Cornell aerospace engineer Mason Peck captured the attention of Centauri Dreams readers recently when Larry Klaes wrote up his ideas on modular spacecraft and self-assembly. Peck has talked about using the technology, which draws on a property of superconductors called magnetic flux pinning, to assemble or reconfigure structures in space without mechanical hardware. These are provocative concepts, and Dr. Peck has been kind enough to provide answers to questions from reader Christopher Bennett, beginning with whether or not his notions bear any resemblance to an idea long familiar in science fiction, the manipulation of objects by force fields. Here’s what Bennett wondered about reconfigurable structures in space:
“It sounds like what’s being talked about here is something surprisingly similar to the old SF idea of building with forcefields. Do I understand this right?” Are we talking about creating clusters of unconnected components that are held rigidly in place by magnetic fields even without touching each other? I don’t quite see how that works — I can get a field causing a continuous pull or push, but I don’t get how that pull suddenly stops pulling and holds an object at a fixed distance.”
And here is Dr. Peck’s response:
[On the issue of force fields,] the applications are similar. In this case, an object with a magnetic field, like a permanent magnet, experiences a force when it’s placed in proximity to the Type II superconductor. What makes the effect really valuable is that the object might feel no force in some particular position and orientation, but when it moves, the superconductor forces it back into place. In that sense, flux pinning acts like a six degree-of-freedom spring (three translations and three rotations) that keeps the object hovering in space.
What happens is that the superconductor dislikes changes in the state of little electrical currents that eddy around inside it. That’s why superconductors conduct electricity perfectly: the current isn’t allowed to change as it passes through. When the magnet moves, it tries to induce a current in the superconductor. The superconductor puts up a fight, resisting that change. If the magnet pushes back hard enough, the superconductor relents and soaks up some current. There’s mechanical work done, and energy transferred in that process. Small displacements of the magnet from its flux-pinned equilibrium result in a little vibration of the magnet, and then the magnet typically returns to equilibrium. Large displacements cause a sort of hysteresis behavior, where the magnet can establish a new equilibrium position. Changing the position can result in so much energy exchange that the motion becomes heavily damped–i.e., vibrations calm down quickly. All this is very useful in building structures in space.
Action at a distance inspires lots of science-fiction ideas. I suppose the reason is that electromagnetism, gravity, and inter-nuclear forces seem somewhat beyond our ability to appreciate on an instinctive level. They don’t conform to our daily experience, in which matter has to touch in order to exchange momentum, i.e. interact via forces. In the case of flux pinning, the superconductor acts like a very high-bandwidth active control system, continually tweaking its internal magnetic field to maintain this stable relative equilibrium. That’s necessary because without this sort of feedback, a theoretical result known as Earnshaw’s theorem proves that no constant electromagnetic forces can produce the kind of equilibrium we achieve with flux pinning. For example, Earnshaw prevents you from being able to devise that 3D hovering magnetic setup you’ve been thinking about. The magnets will collapse onto one another or spring apart unless at least one degree of freedom is constrained somehow. It’s also the basis for your insightful comment that you can “get a field causing a continuous pull or push.”
I’ve never really understood how a force field is supposed to work. I suspect it’s a misnomer. We understand what a “potential field” is, and we know that force equals the negative of the potential’s gradient (or local rate of change). But no physics I’m aware of allows force to hang out in the middle of nowhere, waiting for someone to walk into it and receive an unpleasant surprise. I suppose if one could devise a way to collect force-carrying particles (like gravitons) and cause them to hover in place, such an effect might arise; but that’s completely speculative on my part. You would be better off asking a particle physicist how he might accomplish such a thing. Flux pinning might seem magical, but as Clarke says, any sufficiently advanced technology is indistinguishable from magic. Flux pinning isn’t even that advanced. It’s a well-known property, but our research group may be the first to consider applying it as a technology for building structures in space.
If you want to build, say, a Star Trek style door that confines people to the brig, you’ll have to look further. Flux pinning works only on magnetic materials. Sure, there’s a little magnetic material in your body, but it’s not enough. In fact, your body is also weakly diamagnetic (see http://www.hfml.ru.nl/froglev.html). Unless you can strongly magnetize a prisoner, he’s unlikely to stay put merely with flux pinning.
And how rigid a connection are we talking about?
For a few ounces of magnet and superconductor material, one can achieve about 400 N/m stiffness. More material adds stiffness. If you try demonstrating the effect yourself, you’ll discover that it’s quite difficult to press a magnet about 1” in diameter close enough to a superconductor to make them touch. It requires some surprising strength.
If one component of, say, a multi-piece spaceship were under thrust, would the other components travel with it and stay in the same orientation? Could astronauts push off from one module to the next and have them stay in place? Could you have something like, say, the starship Enterprise without the connecting pylons?
Depends on the thrust. Anything we’re likely to see in the next few decades in space will not experience acceleration above 1g. Most forces in space cause micro-g accelerations. We know that flux pinning can levitate and suspend objects in gravity (1g). So, flux-pinning forces are likely sufficient in the near term in a variety of applications. I don’t know what accelerations the structure of the Enterprise experiences during warp drive. No-one inside seems to feel it. So, I suppose it’s possible.
And is this something that would require power to maintain, like a fictional forcefield, or would it be intrinsic to the materials?
That’s the best part. The connection requires no power to maintain the equilibrium. All that is necessary is for the superconductor to remain cold. And space is pretty cold already. Our favorite material, YBCO [yttrium barium copper oxide], needs to stay colder than 88K. Even colder is better. That’s above the temperature of liquid nitrogen and well above temperatures that can be achieved on spacecraft that use “sun shields” to keep things cool. Beyond the asteroid belt, roughly, a bare superconductor in space would stay cold enough without any other means of keeping it cold. So, a permanent magnet and a superconductor would remain in a flux-pinned equilibrium for a long time. In our lab, we can keep the superconductors cold for about 15 minutes with a thin layer of foam around them–and that’s in a room-temperature environment with convection (heating by wind). Doing the same thing in space ought to be easier.
Needless to say, Centauri Dreams thanks Dr. Peck for taking the time to fill us in on these concepts, which suggest new methods for accomplishing a wide range of potential missions. We’ll keep a sharp eye on future developments growing out of this work.