Argument: Space exploration produces many valuable technological innovations that benefit all of human society:

Space exploration and research have resulted in many major advances in science and technology. Everything from Velcro to more efficient and powerful computers has come out of the space program1. The technological advances produced by the space program would not have been possible were it not for the intensity of focus on the paradigm of exploration. That same paradigm has come to permeate scientific enquiry generally, pushing scientists to seek new answers and to develop new technologies. So long as mankind keeps pushing the barriers of its own knowledge, it will never stagnate, and human understanding of the Universe will continue to grow. Should humanity, however, take an insular view of itself and turn back on a history of pushing of boundaries, the paradigm of progress might dissolve as well. 1 Coalition for Space Exploration. 2010. "Benefits of Space

Candidate 1: "Extraterrestrials might prove unintelligible, or even hostile toward humanity:

Were extraterrestrials to actually be contacted, an unlikely event in itself, the ability to share in any form of meaningful communication might well prove more difficult than might first seem. The extraterrestrials will have evolved on a different world, perhaps along lines so alien to that of Earth's life that it would be utterly impossible to understand what they said, even if they could be heard. Language is built largely on frames of reference, and when an extraterrestrial, evolving on an entirely alien world, perhaps possessing an entirely alien thought process, and certainly having an entirely alien linguistic frame of reference, no level of communication might be possible. With no conceptual similarities or cognitive common ground, the potential for sharing knowledge between species would likely prove impossible to decipher. They could simply be too alien. Furthermore, the hope that technologically advanced extraterrestrials would be friendly towards humanity is entirely unfounded. They might well prove hostile, and if they are more technologically advanced, they could prove an existential threat to the survival of mankind1. This is especially true if the extraterrestrials were actually capable of physically reaching Earth, which would require an understanding of physics and engineering centuries ahead of that of mankind. Welcoming such creatures to Earth could well spell ruin for humanity, as so advanced a civilization might well consider humanity to be of no higher an order of life than we might consider insects. It is far better than humanity not make such a habit of broadcasting its position. It could prove dangerous, especially if the Universe is not as friendly a place as scientists hope.

1 Brin, David. 2006. "Shouting at the Cosmos". Lifeboat Foundation."
Candidate 2: "The benefits accrued from spin-off technology resulting from space exploration are generally overstated. NASA, for example, had claimed that protein crystals could be grown in zero gravity that could fight cancer, as well as numerous other claims of benefits. Most of these benefits have never materialized. With all the billions of dollars wasted on space exploration and trying to contact extraterrestrials, most of the spin-off technologies could likely have been created independently, given the resources, and probably at lower overall expense. As to the paradigm of exploration, efforts to explore parts of space, as well as our own planet would continue. The paradigm is not shattered by the choice to take a cautious approach toward extraterrestrial life, which is likely a waste of effort anyway."
Candidate 3: "The appreciation for complex life by all reasoning should be universal among intelligent species. It seems intelligent life is a rarity in the Universe, and thus it would be unlikely for any civilization, no matter how advanced beyond our own, not to appreciate the advent of complex life on Earth and the value of humanity, flawed and inferior as it might appear to them1. The time energy necessary to traverse the stars in order to reach Earth would only be worth spending if it were to a peaceful end. War would yield only the resources of this small planet, hardly the spoils worth an interstellar war, however one-sided it might prove. Human civilization, as short as its span has been, has developed far in terms of both physical and social sciences. The wealth of knowledge that might be had from contacting extraterrestrials is too great an opportunity to pass up. 1 Sagan, Carl. 1973. Communication with Extraterrestrial Intelligence. Cambridge: MIT Press."
Candidate 4: "Attempts to contact extraterrestrial life are a waste of time and money:

Billions of dollars have been spent by a number of countries, principally the United States, on great projects seeking to make contact and signal extraterrestrials. None of these has received so much as a peep in reply. The reason for this is likely that intelligent life is an extreme rarity, with humanity its only exemplar in this part of the galaxy1. If there were intelligent life within receiving range of Earth-based transmissions, the extraterrestrials would have had ample opportunity to respond, or at least make their presence known. The fact that they have not suggests that there are no extraterrestrials within contactable range, or at least none with any interest in talking to Earthlings. If there were extraterrestrials on more distant planets, efforts to contact them would be pointless, as they would be so far away that communication would take many years and would be unreachable physically. Furthermore, the search for extraterrestrials violates reason. Money should only be spent on projects after phenomena worth analyzing have been detected. There is no evidence that intelligent lie other than our own exists at all. Trying to contact little green men in space is just a waste of time. 1 Ward, Peter. 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Philadelphia: Springer."
Candidate 5: "Contact with an extraterrestrial civilization more advanced than our own could lead to mass existential crises, putting the existence of human civilization at risk:

Almost every human belief system, religious or secular, is based on an anthropocentric outlook. Humanity is the collective center of its Universe; the cognitively aware being that can interact with physical reality not simply by impulse, but by self-aware, conscious agency. Human belief in itself is based upon its conception of dominion over the physical world. Mankind shapes its own environment; while weaker, slower, and smaller than many other species, the intelligence of Man makes Him the apex predator. Mankind's image of itself is compromised by the existence of other intelligent life, especially more advanced intelligent life. It seems that most religious belief systems could not effectively survive with such knowledge, since the existence of intelligent, advanced extraterrestrials seems to imply the nonexistence of a creator God with any active interest in humanity over any other species1. The realization that we are not the center of the Universe could shake many people to their cores, particularly the religious, many of whom would likely find great difficulty coming to terms with that reality. It would be better that humans not seek out such revelations about the Universe. If intelligent life does exist elsewhere in the Universe, better not to invite it to Earth. The cost to people's beliefs and sense of being is too high. 1 Peters, Ted. 2011. "The Implications of the Discovery of Extra-Terrestrial Life for Religion". Philosophical Transactions of the"
Candidate 6: "The focus of states and individuals should be on fixing the problems of this planet, not with exploring other ones:

The Earth is faced with many problems that people should be focusing their efforts on addressing, not on the stars and what may or may not be out there. Global warming, the destruction of ecosystems, rising sea levels, pollution, poverty, and resource depletion are all issues weighing heavily on states and the international community as a whole. Individuals and governments need to rally and fight these growing terrestrial problems1. The resources poured into space exploration and the contacting of extraterrestrials, which will likely serve no lasting purpose, would be better spent in combating the hundreds of serious issues facing the planet today. The search for extraterrestrials serves only as a distraction, keeping people's minds off the pressing concerns of the Earth. To make things worse, governments use manned space flight as a means of distraction quite deliberately. It is often easier to devote attention and resources to headline-grabbing endeavors like efforts to contact extraterrestrials than to address concerns like global warming, which requires extensive international coordination to a degree rarely reached in history. As is shown by developing countries like China and India having space programs while helping to block progress on climate talks and while they still have millions in poverty. Governments may find utility in keeping people focused on such grand projects while doing comparably little to affect change where it is direly needed. Clearly, humanity's concerns should be focused wholly on the survival of its home world, not on trying to get in touch with worlds that might not even exist, and almost certainly cannot sustain human life. 1 Carreau, Mark. 2009. "NASA Urged to Keep Feet On Earth". The Chronicle."
Candidate 7: "Certainly there are many concerns on Earth that must be addressed if mankind hopes to survive. Pollution and climate change are real threats that deserve a great deal of attention from all governments around the world, since everyone is affected. This attention, however, does not need to exclude from the agenda the effort to contact extraterrestrials. Rather, states can focus on more than one science-based issue at a time. It is rather disingenuous, in fact, to suggest that it is a matter of one or the other. Furthermore, the international unity created by mankind's collective search for intelligent life in space, binding people not as members of separate nations, but as members of the human race, can serve as a means of promoting unified action on issues affecting the whole planet. Space exploration and the search for intelligent life among the stars can thus actually help to ameliorate coordination problems between states in their efforts to deal with global issues. Clearly, when people think of themselves as citizens of the world they are more ready to think globally and consider issues affecting other countries. The search for extraterrestrials is a very real means of promoting such thinking."
Candidate 8: "The search for extraterrestrial life does not solve the problem of the human propensity toward group distinctions. Even if such projects succeeded in fostering a sense of universal humanity, it only does so at the cost of creating a new in group-out group dynamic, namely humans defining themselves against the out group that is extraterrestrial life. Such redefinition of identity could cause serious problems in the extremely unlikely event that humans ever actually do make contact with extraterrestrial life."

The benefits accrued from spin-off technology resulting from space exploration are generally overstated. NASA, for example, had claimed that protein crystals could be grown in zero gravity that could fight cancer, as well as numerous other claims of benefits. Most of these benefits have never materialized. With all the billions of dollars wasted on space exploration and trying to contact extraterrestrials, most of the spin-off technologies could likely have been created independently, given the resources, and probably at lower overall expense. As to the paradigm of exploration, efforts to explore parts of space, as well as our own planet would continue. The paradigm is not shattered by the choice to take a cautious approach toward extraterrestrial life, which is likely a waste of effort anyway.
