Since, in the long run, every planetary civilization will be endangered by impacts from space, every surviving civilization is obliged to become spacefaring — not because of exploratory or romantic zeal, but for the most practical reason imaginable: staying alive... If our long-term survival is at stake, we have a basic responsibility to our species to venture to other worlds.

Since, in the long run, every planetary society will be endangered by impacts from space, every surviving civilization is obliged to become spacefaring — not because of exploratory or romantic zeal, but for the most practical reason imaginable: staying alive.

A civilization that is expanding through space will not need our few worlds and will probably be too intellectually advanced to feel the need to batter its way into hegemony here. If, however, we are surrounded by a more lively, a more vibrant civilization, we will wither away by the mere force of the comparison; we will die of the realization of what we have become and of the potential we have wasted.

The choice is with us still, but the civilization now in jeopardy is all humanity. As the ancient myth makers knew, we are children equally of the earth and the sky. In our tenure on this planet we've accumulated dangerous evolutionary baggage — propensities for aggression and ritual, submission to leaders, hostility to outsiders — all of which puts our survival in some doubt. But we've also acquired compassion for others, love for our children and desire to learn from history and experience, and a great soaring passionate intelligence — the clear tools for our continued survival and prosperity. Which aspects of our nature will prevail is uncertain, particularly when our visions and prospects are bound to one small part of the small planet Earth. But up there in the immensity of the Cosmos, an inescapable perspective awaits us. There are not yet any obvious signs of extraterrestrial intelligence and this makes us wonder whether civilizations like ours always rush implacably, headlong, toward self-destruction. National boundaries are not evident when we view the Earth from space. Fanatical ethnic or religious or national chauvinisms are a little difficult to maintain when we see our planet as a fragile blue crescent fading to become an inconspicuous point of light against the bastion and citadel of the stars. Travel is broadening.

No civilization can possibly survive to an interstellar spacefaring phase unless it limits its numbers.
Any society with a marked population explosion will be forced to devote all its energies and technological skills to feeding and caring for the population on its home planet. This is a very powerful conclusion and is in no way based on the idiosyncrasies of a particular civilization. On any planet, no matter what its biology or social system, an exponential increase in population will swallow every resource. Conversely, any civilization that engages in serious interstellar exploration and colonization must have exercised zero population growth or something very close to it for many generations.

On Earth, humankind can step onto another continent, and without a thought, destroy the kindred civilizations found there through warfare and disease. But when they gaze up at the stars, they turn sentimental and believe that if extraterrestrial intelligences exist, they must be civilizations bound by universal, noble, moral constraints, as if cherishing and loving different forms of life are parts of a self-evident universal code of conduct. I think it should be precisely the opposite: Let’s turn the kindness we show toward the stars to members of the human race on Earth

In the vastness of the Cosmos there must be other civilizations far older and more advanced than ours.

Thousands of years ago, tribes of human beings suffered great privations in the struggle to survive. In this struggle it was important not only to be able to handle a club, but also to possess the ability to think reasonably, to take care of the knowledge and experience garnered by the tribe, and to develop the links that would provide cooperation with other tribes. Today the entire human race is faced with a similar test. In infinite space many civilizations are bound to exist, among them civilizations that are also wiser and more "successful" than ours. I support the cosmological hypothesis which states that the development of the universe is repeated in its basic features an infinite number of times. In accordance with this, other civilizations, including more "successful" ones, should exist an infinite number of times on the "preceding" and the "following" pages of the Book of the Universe. Yet this should not minimize our sacred endeavors in this world of ours, where, like faint glimmers of light in the dark, we have emerged for a moment from the nothingness of dark unconsciousness of material existence. We must make good the demands of reason and create a life worthy of ourselves and of the goals we only dimly perceive.

Statistically likely that there are more advanced alien civilizations out there. Hopefully they're good environmentalists and find us cute.

It is pointless to worry about the possible malevolent intentions of an advanced civilization with whom we might make contact. It is more likely that the mere fact they have survived so long means they have learned to live with themselves and others. Perhaps our fears about extraterrestrial contact are merely a projection of our own backwardness, an expression of our guilty conscience about our past history: the ravages that have been visited on civilizations only slightly more backward than we.

Personally, I don’t think there's intelligent life on other planets. Why should other planets be any different from this one?

The great civilizations have to join together and result in new civilizations. The current civilizations will be cast away and they finally die like the disabled aged while they pave the way for the fresh newly born replacements. Similarly, the descendants of these new civilizations will do their best to fill the gap between the internal and external heritages and rescue the long forgotten spirituality; they will bring the past into the future.

Once the threshold is crossed when there is a self-sustaining level of life in space, then life's long-range future will be secure irrespective of any of the risks on Earth (with the single exception of the catastrophic destruction of space itself). Will this happen before our technical civilisation disintegrates, leaving this as a might-have-been? Will the self-sustaining space communities be established before a catastrophe sets back the prospect of any such enterprise, perhaps foreclosing it for ever? We live at what could be a defining moment for the cosmos, not just for our Earth.

Persons such as Isaac Asimov and Carl Sagan say that tens of thousands (maybe tens of millions) of planets will fulfill the conditions for the support of life. And then they take the rather deceptive step from the ‘possibility of life’ to the ‘inevitability of life’ by such connivance as would shame a crooked gambler. They posit towering numbers of ‘civilizations’ on those ‘possibility-of-life planets’, at least half of them to be more advanced than the Civilization of Earth and Humankind.
But there is a strong element of Advocacy Science in this. There is a great and powerful lobby advocating the existence of great numbers of superior civilizations. One reason for this is that the secular-liberal-agnostic-relativistic faction of scientists cannot allow the uniqueness of anything, not of Earth, not of Life, certainly not of Human Life, most certainly not of existing Human Civilization. To allow the uniqueness of any of these things, they would have to cease to be secular-liberal-agnostic-relativistic persons. And the shock of changing their style would kill all of them.
Science Fiction also has a vested interest in there being a multiplicity of inhabited worlds and civilizations. That is one of the small number of things that Science Fiction is about. But Science Fiction is, after all, only a fiction.