Even though Walter Hohmann had already calculated the most energy-efficient trajectory between two similar circular orbits (the Hohmann transfer orbit) at the beginning of the century, an Earth-Mars-Earth round trip utilizing this flight path would have made it a necessity to remain on the surface of Mars for a duration of 425 days, waiting for the planets to align and the next launch window to open, in addition to 259 days for both the journey to Mars and the return to Earth.