The Impulse Drive level is factored into the flight radius.
Each level of the development increases the radius of flight of the Interplanetary missiles.
Why they use Impulse and not Rocket I have no clue, but that is how it is set up.
I have level 10 Impulse Drive and can hit planets about 9k+ away. So it might be set to 1k each level after level 1? just a theory since it increased another 1k after I completed level 10...