My understanding is that it&#x27;s primarily a safety issue: these long-duration missions tend to have to get delta-V boosts by slingshotting around the Earth. (Rosetta did this <i>three times</i>. And a slingshot around Mars once.) While the RTG capsule is robust enough to survive a launch failure intact, it&#x27;s <i>not</i> robust enough to survive reentry at interplanetary speeds if the slingshot goes wrong. It&#x27;s not like we don&#x27;t have tonnes of vapourised plutonium in the atmosphere anyway, but that&#x27;s no excuse for adding more.<p>Plus, the kind of plutonium that goes into an RTG is fabulously expensive, not just because it doesn&#x27;t occur in nature and has to be made via transmutation, but because it makes handling so much more complicated.<p>They&#x27;ll have done the cost-benefit analysis and decided it simply wasn&#x27;t worth it.<p>(Also, RTGs are <i>big</i> and <i>heavy</i>. New Horizons, which is a really small spacecraft, has a 50kg RTG. Philae only massed 21kg.)