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.

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