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Have you heard of the idea of running a current through a long wire to propel a spacecraft against the earth's magnetic field?

I remember reading about it somewhere, but I can never find it again.

Yep. It's called an electrodynamic tether. Proposed for stationkeeping for everything from space stations to skyhooks, among other things.

Might be too late to follow up. (I wish HN had the response alerts like Reddit does)

But how can you have a current in a single wire without a loop?