- Human space exploration is very costly, and far less effective from the point of view of pure planetary and astronomical science. Many scientists (including Carl Sagan) have argued that human space exploration is a waste of science dollars, compared to the payoff from unoccupied probes -- especially as the technology used in those probes continually improves, allowing ever-increasing bandwidth, sensitivity, and autonomy.
- The ocean abyss remains largely unexplored, and is an environment that is in many ways no less unusual than that of space -- but with considerably more interesting life-forms. Self-sustaining ocean habitations would be almost as well-insulated from atmospheric disasters as would off-world colonies.
Human space exploration is needed for reasons other than science, and for scientific reasons other than astronomy and planetary science.
- If we are ever going to learn how to manage our native environment, we need to learn how to live in environments that are less forgiving -- and where waste creates problems on the order of months, days, or hours, rather than centuries.
- If earth were to be rendered uninhabitable by (say) a giant meteor strike, or (more likely) a nuclear war, we would need to have a reserve of humans -- and other species, and independent technological capability -- located off-world, to preserve the most important elements of what had been lost.
- So far, we have only taken from the environment; we have never added to it. Sometimes we find ways of giving back some of what we have taken -- restoring wetlands, replanting trees, saving species from near-extinction -- but we have not made any serious efforts to create ecosystems where none previously existed. A large habitat such as the O'Neill colonies envisioned in the 1970s, or (less expensively) a subterranean colony using natural caverns believed to exist on Luna, could allow enough room for some wild and semi-wild areas. Perhaps these habitats could be entirely automated, but there would be additional benefit in having them enjoyed and maintained by humans (see above points).
All habitable places on earth are currently occupied by existing governments or owned privately; there are no new frontiers. An off-planet colony, while having to bear the costs and risks of not having the larger civilization close at hand, could in theory also have the benefit of being able to make its own laws and regulations with considerably less interference from an established orthodoxy.
The urge to explore is a human universal. The process of discovery is one that enriches and enlivens culture and civilization. As fewer and fewer places on Earth remain unexplored, we are forced to turn that urge inward in ways that are not always productive.