Complex sentence: In NASA vernacular, a Payload Specialist (PS) was a Space Shuttle crewmember who was selected for a single specific mission, by opposition to a Mission Specialist, who is selected as an astronaut first and is then attributed to a mission.
Split into simpler sentences: In NASA vernacular, a Payload Specialist (PS) was a Space Shuttle crewmember who was selected for a single specific mission. In contrast, a Mission Specialist is selected as an astronaut first and is then attributed to a mission.