Can we use interstellar hydrogen as a fuel for interstellar travel? (2014)

My guess is this.

There is a lot of water ice and other volatiles (CO2, CO, CH4, NH4, ...) in comets out between the stars: if you add up all of those comets between the stars and that you can use 100% of the bulk it could be the major resource for life in the galaxy.

Close to a star you can get a lot of energy with thin-film solar cells, probably that is good at the Jupiter-Saturn distance where icy and carbonaceous bodies are the norm.

Once you get to some 1000's of AU you have to have some kind of nuclear energy and I'd imagine you could extract Deuterium from the water and do D + D fusion which is possibly workable if you build something like the ITER but a lot bigger. (D + T fusion is easier but you need somewhat rare lithium to make the T) There might be lithium out there or uranium, etc.

If somebody developed a lifestyle around comets they could hop from one comet to another and make it to the next star in 10,000 years or so. Whether it is something like an O'Neill colony or people just learn to live without gravity I think they could get comfortable and not feel like they were born into somebody else's dangerous mission.

Trouble is that the comet resources, if you can make the energy work out, are much greater than the resources you would get in solar systems. People comfortable living off comets would probably find planets like the Earth and Mars pretty boring from a commercial standpoint. (e.g. you are not going to come steal our water if you have access to generic outer solar system bodies that could be 20-80% water in bulk and small enough to disassemble.)