The Hohmann Transfer Orbit is thought by NASA to be the most energy-efficient way to get spacecraft to Mars. It involves launching the spacecraft into space at the optimum time so that the craft can escape the gravitational pull of the earth and instead intersect with the gravitational pull of Mars' own orbit.
This method allows the object to use less fuel than if it used a conventional trajectory, as it can cruise along the existing orbital pathway. The craft only needs fuel to increase speed and velocity to escape Earth's orbit, and again to decelerate to connect with Mars' orbit until it is ready to touch down on the surface.
Though this mission design is energy and fuel-efficient, it still has a considerable travel time because it is a very indirect route. Unmanned missions with robotic spacecraft have had a 7 month journey time using this method. NASA estimates that manned missions would take 9 months via this route.