Though it seems like it's all downhill, sending spacecraft to Mercury and Venus is just as energy consuming, if not more, as going to Mars and beyond. This is because, whereas trips to the outer solar system requires gaining speed, going to the inner solar system means losing speed. This means that a spacecraft heading closer to the Sun must either use massive rockets or find some other way of braking.

Since the 1970s, the world's space agencies have been perfecting the use of slingshot maneuvers for changing the velocity of spacecraft without the need for rockets. The first slingshot maneuvers were used on the Pioneer 11 mission, where the spacecraft used the gravitational pull of Jupiter to shoot it toward a flyby of Saturn.