NASA's SNAP-10A satellite operated for 43 days before stopping.
Notes on verification
Multiple independent sources including DOE, Wikipedia, and Astronautix corroborate the 43-day operational period and the non-nuclear electrical cause of shutdown. [cascade flags: wikipedia_dropped_other_sources_exist | tier=silver indep_score=0.925 clusters=2 claim_tier=notable]
Sources
- Inside NASA's audacious plan to build a nuclear-powered ... (seed:science_and_space)
- https://www.energy.gov/etec/articles/lasting-legacy-snap-program-laid-critical-groundwork-space-nuclear-power-systems (corroboration)
- https://www.energy.gov/etec/system-nuclear-auxiliary-power-snap-overview (corroboration)
- http://www.astronautix.com/s/snapshot.html (corroboration)