Here is a possible Challenge solution to go Mars (or its Moon) and back using gravity assist kicks !
You send your Mars spacecraft to the Earth Moon for the first kick and direction change back to Earth - speed increases! The second kick is with Earth with a direction change back to the Moon. After a few kicks between Moon/Earth your space ship has reached a very great speed using no fuel and you kick your self away to Mars (or its Moons), where you arrive after two days.
At Mars you use the Mars Moons to kick your speed down to almost 0 again, so you can land on a Mars Moon. You must have enough fuel to land and to get off a Mars Moon, but then you can kick yourself back to Earth using gravity assist kicks ! Gravity assist kicks provide free energy, you know!
But maybe the high g-forces applied at every kick will kill you?
The highest the g-force could ever be will be the acceleration due to gravity at surface of the body you are getting the assist from. This is because you are using that gravitational pull from that body in question, there is no other acceleration present.
You do not understand astrophysics. At a 180°
gravity assist kick swing by around a very solid, great, high density mass body of say 1 m diameter in space, the spacecraft is subject to 10 000 g forces, that will rip it apart into atoms. Pls do the calculations yourself.