[jmyeet]

Historically, PU-238 was a byproduct of (fissile) Pu-238 production for nuclear weapons. That need kinda went away with arms reduction treaties beginning in the mid-to-late Cold War. IIRC the US nuclear weapons stockpile is about 10% of what it was at its peak. AFAIK there is no current Pu-239 production in the US.

The need for Pu-238 was recognized years ago as the stockpile of consumed by various space probes and I believe Oak Ridge now products Pu-238 fuel pellets. I'm not sure if this production could be ramped up.

But Plutonium use has various risks associated with it. Aside from the obvious security risks, you're strapped it to a rocket that may explode while launch and come back to Earth. This is effectively a dirty bomb if the RTG containment is breached.

Solar panels are really the right choice for anything out to at least mars. Here we had a probe fall over. Would that be recoverable with RTG? Maybe. Maybe not.