Can we use interstellar hydrogen as a fuel for interstellar travel?

Question

From my understanding, space is not entirely empty. Infact, between the voids of space are molecules or maybe individual atoms of hydrogen just floating around. I understand that gathering massive quantities of this from space would be a difficult endeavor; however, if we could refine technology to use hydrogen thrusters and create a collection device that would collect the sporadic rogue hydrogen atoms floating in space to refuel our reserves could we travel space on an interstellar level indefinitely?

A second part to the question if for some reason the first is completely implausible. If the intake of hydrogen isn't sufficient enough to replenish the reserves we have, would it be possible to collect what we could from interstellar space and then fill up on hydrogen from a nearby comet, asteroid, star, or planet(oid) when we run low?

Very farsighted questions, I know, but a real question nonetheless.

Answer 1

It's theoretically possible to collect fuel from near empty space, the bussard ramjet is an example of an engine designed to do just that. The principle is that you use magnetic fields to collect and concentrate hydrogen atoms from the near vacuum of space, and then a fusion rocket would turn some of this into propulsion for speed and a fusion reactor would convert the some into power to maintain the fields. The rest would get stored in tanks. In order to make this self-sustaining you would need to get going to a significant proportion of the speed of light so that the amount of hydrogen collected from space would be greater than the fuel needed to maintain the collection fields. So you'd have to have some fuel to begin with to get you going fast enough.

That's where the second part of your question comes in: are there sources of hydrogen fuel to be had in space? Yes, hydrogen is the most abundant element, there's loads out there. All you need is to find some water and split it by electrolysis, although you could also free hydrogen from other compounds. Once you have hydrogen fusion power these actions would be reasonably economical.

So if the technology actually works you could go from one star to another using a bussard ramjet, use your fuel reserves to slow down, explore (trade, lay waste, etc), then find a hydrogen source to replenish your reserves and go to the next star. Just make sure you don't end up in a solar system without enough matter or you'll never get going again.

EDIT: This is all very theoretical, whether it would work or not depends on how efficient you could make hydrogen collection, how much power you could get from the rocket, etc. It's far beyond current technical capabilities at any rate.

Answer 2

The issue is that the collection mechanism tends to produce more drag than thrust. Robert Zubrin and Dana Andrews showed that this makes most Bussard Ramjet-type starship propulsion impractical.

In fact, this sort of mechanism is so good at producing drag, it actually does function well for decelerating a starship once it reaches its destination, meaning that the starship can carry a larger payload or travel faster. This is called the magsail. Bit of history here.

Answer 3

As for the amount of hydrogen in Interstellar space, at present the estimates state that although the density varies. It could be as high as 1 atom per cubic centimeter in some regions. Which would mean that the magnetic scoop would need to be large enough to collect enough fuel to power the various system on board, as well as fueling the vehicle and stockpiling for regions such as planetary systems that will have much lower densities of hydrogen. Once a vehicle reaches interstellar space and the effects of planetary gravity reduce, then the need for hydrogen as a fuel will lessen. Of course at this point its all conjecture as the majority of a galaxies mass is located in regions of interstellar space, and with mass comes gravity. As we cant see this mass,(Dark matter and Dark energy) we have no idea of what to expect.