No matter how you do it you must cancel your forward velocity. The most efficient manner is thrusting in the opposite direction.
No you don't and no it isn't. You don't have to cancel it, just redirect it.
A "banking turn" is a turn that redirects your momentum along a curved path, imparting a SERIES of vectors that add up to a single Delta-Vee. The alternative scenario is called a "single-impulse" transfer; it's where you perform a SINGLE vector change to alter your velocity into a new vector.
The reason the banking turn does not save energy is because the difference between your initial vector and your new vector is the same in either case, the only difference is the path you took to get there. Star Trek style banking turns, on the other hand, would consume MORE energy, since the final vector product is at a zero angle of attack relative to the bow (starships do not appear to "power slide" as such).
And you get there most efficiently in terms of reaction mass expended by using RCS to alter your vector and translate the momentum rather than the brute force approach you are talking about.
But you CAN'T translate your momentum into a different direction, you can only turn your nose around. Your momentum continues along the same vector no matter where you're facing.
No there isn't, but there IS a "reduced price" lunch. Conserving your existing momentum allows you to impliment a new vector with much less reaction mass expended.you give your nose a series of nudges with your RCS thrusters while simultaneously applying the aft RCS thrusters and the vector gradually shifts with less mass expended than if you brute forced it with the main engines.
Your momentum is along one vector and one vector alone. You can't "conserve" it by magically applying it to a new vector, you have to thrust an amount equal to the difference in the two vectors. No matter how or when you apply thrust, at the end of maneuvering, you'll pay the full price.
Not true. In the asteroid diverting scenario I described the mechanism by which the course is changed but not the momentum with relatively small energy inputs.
Actually the momentum IS changed. Any time you impart motion on an object you have caused it to accelerate; the difference between the old vector and the new vector is called "delta-vee"
A vector, by the way, is the combination of an object's direction and its relative velocity. The simplest vector change is forward acceleration, where the direction remains the same but velocity increases. However, changing paths without changing forward motion ALSO constitutes acceleration and requires the same amount of reactant mass to be expended.
Deflecting is always easier and more efficient than flat stopping
That depends on what you're trying to accomplish. If you're trying to reverse directions entirely, it's no different at all.
