Werner Von Braun famously told his secretary that the public threats would be, in order: terrorists, then rogue nations, then asteroids, then extraterrestrials.

If you go down the rabbit hole of inertial propulsion- propulsion by means of gyroscopes or centrifugal force, it seems extremely likely that space propulsion is much cheaper and easier than it is made out to be. I know of three patents in this area: by Robert Cook, Eric Laithwaite, and Harvey Fiala.