This question about Kilopower nuclear reactors got me thinking.

Answer(s) to How many nuclear fission reactors have been launched into space? How many are still there? suggest that most nuclear reactors still in space are pretty darn old.

Question: What was the most recent launch of a nuclear reactor, and what are the current barriers to launching the next one?

note: I'm not asking about RTGs which produce energy from simple radioactive decay, I'm asking about reactors that use chain reactions to produce a high rate of power production.

Nuclear Energy Our Misunderstood Friend


1 Answer 1


From the Wikipedia list, the most recently launched nuclear reactor appears to be a TOPAZ-I on Kosmos 1867, launched by the USSR in 1987. It's parked in a 800km orbit at 65º inclination and appears to be falling apart.

For Earth orbit applications, solar power mass efficiency (~100 W/kg) is much better than RTG (~2.8 W/kg) or reactors like TOPAZ-I (~15 W/kg). Even out to Mars or Ceres, solar still wins for mass. This means that almost all space missions rely on solar, driving R&D and bringing costs down.

For outer-planet missions, RTGs tend to win out even though they're less mass efficient than reactors, because they are simple and relatively cheap.

There are legitimate concerns about launching nuclear reactors on rockets that do, still, occasionally explode; no one wants U-235 scattered over thousands of square kilometers of Earth's surface, even the oceans, and even more so, no one wants to be the person who signed off on the risk assessment that let the launch go ahead, so for a bureaucrat, it's always safer to kick the risk assessment back for one more re-evaluation than to accept it.

For any given mission design, it's probably cheaper and simpler to reduce power requirements to the point that RTGs can handle it than to get a reactor in the design.

  • $\begingroup$ I like the upbeat tone of the first sentence. While there is a lot of talk about the kilopower reactors, perhaps for the Moon or Mars manned missions, it sounds like you think that isn't going to be happening too soon, and instead there's likely to be a lot of solar along with some batteries and other energy storage at night such as fuel cells shown in this Toyota Moon Cruiser youtu.be/1kd2nFHAAtU?t=46 and youtu.be/MfrAD7ctLrY?t=1126 $\endgroup$
    – uhoh
    Mar 20, 2019 at 15:08
  • $\begingroup$ If the nuclear reactor was used for some time, the remaining high activity fission products are more dangerous than the pure uranium of the unused reactor. $\endgroup$
    – Uwe
    Mar 20, 2019 at 15:39

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