Destinations & interplanetary travel times
I've said that for years.
And as to travel times, only because of utter lack of nerve have we not considered nuclear electric VASIMR drive spacecraft, craft that could reach Mars in under 90 days. Years ago, NASA flew a couple ion drive spacecraft, yet wimps out on really utilizing advanced propulsion. Were it not for Chang-Diaz's efforts and private money, NASA wouldn't even try VASIMIR in that pig-in-space ISS.
Instead NASA wastes money on its Battlestar Galactica multi-billion dollar boondoggle rover on steroids to Mars. My prediction, it augers in.