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,