[rbanffy]

> Maybe not on earth, but there are applications in deep space.

Depends on how long the interstellar craft is supposed to travel. If it's under 100 years, fission should be able to do the trick of keeping the craft warm and the lights on for the sealed ecosystem to function during the decades of coasting between stars.

Fusion rockets would be more convenient than fission ones because you can store the hydrogen you need in the form of water and water also acts as a great radiation shield while in deep space. Then, to brake, you use your radiation shield as reaction mass for fission or fusion rockets.

If we are talking about much more than that, fusion is probably a better answer as fission fuel will half-life itself into paperweights over a grand transgalactic tour.

[eru]

> Depends on how long the interstellar craft is supposed to travel.

I didn't have (only) travel in mind. I was thinking of living between the stars.

[rbanffy]

You'll need a constant power supply for the entirety of the trip. After doing the math, it turns out, fission seems quite viable - all usual fuels, in storage, have half-lives of more than 10,000 years.

So, if you have enough fissiles for keeping the closed ecosystem happy for the duration of the flight, you can go quite far.

The ship/colony will need to enter orbit around a star and drop by a rocky planet at some point, to gather more fissiles and reaction mass (and other materials needed for fixes and upgrades), so it wouldn't be able to stay indefinitely in deep space. If it's fusion-driven, a gas giant may be a good option for both fuel and reaction mass, and icy moons may work well for replacing water.

[eru]

I'm not talking about travel. I am talking about permanently living between the stars, and essentially living off hydrogen harvested from the interstellar medium.