Knowledge Zone · Card 9 of 13
Launch windows
Arriving at Mars' orbit is not the same as arriving at Mars. The transfer takes about 259 days, so Mars has to be somewhere else when you leave — about 44° ahead of Earth — in order to be at the far end of your ellipse when you get there.
Earth laps Mars every 780 days or so, which is the synodic period, and that is how often the geometry comes back around. Leave outside the window and the arc is still a perfectly good ellipse; it simply crosses an empty piece of Mars' orbit. Fixing that with fuel instead of with patience is far more expensive than waiting.