing in space and start doing it with more certainty that will actually be viable once it reaches orbit. But here is the thing that makes this announcement feel like deja vu for people like me, people that have been following NASA's deep space plans for the last 20, 30 years or so. NASA has looked at nuclear electric propulsion in far more detail with ambitious mission uh two decades ago with something called project prometheus and that was a grand vision that never left the drawing board but it did the engineering studies right we have the NTRS papers that they published and some of the hardware prototypes that they built to test this they still represent one of the most serious deep dives into space fishing power and electric propulsion the world has ever seen. So I want to take you back to that to project Prometheus and its flagship concept the Jupiter icy moons orbiter GMO and hopefully that'll help explain why nuclear power is such a gamecher compared to the radioisotope thermoelect electric generators and the solar arrays that we actually use in space exploration today. First let's set the stage with what we do in the outer solar system right now. When NASA sent the Galileo to Jupiter in the 1990s, it carried a pair of radioisotope thermmoelect electric generators producing a few hundred watts of electricity from the heat of plutonium 238 decay. RTGs are brilliant for what they are. No moving parts, incredibly reliable, and they keep working for decades in the cold and dark of deep space far from the sun. New Horizons used one, too, and Cassini at Saturn had three of them. But spacecraft heading to Saturn and beyond really need nuclear sources because solar power falls off with the square distance from the sun. At Jupiter, you're already down to 127th the intensity that we get at Earth. So we have Juno out there. It's you orbiting Juno Jupiter right now with absolutely enormous solar arrays. Something like 650 square meters of panels producing like 400 to 500 watts at Jupiter's distance after degradation. Europa Clipper, which launched in 2024 and is currently on route. And Europa's uh sorry, Europe's Jupiter Icy Moons Explorer, Juice, they both rely on similarly massive solar arrays. Jupiter

is right at the limit of where solar panels can be viable for running a spacecraft. But if you need more power or need to operate further from the sun, then it is time to go nuclear. So, as I said, we've already sent missions to Jupiter and Saturn that have gone nuclear using radioisotope thermmoelectric generators, right? And you can make bigger and more powerful ones just by chaining them together. But they become horribly mass inefficient once you start say wanting kilowatts worth of power so you can run ion thrusters as opposed to the hundreds of watts that are needed to power you know scientific instruments like they're powered by uranium 2 sorry plutonium 238 decaying and that is expensive extraordinarily difficult to manufacture to the point that the US actually stopped making it for a long time until NASA began running out but beyond that you can't control when radioactive decay occurs. You can't stop it with for and save it for later. It's just consistently producing power and slowly getting weaker over time and you can't really turn off. You get the power regardless of whether you need it or not. But once you get above a certain power requirement, that's when fishing reactors really start to look like a much better deal. There's a certain minimum size that's needed for the critical mass of the core and the accompanying control equipment, but once you sort of get to that point, then it wins by every metric. A controlled chain reaction using enriched uranium gives you much more energy per gram than radioactive decay. And you can throttle it, you can shut it down, restart it. It doesn't run out in the same way. Prometheus was aiming at a space qualified reactor delivering about 200 kW of electrical power. Roughly 800 to 2,000 times what you know uh existing RTGs on space missions have delivered. and that could run at that power level for 10 to 15 years. That's the kind of juice that lets you run high performance ion engines continuously for years or power a massive science payload. Once you get there, they can hit targets with powerful radars or liars and of course send data home using gigabits uh level transmitters, right? Ultra high power transmitters are made available or made possible by that power. Of course, turning reactor heat into electricity in space is a hard part. On Earth, we would boil the water and then spin turbines. And in engineering terms, that's what's called the Ranken cycle. It's named after William Ranken, a professor at the University of Glasgow who developed it. And we have over a century worth of engineering experience perfecting the Ranken cycle to make our terrestrial power stations as efficient as possible. But it does require separating liquids and gases, which is a bit of a problem in zero gravity. Now, RTGs, as I've mentioned, they use solid state power generators. These are stacks of thermalouples that produce a tiny amount of electrical power through something called the CBEC effect when one side is warmer than the other. These are great because there is no moving parts. America's first fish reactor experiment in space also used these solid state converters. But the downside is the conversion efficiency is really low. Like it maxes out at about 8% efficiency. The Prometheus team working with the Department of Energy's naval reactors people, they evaluated more efficient power conversion cycles in those NTRS reports. The Sterling cycle was one candidate with desirable properties. A reciprocating piston engine driven by external heat flow. It too was developed 200 years ago by a Scotsman, Robert Sterling. And it's great. at small scales. NASA even developed a more efficient RTG that used a sterling engine when they were worried about the supply of plutonium 238 becoming limited. However, Prometheus study baseline settled on a closed Brighton cycle which is named after an American called George Brighton. And although his design was for a piston engine, it's the cycle that's used in jet engines, but instead of being used to generate thrust, this would be running in a closed loop with the helium xenon gas mixture. Reactor heat expands the gas through a turbine that drives both the compressor and a high-speed alternator, and so they get power out. The gas will then go through a recuperator to preheat the incoming flow. It gets cooled in a radiator and all heads back to the compressor. They were targeting 20 to 30% thermal to electric efficiency which is way better than the 5 to 8% that they would get from a static electro thermoelect electric converters in the RTGS. NASA Glenn they even built and hop fired a 2 kW breadboard version which was integrated with an ion thruster watching how the whole system handled sudden load changes, faults, and the weird electrical spikes you get from electrical propulsion. It wasn't flight hardware, but it proved that the whole chain could work together without melting down or shaking itself apart. Imagine trying to keep a jet engine

running smoothly for 15 years in space. That is the kind of engineering flex that makes you appreciate why NASA would spend a lot of time working with hardware and uh theory on test rigs before actually flying something. But anyway, here's the catch. With any heat engine in space, you still have to reject a lot of the waste heat. For a 200 kW electric system, you might be looking at 600 kW of thermal power. The only way to dump that in a vacuum is radiation. And that means big deployable radiator panels. Detailed studies from the Glenn Research Center traded radiator designs with pumped liquid metal loops, water heat pipe panels, lightweight composite surfaces, and then you have to consider that the temperature of the cold side of the loop, the lower that temperature gets, the more thermal energy can be converted to electricity. The more efficient your generator gets, but in turn lowering the temperature means you have to have larger radiators, right? So for the GMO reference design, they ended up with sodium potassium liquid coolant being piped to the radiators at a temperature of about 500 Kelvin and then returning to the heat exchangers at about 400 Kelvin. The coolant pipes would deliver their heat to water-based heat pipes to spread the thermal energy across the radiator surface so that the energy radiates into space. With this temperature modeled, the Stefan Boltzman law tells how much area is required to emit the 600 kilowatts of heat load. And that will be roughly 420 m. And that will of course be spread across deployable wings. That's not small. Like those radiators had to be lightweight, deployable, and carefully positioned so they didn't cook the rest of the spacecraft or get blocked by the radiation shield. And of course, they have to be reliable because losing radiators means higher temperatures and a loss of performance in your power generation. And there's like an important NTRS study that modeled what happens if micrometeoroids punch a hole in the radiator panel. The system had to be designed to isolate damaged sections and keep running. So that's the kind of detail that you would see in these studies and it makes you realize why these studies would run to like hundreds of pages. You're not just building a power plant in Kerbal space program. You know, you're building a power plant that has to survive launch deployment and 15 years of deep space ballet while shedding heat like you know cosmic space heaters, right? Speaking of which, the spacecraft architecture itself was dictated by radiation safety. So like efficient reactor is a serious neutron and gammaray source that would you know fry the electronics and it would ruin your science instruments. Radiation shielding requires a lot of mass. It's more efficient to reduce shielding by moving the reactor far from the sensitive parts of the spacecraft. So GMO is was designed as a long spindly beast like was a main truss or boom that stretched something like 60 m out at full deployment. The reactor module would be at one end uh and that would be about six tons and that would include some shielding over a narrow area uh for that you know that it would for that power source and the science you know and other hardware would be at the other end along with the thrusters and the power processing units and everything in between. So you have this like conical radiation shadow shield uh lith with lithium hydroxide to capture neutrons, maybe a tungsten for gamma rays that would cast a protective cone, a shadow right over the sensitive bits. And you'll notice those radiator panels extend out in a triangle from one end to the other. That's because the shield would create a conical area of lower radiation, right? A radiation shadow. The radiator panels would be arranged to fit inside that. Interestingly, the reactor would be contained inside an ablative heat shield during launch so that if there was a problem, it would survive re-entry. Not because they planned for the reactor to land on another planet, but if there was a launch failure or some problem that caused it to return to Earth. You wouldn't want the reactor breaking up and spreading radioactive uranium dust throughout the atmosphere. The whole thing would land and could be recovered. deploy in space. Solar arrays for initial power startup h before the reactor was used. Radiator panels would unfold on booms. Thruster pods would extend. The total launch mass for the assembled spacecraft was something like 36 tons. All right, including 12 tons of xenon propellant and the spacecraft dry mass would be like about 16 tons plus the reactor module. It was going to be one of the largest and most complex robotic spacecraft ever contemplated. And how did they plan to launch something that big? Well, they wanted three launches. The baseline was three separate Delta IV Heavy launches, although at the time the report was written, there was still talk about Atlas 5 Heavy existing. Either way, one of those launches would carry

the Prometheus spacecraft itself, while the other two could deliver long duration upper stages, which would all dock together in Earth assembly orbit. That's a 407x 28. 5 degree orbit, which is the most efficient launch inclination from Florida. That assembly orbit was chosen as a compromise. High enough for reasonable lifetime against drag. Low enough that the existing heavy lift rockets could put a 37 ton spacecraft there. Once assembled, it would get underway using the two upper other upper stages right from the three rockets to boost the spacecraft into an Earth escape trajectory. While they could absolutely have fired up the reactor and spiraled outward slowly on the electric propulsion, that would actually be more massefficient, but it would mean starting the reactor at a time when a failure could then lead to it falling back to Earth. Also, it would take a whole lot longer and would in turn, like rob the reactor and engines of life. A rapid boost to escape velocity from a chemical rocket would make more sense and save more time. So from that escape trajectory, that's when they would begin the startup. The long slow voyage out to Jupiter on the electric propulsion would require multiple acceleration stages and it would take about 5 to six years. Now all that electricity, yeah, that was great. It meant that you could run these very powerful electric propulsion uh systems, right? A the baseline designs used high power electric propulsion thruster or high pep or NASA's evolutionary xenon thrusters Nexus. Each of these thrusters was capable of swallowing 20 to 40 kW and delivering specific impulses, you know, as high as 9,000 seconds, which is handily higher than any chemical rocket that maxes out about 450. You know, we're talking about like 90 km/s of exhaust velocity. Sure, the thrust is tiny. We're talking about accelerations of micro G's, but you can fire them continuously for months or years without running out of repellent. The GMO spacecraft would carry, yeah, 12 K uh 12 tons of xenon, enough to easily get them beyond the asteroid belt, capture at Jupiter, and then hop between its icy moons, giving each of them some uplose attention. The thrusters would be mounted out on arms that could be rotated to gain attitude control from the low thrust of the propulsion system without having to resort to standalone reaction control systems or for any like required maneuvers. The flagship mission they were building all this hardware for was genuinely ambitious. Once at Jupiter, it would initially capture into orbit around Kalisto for several months of highresolution mapping with ice penetrating radar, liar, magnetometers, spectrometers, and cameras. And then it would once again use its low thrust engines to transfer to Ganymede for another extended stay. And finally, it could head to Europa. Total science time at the moons would be measured in years, not hours. with a 1500 kilogram payload that can actually phone home gigabits of data thanks to 200 kilowatts of spare capacity powering those KA band communications. Compare that to Galileo which could do like flybys of planets or moons and it still ran out of propellant before it really had explored everything. And of course, yes, it had a damaged antenna, but even with a proper antenna, it wouldn't have delivered anywhere near the level of power of data that GMO could deliver. The solar powered Juno is nearing the end of its life and Europa Clipper is expected to do great work. But neither of those can linger or carry the same monster instruments. Jimu would not have been the first proper tour of the Jovian system, but it would eclipse the science gathered by all previous robotic visitors, answering questions about subsurface oceans and habitability that were still chasing today. It was a kind of mission that would make engineers and scientists stay late at the office dreaming about what we might discover and how best to make the spacecraft do that. Right? So, Prometheus, it made real progress in phase A studies, right? Northrup Grin handled spacecraft design. JPL led the science side and the naval reactor, you know, unit. They would do the reactor design. They ran integrated tests, produced the mass budgets, reference trajectories and even some prototypes of the Brighton system and thrusters. But then the budget reality hit. The program had ramped up under the 2004 Vision of Space Exploration Act, right? But by 2005, the space shuttle was still recovering from Colombia. The ISS was needing attention, and the new push to put humans back on the moon meant nuclear efforts got redirected to nearer term surface power and nuclear thermal propulsion. Jimu's estimated price tag was north of $20 billion, according to some internal numbers that leaked later. Plus, the launch vehicle uncertainties and the technical risk of flying humanity's first space fishing reactor on such an expensive mission made it a very easy target for cuts. Funding was slashed, work wound down in

late 2005, and the project was effectively cancelled before phase B. The reactor and power conversion tech would live on in later studies, but the Grand Jupiter tour died on the vine. And this is a classic NASA story. brilliant engineering, stacks of papers and studies full of trades and test data producing a vision that is compelling even a decade later, but a goal so lofty and far off it is incompatible with the attention span of Congress. And so you can see there is a logic in building space reactor one freedom out of something that already exists. uh something that needs to exist, the nuclear reactor and a mission that JPL has already designed and proven with low consequences for its failing. 20 years on from Prometheus, this is a much more modest practical first step. Something that won't stretch across decades of development and make it vulnerable to cancellation. We all know that there are many uses for nuclear power in space and also niches in space flight where nuclear electropulsion is really the best option. So proving that fishing reactors in space is possible and buying down the technological concerns with a test flight using essentially spare hardware removes a massive roadblock to future missions that might actually use the technology in a way that really exploits its unique capabilities and when it does you know exploration of the outer solar system will never be the same again right of course today we do have Europa Clipper and the Jupiter icy moons orbiter both of those will have extended missions at Jupiter using solar power sources. They will visit the big moons for multiple flybys, but the power budget won't support massively powerful radar. But if we're lucky, we'll probably get a comparable amount of science out of these missions that we would get from Jimbo. But look, if we go out to Saturn, we won't be able to use solar power there, right? Or even beyond to Uranus and Neptune, those places, nuclear power is absolutely necessary. We have Dragonfly. It'll be landing on Titan powered by an RTG for its flights across the surface. But hopefully space reactor 1 will fly and it will provide it'll prove that it works. It'll provide concrete data to show that efficient reactors are absolutely viable sources of power for exploration of the outer solar system. And finally, we'll be able to get that orbiter at Uranus and Neptune. I'm Scott Manley. Fly safe.