Original link: NASA still maintains some of the Voyager spacecraft code in a 1970s-era programming language that almost nobody on Earth fully understands anymore, and the handful of engineers who do are now in their 80s / SpaceDaily.
Note that it is not any assembler, but assembler for "purpose-built General Electric interrupt-driven processors, designed and fabricated in the early 1970s".
Fortran would be sligtly easier. There must still be some around that know Fortran 66.