Indeed, scientist-astronauts have an important role to play in planning, postmission analysis, and preparations for future exploration. Since the end of the Apollo program in 1972, humans have not set foot on another body in the solar system. The Apollo experience involved hundreds of scientists in many disciplines. Although science was not emphasized or well planned at the beginning of the program, Apollo evolved a highly successful mechanism to include scientific input that ultimately produced important scientific results. Participation of scientists in a program of human exploration is a sensitive subject in the broad scientific community.
Some individuals fear that any involvement is an implicit endorsement of such a program.
That is, only science that truly takes unique advantage of human presence should be undertaken and then only if it is of competitive quality. CHEX identified these enabling science issues in its first report 9 and classified them according to their degree of urgency. Critical research issues were defined as those for which inadequate scientific data lead to unacceptably high risks to any program of extended space exploration by humans. Items in this first category include, for example, the effects of prolonged exposure of humans to the microgravity and space radiation environments. Optimal performance issues, the second category, were defined as those that, based on current knowledge, do not appear to pose serious dangers to the health and well-being of humans in space.