Don't doubt Elon Musk, says astronaut who spent a year in space

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."

The CEO of SpaceX plans to use a 42-engine rocket, called BFR, to send humans to Mars by 2024. Last year, Musk touted BFR as capable of holding around 100 people, or 150 tons of cargo.

"I feel fairly confident we can build the ship and be ready for the launch in five years. Five years seems like a long time for me," Musk said.

Kelly said current technology means a one-way trip to Mars takes over 200 days. One of the reasons for his year-long space mission was to study the effects of space on the human body. Kelly likened being in space to aging at an accelerated rate.

"In this weightless environment, we lose a lot of bone mass and muscle mass. We'd lose one percent of our bone every month if we didn't really do anything about it," Kelly said.

A trip to Mars is only possible if scientists know how to prevent those losses from happening. Otherwise, a human "would be like jelly" after 100 months, Kelly said, because there would be no bones left. To combat this, Kelly says he worked out 2½-hours each day in the International Space Station.

Kelly thinks the the space industry will reach mass adoption in his lifetime. Safe and regular space transportation is 30 years away, Kelly said, which would revolutionize the speed earthly travel happens at as well. Suborbital transportation means one can imagine going from Los Angeles to London in 45 minutes as a realistic possibility.

"I think at first what we'll have is something similar to the early days of aviation, where the barnstormers took people for rides, and that developed into a transportation system," Kelly said, before adding that "the more we do it, the better we get at it."