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."
Bone mineralization is one of those lack-of-gravity problems. For each month in space, Logan says the human body loses 1 percent of its bone mass. And, once back on Earth, the bones don't grow back.
"I'm not trying to throw cold water on the idea of going to Mars. I want to make sure we can survive if we do go," Logan said.
He says, if putting humans on Mars is truly a near-term goal, it means studies today must focus on Martian gravity, or 0.38 Gs.
"If we study the effect of 0.38 on the body around the clock, and it doesn't work, you can cross off Mars as a human settlement site until we do sort out the medical diagnosis," Logan said.
After medically serving 25 space shuttle missions, Logan stated current measures are stop-gaps that do little to actually counteract the effects of weightlessness on the body.
For "each deleterious effect," Logan says there's a new mechanical or pharmaceutical countermeasure that only "retards the deconditioning. They don't neutralize it and they don't reduce it," Logan said.
"We need to start sending more people into space if we're actually going to get somewhere, with even problems we know like the gravity prescription," he added.
The scientist also said that it is critical to use the words "outpost" and "settlement" distinctly when talking about space exploration.
"Let's stop playing fast and loose with this idea that the ISS [International Space Station] is our first settlement in space. We have a permanent presence, but with rotating crews," Logan added. "Settlement is men, women, children, over multiple generations."