TVDad Jim wrote:

Also, what's the etymology of Agena as being used for the upper-stage
name? As far as I know, Agena is a 1st-magnitude star in Centaurus,
and its name means "the knee."

ARPA proposed to name this vehicle for a star because, as an upper
stage, it didn't ignite until it reached high altitude and would look
like a new star. Lockheed liked the idea because many of their other
projects were named after stellar phenomena.

The various stages of the Scout launch vehicle were also named after stars.

--
Dave Michelsn