Spacecraft May Surf Magnetic Fields

Maggie McKee writes "Future spacecraft may surf the magnetic fields of Earth and other planets, taking previously unfeasible routes around the solar system, according to a proposal funded by NASA's Institute for Advanced Concepts. The electrically charged craft would not need rockets or propellant of any kind. To get charged up, the spacecraft could either fire a beam of charged particles into space or simply allow a radioactive isotope to emit charged particles. Because long, thin filaments have large surface areas for holding charge, the spacecraft might look like Einstein's head — with wiry filaments sticking out at all angles — or a weird space 'stocking.'"

Umm... capacitance of the ball made of filaments?

[Alex Belits]

But the amount of charge held by a sphere at a given voltage, a quantity known as its capacitance, is not very large. Long, thin filaments, on the other hand, have a lot of charge-holding surface area, so one possible design involves many filaments attached to the spacecraft. The setup would have a rather comical look - because of the static charge, the filaments would stick out in all directions, like newly brushed dry hair.

Where did those people study Physics? It doesn't work that way.

The only surface area that matters is an equipotential surface, so in the case of "filaments sticking in all directions" it will be a roughly spherical surface formed by the ends of filaments. Within this sphere there is almost no electric field -- filaments can be seen as a kind of lightning rods, except there is no lightning because they are in vacuum. So at best they will have a larger sphere, at worst a cigar or other shape with less surface area. If one has to build a large but light sphere, he can make it out of the wire mesh -- in vacuum it won't discharge like it would in the air, where those spheres have to be smooth. Filaments or spikes can be useful for acceleration of charged particles.

As for usefulness of the whole thing, I guess, you can use this for steering the spaceship, however the analogy to surfing is very poor. Surfboards can accelerate by absorbing the energy of waves moving from deep to shallow water. This thing flies through a stable magnetic field, steering by changing its electric charge. A better analogy would be a sailboat changing tacks, with gravity acting as a wind and magnetic field as water resistance.

Re:why wires don't work

[frostband]

"induced current was about 9600 volts"

Why do I feel the need to point this out?

I know what he meant, but why did you make me reply? Damn you, EE degree, stop controlling my life!

Re:Umm... capacitance of the ball made of filament

[Alex Belits]

It will be the same as in vacuum tubes and CRTs -- spikes themselves won't do much, but you can heat up the cathode, and create some electric field with anode -- electrons will be emitted, some of them will be captured by anode, the rest will escape the device, leaving it positively charged. As long as anode is positive relative to cathode, electron cannon will work. You can lose electrons until the point when you are so positively charged that all electrons that left the cannon have less energy that it is necessary to leave the spaceship.

Or you can make a source of positively or negatively charged ions -- they can be accelerated in a similar way. Again, this probably can be much easier achieved by messing with an ion engine, under/over compensating the positively charged ions that leave the engine.

Re:Umm... capacitance of the ball made of filament

[Ceriel Nosforit]

With a radioactive source emitting alpha and beta radiation it might be possible to use two ion drives; one ejecting the positively charged alpha particles and the other ejecting the negatively charged beta particles. That way you won't build up a total charge in the craft. Plus, there won't be any charged cloud tracing the craft since the sum total charge of the ejecta is zero.

Though apparently, the NASA people WANT to build up a charge... Meh.