NASA's Payload Specialist program has been criticized for giving limited Shuttle flight positions to civilian aerospace engineers such as Cenker and Greg Jarvis (killed aboard "Challenger"), politicians such as Bill Nelson, and others civilians such as Teacher in Space Christa McAuliffe (also killed aboard "Challenger"). Even the flight of former Mercury astronaut and US Senator John Glenn was questioned. The concern was that these people had replaced career astronauts in very limited flight opportunities, and some may have flown without fully understanding the level of danger involved in a Shuttle mission.