One discussion that has surfaced repeatedly since the dawn of the Space Age is the worth (and role) of space travel, especially in the context of all the other priorities that demand attention, such as ending world hunger, developing flying cars, and creating a new energy infrastructure. We have no real shortage of challenges and crises on our doorstep- why should we take some of our hard-earned resources and spend them where they may or may not do any good? A number of questions seem pertinent to this question. First, how much do we actually spend on space exploration in a single year? Following on the heels of this question is one of what benefits we derive from such exploration and whether these benefits could be acquired through other means. After these questions have been addressed, we need to look at the long-term question of where we want to go as a species and a planet, and what we'll need to get there. This first article addresses those short-term questions, while the second makes a start at discussing the long-term one. The third addresses societal and systems issues that have a bearing on the success of space exploration, and the fourth addresses the challenge of maintaining public awareness of exploratory efforts.
How much money do we spend on space exploration? Either a whole lot or precious little, depending on how you do the reckoning. The following list of space program budgets is by no means comprehensive- it is intended to give a sense of the size of the global space program budget pool and the proportion of that pool that each program accounts for. Based on a Wikipedia table of space agency budgets, the total global expenditures are approximately equal to $35 billion, a little over twice NASA's budget and about half again greater than what the United States Department of Defense spends on space activities annually. (While the Department of Defense is not included in the table, its space activities are an integral part of the overall picture, given the fact that many of NASA's payloads ride into space on rockets developed under Department of Defense funding.)
Put into context, this amount is dwarfed by the amount spent on recreation, health care, or routine household expenditures. The 2001 Gross World Product was ~ $45.9 trillion dollars, making the ratio of gross world product to space program expenditures of the same order as that of US gross domestic product to NASA's budget.
There seems little question whether the industrialized nations can afford such spending on space exploration- the critical question is whether we will choose to or not. Yes, there are other things that we could spend that money on, but there are also a lot of other things that we don't have to spend money on (or could reorganize to spend less money on with no loss in the end products or services), as well. Energy is a good example of this- maintaining our supplies of fossil fuels relies on continual mining and drilling (and, thus, continual resource usage on top of the associated environmental damage). Solar power is a very strong member of the renewable energy portfolio and a contender in many energy usage scenarios, even though it does have the drawback of, with the exception of solar thermal power systems, being heavily dependent on weather conditions. Thus, replacement of fossil fuels with renewable sources in a given application eliminates that ongoing energy overhead that mining and drilling require, though raw materials are still required to construct devices. Turning the space debate into a matter of either-or is misleading, since it's actually a matter of relative priorities.
As for what the space exploration budget buys, a thumbnail sketch can be seen by a glance at the following graph, which shows a breakdown of NASA's budget by category for the fiscal years 2004 through 2020. [ad name="Adsense Small Horz Banner"] At a glance, it is clear that manned space exploration, represented by the Space Shuttle program and International Space Station, consumes approximately half of the budget, while another third is appropriated for "aeronautics research and other science activities". NASA's robotic exploration missions make up the majority of the remainder. Few other countries have manned space programs, and most lack the level of funding or long-term commitment required to support them; thus, they make up a relatively small portion of overall spending. That collective spending includes activities such as earth observation, planetary exploration, communications & navigation, and astronomy. Without a doubt, many space agencies have duplicated each others' efforts, partially due to the dual-use nature of a number of space-related technologies; the majority of rocket development has been for military purposes first and foremost with the fortunate side effect of enabling space exploration.
Some would argue, and with good reason, that the ratio of volume of scientific data to cost is much higher for robotic missions than their manned counterparts, and that they have the added attraction of not endangering human lives. However, manned missions produce critical data on human biology, performance, and effectiveness in a microgravity environment, data that is invaluable if we are to one day work and live in space. Further, direct control of robotic missions imposes line-of-sight restrictions and communications lag limitations, factors that can give a manned mission an edge in rapidly changing or unforeseen circumstances.
Perhaps one of the most convincing arguments for maintaining space programs, both manned and unmanned, is the "capacity-building" argument. Whether manned or unmanned space exploration is the preferred method of pushing our boundaries of knowledge outward, it is certain that humans will be designing and building the systems and running the organizations that will be doing that exploration. If we want to have that expertise twenty years in the future, we need to be nurturing and developing it right now.