When planning a new architecture for lunar missions, one always faces the potential of nuclear thermal engines. Having a much higher $I_{sp}$, they offer a much higher propellant efficiency, causing a more elegant design over all. NTRs have their pros and cons, but also a question: "Where do you get rid of the engine after it is used?" It would be very relaxing if you could just let the engine crash into the Moon, but is that a good idea? The Moon is an interesting place, and some people have great plans for it. As a rule of thumb, it is not a good idea to cover something that may come in handy with radioactive dust.

What happens when a spacecraft crash into the Moon? In the case of a nuclear reactor, it does really matter if all the debris is limited to a few kilometres, making a "nuclear graveyard" possible, or not. High velocity impacts are hardly intuitive, and the velocities here are high. (2500 m/s in case of a direct impact from a transfer orbit, 1700 m/s from LMO). There should at least be some data available, as a lot of probes have been crashed into the Moon, both intentionally and by accident.

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    $\begingroup$ During the moon landings, ejecta from the landing site were seen flying off over the horizon. It was calculated some fragments could end up halfway across the Moon. I suspect impact debris has a similar radius. $\endgroup$
    – Hobbes
    Mar 15 '16 at 9:35

Smashing into something at those velocities, even metals can behave pretty much like putty or a really wet snowball, as exemplified here. Judging by the appearances of previous impacts, I would hazard to say that an impact on the Moon is pretty much like that: like a wet snowball smacking into an inch thick layer of wet snow/slush.

Also on the Moon you have no atmosphere, meaning that anything that gets tossed up and out go in a nice Newtonian parabola without any air getting into the way. With initial speeds of 1000 m/s or more, some particles may go very far.

Hence: your nuclear engine would be pulverized and throw "hot" dust and chunks wide and far. This is a bad idea.

And since we cannot expect perfectly uniform distribution, but instead that the spent nuclear fuel may come out in sizable chunks ("sizable" in this context meaning a few millimeters), you may end up with some very nasty "hot-spots".

This is exacerbated — again — by the fact that the Moon has no atmosphere, meaning that the radiation intensity of these hot-spots will not diminish by the cube of the distance as on Earth but "only" by the square of the distance.

Better to just put the nuclear engine in a parking/disposal orbit until you feel like bringing it home in a controlled manner.

  • $\begingroup$ Nice if you have some date for actual crash landings too. $\endgroup$ Mar 15 '16 at 8:57
  • $\begingroup$ I am afraid I do not... and I doubt anyone else has because we have not actually sent probes to try to examine the ejecta of man-made high speed impacts. The most we have to go on I would expect is visual assessments of impacts, and these cannot distinguish the most extreme extents of such an impact. In any case my gut instinct is the same as yours: it is a bad idea to contaminate this environment this way for very questionable gains. I would also like to add that most previously made impacts were done for scientific gains. $\endgroup$
    – MichaelK
    Mar 15 '16 at 9:01
  • $\begingroup$ I wonder if allowing the reactor to run so hot that it turns into a sphere of overheated corium before impact would help the dissipation. OTOH, these things will have so much unspent fuel, that parking them in a medium Earth orbit for reuse sounds like a reasonable idea. $\endgroup$
    – SF.
    Mar 16 '16 at 2:44

Why do you want to get rid of your engine in the first place? A trip to the moon would hardly use all the nuclear fuel you got on board. Of course it depends on the intensity of your burns, but current wet-navy ships (submarines or carriers) reactors are only refueled every year or so. So your engine should last for several trips. After that you can probably afford to park it on some remote orbit.

  • $\begingroup$ When you say current ships, are you referring to nuclear subs? It might be better to clarify that. $\endgroup$
    – kim holder
    Mar 16 '16 at 17:21
  • $\begingroup$ Current reactor designs for the US Navy are refueled once every 25 years or so. Once a year applied only to the first generation of shipboard nuclear power plants. $\endgroup$
    – Hobbes
    Mar 16 '16 at 19:05
  • $\begingroup$ I thought I could get away with handwaving since a submarine reactor does not really compare to the one used e.g. in a torchship. It is the order of magnitude that matters IMO. $\endgroup$
    – choeger
    Mar 16 '16 at 19:19
  • $\begingroup$ How do you intend to return it to LEO? Big heat shields? (heavy). Propulsive? (heavy too) $\endgroup$ Mar 16 '16 at 19:29
  • $\begingroup$ Note also that the question is "What happens to a spacecraft crashing into the Moon?". This is not an answer to that. $\endgroup$ Mar 16 '16 at 19:31

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