Last July at the Aosta conference Greg Matloff presented a paper on using near-Earth objects for transportation. It’s an interesting concept (discussed here), one that takes advantage of the fact that there are a few such objects that pass close by the Earth and then go on to cross the orbit of Mars. Greg was able to show that it would be possible to exploit this trajectory to use the NEO as what Buzz Aldrin has called an ‘orbital cycler,’ hitching a ride at least one way and disembarking upon arrival.

Reducing Starship Mass

The idea is useful because space travel requires so much energy. Put all this in the interstellar context, as science fiction writer Karl Schroeder does in this interesting essay, and you realize that whether we’re talking about beamed sails or antimatter or nuclear fusion, most of the mass of the vehicle is involved with accelerating and decelerating it. Schroeder pondered the question of using the cycler idea on an interstellar level. All you decelerate at destination would be your payload, while the cycler vessel simply keeps in motion, available for re-use at a much lower cost.

Schroeder likens a cycler to a generation ship in that it is intended to be self sufficient, and imagines using magnetic or plasma sails and particle beam propulsion for acceleration of rendezvous craft and their deceleration upon arrival in the destination system. A cycler is:

…the way-station for travelers, 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.

Thrustless Turning Between the Stars

Read Schroeder’s novel Permanence (Tor, 2002), for a look at cyclers in the context of a vividly imagined future universe. Cyclers stay in motion, using a combination of Lorentz-force turning and, if manageable, gravitational slingshot to alter their trajectory to pass by a number of stars before returning to Earth to begin the same journey again. Each cycler, even at fifty percent of lightspeed, takes a long time to make the rounds, but a network of such cyclers could sustain communications and transport needs for colonists on planets around nearby stars.

The cycler becomes a way station for cargo or travelers who rendezvous with it, ride the cycler to destination, and then use a magsail and particle beam propulsion from the destination system to decelerate once they’ve left the cycler upon arrival. This presupposes, of course, the ability to build these resources in the destination system, which Schroeder imagines occurring through a series of cargo drops involving robotic and perhaps nanotech tools to create the needed infrastructure.

How does Lorentz-force turning work? Here’s Schroeder on the subject:

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… 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.

Is Lorentz-force turning sufficient to manage such a trajectory? It’s possible we might need to use forms of propulsion like ion engines or beamed energy from the systems the cycler passes through to help turn the vehicle, so part of the cargo sent to any cycler might include the necessary fuel. But the advantages of the cycler are still notable. In Permanence, Schroeder writes about the ‘lit’ stars like our Sun, contrasting them with the much harder to find brown dwarfs, noting that there may be more brown dwarfs than any other spectral type in the galaxy. Add brown dwarfs into a cycler network and the power question changes as we exploit their magnetic fields. Schroeder again:

…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. 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.

Growth of an Interstellar Network

What Schroeder imagines is a ring of connected colonies using a network of cyclers to promote commerce and trade, a network encompassing both ‘lit’ stars and brown dwarfs. A new solar system is ‘seeded’ with robots programmed to build a particle beam system to decelerate incoming traffic. While colonization time frames are still large, the development of regular transport into such a system would allow regular cycler visitation and cargo delivery. A human colony could first be established by settlers leaving a passing cycler on a magsail rendezvous ship, knowing they would be part of the growing interstellar network.

Read Permanence for a look at cyclers in action. Thrustless turning using the interstellar magnetic field has been discussed in the scientific literature. Both Robert Forward and P. C. Norem considered Lorentz-force turning of an electrostatically charged spacecraft, and Greg Matloff has studied an approach to electrodynamic thrustless turning involving a partially sheathed superconductor. For more on that one, see Matloff’s Deep Space Probes (Springer, 2005). The Forward paper is “Zero Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889.

The Norem paper, meanwhile, is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Velocity Capabilities,” AAS paper 69-388 (June, 1969). Cyclers are a fascinating scenario for in-system travel, but driving even stripped-down cargo vessels to a rendezvous at half the speed of light, much less getting the cycler accelerated in the first place, remains a mammoth challenge. While there are no easy propulsion solutions, a far future society working with cycler principles could indeed create an interstellar network. All of which leaves those of us in the 21st Century to ponder the continuing conundrum: How do we push that much mass up to such speeds?