The Dimming of Starlight
Supporters of space exploration are often perplexed by the failure of their fellow citizens to grasp the significance and fascination of exploring the heavens. In the words of Arthur C. Clarke: "The urge to explore, to discover, to follow knowledge like a sinking star" is its own justification. This self-justification is presumably rooted in human nature or human destiny. As the Norwegian explorer Fridtjof Nansen once said, "The history of the human race is a continuous struggle from darkness to light. It is therefore of no purpose to discuss the use of knowledge - man wants to know and when he ceases to do so he is no longer man." 
For supporters of exploration such as these, the matter is quite simple. Since knowledge is of the essence for man, man cannot help wanting to explore. Thus it is destiny that propels us forward into the cosmos. Under such circumstances there is little point in bothering with a more extensive justification. We need only grab history by the tail and make it go where we wish to take it. Why should philosophy force us into reflection where action is so tempting?
One reason for reflection is that appeals to human nature and destiny are not convincing enough. In saying that it is in our nature to explore or that man's destiny is in the stars, we can mean several things, but some interpretations are more central than others. We can mean that
(1) We are going to explore space come what may,
(2) Because of our nature we have a strong tendency to explore space,
(3) Our nature is such that in some important sense we will not fulfill ourselves if we do not explore space.
Let us see how these interpretations fare in the task of making clear that space exploration is a worthwhile undertaking.
From the mere fact that we are going to explore come what may it does not follow that exploring space is to be recommended. We are all going to die come what may, but many of us do not think highly of the prospect. That something is inevitable does not make it good.
The claim that humans have a strong disposition or inclination to explore space does not fare much better. If the claim is true, and I suppose that it is, we may be interested to know why. But if we wish to know whether the exploration of space is wise, we have not yet moved from our starting point. After all, suppose that men had a strong disposition to rape. Should we condone rape then? Should we yield to it or, if no longer able, encourage its exercise by the strong of body and heart? Surely not!
Appeals to nature or evolution do not get us very far. For it would be a mistake to think that any trait with which nature has endowed our species cannot fail to be commendable. The trait in question might be an adaptation to an environment that no longer exists, and thus it may no longer serve us well. The fate of most species that ever lived has been marked by the development of characteristics well suited to one environment but woefully inadequate when the environment changed. Hence they became extinct. Imagine for the sake of argument that our species has a disposition toward war. In these days of nuclear weapons, that is clearly a bad disposition to have. Thus, for the purposes of justification, it is not enough to show that a natural disposition exists: We must show that it is a good disposition. Therefore, to justify space exploration, it does not suffice to learn that man's nature tempts him to the heavens.
This third claim, that because of our nature we will go unfulfilled unless we explore, is perhaps more promising. It would have to specify, however, in what important ways space exploration might fulfill us. To become convincing, it would have to go beyond slogans and platitudes: it would have to supply strong evidence for the conclusion. It would require, that is, a good argument after all. This result thus returns us to the problem the supporters of space exploration face.
If those supporters hope to win over their fellow citizens, they should pay greater attention to why their opponents resist their entreaties. True argumentation cannot take place in a vacuum: You may not always be able to convince open-minded people, but you improve your chances when you consider seriously what keeps these people from coming over to your side. This is why impatience tends to be rhetorically self-defeating. Supporters should start, then, with the objections critics offer against space exploration.
. Arthur C. Clarke, "The Challenge of the Spaceship," Journal of the British Interplanetary Society, December 1946, p. 68. With great honesty he wrote: "Any ‘reasons’ we may give for wanting to cross space are afterthoughts, excuses tacked on because we feel we ought, rationally, to have them. They are true but superfluous - except for the practical value they may have when we try to enlist the support of those who may not share our particular enthusiasm for astronautics. . . . The search for knowledge, said a modern Chinese philosopher, is a form of play. Very well: we want to play with spaceships."
. As quoted in The Impact of Space Science on Mankind, T. Greve, F. Lied, and E. Tandberg, eds., Plenum Press,1976., p 13.
. Actually, if most men, let alone all men, had a disposition to rape, we might suspect that they had it by virtue of being (male) humans. This, of course, would not justify it.
. Some philosophers may be disappointed by my failure to accuse these supporters of exploration of committing the "naturalistic fallacy." In saying that since exploring is part of our nature we need no further justification, the supporters might be thought to claim that "being in one's nature" is sufficient justification to show that exploration is a good thing. And this claim the philosophers would interpret as deducing values from facts (or prescriptions from descriptions, or an "ought" from an "is") – a deduction that presumably amounts to a fallacious inference: the naturalistic fallacy. Some ideological critics also can be thought to commit the fallacy when they charge that the practice of science disrupts the harmony of nature. I am not quick to make such accusations, however, because they fit only the most simplistic and often distorted version of the position under discussion. My impression is that most of the "classic" examples of the naturalistic fallacy are nothing but arguments that were taken out of context, oversimplified, and distorted so as to serve as illustrations of a neat "logical" point. A more detailed discussion of this claim belongs in a different kind of work (see my "Review of Peter Singer's The Expanding Circle," Explorations in Knowledge, Spring 1987, p.43; “Evolution and Justification,” The Monist, Vol. 71, No. 3, 1988; and especially “The Morality of Rational Ants,” Ch. 11 of my Evolution and the Naked Truth, Ashgate, 1998. At any rate, we can see in the text below that the supporters of exploration can, with some thought, express their insight in terms of fulfillment – a move that does not strike me as fallacious.
 I am sympathetic to this intuition, as will be seen throughout this book, although I am critical of some of the ideas involved in what some have called the “space imperative,” which is discussed by G. Genta and M. Rycroft in their book Space, the Final Frontier? Cambridge University Press, 2003. Indeed, some versions of the space imperative fall under the interpretations 1 and 2 criticized above.