Do you have an answer? A discussion of all the myriad economic benefits that will come from space travel and exploration won't make a dent is such a simplistic anti-space sentiment. Pointing out all the benefits (read spin-offs) that have come from the space program to date will do no good either. In fact, spin offs, no matter their economic worth, are countered by the observation of many anti-space people, "Well, if we had researched that directly, that benefit could have come about more directly and cheaper." On the face of it, that is a very valid counter argument, for virtually every spin off from space research that has occurred would have come about cheaper and faster if there had been directed research to go after the particular benefit or spin off. The mere fact that such directed research would not have taken place doesn't mean much; it could have.
Most anti-space arguments are broad and general, and somewhat philosophical in nature, as the above. Economics, spin offs, and other such materialistic arguments can't sway someone who believes that money is being wasted in space while people are starving on Earth.
Civilization will collapse and the human race will become extinct if we don't expand into space. That bears repeating, so, CIVILIZATION WILL COLLAPSE AND THE HUMAN RACE WILL BECOME EXTINCT IF WE DON'T EXPAND INTO SPACE. Shout it out; a pro-space argument cannot get much more powerful than that, and many anti- space advocates provide us with supporting arguments.
To fully delve into this line of reasoning, you first need to be familiar with two related items: Drake's Equation (Greenbank Equation in Britannica) and The Fermi Paradox. Drake's Equation is a scientific way of guessing how many intelligent civilizations might exist in a particular galaxy at any one time. (I use the word "guessing" rather than "estimating", because estimating implies that different groups of people using the same starting point ought come up with answers that are close in magnitude. Such is not the case here.) The Fermi Paradox results from believing that Drake's Equation provides a solution that at any one time for a galaxy is greater then or equal to one. If even one predecessor civilization more than five or so million years older then our own had developed space travel, there would (or should) not only be abundant evidence of them in the skies, but they would (or should) physically be here by now. They are not here. That's the crux of The Fermi Paradox. If you believe that intelligent life is common, then you run into the difficult problem of trying to explain where the extraterrestrials are. Whereas the absence of evidence isn't usually evidence, the absence of evidence in this case where there should be overwhelming presence of evidence is at the least, worrisome.
If you head towards the SETI section of your local library and start researching this area, you'll find that scientists of all stripes fall into two general groups regarding ET's- Those who believe that life is common and that intelligence naturally arises from life, and therefore, ET civilizations are common, and those who believe that intelligence is so exceedingly unlikely that there may even be fewer then one intelligent species per galaxy per universe lifetime. There really is no middle ground; life, and intelligent life, is either common, or very, very rare. Those who believe that intelligent life is common concoct all sorts of explanation, ranging from the truly bizarre zoo hypothesis, in which we, the human race, are a nature preserve to be left in its natural wild state, to the only slightly bizarre, where intelligence and technology are decoupled, and intelligence without technology is postulated.
In all the readings I have recently done on the subject, both camps failed to mention something that is commonly believed and talked about in the pro-space movement, that I think was first put into print by Robert Heinlein: "A civilization or species that fails to develop space travel becomes extinct." This is a self-evident and easy to prove axiom. What is not so evident, and a thought that I haven't seen before in print is this: a technological civilization on the verge of expanding into space is close to the point where it runs out of resources on its home planet. If it runs out of resources before establishing itself in space, the civilization collapses, never rises to the same heights, and soon thereafter becomes extinct.
Civilization is now close to the point of collapse. How close is open to debate. Even if you don't believe this yourself, most national and international leaders believe this, and their actions are shaped by their beliefs. In fact, the coming collapse of civilization is required reading in most school systems. Limits to Growth is the original tract outlining future chaos. The Population Bomb and The Population Explosion outline the same doomsday scenario of the collapse of technological civilization. Anything by the widely-quoted-in-the-press-as-a-scientific-expert Jeremy Rifkin contributes to this belief. Our very own vice-president's book, Earth in the Balance, and all of its 100+ listed references all outline the coming collapse. Not one of these books outlines an optimistic future in which space travel has become routine, In fact, they all offer the same prescription, one which would doom space travel for the human race forever. Reduce the world's population, immediately. Reduce the first world's standard of living, immediately. Create a one world government, even if you call it something else. Deindustrialize, and return to a more balanced way of living with Gaia, the Earth Mother. If you don't believe that these are the universal panaceas, read the books. Most of our political leaders have. They have not read High Frontier, nor are they familiar with terms like single stage to orbit, solar power satellites, or generation ships.
The doomsday argument: space travel or extinction. It may sound extreme, but it is really the only choice of futures. Furthermore, we have only a limited amount of time left to achieve space travel. Extinction is the default choice. If the human race does not actively pursue space development in the near future, the choice of extinction has been made. I have run this argument through several dozen people who are not rabid pro-space advocates like me. They have all understood it; it is a simple argument. None have found serious fault in it; there is none.
If you have read this far and remain unconvinced, educate yourself. Read Al Gore's treatise, and some of its references to see the future being planned for us. Then head towards the SETI section of your local library and read up on Drake's Equation and the Fermi Paradox. The conclusions are obvious, and waiting to be drawn. Let us hope we can convince the rest of the human race before civilization collapses. Monday morning quarterbacking on an issue this big isn't going to achieve a happy future for the human race, and we are not going to get a second chance.
