Since last decade, NASA has turned repeatedly to Colorado companies to produce the technology it needs to not only send astronauts on new lunar missions but also to Mars and into the depths of space. Above, the International Space Station.

Plans to rocket humans to Earth's closest neighbor continue to advance, with the year 2024 a near-term goal — at least if Elon Musk has his way.

Yet space medicine expert Jim Logan said recently the effects on the human body from spending extensive time outside of Earth's gravity remain unresolved.

"We need a huge sample size and right now we have a sample size of one, and soon maybe two," Logan told CNBC at the New Worlds conference in Austin, Texas. Logan referenced astronauts Scott Kelly, who spent 340 days in space; and Peggy Whitson, who recently returned from the International Space Station after 290 days.

"We need to fly a lot of people, for long durations, if we're going to make any progress. You cannot talk about human colonization without that," Logan said.

Logan spent 20 years helping diagnose and treat NASA's astronauts. He said the main obstacle to sending humans to Mars is what he calls "the gravity prescription."

He said: "We know what Earth's gravity – one G for 24 hours a day – does. And we know that zero gravity, after four months, begins serious health deterioration."