For Mars the ion drive will be too slow. It would need fossil fuels in the first place to get into earth's orbit (of which in 50 years there would not be much left - end of the space era?) and then it would need to speed up to at least 40.000 km/h to escape the earth's gravity well. Imagine a spaceship for a crew of 4 astronauts: habitats, exercise rooms, lots of storage for food and oxygen, the lander- and take off module with its own load of storage for fossil fuels to escape Mars' gravity well (something more than a quarter of the earth) and for braking the speed at Mars and reentry back at earth and the equipment for various experiments. You can cut down on some of that by making it a suicidal one-way mission but it will be a lot of weight. More than the Apollo missions for sure. Now imagine putting a a roll of toilet paper on top of that; that'll be the amount of thrust an ion drive would give it. We can go to Mars but it will require a lot of willpower, a lot of money, a whole lot of energy, not enough time and many sacrifices. Personally I think there will never be manned Mars missions.