If you accelerate a body to 0.5c in space and turn off the engines, how long would it take for the "friction" of space to slow it down? Or why does space travel require continuous burn of engines (which I have read elsewhere)?
The space probes we have headed out burned for a very short while and have coasted ever since. I am unfamiliar with the "friction of space" concept that would slow them down over time.
Travel with continuous (low level) acceleration is better if you can do it. The burn at the beginning and end is what we are stuck with until we get better engines, but they're working on it.
Sails are good because they provide continuous acceleration without having to carry fuel or engines, but their effectiveness decreases with distance from the sun. They are experimenting with that too.
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