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I've heard from a number of different places that the density in the interstellar medium can have an average of 1 atom per centimeter cubed. Perhaps I have a wrong understanding of what 'nothing' is (I am very knew to astronomy/astrophysics), but how would a vehicle, like the Voyager for example, be able to travel through it?

My (perhaps very flawed) thinking behind this is that if there are so few atoms not in close proximity, there couldn't be any (meaningful) forces interacting between them to provide any sort of way to travel through it.

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The answer is in your question.

You said

there couldn't be any (meaningful) forces interacting between them to provide any sort of way to travel through it.

That's the great thing about outer space. There normally aren't many particles around that could slow down a spacecraft via drag or friction. If you give something a push in outer space - somewhere that unrealistically is free from any gravitational interaction with other objects - it will travel forever. Objects travel through the ISM via Newton's first law - an object in motion tends to stay in motion.

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  • $\begingroup$ So, a spacecraft getting to interstellar space would travel exponentially more efficiently? Would propulsion still have an effect? $\endgroup$
    – galois
    Dec 17, 2014 at 23:53
  • $\begingroup$ Rocket propulsion certainly does. The ISM isn't too dense, which means that there isn't much of a difference between interstellar space and the space inside the solar system - in terms of things that could slow you down. $\endgroup$
    – HDE 226868 ♦
    Dec 17, 2014 at 23:55
  • $\begingroup$ Propulsion only affects acceleration. If Voyager puts out zero thrust, it will still keep moving due to its momentum. It would be perfectly efficient... but it's still going to take forever to get anywhere. $\endgroup$
    – Jonny D
    Dec 17, 2014 at 23:56
  • $\begingroup$ @JonnyD Exactly. You need a little bit of propulsion to keep moving at a constant speed because of the generally-negligible interactions with matter such as the ISM, but it's next to nothing. $\endgroup$
    – HDE 226868 ♦
    Dec 17, 2014 at 23:58
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For certain methods of propulsion, you need a medium. In space, you can use a medium that includes cosmic wind in order to drive something like a solar sail. In that case, the energy of particles is important even if they are sparse.

However, often you use a rocket engine in space and it doesn't matter what you are moving through. Not to mention that inertia is a valuable way to get around in low friction environments (once you get moving, keep going).

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