I was looking for this everywhere and realized that the site hosting this text had went down (I don’t know if permanently), so I’ll place it here for anyone who wants to share it.
Originally from: http://www.spaext.com/info/sagan/index.html
Carl Sagan (1934–1996) was a professor of astronomy and space sciences at Cornell University.
Extinction hazards force civilizations to pursue spaceflight
Since hazards from asteroids and comets must apply to inhabited planets all over the Galaxy, if there are such, intelligent beings everywhere will have to unify their home worlds politically, leave their planets, and move small nearby worlds around. Their eventual choice, as ours, is spaceflight or extinction.
Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space (New York: Random House, 1994), 327.
We have the technology to cause extinction or ensure survival
Due to our own actions or inactions, and the misuse of our technology, we live at an extraordinary moment for the Earth at least—the first time that a species has become able to wipe itself out. But this is also, we may note, the first time that a species has become able to journey to the planets and the stars. The two times, brought about by the same technology, coincide—a few centuries in the history of a 4.5-billion-year-old planet.
The rationale for spaceflight is survival
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.
The more places we inhabit, the safer we will be
If we were up there among the planets, if there were self-sufficient human communities on many worlds, our species would be insulated from catastrophe. The depletion of the ultraviolet-absorbing shield on one world would, if anything, be a warning to take special care of the shield on another. A cataclysmic impact on one world would likely leave all the others untouched. The more of us beyond the Earth, the greater the diversity of worlds we inhabit, the more varied the planetary engineering, the greater the range of societal standards and values—then the safer the human species will be.
Human spaceflight is relatively inexpensive
A serious effort to send humans to other worlds is relatively so inexpensive on a per annum basis that it cannot seriously compete with urgent social agendas on Earth.
Earthlife is the only life in the solar system
But as nearly as we can tell, so far at least, there is no other life in this system, not one microbe. There’s only Earthlife.
In that case, on behalf of Earthlife, I urge that, with full knowledge of our limitations, we vastly increase our knowledge of the Solar System and then begin to settle other worlds.
Survival is the key argument for human missions
These are the missing practical arguments: safeguarding the Earth from otherwise inevitable catastrophic impacts and hedging our bets on the many other threats, known and unknown, to the environment that sustains us. Without these arguments, a compelling case for sending humans to Mars and elsewhere might be lacking. But with them—and the buttressing arguments involving science, education, perspective, and hope—I think a strong case can be made. If our long-term survival is at stake, we have a basic responsibility to our species to venture to other worlds.
There is a race between harmful technologies and beneficial technologies
The technologies that threaten us and the circumvention of those threats both issue from the same font. They are racing neck and neck.
It is the beginning of history, not the end
In more than one respect, exploring the Solar System and homesteading other worlds constitutes the beginning, much more than the end, of history.
Colonization of space is the next step in evolution
When we first venture to a near-Earth asteroid, we will have entered a habitat that may engage our species forever. The first voyage of men and women to Mars is the key step in transforming us into a multiplanet species. These events are as momentous as the colonization of the land by our amphibian ancestors and the descent from the trees by our primate ancestors.
Colonization of new environments is usually difficult and dangerous
Fish with rudimentary lungs and fins slightly adapted for walking must have died in great numbers before establishing a permanent foothold on the land. As the forests slowly receded, our upright apelike forebears often scurried back into the trees, fleeing the predators that stalked the savannahs. The transitions were painful, took millions of years, and were imperceptible to those involved. In our case the transition occupies only a few generations, and with only a handful of lives lost. The pace is so swift that we are still barely able to grasp what is happening.
The beginning of new worlds does not mean the end of Earth
But inhabiting other worlds does not imply abandoning this one, any more than the evolution of amphibians meant the end of fish. For a very long time only a small fraction of us will be out there.
Peopling other worlds is a selfless act
But as for a long-term goal and a sacred project, there is one before us. On it the very survival of our species depends. If we have been locked and bolted into a prison of the self, here is an escape hatch—something worthy, something vastly larger than ourselves, a crucial act on behalf of humanity.
Future generations will remember our actions
They will gaze up and strain to find the blue dot in their skies. They will love it no less for its obscurity and fragility. They will marvel at how vulnerable the repository of all our potential once was, how perilous our infancy, how humble our beginnings, how many rivers we had to cross before we found our way.