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In the atmosphere, craft may use aerodynamic effects to their benefit. As you start getting higher in the atmosphere, the atmosphere becomes rarified, requiring you to model the atmosphere not as a fluid but as the interaction of a bunch of individual air molecules probablistically impacting your vehicle.

Until you get going fast enough, at which point you start getting aero forces again.

This got me thinking: since space is not actually a pure vacuum and there is a little bit of "stuff" everywhere, how fast does a body have to travel to start making use of aerodynamic effects? I expect the answer may be different within a solar system vs outside one, though clarifying this would be appreciated.

My first visual w.r.t. this question is from Stargate where they fire an AMRAAM in space... but AMRAAM doesn't have vectored thrust and its fins wouldn't work. But what if it was traveling really, really fast?

A side question which you may choose to add to your answer or not if it is relevant: will other forces dominate before aero forces become significant?

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    $\begingroup$ @OrganicMarble - Ah, you're right! Thank you - that one didn't come up when I searching for the answer! $\endgroup$ – iAdjunct May 8 '19 at 2:46
  • $\begingroup$ I'm not an expert on fluid dynamics, but don't think that "How fast..." is the right question. I think what is important is the scale of the craft relative to the mean free path of the particles. To be treated as a fluid medium they need to be able to interact on scales shorter than the size of your spacecraft. I think you need to adjust the wording of your question, and avoid asking about speed. $\endgroup$ – uhoh May 8 '19 at 3:14

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