Interstellar medium as a filling station – a model calculation

In my book “Proxima Rising,” I describe how a spaceship, which at first is the size of a needle, is accelerated by powerful lasers to 20% of the speed of light. Then it increases its size by collecting material from its surroundings. But isn’t that totally unrealistic? Isn’t space just empty between the stars?

No. I’m sure you had already guessed that answer, because hopefully you know I’m trying hard to write scientifically possible science fiction. The vacuum in space is not empty. From a quantum-physics perspective, it is anything but, however I’m not referring to that. Normal interstellar space is full of matter. For the most part, this is hydrogen, but also any other element that was formed in a star and released in a supernova occurs in trace amounts.

How large are these trace amounts? Our Sun is moving, as far as we know, within a local interstellar cloud that is, in turn, located in a relatively empty region of the Milky Way. In comparison with its surroundings, there is a relatively large amount of matter in the local cloud (namely, 0.3 atoms per cm3). The average value in the Milky Way, however, is higher, at 0.5 atoms per cm3. Thus, in the volume of three sugar cubes, there would be approximately one atom in the space outside of our Solar System.

That’s not a lot. But let’s convert that to cubic meters – then there are already 0.3 * 1000 * 1000 = 300,000 atoms per m3.

Now imagine that the ship is pulling a circular net with a diameter of 100 meters behind it (in the novel, the net is even bigger). Thus, it has a surface area of approximately 7500 m2. This surface area is moving through space at 20% of the speed of light, that is, 60,000 km/s (which is 60,000,000 m/s). Thus, every second, the net sweeps through a volume equal to 60,000,000 meters * 7500 m2 = 450,000,000,000 m3. Every single one of these cubic meters contains 300,000 atoms. Thus, if it had a capture efficiency of 100%, the net would collect approx. 1.3 * 1017 atoms every second (!). That would be much more than we’d need. In practice, however, the efficiency would certainly be much lower. If we assume one percent efficiency, then it is still “only” 1.3 * 1015 atoms every second.

Most of those would be hydrogen nuclei or molecules. If only one atom out of every ten thousand atoms is a carbon atom, the net would still collect 1.3 * 1011 atoms of this element every second, thus, more than 100 billion every second, and the ship has a journey of many years.

Granted, constructing such a net that has the ability to capture, sort, and deliver such a quantity of atoms so that they can be used for other purposes is the real trick here. In my novel, those tasks are done by tiny machines. What they will be able to do is unknowable today – they don’t even exist yet. But the idea won’t fail because of the emptiness of space in any case. Plus, the ship could even continuously expand the surface area of its net to capture even more atoms.

PS: 1011 atoms per second sounds pretty impressive. However, one gram of carbon consists of approx. 1022 atoms. The task is thus a little more challenging than described in the simplified model. It would be significantly easier if the nanomachines were also able to combine hydrogen nuclei to form carbon nuclei. They are not able to perform nuclear fusion in my books, however.

PPS: a similar concept is the (hypothetical) Bussard Ramjet proposed in 1960 by the physicist, Robert Bussard; it collects hydrogen from space and uses it as a fuel for a fusion propulsion system.

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