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Comment by Robotbeat | Hacker News Reader

Beamed propulsion of some sort (not necessarily using lasers) relying primarily on solar energy is the best option. Nuclear is useful for ancillary propulsion. Chemical not useful except for launching all the stuff around the solar system to build such a system.

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But how do you stop?

Well... I like the beamed energy idea. It works to get you out of the solar system. But how do you continue to accelerate from other stars? You fire ahead of you a construction kit for a solar-powered beam-generating station. That beam station can be used either for acceleration or for braking.

But how do you get the kit to decelerate? (Remember, you fired it ahead of the ship.) And this problem gets worse as the velocity of the ship goes up. If, say, your construction kit enters a solar system at 0.1 c, it's going to cross it in maybe twenty days. I don't think that's enough time for a solar sail to effectively decelerate it. It's probably going too fast for using planets' gravity to slow it down. And using atmospheric drag is an explosively bad idea.

I don't have an answer...

Your braking laser kit is fired out 1000 years ago.... and you hope it makes it there on time

Braking against the interstellar medium (and possibly the stellar wind as you get closer) using magnetosail-like devices… between the stars isn’t completely empty but has a diffuse plasma that can be pushed against magnetically or electrostatically. This is how you’d slow down in pretty much any scenario as it is “propellantless.” It’s a big challenge to get the superconducting coils light enough, but it is a critical piece to interstellar flight, particularly if crewed.

I actually ran numbers on electrosails to decelerate, they do not work, at all - many orders of magnitude. Email me for spreadsheet. Pulsed nuclear propulsion is the only tech we have that works

Electrosails aren’t as good for this as magnetosails; I just included electrosails for completeness.

Could a black hole be used?

There is apparently a BH with 3x solar mass about 1,500LY away - escape velocity at 830 radius (about 820km from event horizon) is about 0.1c.

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