anticipate; their thought processes allow them to distinguish the trivial from the important. Humans are capable of intuitive leaps based on incomplete information. Such an ability enables us to sort out logical from illogical or contradictory information. Humans experienced in field studies can synthesize diverse and disparate field observations, thereby expanding the opportunity for further discovery.

The value of human presence in conducting field work will depend on the inclusion in crews of experienced scientists with relevant scientific judgment and intuition. Their participation is, however, insufficient if they are not given the opportunity to perform as scientists. For example, the plans, procedures, and schedules of geological traverses must be sufficiently flexible to allow scientist-astronauts to modify sampling procedures, time on site, traverse routes, and so on, on the basis of their real-time assessment of in situ observations. To restrict this flexibility is to relegate the scientist-astronaut to the role of a human robot controlled from Earth.

The discussion of the advantages of human presence in planetary exploration is not theoretical: it has been demonstrated on the Apollo lunar missions. 1 Twelve astronauts, in six missions of increasing complexity, conducted tasks ranging from surface sample collection, with associated observations and photographic documentation of the geological context, to drilling and coring of the regolith, to emplacement of geophysical instruments. Photographic documentation of the sample sites proved invaluable in the interpretation of analyses of the returned samples. The astronauts, despite being encumbered by the spacesuits, proved adept at dealing with unforeseen problems such as repairing their roving vehicle and wrestling stuck drill bits and core tubes out of the ground. The geological training of the crews and the (relayed) interaction with the science teams in the Houston “back room” were sufficiently good to prove that excellent science can be accomplished in human exploration. Although the last Apollo mission included a scientist, many of the potential advantages of his presence were negated by the short duration of the mission and its rigid timeline.

As illustrative examples of human exploration activities, four diverse applications are examined that are particularly enhanced by the techniques of field investigation. In no particular order, these are the study of the lunar regolith as a probe of solar history, the search for martian fossil and extant life, determination of the meteorite bombardment history of the inner solar system, and the study of martian climate history. It can obviously be argued that, in theory, any of the discussed field activities could be accomplished robotically given sufficient advances in robotics and an adequate budget. That possibility is not examined here; CHEX's sole purpose is to look at the more useful activities that human explorers might conduct given their presence on the Moon or Mars for reasons other than science.

The committee hastens to note that it does not expect that a few mis-



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