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A useful document I found - spacecraft power systems
Interesting that the RTGs have power densities of 3-4 W/kg, but are only 5%. Stirling generators are being developed that offer far higher efficiencies, 5-6 times this - the worry is maintenance, which shouldn't be as much of an issue for a crewed base or vessel. We're talking about 20 kWe/tonne, so using an RTG should be viable for a base, and gives us decades to develop in-situ power sources.
Now we just have to persuade people to let them be launched...
Use what is abundant and build to last
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If you take a look at the cost of these systems, you'll see why they're impractical at higher output levels. The Pu238 filled RTG's cost too much because we stopped making that Plutonium isotope many years ago and only recently restarted production. Even if we had tons of the stuff sitting around, the cost to manufacture the isotope is much higher than the cost to make U235 and the potential power density will always be quite low. Current work focuses on using highly enriched U235 to make small, power dense cores for fission reactors. Fission reactors scale from devices weigh about 100kg to devices weighing many tons.
The output level of fission reactors is much higher than RTG's and the only reason the output levels aren't even higher than they already are is the mandate for utter simplicity in space reactor design. A 100We fission reactor has one moving part, which would be the neutron attenuating rod inserted into the center of the core for startup and shutdown. This is one more moving part in comparison to what a RTG has, not counting the Stirling generators that produce electricity. The ASRG RTG's also used Stirling generators to increase electrical power output, so apart from radiation levels that's the only real difference between an advanced RTG and a low-output fission reactor. The fission reactor has far fewer total parts count and is far less expensive to manufacture than a RTG, as a function of the abundance of U235 (even highly enriched U235) in our supply chain and the low cost of the rest of the components (shielding, heat pipes, Stirling generators).
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Why are you talking about Plutonium? The Russians use the far more plentiful and much cheaper Strontium.
Use what is abundant and build to last
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I'd take no issue at all with NASA using Strontium, but there's this little NIH issue NASA must first move past. For them, NIH is a "thing".
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Well, if NASA isn't the one leading the project, will that be such an issue?
Use what is abundant and build to last
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In most countries, nuclear things are government monopolies. It takes an act of congress in the US to allow a private company access to nuclear power, excepting only those entities that build and operate Earthly power plants. Even for them, the permitting process is extreme.
It is rules like that which prevent Spacex or any other outfit, essentially nobody but NASA, from fielding nuclear power supplies in space. Fix THAT, and things like Safe-400 will immediately get used and incorporated into many projects.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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I'd take no issue at all with NASA using Strontium, but there's this little NIH issue NASA must first move past. For them, NIH is a "thing".
The two isotopes that have been most frequently used are Pu-238 and Sr-90. Each has their advantages and disadvantages that make them preferable for certain types of applications.
This web page answers the question for the use in an RTG: https://atomicinsights.com/rtg-heat-sou … materials/
In nature, strontium is present in igneous rocks as celestite (SrSO4) and strontianite (SrCO3) but no Radioactive strontium, Sr 89 and Sr 90, as they do not occur in nature.
Still looking up reactor uses....
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This topic is the only one of many we have that seems close to "spacecraft power". The report below is about Chinese development and testing of 400 volt power for spacecraft. This is a step up from 28 Volts and 100 volts that are (I gather) used in existing spacecraft for human use.
https://www.yahoo.com/science/articles/ … 00586.html
World’s first: China completes 400V space power test aboard Tianzhou-10 for moon landing
Munis Raza
Updated Wed, July 29, 2026 at 5:54 AM EDT
3 min readWorld’s first: China completes 400V space power test aboard Tianzhou-10 for moon landing
China has completed the world's first in-orbit test of a 400-volt bus power system for spacecraft. The system was tested aboard the Tianzhou-10 cargo spacecraft. All validation requirements were met. No 400V bus power system has ever been operated in orbit anywhere in the world before. The developer is the Institute of Manned Space System Engineering under China's Academy of Space Technology (CAST).
Most spacecraft run on 28 or 100-volt bus power systems. These were adequate for earlier generations of spacecraft. The 400V architecture targets next-generation missions with far higher energy demands. These include crewed lunar landings, high-power electric propulsion systems, and permanent lunar research stations.
400V vs 28V vs 100VHigher voltage means lower current for the same power output. Lower current means thinner, lighter cables. On a spacecraft, every kilogram saved affects launch cost and payload capacity. Traditional low-voltage systems lose more energy as heat in the cables over longer runs. The 400V system significantly reduces those losses.
The 400V system uses silicon carbide (SiC) semiconductors. These are more efficient than conventional power electronics. That makes the overall system smaller and lighter. It can still support megawatt-level continuous power delivery, freeing up more space for payloads.
CAST also achieved independent control over the full technology chain, from power generation and energy storage through to control, distribution, and transmission. The system also features solid-state power distribution and high-voltage insulation protection designed to survive space radiation. All key components are domestically developed.
What the in-orbit tests confirmed
After Tianzhou-10 entered orbit, the verification system underwent two dedicated test cycles. Both confirmed the system's stability and reliability in orbit. They validated its resistance to space radiation and extreme temperature swings between sunlit and shadowed orbital phases. The system ran stably throughout both cycles.
"The world's first in-orbit verification of this technology has overcome a key bottleneck in high-power supply for deep-space missions," said Pang Zhihao, a retired CAST researcher. He said it lays the energy foundation for crewed lunar exploration and lunar base development. It also establishes the power supply framework for future Mars missions.
Power for the Moon and beyond
China plans a crewed lunar landing before 2030. The mission involves two separate launches on Long March 10 rockets. One carries the lander, the other carries the crew spacecraft. Both require far more power than current missions demand. High-power electric propulsion is needed to reach the Moon efficiently. Sustained power is also needed for landers, rovers, and life-support equipment on the surface.
"It can provide efficient and sustained power for high-power electric propulsion systems on lunar spacecraft, lunar lander equipment, and lunar rovers," Pang said. "Its weight reduction lowers rocket launch costs and increases payload capacity for lunar missions."
China is also developing a long-term lunar research station with Russia. That facility will need continuous high-power supply for scientific instruments and crew life-support over extended periods. The 400V architecture is designed to meet those demands. CAST said the team will continue optimizing the system in preparation for future missions, including crewed flights to Mars.
(th)
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