# Launch windows to Mars

I just watched a National Geographic documentary on ExoMars. In it, they mentioned there were 2 launch windows in early 2016 for the TGO/Schiaparelli launch, one in January and one in March.

Quote from ESA site showing this is not just a gaffe by NatGeo:

The later window is open 14-25 March and, thanks to the relative orbital positions of Earth and Mars, the mission will still arrive at Mars in October, just as if launched in January.

How does that work? As I understand it, there is one optimal date every 780 days, with the required launch energy going up slowly either side of the optimal date. So where do two launch windows, 2 months apart, come from?

• I'm not certain, but I think it has to do with the eccentricity of Mars's orbit (about six times Earth's, small but not negligible). If both Earth and Mars were on perfectly circular orbits there would be one optimum, but depending on where Mars is in its orbit there's a tradeoff between delta-v to intercept and delta-v to enter Martian orbit, and trajectories other than pure Hohmann become more advantageous, yielding two different local optima. – Russell Borogove Nov 3 '16 at 21:07

This can be seen in the traditional "pork chop" plots, which show various trajectory characteristics, such as departure $C_3$ or arrival $V_\infty$, as a function of departure and arrival dates. Here is an example from the 2005 opportunity: