There is still a problem we haven't solved - solar radiation. The ISS astronauts may have been up there for long periods of time, but they are protected by the

There is still a problem we haven't solved - solar radiation. The ISS astronauts may have been up there for long periods of time, but they are protected by the earth's magnetic field. A trip to another planet doesn't have this solution. Even for the Apollo missions, we got lucky that no bad solar activity occurred, and that mission lasted eight days.