u/ActuaLogic

Shouldn't the focus of space development should be neither on pure science nor on human settlement but instead on profit-generating resource extraction? Scientific discoveries and human movement into space will necessarily accompany the need to solve engineering problems associated with locating resources and making them usable (typically by bringing them to the vicinity of the Earth-Moon system), but science and settlement don't necessarily produce economic development.

There are several problems associated with profitably extracting and transporting resources in space, but the key problem for profitability is that financing space ventures requires a turnaround time that is very fast in comparison to the timetable imposed by available propulsion systems. In brief, because of the time value of money, hypothetical investors in space resource development ventures are going to want to see return in 5 to 10 years rather than the 20 to 50 years which would be optimistic given current trajectories. The present value of a dollar received 25 years from now is effectively zero.

Because the problem with financing resource development in space is the lack of transportation fast enough, in view of the time value of money, to produce a positive net present value, and because that lack exists because of inherent limitations of reaction-mass propulsion systems (such as rockets), the solution is the one method of reactionless propulsion that actually exists: the light sail, and specifically the laser-pushed light sail.

The utility of laser-pushed light sails necessarily depends on the power of the lasers, and developing the lasers would be no mean feat. It would be more practical to develop a network of moderately powerful lasers arranged in a series of stations along a network — in order to bring a vessel up to speed in stages and then slow it down in stages — than it would be to develop one superlaser at each end. Laser stations would be permanent infrastructure that would be used repeatedly, and the utilization rate would determine the profitability of the network. The up-front cost of building the network would be high, but the marginal cost of using the network for one trip would be relatively low. The network would resemble a railroad in that travel would be possible only along the route of the network and only by vessels authorized to use the network (some may be owned by the network, while others may pay to use the network, much as Amtrak pays to use freight rail track for passenger service). One solar system-wide transportation network could serve multiple ventures (as long as they pay). How many intermediate laser stations are needed for the network would be a cost-benefit question, balancing the cost of laser stations against the power of the lasers. Intuitively, it seems that three lasers per station and three light sails per vessel would enable the use of less powerful lasers, would improve the handling of the vessels by adjusting the sails, and would make it easier to avoid hitting the vessels with the lasers. The light sail apparatus could be detached from the vessel at the destination, with a new vessel being attached, either for a return trip or to go on to a new destination. The laser stations would be held in place using reaction-mass thrusters that could be resupplied with fuel by using the network. That would make it possible to reposition laser stations to accommodate the movements of destinations (planets, asteroids). Initially, the vessels would almost certainly be completely automated, because accommodating humans with life support and so forth increases total mass of a vessel by something like a factor of 10. Automated systems should be very capable of identifying and extracting valuable minerals and returning them to the Earth-Moon system. Over time (how much time would depend on the profitability of the ventures using the network), the network would extend to Uranus and Neptune (sources of deuterium and He-3, as well as hydrocarbons from which composite space hulls could potentially be made), and even to the Kuyper Belt if there's something there worth extracting. (In theory, the network could go interstellar — it's reactionless — but the nearest resource-rich star system is Epsilon Eridani, which is 10 light years away.) Over centuries, the number of laser stations could number in the thousands, enabling reactionless travel throughout the solar system. (Development of plasma propulsion would benefit from the development of systems to power the lasers).

An appropriate roadmap could begin with a Moon base that includes facilities to produce fissionable materials using locally mined inputs, which would serve as fuel for laser stations and for ion and plasma reaction-mass engines (which would also be necessary). It seems unwise to rely on terrestrial sources of fissionable materials, because the public outcry from one launch accident could shut the whole thing down. Ultimately, the laser stations would have to be mass produced in order to keep unit costs as low as possible.

Slower-than-light travel is the only kind of interstellar travel that is feasible using foreseeable terrestrial technology. Slower-than-light interstellar travel at speeds that could be accomplished using foreseeable technology imply travel times that exceed a human lifetime. As a result, interstellar travel using foreseeable technology necessarily implies multiple human lifetimes, namely, generation ships.

A generation ship is an artificial human habitat specialized for interstellar travel. Artificial habitats are likely to be developed to provide places to live within the solar system in order to accommodate a human population that might theoretically reach 100 billion people. (Imagine mass production of kilometer-scale habitats at Uranus and Neptune with plastic hulls made using atmospheric hydrocarbons, a kind of Levittown in space.) Therefore, it seems likely that interstellar generation ship designs will be artificial habitat designs adapted to the environment of interstellar travel (perhaps using an “interstellar railroad” network of lasers placed between star systems to accelerate and decelerate ships using light sails, because light sails are the only foreseeable means of reactionless propulsion and the rocket equation tells us that reaction-mass propulsion is unavailable for interstellar travel).

On a separate but related note, in my opinion, common sense tells us that pervasive artificial intelligence would be integral to such artificial habitats and generation ships. In fact, the habitats and ships may ultimately resemble a kind of symbiosis between humans and artificial intelligences. One corollary of that symbiosis would be that putting people into suspended animation for interstellar travel (arguably not a foreseeable technology) wouldn't work in practice. The smooth functioning of a generation ship would be an ongoing collaboration between the humans on the ship and the artificial intelligences that comprise the ships’ systems. If the humans are in hibernation while the artificial intelligences are actively operating the ship for, say, a century of interstellar travel to Alpha Centauri (even if a top speed of 0.10c could be reached, acceleration and deceleration imply an average speed of 0.05c or less), then there would be only a limited basis for the necessary symbiosis upon arrival. Artificial habitats and generation ships would, in effect, be living things.

There's a joke that practical fusion is at least 30 years away — and it always will be. Meanwhile, fission exists, though it's less efficient. And even if fusion is commercialized, there are likely to be applications where fission is a better option (just as we still use coal for some things, even though other fuels are now available). The solar system is surprisingly low in fissionable materials (including among the asteroids), with the Earth and Moon being the only known sources of significance. By contrast, the Alpha Centauri system is higher in metals and is predicted to be higher in fissionable metals. If that is confirmed by exploratory probes, then fissionable materials could be exported from the Alpha Centauri system to the Sol system.

I didn't come up with the idea of interstellar highways comprised of laser stations to push laser-sail-equipped vessels between star systems, but it seems to me that a network of laser-driven light sails through interstellar space would be the best way to get around the limitations of the rocket equation. The lasers would probably have to be fusion-powered lasers similar to rocket dynamic lasers. The first step would be to build the network by using a solar system-based laser to push another laser platform to a predetermined location in interstellar space and then push a third laser platform further out, and so forth until the destination star system is reached. After the network was in place, ships could use it to move through interstellar space, paying a fee. Fuel and other supplies needed by the laser platforms (nodes) could be shipped using light sails, and the platform infrastructure could serve as waystations to resupply ships moving along the network.

The virtue of such a network would be that it wouldn't require reaction-mass space drives and therefore wouldn't be limited by the rocket equation. It would be more analogous to a railroad than to ocean-going vessels, because it would facilitate movement only along the network infrastructure. Such a project might seek out ice giants among the exoplanets, which are among the most common type of exoplanet, because they have valuable resources. Such infrastructure would not be cost effective to build to enable a single vessel to move through interstellar space, but it may be cost-effective to build to enable thousands of vessels to move through interstellar space. If rogue planets with valuable resources are identified, they could be sites for laser stations. At some point, maybe the Lagrange point between the Sun and the galactic center (about 4 light years towarda the galactic center from the Sun) could also a site for a laser station on the network. Vessels and laser stations could be mass produced, and new manufacturing sites for vessels and laser stations could be established in destination star systems using local resources.

Unfortunately, one essential thing which is currently lacking is the economic justification for traveling between the stars. That's likely to change before the technical capability exists to build an interstellar railroad, but it's essential because something on such a large scale can't work unless it generates a profit.