If rising fossil fuel costs were all that mattered they would eventually make space travel prohibitively expensive, but such decisions are more political than economic. Still, as fossil fuels become increasingly scarce and expensive, traveling off-earth will become accordingly more costly, especially for human beings, whose need for as bulky life support equipment greatly increases the energy requirements for space transportation systems. As a result, cost will become an increasing concern in space-related projects, and take on an increasingly high profile in the political and scientific debates that govern space programs. Recent cutbacks in funding to NASA shows evidence of this. Increasing costs and decreased funding may result in an increasing proportion of robotically-manned space expeditions, and could even reduce the human presence on the space station in the future and see it refitted for increasingly automated operation. Eventually a new, cheap, and powerful energy source (fusion?) will be developed and a new era of space travel will come about. How soon that happens, however, depends on us.
Energy cost will continue to increase in importance until a new, cheaper source of energy is available. This trend will not be reversed until a new, truly inexpensive, and high capacity energy source is available. For now the only future candidate for such a source is nuclear fusion. No other power source even suggested appears to have the potential to provide the amounts of energy required for lifting an object into orbit. Unfortunately, fusion power appears to be decades from practical availability. Even if propulsion could somehow be derived from non-fossil chemical fuels found in the earth, the mining, transportation, and processing of those fuels would still require more energy than can be obtained through any means except fossil fuels. Our current dependence on fossil fuels essentially puts us in an increasingly risk and problematic position, and not just with respect to space programs.
Our other source of large volumes of energy, nuclear fission, will not support space travel. The only other source of energy in sufficient volumes is fission power, but, even beyond the issue of how that energy could be employed to propel a vehicle into orbit, to say that we could dedicate sufficient fission-based power to the space program would be silly, as fossil fuel would be required to replace the energy previously provided for other uses by those fission reactors. Building more fission reactors solely for the purpose of space travel would be silly as well, as the energy to build fission reactors itself comes from fossil fuel, and anything that increases our demand for fossil fuel only increases our costs and risks.
Natural energy sources such as geothermal and storm-generated energy aren’t available for use because we haven’t figured out how to control them. How do you collect, contain, and then use a lightning bolt, hurricane winds, or the intense heat of a volcano? The energies the earth provides in these forms are sufficient to put a vehicle in orbit, but we haven’t got technology capable of bottling them and then releasing them in a controlled way for our own use.
The energy-density of the power source used is critically important to space travel. Too heavy or bulky a power source would be self-defeating, as there is a set amount of accelerating energy needed per unit mass to achieve escape velocity. This is why the space programs use extremely powerful solid and liquid chemical propellants that have a very high energy-to-weight ratio. A fission reactor might be built to provide enough energy to put something into orbit, but the energy would somehow have to be stored and packaged in a self-contained propulsion system that could be included in the object to be sent, as the shielding and support equipment for the reactor would be far too heavy to lift. In essence, a fission reactor doesn’t have sufficiently high energy-to-weight ratio to permit it to fly, and it is probable that this could not be achieved at any scale (at least safely). Whether this condition can be achieved with fusion power is not yet clear, but eventually research in this area will answer the question.
The justification for putting humans in orbit, as opposed to “robots”, will continue to be questioned. Putting a human into orbit is a great deal more expensive than putting remote controlled equipment there, and the justification for doing so has been rightly questioned for decades, though, so far, it appears the question has been answered affirmatively at least by decision makers capable of wielding the needed resources, who perceive that the romantic concept of humans in space sells programs to the public and secures their supporting votes.
Moral tradeoffs will increasingly enter into the ongoing debate over the funding of space exploration. Can we gain enough real value from a trip to Mars or the Moon to justify the huge expense? In the coming decades we will increasingly be struggling to feed the huge and burgeoning population of humans on earth, at least until it declines or is somehow reduced. What could we possibly bring back that would have such huge value as to be worth the cost of the trip? At some point we must admit that it is questionable whether the knowledge that some form of life ever existed on Mars, for example, provides enough benefit, even including the intangibles, to justify the cost of finding that out. Much as we’d passionately like to know, finding out for sure may have to wait until we have cheaper energy.
Lower energy costs in the 20th century were a factor in previous space ventures. We learned a great deal from the voyages to the moon of the 20th century, but the energy cost was much lower because we were nowhere near the limits of our fossil fuel supply. Even then, did we make those investments by ignoring the moral trade-offs with feeding the masses of earth? Did the things we learned in the process really justify the investment? Those are tough questions to answer, as many of the benefits were intangibles such as the confidence we gained from proving we could do such incredible things. Could the same amount of money and resources, applied to basic scientific research on Earth, have provided greater value? Some would undoubtedly say yes. The cost of putting a human in orbit is already so great as to drive cost reduction measures in all national space agencies. As costs will continue to rise until a much cheaper energy source is available, pressure to look at the cost-benefit ratio of putting people in space will increase in the coming decades
It is probable that cheap, high-volume energy will return in the future. Someday it is probable that large volumes of cheap energy will be available, quite possibly through the development of practical fusion-based power sources. New technologies will be developed that will be more efficient, reducing the energy required, but it appears extremely unlikely that conservation alone will ever come near satisfying the energy requirements for space travel (though it will be of critical importance until such time as a cheaper energy source is invented and implemented). Using a fusion-based power source, the tiny amounts of material needed to generate that power will be easy to acquire and transport, and the packaging and mass of the equipment involved will be the gating factor in space travel applications.
Fusion-powered interstellar travel was envisioned decades ago. The science fiction-like vision, first conceived by Robert Bussard in 1960 (link), of ships that travel interstellar space at near-light speeds and sweep hydrogen atoms from space to fuel their engines is a debated possibility, though technology capable of proving out the theory may not exist for a century or more. From here it appears that fusion power may be the ultimately efficient energy source of the future, and it is probable that it will only be transcended by a technology undreamed of today.
Long-term research is needed if a new era of cheap energy is to be realized. More resources are needed for fusion research and related scientific work if we are ever to again have the cheap energy we have enjoyed in the current fossil fuel era. We should each be making this fact clear to our elected representatives, and frequently, as fusion research currently ongoing is not getting enough funding.
As always, I welcome your comments. — Tim