A while back, we had a thread on sailing a lighter-than-air vehicle, the consensus being that you really need two media to push against if you want to be able to steer. Ok, so that kills my whole sky-pirate dream, but hey, laws of physics and all that.
I just recently read The Algebraist, by Iain M. Banks, in which gas-giant dwellers do some sky-sailing by using the rather hefty magnetic field of the planet as a second medium to push against. So I’m wondering, how feasible would that be? From what I remember of high school physics, a wire carrying charge can produce force in a magnetic field, but I don’t know how controllable that would be - presumably, you’d need a force opposing the direction of the wind. Are there other ways to use a strong magnetic field to push against? Would you need electricity flowing, or would strong enough magnets do the trick?
In principle, permanent magnets would work, as long as you have some way to move it around and point it in different directions. It’d still be pretty impractical on Earth, but on a gas giant, maybe.
How would you do it with permanent magnets? I thought permanent magnets only twisted in magnetic fields, and I can’t think of a way to sail using just torque. Am I wrong about how they work, or is there some way to use twisting?
IIRC, in order to get a magnetic field, you need liquid metal sloshing around your core. The Earth isn’t particularly large as planets go, and it’s lucky enough to have a molten core (tho’ I don’t think Venus does. Not certain why.)
Underneath all that gas, Jupiter’s got a core of rock and metal. And because it’s so huge, the pressures are enormous - enormous enough to not only (likely) keep its metal core molten, but also to cause the helium and hydrogen it retains to exist in a metallic state. I believe it’s the combination of the two that allows for Jupiter’s massive electric field.
You don’t need molten metal, per se, just any conductive fluid. And in fact a less viscous fluid will work better, which is part of the reason why Jupiter has a stronger field than the Earth.
Venus probably does have a molten core, but it (and Mercury) lose out in the magnetic field department because you also need rotation, and they both rotate extremely slowly. The gas giants all rotate faster than the Earth, and have a better fluid and are larger, so they all have stronger fields than Earth. Mars rotates at about the same rate as the Earth, so you’d expect it to have a reasonable field, but for reasons that aren’t well understood, Mars does not have a well-differentiated iron core.
It’s interesting to note that the Hubble Space telescope does something along this line:
It uses electrically powered reaction wheels to control the direction it points. Because these can be spun only up to some limiting RPM, there needs to be a way to dump energy without causing unwanted torque. The solution is to have electromagnets and use the Earth’s gravitational field - with care, you can slow a reaction wheel without any net torque on the satellite.
So, it looks like Jupiter’s magnetic field is around 14 times stronger than Earth’s - is that enough to give you something to push on? Also, is that roughly what one would expect in a gas giant? For that matter, do we have enough data points to estimate that?
Now that you mention it, possibly no. Not unless you make a really big magnet, of a size on the scale of the non-uniformities in your field. (Right guys? There’s no magical relativistic spinning phenomenon that could work?)
The thing you could do, which is what NASA is doing, is to use the property of a charge moving through a magnetic field. But the trick is to have the charge only move in one direction. (Figuring that out is an exercise for the reader.)
I’m still not sure how you could travel along the direction of the field, though. Oh, I know, monopoles! The cause and solution to all of life’s problems…
Is the total magnetic field 14x stronger, or is the magnetic flux density 14x greater at the surface?
Anyway, the equation for the electromagnetic thether is this:
Current in Amps x Field in Teslas x Wire length in meters = Force in Newtons
For a 10-foot wire carrying one amp in the Earth’s field, you get 3 x 1 x 0.000040 = 0.00012 Newtons = 12 milligrams equivalent. Multiplying that by 14 doesn’t add up to much, but maybe you could signficantly up the current if you get a nice plasma channel to conduct the return. (Note: you can’t just use multiple conductors to increase length.) But guess what this whole electro-tether / atmospheric return current set-up will really do for you: it’ll be an incredibly energy-expensive way to make a fan.
A wire carrying a current (flowing charge, as opposed to a simple static charge) will produce a magnetic field whose lines encircle the wire. If the wire is arranged into a coil, the field lines form a torus. Now you have an electromagnet (EM), and this does interact favorably with the planet’s magnetic field.
To fly toward the equator, you would orient your EM so that its north pole faces the planet’s north pole (and EM’s south pole toward the planet’s south pole); the two fields will repel each other, and your vessel will be pushed toward the equator. Any slight misalignment will tend to cause the electromagnet to want to flip so that its north pole swings toward the planet’s south pole; some sort of automatic control system would be required to make constant minute corrections, the equivalent of a modern fighter plane’s “fly by wire” control system. That tendency for the EM to rotate its north pole toward the planet’s south pole could also be used purposefully to reorient the ship as needed.
To fly toward the nearest magnetic pole (e.g. the north pole if you’re in the northern hemisphere), you would orient your EM so that its north pole faces the planet’s south pole. Your EM (and vessel) will be drawn toward the direction of increasing magnetic flux, i.e. toward the planet’s nearest magnetic pole.
I don’t think you could magnetism alone to propel the vessel east or west. But you could combine it with aerodynamics to “tack” east or west, sort of like a sailboat.
I’m not following the question: Magnetic field is the same thing as magnetic flux density.
And come to think of it, runcible spoon might be right about needing a very large magnet (comparable to the size of the planet, if the planet’s field is dipolar). Lemme think about this some more.
The cause and solution to all of life’s problems…