> I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light.
It's not that bad. With currently known science, your fuel would most likely be hydrogen so you can run a fusion reactor.
The rocket equation tells you that most of your starship by mass would be fuel, if you want to go fast.
Of all the stuff on your ship that's not fuel, you'd probably need quite a bit of water for survival needs.
So you would make your spaceship relatively long and thin (to maximize internal volume for a given frontal area), and you would store your fuel (and water) in front of you to serve as exactly that shield.
Bussard ramjets will have problems at higher relativistic speeds. The fundamental issue[1] is that from the perspective of a near light speed system, everything else is near-frozen - which includes things like neutron capture and electromagnetism.
It's sort of funny, but dozens or hundreds of orders of magnitude below, the same sort of dynamics are at work in air-breathing ramjets. The impact velocity of the medium is starting to tell, and the exhaust velocity isn't particularly more energetic than what's threatening to ionize the air around your leading edges.
[1] Well, aside from the fact you're exceeding the average velocity of your exhaust mass
Your fuel is the outermost layer of your shields (modulo whatever is necessary to keep the fuel in place. But you might use magnetic fields perhaps).
That's basically free shielding: you have to carry the fuel around anyway, so you might as well put it to good use. If you run a nuclear fusion reactor, you won't really lose much of the mass of your fuel, unless you want to. Eg you could use the helium you produce as the reaction mass for your ion drive. (I haven't done the numbers to see how the required mass per second for your ion drive compares to the helium mass per second a nuclear fusion reactor would spit out.)
Because it's a free shield, you don't really get to complain about your shield being gradually consumed.
Of course, you can have some extra shielding further inside. You would keep your water forward of your people, but behind your fuel. So your water would not bear the brunt.
Hydrogen doesn't really get all that radioactive: you can use chemical means to remove any helium or so you might accidentally produce; and hydrogen's isotopes are both pretty short lived and relatively easy to separate. (At least much easier than eg enriching uranium.)
Your water and food is also only a very small fraction of the overall mass of your rocket: as always, the vast majority is made up of fuel.
Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel.
And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually impossible to isolate tritium out of water. So if any tritium forms (which is an extremely common by-product of any fusion reactions which might happen, and the most common decay product of heavier hydrogen isotopes), it will render your water quite poisonous for human consumption, virtually irrevocably.
> And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually impossible to isolate tritium out of water.
On the contrary, it's quite easy to do if there is any significant fraction of tritium present. The proportional mass difference of ³H vs. ¹H is x3, which alters the chemistry enough to make separation easy. You can use fractional distillation or electrolysis even for ²H — even mere hobbyists involved in the DIY fusion reactor scene sometimes extract deuterium from water this way, tritium would be easier.
With a half-life of 12 years and the time interstellar voyages take, you can just let your tritium sit around for a while. (In addition to what the other comment said about separation actually being relatively easy.)
Depending on your fusion reactor, you might actually highly value any tritium produced, instead of seeing it as a nuisance.
> Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel.
With a nuclear engine you make a difference between fuel and propellant. When you fuse your hydrogen into helium, you still have the helium afterwards.
I haven't run the numbers to see how your fuel consumption would compare to your propellant consumption for a reasonable fusion powered rocket. Though I suspect that you also need oodles of propellant, given how the rocket equation works.
For the first part of your journey you could rely on passive shielding via your propellant. For the latter part, you could use more costly active shielding like a big magnetic field or ablative shields in front of your main rocket etc.
Basically, you would still want to use your propellant as free shielding as much as possible.
It's not that bad. With currently known science, your fuel would most likely be hydrogen so you can run a fusion reactor.
The rocket equation tells you that most of your starship by mass would be fuel, if you want to go fast.
Of all the stuff on your ship that's not fuel, you'd probably need quite a bit of water for survival needs.
So you would make your spaceship relatively long and thin (to maximize internal volume for a given frontal area), and you would store your fuel (and water) in front of you to serve as exactly that shield.