Interplanetary Travel in the Solar System of the Near Future

What would a truly advanced interplanetary transportation system be like? After all the near-future flags-and-footprints expeditions, research outposts, experimental space habitats, and space tourist centers we hear about so much are up and running, the build-out of space infrastructure will begin in earnest. In this post we will focus on the transportation options for a spacefaring civilization of the near future, rather than the details of space colonization.

Chemical Rockets to Near-Earth Space

The earliest space infrastructure will likely be sent up from Earth using conventional chemical rockets, as that is the only way large payloads (not to mention human beings) have been sent from Earth to outer space so far. The maturity and ubiquity of the technology as well as the relatively low cost means that the earliest point-to-point space transportation will almost certainly be done with this method. Substantial cost reductions compared to the Apollo-Shuttle era have already been achieved, and this raises the question of low low costs can get over time. Propellant costs per pound of payload range from $10-25 per pound, and if rockets achieve the same level of maturity as jets, fuel costs should be 25-50% of the cost of launch. This implies $20-100 per pound as the realistic minimum for launch costs, assuming the cost of fuel doesn’t decrease substantially. Considering that very large next-generation nuclear reactors could provide very cheap reactor heat to separate water into oxygen and hydrogen, this may not be a safe assumption; a plan along similar lines to turn carbon dioxide into hydrocarbons with nuclear power was actually proposed by Los Alamos in 2008.

Nevertheless, even going with the more conservative $25-100 per pound figure, this is extremely cheap compared with the $10 000 per pound or so for Saturn V and the Space Shuttle, amounting to a 99-99.7% savings. So much for “every gram counts”: quite large payloads become more viable to the extent rocket technology matures. For perspective, to launch a man to orbit on the Space Shuttle cost (assuming 400 pounds total body and cargo mass) about $4 million; for a fully mature rocket the cost is brought down to $10 000 to $40 000. So the equivalent of a halfway-round-the-world first-class flight in cost is about the best one can hope for with chemical rocket technology in the near future.

This may still seem expensive to most people, but even a small fraction of this 99% cost reduction will enable a large and diverse number of private individuals to go to orbit, especially if they will be relocating permanently (thus making it a one-time cost). Any number of businesses and organizations will be able to affordably lift cargo into orbit or beyond. The most well-developed plans for this are space stations intended for tourists, which will likely be among the first private-sector structures in space. For all the hatred they get from some quarters, chemical rockets may easily secure near-Earth space as a human habitat all by themselves.

Of course, once these teeming masses reach orbit with their large payloads at an affordable cost, what will further transportation look like? Outside hard science fiction circles one hardly ever sees “delta-v maps” (delta-v being change in velocity) of the solar system, but anyone interested in the future of spaceflight should look one over (I recommend this one made by CuriousMetaphor at Reddit) If you’re astute you might notice that the saying “Earth orbit is halfway to anywhere” is more or less true. Once you are already in even a low orbit around Earth, you no longer need to expend the 9.4 km/s of delta-v just to get into space. Landing on the Moon only takes 5.67 km/s from low Earth orbit, compared to 15 km/s from the Earth’s surface; only a third of the cost!

By the same token, landing on Mars takes 9.5 km/s, about the same delta-v as it took you to get to low Earth orbit in the first place; in terms of energy cost low Earth orbit is halfway to Mars. Much of the cost to land on the Moon or Mars is actually from low orbit to landing; from low Earth orbit to low Lunar orbit takes 3.94 km/s, and from low Earth orbit to low Mars orbit takes 5.7 km/s. The numbers for the nearby asteroids are similar, one reason why it may be more efficient to mine asteroids than an object as large as the Moon. These delta-v figures assume a low-energy route using Hohmann transfer orbits. These trajectories are cheaper but take a much longer time than a more brute-force method. The often-quoted transit times to Mars of 150-300 days are for a chemical rocket using a Hohmann transfer orbit. That’s an awfully long time to spend on a spaceship, especially from a civilian or tourist point of view; astronauts could certainly take it, considering there have already been some space station stays that were longer, but more common denizens of any spacefaring culture will want to get to Mars or other comparably-distant destinations much faster.

Fast Passengers, Slow Cargo

This disconnect between time and cost means the techniques used for businesses’ bulk cargo shipments and the techniques used for passenger transportation will diverge relatively early in spacefaring civilization’s development. Ore mined from asteroids doesn’t complain about being in transit for months on end, so launch costs will be kept as cheap as possible. This means cheap fuels and Hohmann transfer orbits, at least in the early stages of spacefaring in the inner solar system, which is our focus here.

The cheapest fuel (or more accurately “working fluid” in this context) for an asteroid mining operation would most likely be water, as odd as it sounds. Most asteroids have water ice in them, and since the rock is being extracted anyway in these sort of missions extracting the ice and loading it into a tank would be relatively simple. The water ice in the tank could be heated by sunlight using concentrating mirrors, subliming into water vapor and being channeled out the nozzle for thrust. This is known as a “solar thermal rocket” or steam rocket. It seems like a cheap approach since aside from the mirrors it is all “in-situ resource utilization”, minimizing launch mass. In the future of cheap spaceflight the added cost of, say, using a nuclear reactor and hydrogen for heat wouldn’t be much, but in business every dollar saved makes your product more competitive. The only reason we don’t use such rockets now is because their performance is too poor to lift anything into low Earth orbit, but remember: the delta-v requirements from nearby asteroids to Earth orbit are much more forgiving, and can be accomplished by steam rockets.

For passengers willing to pay a premium for speed, nuclear thermal rockets are a close-at-hand technology that can get them across interplanetary distances much faster. Nuclear thermal works the same way as the solar thermal rocket, except instead of heating up the working fluid using solar energy it is heat from a nuclear reactor. This achieves much higher power density, particularly for larger spaceships. The lighter the working fluid the higher the thrust, so hydrogen would produce the greatest speed of any material and would most likely be the standard. The working fluid could in principle be anything, not just hydrogen, and this may attract spacecraft owners that wish to maintain maximum flexibility in suppliers to (nuclear) thermal propulsion.

In addition to the lightness of the working fluid, temperature also determines thrust: the higher the temperature, the higher the thrust. Thus gas core nuclear thermal rockets (where the reactor is kept in a gaseous state) have the highest thrust, solid cores have the lowest, and liquid cores are in between. Although liquid and especially gas cores are well beyond current capabilities, a solid core rocket (NERVA) was tested very successfully during the Apollo era. This is what we will be using in the near future; although thrust-to-weight is very poor compared to chemical rockets, the specific impulse (a measure of performance) of nuclear thermal is well above any chemical rocket, thus making nuclear thermal ideal for beyond-Earth-orbit spaceflight.

This was recognized as early as the 1960s, leading to the development of NERVA as an upper stage for the Saturn V. This was intended for a manned Mars mission; nuclear thermal upper stages are currently being studied by NASA for the same purpose. Interplanetary flight is the only real use of these engines, since you can reach the Moon in a few days with ordinary chemical rockets (as Apollo did). The reason for the interest in nuclear thermal is, of course, the high performance; with twice the specific impulse of a chemical rocket, a nuclear rocket can reach Mars (or any other destination) in half the time. This cuts transit times from around 200 days to 100 days on average. Much better, and something passengers will likely be willing to pay a premium for. More importantly, nuclear thermal has sufficient thrust to enable trips to Mars at any time (albeit ranging up to twice as long travel time) rather than only at the once-every-other year transfer-orbit window. The same principles apply to other destinations in the inner solar system, and would be even more important in the outer solar system later on, with longer distances and longer times between launch windows.

The Martian Cruise

There will be, once spacefaring civilization develops, a lot of traffic between popular destinations, the most prominent in the human imagination being perhaps Earth and Mars. There is an entire class of orbits known as “cycler orbits” that enable spaceships to appear at each destination (these sort of orbits exist not just between Earth and Mars but any two points) at regular intervals without needing to use any propulsion. Wikipedia has a helpful page detailing the Earth-Mars cycler orbits. The Aldrin cycler, an orbit discovered by Buzz Aldrin, takes 146 days from Earth to Mars, 16 months beyond Mars orbit, and then 146 days from Mars to Earth, returning at each planet every 2.135 years. That’s reasonably peppy to not be using any fuel.

It gets even better: another orbit enables transfer time of only 88 days each way, but it swings so far out that it only comes back every four synodic periods, or 8.54 years; thus four spacecraft would be needed to ensure one appears every two years at each destination. The fastest cycler in the table takes only 75 days, and returns every five synodic periods, thus requiring five spacecraft. That’s really peppy.

The key advantage of cycler spacecraft is that the mass of the spacecraft itself only needs to be lifted into the appropriate orbit once; instead of propelling the entire ship, only the people and whatever cargo that accompanies them would need to be moved. This saves a substantial amount of fuel, even if the space shuttles need to accelerate rapidly to catch the cyclers as they pass by (these orbits, especially the 75 and 88 day orbits, are pretty fast). This may enable much larger ships to be constructed at a given cost, perhaps including centrifuges sufficient to provide comfortable artificial gravity and naturalistic habitats that would otherwise be uneconomical. After all, wouldn’t you want as much comfort as possible if you were traveling for several weeks at a stretch? The first interplanetary equivalents of luxury ocean liners or cruise ships may well travel these routes between Earth and Mars.

The Elegance of a Solar Sail

Luxury liners cycling between Earth and Mars with their passengers enjoying resort-level accommodations in an Arcadian centrifuge with no propellant needed is a beautiful vision, but is it the most likely in an age where in-situ resource utilization from small bodies like asteroids predominates? After all, the primary justification for a cycler is launch cost, which, with cheap rockets available and the delta-v requirements from Earth-orbit factories to Mars orbit being light anyway, would likely be relatively small compared with building costs. Especially later on in the near future the finished goods to furnish such a ship will be able to be manufactured in orbit, reducing costs still further. Most importantly, the fuel cost needed to turn around at Mars and head back to Earth may be low compared to the cost of leaving a luxury liner unoccupied for years. Perhaps, at least later on, cyclers may appeal more to economy-minded passengers that are more sensitive to small fuel costs.

Romantics that dream of propellantless luxury spaceflight are unlikely to be disappointed, though. There is another class of spaceship even cooler than the cycler: the solar sail. Solar sails work by using the radiation pressure from sunlight exerted on large mirrors for thrust, in the same way a normal sail uses the pressure from wind. No fuel is needed, as the thrust comes from sunlight acting against the structure. Solar sails thus have unmatched endurance among near-future propulsion systems; the ship can propel itself without resupply as long as the sail is reflective.

The only drawback is that solar sails are a very low-thrust propulsion method, thus it takes a long time to get anywhere. Final speeds, though, can be very fast, since the low thrust, unlike a rocket engine, operates constantly over a long period of time. This characteristic is shared with ion propulsion, commonly used by space probes today. Unlike ion drive, however, solar sailing offers a way to accelerate much faster than normal: swing in close to the sun where there is much stronger radiation pressure and get up more speed. How much more speed? So much speed that such close flybys (or “frybys”) of the sun have been studied as a method of interstellar propulsion. The closer one can get to the sun the more acceleration will result. The sail itself being more reflective, larger in area, and lighter in weight will make it more effective everywhere it travels.

As outlined here, a close approach of 0.019 AU (around four solar radii) with high starting acceleration can get a graphene solar sail up to 13% of light speed, fast enough to reach Pluto in less than two days and Alpha Centauri in 32 years. Approaching to a much less strenuous 0.2 AU, some way inside the orbit of Mercury but well short of fryby range, can get one to Pluto (and hence to any point in the inner or outer solar system) by solar sail within a few months. Thus advanced solar sails can reach anywhere in the solar system you would ever want to go on a timescale comparable to terrestrial sea voyages at worst. This is much better than any propulsion technology discussed so far. It’s also much better than existing solar sails that have been tested, but nevertheless does fit into near future technology.

Another interesting property of solar sails is that they are by far the most effective method of propulsion for “statites”, satellites that would use propulsion to maintain a station that would be impossible in an actual orbit, such as staying permanently above one spot on the Earth at an altitude well below geostationary orbit. Solar sails, since they use no fuel, would be by far the cheapest method to do this, especially over long timescales and for smaller craft.

A variant on the same concept is the currently very fashionable beam propulsion, where instead of sunlight the energy imparted onto the sail comes from a laser beam. In principle this could provide even more power than sunlight, or supplement solar power after a close solar flyby is completed and solar energy diminishes further out. Lasers can also be used to power a thermal rocket in the same manner as sunlight. Near-future laser technology could even be used to propel a very light spacecraft from Earth’s surface to orbit. This method may be used as a way to cheaply transport small cargo from Earth to space in this interplanetary near future.

The Unmatched Power of Nuclear

Of all near future space propulsion technologies, I have saved the best for last. Aside from using a nuclear reactor to heat a working fluid, there is a much more radical method to use nuclear energy: light a nuclear bomb behind the spacecraft and use the blast for thrust. This was the subject of the famous Project Orion, and is properly called nuclear pulse propulsion. Although the idea might sound crazy, the calculations work out, and there have been proofs of concept of some parts of the system, including the pressure plate which transfers energy from the blast to the spaceship. The blasting in question would be done by a large number of small hydrogen bombs, being detonated at a rate of around one every second. “Shaped charges”, designed so that the vast majority of the blast is directed toward the spacecraft, would be more efficient than ordinary bombs.

Nuclear pulse is by far the highest performance propulsion system for large payloads. How high is this performance? Like solar sails, nuclear pulse has been studied for interstellar propulsion; the theoretical maximum speed that could be attained is 10% of light speed, and this without any bulky sails or needing a close flyby of the sun. As far as I know it should be possible to combine the two methods, in effect using nuclear pulse as a booster for a solar sail, thus achieving up to 23% of light speed. Any velocity in this range puts the nearby stars within reach in a human lifetime, and makes even the outer solar system reachable within hours. Of course, this is performance at the theoretical maximum; actual engineering practice is not likely to approach this for a long time after initial introduction. Nevertheless even a small fraction of this capability would open up the entire solar system to human activity on any time scale, and provide a solid basis for eventual interstellar missions.

Where nuclear pulse really shines far beyond any other current or near-future technology is the sheer scale of craft that can be propelled. Wikipedia provides a helpful table of designs that were considered during Project Orion, ranging in size from 4000 tons for an early interplanetary ship to 10 000 tons for an advanced model. The latter would sport a diameter of 56 meters and a length of 85 meters, a large spaceship indeed. And from one launch! A “Super Orion” was studied that would be a whopping 400 meters in diameter and eight million tons in mass. This monstrosity was designed with materials available in the 1950s in mind; with more exotic modern materials much larger craft could be constructed. For comparison, while the Advanced Interplanetary Orion could transport 5700 tons from Earth to the Moon, the Saturn V could only transport two tons. For space-to-space transportation without the delta-v cost of planetary gravity wells, very large ships could easily be constructed and propelled quickly. This is a technology that opens up space on a truly mass scale, well beyond anything else mentioned in this post.

Six Dollars to Earth Orbit

Nuclear pulse has the potential to be very cheap as well. The amount of fissile materials required is not significantly greater for larger models than it is for smaller ones, so the fuel cost increases much more slowly than the mass increases, meaning the launch cost per pound decreases. Hydrogen bombs of the appropriate type could be mass-produced, reducing the cost per pulse by an estimated factor of twenty relative to current bombs, to around $250 000. 800 bombs to low Earth orbit mean cost (by my back-of-the-envelope calculation) of around $200 million for the bombs, for the Advanced Interplanetary ship equaling $16 per pound of payload. And this isn’t even close to the theoretical maximum efficiency.

The eight million ton Super Orion by the same calculation would cost $0.017 per pound of payload; that’s around $33 per ton to low Earth orbit. That’s really cheap, so much so that if payloads of such size were routinely being transported along the same route nuclear pulse ships would edge out all competition and become the dominant method of spacecraft propulsion. Assuming 400 pounds of payload for a human, a ticket to orbit on such a craft would come to somewhere around $6.80. That’s cheap even by contemporary terrestrial standards, let alone contemporary space launch standards. For comparison, this cost is not only competitive with airplanes, but also with trucks over long haul routes, according to my back-of-the-envelope calculation ($0.01 per pound in fuel per thousand truck miles). Of course trucks don’t carry millions of tons of cargo, so it’s sort of an apples-to-oranges comparison. Nevertheless this shows the way to a future, which we have the technical ability (but not the available funds) to build right now, where spaceflight is as routine and affordable as taking a train or a bus.

This future may not be too distant, and is not often considered even in harder science fiction, let alone popular culture. A future where whole cities worth of people and cargo are traveling through space would certainly support such an infrastructure, as well as provide a large enough economy to finance such vehicles. To reach the full maturity needed for the $6 ticket to orbit will likely take an extended period, but once again, even a small fraction of these savings (through any propulsion method) would be revolutionary, kicking off the process of extraterrestrialization, well-described by J.N. Nielson as being as historically important as the agricultural and industrial revolutions before it.

I have neglected to include the fallout issue here, because nuclear pulse works just as well outside Earth’s atmosphere as it does within it, and would accordingly be revolutionary for the space-based or extraterrestrial part of human civilization even if it were forbidden to use on Earth. However, the fallout issue even on Earth has been greatly exaggerated; as only the first few pulses would have any effect on the ground whatsoever (which is where fallout comes from) radioactive fallout is minimal compared to the energy involved. 0.1 to 1 excess fatal cancers worldwide are predicted to result from the additional radiation from each launch under the linear-no-threshold model of radiation’s effects on human health, which is unsupported by the inconclusive research. So the effect on human health of such low doses may actually be zero or even positive (“radiation hormesis”). In any case, your dose of natural background radiation varying by location (not to mention chemical pollution that gets far less attention than nuclear does) has a much larger effect on your dosage than nuclear pulse launches ever would. Even assuming linear-no-threshold is true, environmental remediation can be easily undertaken to offset the 0.1-1 fatal cancers per launch, and then some, at negligible cost compared to what the launch would cost anyway.

Much of this assumes that “clean” hydrogen bombs of the appropriate type could be developed (cleaner meaning a higher percentage of the yield being from fusion as opposed to fission) that would have less fallout per ton of yield. The Russians have achieved 95% clean or better in the hydrogen bombs they used in civilian projects; even cleaner bombs could likely be developed. On the more speculative end, antimatter could substitute for the fission primary, eliminating fission decay products and thus the remaining fallout, as well as enabling much smaller nuclear pulse ships to be engineered. However, antimatter is so difficult to manufacture that this method is not likely to be economical for some time to say the least.

Another variant of nuclear pulse propulsion is the Medusa design, where instead of a pusher plate at the back of the spacecraft the energy is transferred by means of a sail in front of the spacecraft, the pressure of the blast on the sail propelling the craft forward in the same way sunlight propels a solar sail. This design may have higher performance than conventional nuclear pulse ships, though the maximum specific impulse for nuclear pulse in both instances tops out at 100 000 seconds (compare to around 1000 seconds for solid-core nuclear thermal).

Nuclear pulse propulsion is the ultimate space propulsion technology in any near-future setting, because it provides the highest speeds attainable with current or near-future technology to the largest ships. As human civilization expands into space and the fear of nuclear technology instilled by World War II and the Cold War ebbs, advances in this technology will first introduce it and then spread it far and wide across the solar system of the near future.

Conclusion

Nuclear pulse’s great weakness is that it needs large scale to be cheaper than other methods, so it will only dominate among large spaceships. Small to medium size interplanetary ships will likely stick to solar sails, nuclear thermal rockets, or perhaps riding laser beam; if economy-minded they will use solar thermal or ordinary chemical rockets. Within one planet’s sphere of influence, like Earth-Moon journeys, chemical rockets may still prevail on small to medium size ships well into the future. For very small ships, the near future’s equivalent to CubeSats, solar sails or laser beams seem likely to prevail.

So, these are the technologies the denizens of the spacefaring civilization of the near future will use to travel through the solar system. Not only from planet to planet, but to and from space stations, space factories, space habitats, spaceships, asteroids, and comets too. Destinations abound: Freeman Dyson once pointed out that the asteroids collectively contain at least ten thousand times as much surface area as Earth. Together with the planets like Mars and Saturn, as well as man-made destinations that will be built in the future, our solar system once opened will be the greatest frontier in human history. Anyone contemplating our real future or worldbuilding a work of hard science fiction set in the solar system of the near future would be wise to consider these facts and speculations about the transportation technologies that will likely be in our hands at the dawn of the extraterrestrial age.