Impulse, denoted as J, is defined by the change in momentum. Since we have our initial and our final, we can solve for the change in momentum.
This means that the farther away the astronaut is from the Earth, the less force gravity will be applying to his/her spacecraft. With this being said, once the astronaut breaks Earth's atmosphere, he can reduce the throttles a little to save on fuel, because he doesn't need all of that power to escape such a weak gravitational pull :D.