[netfire]

Question: Why do the autonomous cargo missions need to be done in the ideal launch window? It doesn't seem like the systems required to operate and navigate in space should require much energy, once your are up to cruising speed (and there's no air resistance in space to slow you down). Does it matter if the cargo takes 6 months to deliver the cargo instead of 3? Am I missing something?

[anvandare]

It is indeed an energy issue, but for the orbital change, not system operations. Every kilogram you have to spend on fuel is a kilogram you can't use for cargo instead. You can use porkchop plots[1] to determine ideal launch windows (least ∆v = least fuel needed). Late summer of 2022, for example, is a good window[2].

For manned missions, you're more interested in the least '∆t' (time spent traveling) since crew sanity/health is more important than fuel. (Yes, astronauts are exceptional and willing to endure a lot of discomfort, but the less time they have to spend in a small tin can, the better.)

[1] https://en.wikipedia.org/wiki/Porkchop_plot

[2] http://www.amssolarempire.com/Programs/porkchop_plot.png

[ashark]

Longer exposure of components to radiation? Deterioration of fuel and fuel-touching components (wouldn't be able to get by on relatively inert monopropellant like on tiny probes, probably)? Best guesses.

[greedo]

It saves you fuel. More space spent on fuel reduces your payload, driving costs up.