S/360 I knew you well
The S/390 name is a hint to its lineage, S/360 -> S/370 -> S/390, I'm not sure what happened to the S/380. Having made a huge jump with S/360 they tried to do the same thing in the 1970s with the Future Systems project, this turned out to be a huge flop, lots of money spent on creating new ideas that would leapfrog the competition, but ultimately failed. Some of the ideas emerged on the System/38 and onto the original AS/400s, like having a query-able database for the file system rather than what we are used to now.
The link to NASA with the S/360 is explicit with JES2 (Job Execution Subsystem 2) the element of the OS that controls batch jobs and the like. Messages from JES2 start with the prefix HASP, which stands for Houston Automatic Spooling Program.
As a side note, CICS is developed at Hursley Park in Hampshire. It wasn't started there though. CICS system messages start with DFH which allegedly stands for Denver Foot Hills. A hint to its physical origins, IBM swapped the development sites for CICS and PL/1 long ago.
I've not touched an IBM mainframe for nearly twenty years, and it worries me that I have this information still in my head. I need to lie down!