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I had a thought a while back about interstellar flight:

If there are a lot of massive objects out there (rogue planets, maybe primordial black holes) then these could be used to aid acceleration and deceleration through gravity assist and the Oberth effect. (The latter describes how you get more acceleration thrusting near a massive body.)

You could fly along with an ultra sensitive gravity telescope / sensor and hop between massive objects.

You could also perhaps harvest propellant from some of these.

Long duration interstellar flight would be easier if the space between solar systems was not as empty as we think.

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I think gravity assists only really make sense when you're talking about orbits - a solitary object floating through the void won't help you accelerate, though it can help you make a cheaper "turn" in space.

Doesn’t the Oberth effect still work?

Actually, hm, I don't know. If you're approaching a mass in space, and firing your engine close to it - do you end up with a faster relative velocity than if you'd just fired your engines for the same amount of time in empty space?

edit: I did the most basic possible amount of research by reading the first paragraph of the wikipedia article (can we call that a milliHbomberguy?), and you're right, you do gain more velocity than just firing your engine in space. Although it puts a limit on where you'll be going after your approach.

The Oberth effect is fascinating. It sounds like a violation of conservation of energy but it’s not. You can get multiples of thrust.

An Oberth effect flyby of a primordial black hole with a high impulse drive like nuclear pulse propulsion would be pretty spicy. It’s a feasible way to achieve speeds close to the speed of light. Acceleration would kill you though.

If we did find such an object anywhere within a few light years of the sun it would be a way to send probes out at significant fractions of the speed of light.

Interstellar travel is only going to work if we're using much more powerful propulsion techniques than we currently are. I don't think gravity assists will help much in that case. I still think it's a boon to have a lot of interstellar bodies though, they are certainly easier to reach than the nearest stars and they could be just as interesting. Who knows what kind of weird, icy worlds might exist out there heated only by radioactive isotopes and tidal forces?

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