American plutonium 238 was formerly produced using Savannah River Site reactors that primarily produced materials for nuclear weapons. With the retirement of those reactors in 1988, and the American nuclear weapons program going to maintenance mode, NASA lost the side-benefit of Pu-238 production using the weapons infrastructure. Deep space missions requiring RTGs had to subsist off of historical Pu-238 stockpiles and additional material purchased from Russia. But the Russian supply has run out too now -- apparently they aren't producing more of it either.
American plutonium 238 production efforts resumed in 2013:
http://www.spacesafetymagazine.com/aerospace-engineering/nuc...
With NASA now paying the full cost, including fixed startup costs, Pu-238 is extraordinarily expensive:
NASA and DOE have estimated the rebooting will cost between $75 million and $90 million over five years. According to NASA officials, the agency expects to have 1.5 to 2 kilograms produced per year, starting 2018.
The high cost and limited supplies of Pu-238 have spurred the search for alternatives to Pu-238 RTGs. A few years ago I remember reading that the European Space Agency was going to try to design its own RTGs using the less powerful but more abundant americium 241 instead of plutonium 238. But a quick search just now doesn't show any concrete development effort.
This Kilopower reactor is an alternative to RTGs for some mission profiles -- in fact offers more power than RTGs and uses cheaper nuclear materials. (Highly enriched U-235 isn't cheap in an absolute sense, but it's far less expensive than Pu-238. And the security of supply is effectively backstopped by its use in reactors for the US Navy.) It can offer ample power for very deep space missions at a cost significantly less than using years' worth of Pu-238 production.
Improved photovoltaic cells have also enabled missions further from the Sun than they could have supported in 1988. The Juno mission, which entered Jupiter orbit in 2016, relies on PV instead of RTGs. It seems plausible that further evolution will eventually push PV's reach out to Saturn missions. But PV is not currently plausible for missions beyond Jupiter, and it is reliant on slowly-evolving battery technology for surface missions on Mars. Martian missions using PV also face significant problems from cell-obscuring dust. This reactor seems too large to be conveniently integrated in a Martian rover, but enabling non-surface missions to avoid Pu-238 use may mean more can be reserved for future rovers akin to the RTG-powered Curiosity.