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Say there's a powerful magnet in an orbit between those of Earth and Mars. Could a spacecraft traveling between those planets save propellant by performing a slingshot maneuver using its magnetic field?

If so, how reasonable and useful would it be to build space infrastructure in the form of a ring of magnets in the same orbit, so that there's usually a magnet in position to assist any transfer you want to do through that orbit? Would the spacecraft need an electromagnet to take proper advantage of the magnetic field? (Would that cost too much mass / energy to be worthwhile?) Would a magnetic field powerful enough to be useful for this purpose pose a risk to equipment or people on the spacecraft?

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    $\begingroup$ I think I remember writing an answer to a different question that might also apply here as well. Yes magnetic fields allow for two objects to give each other a kick, and if the orbital magnets were heavier than the spacecraft the spacecraft could get the bigger kick. The problem is that force between two dipoles falls off as $1/r^5$ outside of a range equal to the size of the dipole, so if both magnets were 100 meters you'd only feel much force over less than 1 km travel nearby, and it would be pretty weak for known magnet technology, even if huge and incredibly expensive to put in orbit. $\endgroup$ – uhoh Nov 30 '20 at 22:49
  • $\begingroup$ this answer to Could a CubeSat be propelled by this low-tech electromagnetic propulsion system? and this answer to Could a horseshoe magnet help a satellite stay in orbit? $\endgroup$ – uhoh Nov 30 '20 at 22:54
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"Give me a place to stand and with a lever I will move the whole world." (Archimedes of Syracuse)

In a slingshot maneuver:

  • The planet is that place to stand: Its mass is so huge compared to your spaceship that you don't have any (realistic) chance of moving it.
  • The force of gravity is your lever: You use gravity to pull your spacecraft as it passes by.
  • Your spacecraft is "the whole world" here: It means the whole world to you, so what I've just told you is true from a certain point of view.

You plan on using magnetic forces as lever instead of gravitational, but you still need a place to stand. Unfortunately, no such place exists between Earth and Mars; the magnet's orbits would be disturbed the same as yours. The bigger they are, the less affected they will be, but unless they are really massive (like, Death Star massive) they will need constant orbital corrections.

In the end, you will spend a lot of energy putting the magnetic stations in place, and will need to keep them supplied with propellant just to provide a small kick to every passing ship. It would be more economical to just add a little extra punch to every Martian launch.

From the above linked Wikipedia page on slingshots:

A close terrestrial analogy is provided by a tennis ball bouncing off the front of a moving train. Imagine standing on a train platform, and throwing a ball at 30 km/h toward a train approaching at 50 km/h. The driver of the train sees the ball approaching at 80 km/h and then departing at 80 km/h after the ball bounces elastically off the front of the train. Because of the train's motion, however, that departure is at 130 km/h relative to the train platform; the ball has added twice the train's velocity to its own.

You would be bouncing tennis balls off supermarket trolleys, after a few bounces the trolley's movement has been disturbed. The fact that you use magnetic forces has little effect on the simple arithmetic of conservation of movement.

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  • $\begingroup$ +1 for Syracuse; had I known that Archimedes lived there when I'd visited I would have looked them up! $\endgroup$ – uhoh Dec 4 '20 at 2:01

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