Why do missions to Mars launch only every 26 months?

Because Earth and Mars are both circling the Sun, and the cheapest way to go from one to the other only works when they are in the right places. A spacecraft on a Hohmann transfer, an elliptical path that touches both orbits, must wait for that alignment, which opens a launch window. For Earth and Mars those windows come around every 26 months.

Think of two runners on a circular track. Earth is the faster one and Mars is out in an outer lane. A spacecraft is thrown on a path that leaves Earth's lane and meets Mars's lane half a lap later, so the planet must be at the meeting point when the spacecraft arrives. Mars's year is 1.88 Earth years long, so Earth keeps gaining on it, and the right layout comes back every 780 days, which is about 26 months.

The trip itself then takes about nine months. A Hohmann transfer is named after Walter Hohmann, a German engineer who described it in 1925, and it often uses about the lowest possible amount of propellant, though it is slow.

Miss the window and you wait two years or change your plan. ESA's Rosetta spacecraft was first meant to fly to one comet, but a launcher problem delayed it past its window and it was sent to a different one. Japan's space agency, JAXA, launched its MMX probe to the moons of Mars on 20 October 2026, Japan time (19 October in UTC), during this year's Mars window.