Objections

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A corollary to the Fermi paradox—"nobody else is doing it"—is the argument that, because no evidence of alien colonization technology exists, it is statistically unlikely to even be possible to use that same level of technology ourselves.

Colonizing space would require massive amounts of financial, physical, and human capital devoted to research, development, production, and deployment. Earth's natural resources do not increase to a noteworthy extent (which is in keeping with the "only one Earth" position of environmentalists). Thus, considerable efforts in colonizing places outside Earth would appear as a hazardous waste of the Earth's limited resources for an aim without a clear end.

The fundamental problem of public things, needed for survival, such as space programs, is the free-rider problem. Convincing the public to fund such programs would require additional self-interest arguments: If the objective of space colonization is to provide a "backup" in case everyone on Earth is killed, then why should someone on Earth pay for something that is only useful after they are dead? This assumes that space colonization is not widely acknowledged as a sufficiently valuable social goal.

Seen as a relief to the problem of overpopulation even as early as 1758, and listed as one of Stephen Hawking's reasons for pursuing space exploration, it has become apparent that space colonization in response to overpopulation is unwarranted. Indeed, the birth rates of many developed countries, specifically spacefaring ones, are at or below replacement rates, thus negating the need to use colonization as a means of population control.

Other objections include concerns that the forthcoming colonization and commodification of the cosmos may be likely to enhance the interests of the already powerful, including major economic and military institutions e.g. the large financial institutions, the major aerospace companies and the military–industrial complex, to lead to new wars, and to exacerbate pre-existing exploitation of workers and resources, economic inequality, poverty, social division and marginalization, environmental degradation, and other detrimental processes or institutions.

Additional concerns include creating a culture in which humans are no longer seen as human, but rather as material assets. The issues of human dignity, morality, philosophy, culture, bioethics, and the threat of megalomaniac leaders in these new "societies" would all have to be addressed in order for space colonization to meet the psychological and social needs of people living in isolated colonies.

As an alternative or addendum for the future of the human race, many science fiction writers have focused on the realm of the 'inner-space', that is the computer-aided exploration of the human mind and human consciousness—possibly en route developmentally to a Matrioshka Brain.

Robotic spacecraft are proposed as an alternative to gain many of the same scientific advantages without the limited mission duration and high cost of life support and return transportation involved in human missions. However, there are vast scientific domains that cannot be addressed with robots, especially biology in specific atmospheric and gravitational environments and human sciences in space.

Another concern is the potential to cause interplanetary contamination on planets that may harbor hypothetical extraterrestrial life.

Colonialismedit

Space colonization has been discussed as continuation of imperialism and colonialism. Questioning colonial decisionmaking and reasons for colonial labour and land exploitation with postcolonial critique. Seeing the need for inclusive and democratic participation and implementation of any space exploration, infrastructure or habitation.

The narrative of space exploration as a "New Frontier" has been criticized as unreflected continuation of settler colonialism and manifest destiny, continuing the narrative of colonial exploration as fundamental to the assumed human nature. Also narratives of survival and arguments for space as a solution to global problems like pollution have been identified as imperialist.

The predominant perspective of territorial colonization in space has been called surfacism, especially comparing advocacy for colonization of Mars opposed to Venus.

It has been argued that the present politico-legal regimes and their philosophic grounding advantage imperialist development of space.

Physical, mental and emotional health risks to colonizersedit

The health of the humans who may participate in a colonization venture would be subject to increased physical, mental and emotional risks. NASA learned that – without gravity – bones lose minerals, causing osteoporosis. Bone density may decrease by 1% per month, which may lead to a greater risk of osteoporosis-related fractures later in life. Fluid shifts towards to the head may cause vision problems. NASA found that isolation in closed environments aboard the International Space Station led to depression, sleep disorders, and diminished personal interactions, likely due to confined spaces and the monotony and boredom of long space flight. Circadian rhythm may also be susceptible to the effects of space life due to the effects on sleep of disrupted timing of sunset and sunrise. This can lead to exhaustion, as well as other sleep problems such as insomnia, which can reduce their productivity and lead to mental health disorders. High-energy radiation is a health risk that colonizers would face, as radiation in deep space is deadlier than what astronauts face now in low Earth orbit. Metal shielding on space vehicles protects against only 25-30% of space radiation, possibly leaving colonizers exposed to the other 70% of radiation and its short and long-term health complications.