Defining the Problem

The huge distances between Earth and the nearest star make it necessary for us to conceive of extremely high-velocity starships if interstellar travel is to be possible with durations less than a human lifetime. In practise this means accelerating the starship to some percent of lightspeed. The problem with doing this, of course, is that truly phenomenal amounts of power are required to boost a ship to such velocities.[1]

Various propulsion schemes have been proposed, from nuclear fusion to antimatter to laser sails. Until recently, laser sailing seemed like the most economical and easiest way, even though it still requires that we build lasers that draw more power than all of human civilization is now capable of producing.

Lately, a new candidate propulsion system has been suggested: particle beam propulsion using magnetic or plasma sails.[2] Magnetic sails can be made much smaller and lighter than laser or solar sails; a magnetic sail is essential just a loop of wire with a current running through it. This mass reduction for the starship is the first reason particle beam propulsion would be cheaper. The second reason is that particle beams can be produced at a much higher efficiency than lasers. A neutral beam projection system could draw one-sixth the power of a laser and get the same results.

Best of all, from the point of view of any initial mission to another system, magsails can be used to decelerate (albeit slowly) and even to move around within the target system.
All of this sounds great; however, the costs and energies involved are still huge. And what's the payoff? You spend years pumping energy into an interstellar probe that you never recover, because the probe has to stop at its destination and the next time you want to send a payload, you have to pay the same energy cost to accelerate it. Star travel thus requires an ongoing, high energy cost that can only increase with any amount of traffic.

One of the traditional solutions to the energy problem has been the "generation starship" or slowboat. Slowboats are conceived of as giant self-sufficient arks that may take centuries to travel between stars. The attraction of slowboats is that they don't require the same set of engineering solutions, and presumably would be much cheaper. However, I don't think anybody's ever written a happy "generation starship" story. Slowboats raise the spectre of social decay and hopelessness more than inspiring a spirit of adventure.

My Solution

To me, the idea that you should expend billions or trillions of dollars to accelerate a starship, only to decelerate it again, is pure lunacy. 90% of the ship's mass is support structures--either power or life support systems. The key to viable interstellar transport, in my view, is simple: if you've got it all in motion, keep it in motion and re-use it. The only thing you want to decelerate at your destination is your cargo.

Apollo Astronaut and orbital mechanics whiz Buzz Aldrin has proposed a mechanism for travel within our own solar system that he calls an "orbital cycler". It turns out that there are stable orbits that cross both Earth's and Mars' orbits. Aldrin suggests placing a large permanent station in such an orbit. Travellers would embark on the station when it passed the Earth, and disembark as it passed Mars. The energy cost for the traveller is the cost to do the rendezvous at each end. The rest is free.

The system I am proposing works similarly, except that while Aldrin's cyclers are passive, using celestial mechanics to move them in a grand circle, my cyclers rely on active course correction. For this reason I'll refer to his cyclers as Aldrin cyclers or orbital cyclers; mine I'll call, for lack of a better term, Schroeder cyclers.

The Schroeder cycler is initially accelerated from the Earth or a colony star to some percentage of lightspeed. The velocity would be determined by how long the cycler can survive the battering of hard radiation at speed, and also by the turning radius required by its course. Once in motion, we leave the cycler in motion. It uses a combination of Lorentz Force turning and gravitational slingshot (if feasible) to alter its trajectory so that it passes by a number of stars in succession, finally returning to Earth to begin the cycle again. Even at half lightspeed, a typical cycler might take a century or more to make such a grand circuit of local interstellar space; however, if new cyclers are being launched every few years, there could eventually be more than enough of them to supply all the traffic that the solar system can afford to send out.

The key to making cyclers work is our ability to use the magnetic fields of the interstellar medium as a way of turning the craft. In Lorentz Force turning, you unreel several extremely long wires (tethers) and give them a high electric charge. Their interaction with the galactic magnetic field results in a slow, constant course correction for the ship. Over time, it can be enough to change the trajectory from one star to another. (In practise, since stars are distributed three-dimensionally and not like dots on a piece of paper, destinations may not line up so neatly. But there may be an answer to that problem; see below.) A Schroeder cycler would use this active interaction with the galactic field to change its course; hence it is using different principles than an Aldrin cycler, which relies on orbital mechanics and is essentially (and preferably) passive.

I should note that Lorentz Force turning might not be able to provide the "turning radius" we need for these cyclers. There are other ways of turning the ship, including using ion engines, or even a boost from the power beams of the system the ship is passing. Since cargoes can be sent to the cycler, fuel in the form of hydrogen or antimatter could be "shipped up" to aid trajectory changes.

A Schroeder cycler is like a generation starship in that it is intended to be self-sufficient. It is the way-station for travellers, who embark and disembark at the solar systems it passes. Since it supplies life support, passengers need only carry supplies necessary for them to make the rendezvous, which would probably take a few months' time. Even more dramatically, a non-living cargo sent to rendezvous with a cycler can be very light. Instead of accelerating an entire starship, you'd only accelerate the cargo, plus a wire to form the magsail and some attitude jets to make the rendezvous and docking. In other words, a cycler rendezvous craft is almost all cargo.

If a cycler is going be passing the solar system, long-term communications between it and Earth will establish a rendezvous, negotiate supplies, number of possible passengers etc. Then the solar launch mechanism goes into effect: the cargo is attached to a magsail and sent at a high acceleration to rendezvous with the cycler as it passes by. With small cargos, it is not unreasonable to anticipate 3 gs or more of acceleration on the cargo, giving it matching velocity in 100 days or less.

With cyclers in the picture, the energy and dollar costs to send a cargo to, say, Alpha Centauri need to be recalculated. Why send a 10,000 tonne starship, when we can accelerate 100 tonnes and get the same amount of cargo there? --And what's more, turn around and immediately do it again with the next passing cycler?

It should be quite possible to do this; however I've so far identified three technical difficulties with cyclers that other starship designs don't have:

Scaling the Heavens

The above title is a deliberate pun, because here I want to talk about the issue of scale in interstellar travel. Because Schroeder cyclers travel from station to station, they are most useful if we can scale up the number of stations they pass by, as mentioned above. This is where they open possibilities that other starship designs have not. Cyclers become feasible if we include in their rings not just the lit stars, but the invisible worlds of interstellar space: the brown dwarfs and trans-Jupiterian planets now known to exist between the stars.

There is evidence that there may be many more brown dwarfs than any other spectral type in the galaxy[3]. Certainly, red dwarf stars are in the majority of lit stars; why shouldn't there be even more substellar objects? The intriguing possibility has been raised that there might be one or more brown dwarf between Earth and the nearest star, and perhaps rogue planets as well.[4]

Such knowledge would be a mere curiosity if we were committed to the hell-bent-for-leather once-and-for-all approach to interstellar travel. It would also be academic if brown dwarfs were not a sufficient energy source to launch or receive cargoes. However, particle-beam propulsion systems use essentially the same mechanism as powers the van Allen belts of the Earth; and both Jupiter and the sun have vastly bigger energy belts than Earth. The radiation fields at Jupiter are intense enough to cook any human not shielded by several feet of lead. That energy, in the form of neutral beams, is what we would be using to propel our cargoes, or to initially launch a cycler.

Perhaps more importantly, Jupiter and the sun both display prodigious magnetic fields. A brown dwarf could be expected to do the same. This means brown dwarfs can probably supply the kinds of energy required to launch starships; instead of using solar power, as we might do near Earth, at a brown dwarf we would directly generate electrical power by putting long wires (tethers, like the Lorentz Force cables) in orbit around the dwarf.[5] A wire in a moving magnetic field produces electricity; in the kind of all-encompassing and intense field a dwarf might have, a lot of current would be produced; and if you orbited a million wires... again, things scale up nicely.

It turns out that brown dwarf stars possess a set of features that make them ideal way-stations on our trip to the stars. It is reasonable to expect that dwarfs would have satellites, some perhaps the size of Earth (Jupiter's moon Ganymede is almost as big as Mars). The kinds of energies available from a brown dwarf with a reasonable magnetic field would be more than enough to supply colonies in orbit or on the surfaces of these moons. The moons would be cold, but probably not as cold as Pluto or even Neptune; brown dwarfs radiate infrared energy, and as discoveries at Jupiter have taught us, moons of such large bodies can be extremely hot because of tidal forces (for an example, take the volcanoes of Io). In fact, it would probably be easier to manufacture and orbit a million power cables around a brown dwarf than it would be to manufacture and orbit a thousand square kilometers of solar cells at a newly colonized star; wires are just easier to make than solar cells.

This, then, is what the Schroeder cycler makes possible: a vast ring of interconnected colonies, some on lit stars, some at brown dwarfs, all joined by commerce and trade through numerous cyclers. The rendezvous systems could be working full-time if there were enough cyclers, bringing in cargoes and sending out new ones.

Colonizing New Stars Using Cyclers

The Schroeder cycler doesn't solve the problem of how to stop the first time at a new star. I would expect a combination of magsails and fusion or antimatter rockets could do the trick. What the cycler does permit is "ganging" the effort, instead of putting everything into one colonization ship. The problem the cycler solves is actually much more important: how to explore and settle a new stellar system without putting all your eggs in one basket.

If it turns out that a cycler can make a course correction to a new star and then return to its normal course, it will pay off for it to do so. Even if the cycler can't be spared, local systems that want to explore a new star can invest by sending up men and material to build a "research cycler" by the first one. That research cycler then splits off from the first and investigates the candidate star on its own. It can be a smaller-scale ship if it is intended to rejoin the larger cycler after its investigation (although we're still talking trip-times of years for it).

The first time a cycler passes by a candidate star, it can do close observations and even drop small probes to thoroughly investigate the system at low cost. If the system was on its route anyway, it would investigate as a matter of course. If things look promising, it and other cyclers can target the new system with magsail-dropped cargoes over a small or long period of time. You might picture the cyclers as swooping in like bombers, dropping their cargoes and returning to their grand tour. Some of the cargoes could be small--eg. robotic mining units, even nanotech if that turns out to be possible. Others could just contain supplies. They might loop through interstellar space for years or decades using the magsail to reduce their velocities so that they can settle into orbit around the star. When they do, though, it'll be in quantity.

The ideal situation is to seed the system with robots that can mine its resources and build a particle beam system. Power is easy, especially at brown dwarfs, as mentioned; particle beam generators are fairly simple devices as well, at least compared to lasers. The first one doesn't have to be sophisticated or even very powerful. As soon as it can decelerate a tonne from relativistic speeds the cyclers will start throwing cargo at it. After a few upgrades, human technicians would ride in the beam and prepare the ground for a full-scale colonization effort.
The main point here is that the robots building the initial beam system don't have to be fully-intelligent autonomous self-reproducing entities. After all, if cyclers are passing by regularly; the robots can work until they hit a snag and then wait for new orders, just like Pathfinder did at Mars. If they break down, they don't request resupply from Earth, they just wait for replacements. (It will probably be more efficient to just keep bombarding the system with new robots than to wait for the old ones to break down and replace them.)

So cyclers can be used to make the process of colonizing other stars much less a "one-chance" deal. The time frames involved are still large--on the order of decades to establish the initial particle beam system--but fairly predictable and most of all, controllable. Star travel will still require long years, but much exploration can be done by cyclers observing at the far end of their "orbits" or by probes they drop off.

Each cycler might take a century or more to make its complete circuit, and a given new world might be visited by dozens of them in the initial phase; each cycler might only participate in a single action before hurrying on in its millenial course. For the crews, however, time frames would be shorter, since they could be embarking and disembarking at the stars on either side of the candidate star; each crew might experience only one epic trip between the stars, lasting a few years, or they might criss-cross back and forth on cyclers going both ways, overseeing the crafting of a particle beam system that they or their backers have funded. Although the giant cycler support systems they use to do this would cost more than any individual or single government could afford, the travel price for these dreamers and their cargo might be realizable for corporations or extremely wealthy individuals. And once the beam system is up and running at the candidate star, they could be the first privately funded interstellar settlers, setting out from a passing cycler in a tiny magsail rendezvous ship, trusting their backers or business arrangements to keep them in supply long enough to make it good. And then they'll own worlds.

Research To Do

Although I've outlined a few ideas to do with interstellar cyclers, I don't have the technical expertise to follow through on some of the issues the ideas raise. If anyone would like to explore the math and engineering issues raised here, be my guest. Just some of the questions I think need answering:

• Just how long can a starship survive travelling at relativistic speeds? What shielding technologies are possible?
• What is the "turning radius" of a starship using Lorentz Force turning for a given velocity %c?
• Are there other practical turning mechanisms that would make cycling more efficient than direct source-to-destination trips?
• What kind of communications and coordination are possible between a relativistic starship and a planetary system?
• Is a rendezvous possible between bodies travelling at such velocities?

Conclusion

Travelling between the stars is undeniably the most daunting project humanity has ever concieved. The distances involved are literally impossible for the imagination to grasp, and the velocities required to traverse them are many orders of magnitude greater than anything we've been able to attain to date. One thing remains true, however: the more we analyze the problems, the more they seem to be tractable, if not now then within the relatively near future. Much of what we have to do in order to conquer these distances is to think about them, and about how we will travel, in innovative ways. Each innovation will later look obvious in hindsight.

I confess that the idea of the interstellar cycler seems so obvious to me now that I cannot help but think that it must have been thought of, and dismissed, many times. I can't find any references to such a concept in the literature, however, and for the life of me I can't see why it won't work. Is there an obvious problem (beyond those that plague all starship designs, that is)? Let me know. Meanwhile, I've used the idea of interstellar cyclers in one short story ("Halo", first published in Tesseracts 5, Tesseract Books, 1996, reprinted in the David Hartwell and Katherine Cramer anthology The Hard SF Renaissance for Tor Books). These ideas also form the basis of my new novel Permanence. If they turn out to be wrong, I'll have egg on my face.

Meanwhile... it's always fun to speculate.

Appendix: Linear Cyclers

If turning a cycler is prohibitively expensive, can the system still work? Imagine a set of colonies, on a mix of stars and substellar objects that lie on a line across a span of say, ten light years (stars rarely line up so conveniently, but if you add the substellar objects, the chances increase). Suppose that the worlds at the ends of the span launch proto-cyclers (basically life-support systems and nothing else) along the line. For each of the other worlds on the line, travel costs are now reduced; they merely rendezvous with the passing life-support package, without having to ship all of it up themselves. It might seem that the worlds on the ends of the line get shafted, but provided each world leaves a bit of surplus on board the cycler when they disembark (fine art, fossils, frozen embryos) then when the cyclers reach the ends of the line, each is decelerated and both worlds that launched one get a substantial cargo in payment; in fact, with one starship launch they each receive cargos from a number of worlds. For the other worlds along the line, economics work exactly the same as in ring-shaped cycler systems.

We can conclude from this that the basic concept of the cycler is that of a cooperative energy-saving system among established colonies, and that it does not rely on having a starship be able to revisit the same ports.

References

[1] Starship Designs: Sailing Ships -- Solar and Laser Driven Lightsails.

[2] Starship Designs: Sailing Ships -- Relativistic Particle Beam Propulsion.

[3] A New Class of Star, by Robert Naeye (Astronomy Magazine).

[4] Hubble Takes First Image of a Possible Planet Around Another Star and Finds a Runaway World, Hubble Space Telescope News, May 28, 1998.

[5] Tethers Unlimited home page.

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