Re: how is assembler outdated and by what?
General Electric provided the CPU for Voyager, as they had for the earlier Viking system http://www.cpushack.com/space-craft-cpu.html
The RCA 1802 was launched barely a year before Voyager, so could never have been used. It did see use in NASA in the Magellan and Hubble projects over a decade later, which gives you an idea of how long these projects take to design and launch.
The CPUs in Voyager were 18-bit machines with 64 instructions. They seemed to be a one-off custom job for NASA, as GE never commercially offered a CPU, nor did any other GE minicomputer had such an odd word-length (although its 600-series was 36-bit). This would definitely confirm the "obsolete assembly language" part of the story.