Second, human manipulative and observational skills can rarely be used effectively without the support of a large array of sophisticated instruments, machines, and facilities. Neither of these two conditions is generally met by brief forays of human beings into regions far from the facilities that support their sustained presence. This applies to manned excursions to Mars, for example, if the human-inhabited sphere is restricted to space near the Earth. Further, it is important to recognize that the limitations on human survival in space are not well known. At present, we are not certain that mission times can be extended greatly beyond those already experienced, even with considerable technological progress.
Low gravity leads to loss of bone mass and other physical effects.
High-energy, heavy ion radiation causes irreversible damage to cells, including brain cells. Human relationships in a small, isolated group can badly deteriorate and lead to the loss of functional capabilities. We have not demonstrated the feasibility of a closed ecological system yet, and resupply at a great distance for a long period could be formidable. We must address these issues before we can reach a final decision about the nature and extent of human involvement in expanding the frontier of space.