Interstellar Space: Uses of the Medium

by Paul Gilster on March 20, 2012

One of the first things we need to do in terms of interstellar exploration is to get a spacecraft built for the purpose to travel outside the heliosphere and give us solid measurements on the interstellar medium. The Voyagers are doing their best but they were never designed for what has become their interstellar mission, and while we can marvel at their longevity, it’s with the knowledge that their resources are few and their years of useful data gradually drawing to an end. Something along the lines of Ralph McNutt’s Innovative Interstellar Explorer would do the job nicely, allowing us to sample the environment that much longer missions will have to work in.

Lorentz Force Turning

The interstellar medium (ISM) is important not just because we have to learn about things like shielding a fast-moving spacecraft and cosmic ray flux but also because we may be able to use some aspects of the medium for deceleration. Yesterday’s discussion of magsails reminded me of a 1969 paper by P. C. Norem that took a truly round-about way to stop a laser-beamed lightsail at a destination star. Norem was interested in what we can call ‘thrustless turning,’ an idea Robert Forward explored in a 1964 paper (thanks to Gregory Benford for sending the Forward document, my copy of which had disappeared somewhere in the wilds of my office).

Norem’s notion was to send his spacecraft on a trajectory taking it far beyond the target star, using long wires and an electrical charge induced on the spacecraft to allow interactions with the interstellar magnetic field to cause it to turn. The vehicle would actually approach the star from behind (as seen from Earth), allowing a laser beam from Earth to slow it for system entry and exploration. The idea takes advantage of the fact that a charged object moving through a magnetic field experiences a Lorentz force at right angles to its direction of motion and the magnetic field itself. You can see why this would be attractive to those hoping to use the local interstellar medium to accomplish what would otherwise require massive propulsion systems.

Norem was able to extend the idea into a concept for a round-trip mission, because he realized that he could once again accelerate his laser-sail by turning on the beam from Earth. Coming up to cruise, the craft (now moving away from Earth) would again use Lorentz turning to make the needed 180 degree maneuver that would put it on a trajectory back to our planet. Final deceleration into the home system would be with the sail deployed against the same laser beam.

Image: An early concept for the Innovative Interstellar Explorer probe to the interstellar medium. Credit: JHU/APL.

Forward hadn’t worked out the laser sail ramifications as early as 1964, but he thought thrustless turning was a workable mechanism, one powerful enough, by his calculations, to allow not only for mid-course corrections but in some cases to return a small probe to Earth after its journey, in which case we have the odd situation in which the energy required to launch a flyby probe to a star is also the energy needed to fly a round-trip probe. Inspired by Norem, he might have considered deploying a thrustless turning system on some of his own designs, but I imagine that the idea of tripling the mission time, which is what would have happened, for instance, on one of his hypothetical Barnard’s Star laser sail missions, may have led him to drop the idea.

Powering Up the Spacecraft

But the notion persists that the interstellar medium is supple and useful if we can learn how to take advantage of it. We also need a lot more data — Forward noted that his 1964 calculations could be carried out only to 20 percent accuracy because parameters like the strength of the interstellar magnetic field were not yet known. But the physics of thrustless turning as applied to an interstellar mission are well worth considering as we continue working on future missions to the ISM. Here’s Forward’s overview of the idea in terms of technology:

In order to use this force in space effectively, it is necessary to find an efficient lightweight method of maintaining a substantial charge on a space vehicle in spite of the discharging effects due to field emission and ion capture from the surrounding regions. It is shown in the following sections that the concept is quite feasible for probes or vehicles in interstellar space, whereas it would not work in interplanetary space because of the high ion densities near the sun. By using a long, thin quartz fiber to increase the capacitance of the probe, the charge-to-mass ratio can be made very large without having to use high voltages. This, in turn, means that the necessary voltage and current can be obtained from a few grams of a suitable radioisotope or a very small charged particle acceleration.

Gregory Matloff considers thrustless turning in his book Deep Space Probes (2nd edition, Springer, 2005), where the charge carried by the spacecraft is generated by the decay of radioactive isotopes. He notes that a starship of any substantial size would demand an enormous electrostatic charge to make the turning maneuver feasible within decades. But in a 2005 paper with Les Johnson (also kindly sent by Gregory Benford), Matloff examined the use of an electrodynamic tether (EDT) to supply power to an Alpha Centauri expedition that would take 1433 years to reach its destination. In the conclusion of that paper, the authors make the case:

Electrodynamic tethers have a number of applications to interstellar travel. Consideration of a model for a sample world-ship mission through the local interstellar medium reveals that the interaction between an EDT and the interstellar magnetic field can satisfy on-board starship power requirements without an inordinate amount of starship deceleration [in other words, magnetic braking induced by the tether is found to be a minimal consideration].

Thrustless turning using an EDT’s interaction with the interstellar magnetic field will allow for course correction and rendezvous of solar sail-launched modules in interstellar space. It will not, however, allow rapid thrustless circling to allow a starship to re-enter a power beam or make numerous solar passes.

Lorentz force turning turns out to be slow and power-demanding, and maintaining the charge is also an issue because interstellar ions of opposite charge will be attracted to the spacecraft, thus reducing the effective charge. But the work of Forward, Norem, Matloff and Johnson on thrustless turning reminds us that interactions with the medium itself may become a component of starship design, just as the magnetic sail idea — braking against a stellar wind — uses the ambient environment to do something that would otherwise demand onboard fuel. Tomorrow we’ll look at a novel way of taking advantage of a star’s own interactions with the interstellar medium to slow a starship, a kind of solar sailing that may reduce overall travel times.

The Norem paper is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Capabilities,” AAS 69-388 (June, 1969). Robert Forward’s paper on Lorentz force turning is “Zero-Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889. Matloff and Johnson write about electrodynamic tether possibilities in “Applications of the Electrodynamic Tether to Interstellar Travel,” JBIS 58 (June, 2005), pp. 398-402.

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Rick Pikul March 22, 2012 at 0:41

Hmmm, an obvious extension of using Lorentz Force Turning to convert a flyby into a return is to spend a bit more time turning to convert the trip into a double flyby, (with or without a return).

The second pass could very well be worth the extra time. This would be particularly true if the distances and times involved allow for a command set to be sent for the second pass based on the data from the first.

Christopher L. Bennett March 23, 2012 at 11:29

The use of an electrodynamic tether to generate power from the ISM intrigues me. In my first published story in ANALOG, “Aggravated Vehicular Genocide,” I featured an alien race that lived in O’Neill-cylinder habitats in interstellar space, and I’ve been wondering recently where they’d get their power from. I figured they might do flybys of brown dwarfs and stockpile hydrogen and thermal energy, but ISM power tethers sound like a more reliable energy source.

I wonder, given that their habitats rotate for gravity, would there be a benefit to, say, surrounding the habitat with a wide rotating ring of conductive wire? Would you get a decent dynamo effect from its rotation in the galactic magnetic field? Or would you need an impractically wide ring to get any reasonable power generation from that?

Eniac March 23, 2012 at 15:28

Christopher: Any energy you’d get that way would be taken from the ship’s motion, which was presumably generated by a drive of some sort. These “sources” of energy are thus secondary to the star drive, similar to the alternator in an automobile.

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