Complex sentence: This was the first in what would be a long career for the Atlas as a satellite launcher, most with an added spin-stabilized solid rocket motor upper stage for polar orbit military payloads.
Split into simpler sentences: This was the first in what would be a long career for the Atlas as a satellite launcher. Many retired Atlas ICBMs would be used as launch vehicles, many with an added spin-stabilized solid rocket motor upper stage for polar orbit military payloads.