There is no aerodynamics in space.

But when it is flying in earth atmosphere.

Yes, nothing stops you moving because no aerodynamics. You just continue to go forward until something propels you back. Not sure if just trolling or not.

Yes, there's no aerodynamics in space, but the characters are flying from planet to planet, and obviously when they leave or enter planets, aerodynamics become a problem in atmosphere with this particular design. I didn't expect this to be a full on discussion, I really like the design, but just thought that the design is interesting considering that flying in and out of earth (or other planet depending on the composition of gas or density) could be a potential problem, and obviously it is a Sci-Fi game and many things can explain the rationale of the design... I am just pointing out that I found it interesting. Edit: Not trolling at all, just a comment that's all.

I assume the guardians worthy of the space bus are badass enough to drop from orbit

True, like I said it's Sci-Fi, too many explanations are there for us to imagine or explore. They can probably warp from one area to another, and I think it is what they do in the game when you travel from one area to another.