I remember @Hobbes's answer mentioning that the US has put one nuclear fission reactor in space, and that not much was known about Russia's program.

The Bloomberg article NASA Is Bringing Back Cold War-Era Atomic Rockets to Get to Mars; Nuclear-powered spacecraft would cut travel time to the Red Planet says:

Russia’s Rosatom Corp. has said it plans this year to test a prototype nuclear engine for a spacecraft that can go to Mars. Russia so far has led research in the field and has deployed more than 30 fission reactors in space, according to the World Nuclear Association. China aims to use atomic-powered shuttles as part of its space exploration plans through 2045, according to state Xinhua News Agency.

I'd never heard of the "World Nuclear Association" before today.

Question: So is the answer to "How many nuclear fission reactors have been launched into space?" then 31? The answer to "How many still there?" 31 as well?

note: don't confuse fission reactor with alpha-decay-based RTGs.

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above: "A mock-up of a full size Nuclear Engine for Rocket Vehicles Application, or NERVA, in 1967." Source: Bettmann Archives via Getty Images. From Bloomberg


There's a new list on Wikipedia that has a better breakdown of Russian nuclear power systems in space. It lists 32 BES-5 thermoelectric reactors, 2 Topaz reactors, 5 RTGs and 2 '2 kWe' reactors.

At least 6 of those have reentered. The Wikipedia article does not give current status for many of these missions. Its source lists those missions as still in orbit. So 36 reactors and 5 RTGs have been flown, 6 of those have reentered for a total of 31 reactors and 4 RTGs still in orbit.

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    $\begingroup$ The Wikipedia article does not give current status for many of these missions, which is why I declined to put a current number in my answer. List answers are not a good fit for SE as the next launch of orbital decay will make the answer incorrect again. $\endgroup$ – Hobbes Jun 15 '18 at 15:37
  • $\begingroup$ Ya I understand. I can add "roughly" to the question if you'd like. I'm only interested in the quantities, not any kind of list. It's pretty amazing to thing that almost a half-dozen kW-sized nuclear reactors have reentered the atmosphere. I had no idea! $\endgroup$ – uhoh Jun 15 '18 at 15:40
  • $\begingroup$ "have been flown". Shouldn't you add "by Russia" ? Just Apollo program alone flew more that those 5 RTG. $\endgroup$ – Diego Sánchez Jun 15 '18 at 16:07
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    $\begingroup$ Already in the first sentence. 'breakdown of Russian nuclear power systems' $\endgroup$ – Hobbes Jun 15 '18 at 16:36

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