Elon Musk is racing to land SpaceX on Mars in five years, a vision he unveiled late last month at the 2017 International Astronautical Congress. One man not among Musk's critics is Scott Kelly, a retired astronaut who set the record in 2015 for total accumulated days in space, during the single longest mission by an American. "When Elon Musk said he was going to launch his rocket and then land the first stage on a barge, I thought he was crazy," Kelly told "Squawk Box" on Tuesday. "And then he did it. I'm not going to ever doubt what he says, ever again."