Going to the Moon takes three days. Going to Mars and back could take three years. The incredible length of the journey raises technological issues involving life support systems and supplies that people have never had to deal with before.
Of course, history is full of other remarkably long trips. Columbus's first voyage to the Americas and back took about a year. Captain James Cook's first scientific expedition for the Royal Geographic Society in 1768 took three years. But dangerous and long as these early journeys were, they didn't involve leaving the planet and living inside a small pressurized can in zero gravity.
Because of the phenomenal problems of traveling through deep space--the dangers of radiation and weightlessness, the risks of mechanical failures, the costs of supplies--the technological challenge is to make the trip as short as possible. Meeting the challenge requires careful timing (in order to reach Mars when it is closest to Earth--35 million miles away), innovative ship designs, and, potentially, new propulsion systems.
The longest mission under consideration, one that would last one thousand days, would use traditional chemical propulsion to get to Mars. Half of the time in this scenario would be spent in travel and the other half on Mars. The quickest round-trip mission currently imaginable, made by nuclear thermal rocket propulsion and requiring a relatively short thirty-day stay on Mars, would still take more than a year.
