Your question got me curious so I did some googling. The current international agreement is (best I can tell) UNITED NATlONS: GENERAL ASSEMBLY RESOLUTION AND P

Your question got me curious so I did some googling. The current international agreement is (best I can tell) UNITED NATlONS: GENERAL ASSEMBLY RESOLUTION AND PRINCIPLES RELEVANT TO THE USE OF NUCLEAR POWER SOURCES IN OUTER SPACE [December 14, 1992] https://csps.aerospace.org/sites/default/files/2021-08/Principles%20on%20Nuclear%20Power%20Sources%20in%20Space.pdf https://csps.aerospace.org/sites/default/files/2021-08/Princ... I only skimmed, but it says this: > In order to mlnlmlze the quantity of radioactLve materzal in space and the risks Involved, the use of nuclear power sources in outer space shall be restrxcted to those space mLssions which cannot be operated by non-nuclear energy sources in a reasonable way. and > (a) Nuclear reactors may be operated: (i) On interplanetary missions; (ii) In sufficiently high orbits as defined in paragraph 2 (h); (iii) In low-Earth orbits if they are stored in sufficiently high orbits after the operational part of their mission. So it seems like nuclear reactors are restricted, but not completely banned. For an interplanetary mission to Jupiter, it would probably be allowed.