Exploration of Mars - Launch Windows

Launch Windows

The minimum-energy launch windows for a Martian expedition occur at intervals of approximately two years and two months, i.e. 780 days (the planet's synodic period with respect to Earth). In addition, the lowest available transfer energy varies on a roughly 16-year cycle. For example, there was a minimum in the 1969 and 1971 launch windows, rising to a peak in the late 1970s, and hitting another low in 1986 and 1988.

In addition to these minimum-energy trajectories, which occur when the planets are aligned so that the Earth to Mars transfer trajectory goes halfway around the Sun, an alternate trajectory that has been proposed goes first inward toward Venus orbit, and then outward, resulting in a longer trajectory that goes about 360 degrees around the Sun.