About ten years ago I sat next to a guy on a plane who worked for NASA. He had been an USAF officer during Vietnam. Then an alternate astronaut for the Space Shuttle. He never got his shot to go into space. He was in charge of grant money for NASA. He was returning to Houston after a trip to Dartmouth.
He told me the biggest issues in sending people into space was what it did to the human body. Everyone who spends times on the International Space Station returns NEAR SITED. Everyone has bone density loss.
So, they have no problem building a rocket and sending it to Mars, but the issue is would the people on it survive?
That has always been the main problem.
Water can be recycled, and is, but food is a different matter, along with the long term health effects of space travel...............