[ncmncm]

Probably you would use strontium-90, instead, if weight was important. Strontium is 7.5 times less dense than plutonium.

Strontium-90 produces 0.95W/g, where Pu-238 produces only 0.57 W/g. So, it takes only 3/5 as much, by weight, to produce the same heat as a Pu-238 RTG, although it takes 4.5x as much room. Room is relatively cheap on spacecraft, vs. mass.

This makes me wonder why they use Pu-238 RTGs in spacecraft at all. Maybe because it lasts a bit longer?

[ncmncm]

... Probably just because of the industrial process availability of plutonium, as spinoff from weapons work. Sr-90 has no local infrastructure. Which is just dumb.