Passive decay is still fission, it's just not artificially sped up fission :)
it's probably more complex than that. Is high speed natural decay of an artificially created or enriched heavy element natural or artificial? I'm not sure how that gets classified.
Decay is not fission. Some time fraction of decays may be spontaneous fissions but as a rule generally nobody calls decays fission reactions unless they are specifically mentioning spontaneous fission reactions.
Artificial and natural is probably not the best terminology to use [0]. What is true is that this will almost certainly contain enrichment of one or more isotopes (NASA uses Pu-238 for some of their missions).
My bad, I had assumed fission was splitting atoms by adding neutrons, but actually fission is just the "splitting" part, so both cases are indeed fission....
The fission process releases neutrons, so you effectively "add neutrons" by concentrating self-fissioning material in a small space.
To put another way:
If you increase the size of a sphere of fissionable material, its rate of "natural" decay stays the same, but more of these decays lead to neutrons triggering additional fission reactions. So doubling the size of the sphere leads to >>2x fission events due to neutrons having more fissile material to hit on the way out.
A large fission reactor is just a scaled up version of this with control rods to adsorb extra neutrons to control secondary "induced" fission.
