Living on Mars will have to wait, at least according to SpaceX CEO Elon Musk. Last month, Musk announced on X that the company has “shifted focus to building a self-growing city on the Moon.” This appears to be a major change of course for the company, which Musk founded with the stated goal of building a human settlement on the red planet.
At face value it sounds like a practical decision. As Musk noted in subsequent posts, people and supplies can be sent to the Moon much faster and more frequently than Mars, making it conceivable that a foothold could be established there more quickly. But Musk’s lunar and Martian ambitions both suffer from the same fatal flaw: they ignore the human element.
To achieve a “self-growing” human settlement on any celestial body would require reproduction. Yet, our knowledge of how pregnancy and fetal development are affected by the conditions on the Moon or Mars is minimal, at best. While radiation is a major challenge of deep space travel, people living on the Moon or Mars could theoretically be shielded from much of it. Gravity is a more vexing concern.
The Moon has one-sixth the gravity of Earth, while Mars has one-third. While we know a lot about how weightlessness (zero-g) affects the body, essentially no information exists on the effects of partial gravity. And what we know about weightlessness is limited to its effects on adults. To achieve a self-sufficient city we must consider how gravity affects humans of all ages.
Questions about reproduction beyond Earth begin with fertility and conception. Studies on the movement patterns of human sperm and sperm of other species have had mixed results. In a series of experiments conducted on rockets launched from Sweden in 1998 and 1990, German researchers found that bull sperm have somewhat improved movement abilities in microgravity. Yet, Japanese researchers that examined the movement of sperm in mice under simulated microgravity– achieved by suspending the mice from their tails for 7 days– found that sperm motility decreased. Likewise, a study on the effects of weightlessness on human sperm done by Japanese researchers using parabolic flights also found that sperm motility during the brief weightless periods decreased.
The effects of space on embryo development are similarly unknown. So far, the only species that has successfully completed the entire reproductive process from conception to birth is the medaka, a type of fish. Studying how fish reproduction is affected by microgravity is convenient, because fish lay clear eggs so researchers can easily monitor how embryos develop. But like most mammals, human babies develop internally, making it much harder to track their growth. Research on rodent reproduction in space has been limited and largely inconclusive. In 1980, Soviet researchers sent pregnant rats to space for five days then returned them to Earth to give birth. Four out of five were successful. The fifth had complications that led to the death of all the pups in the litter suspected to have been caused by the stress of weightlessness.
Indeed, prolonged exposure to microgravity has a profound effect on the body including muscle atrophy and bone density loss. During each month in space astronauts lose about 1.5 percent of their bone density, the equivalent of about a year for older adults on Earth. The rate of bone density loss in the partial gravity of the Moon or Mars is unknown. The Apollo astronauts were only on the surface of the Moon for a few days, not long enough to measure the rate of bone density loss. Yet, extrapolating from the effects of microgravity suggests that some bone density loss is likely.
Bone density loss could be especially problematic for pregnancy and childbirth. A woman who has been in a lower gravity environment for years would likely have significantly compromised bone strength. This could increase the risk of fractures, especially in the weight-bearing bones of the back and pelvis. This is precisely where bone density loss is greatest for astronauts.
A child born and raised in a lower gravity environment would likely develop progressively weaker muscles and bones as they grow and mature. This would make returning to Earth extremely challenging. A child born on Mars would weigh three times as much on Earth. For a child born on the Moon, Earth’s gravity would make them feel six times heavier. Having weak bones might very well make such a trip impossibly dangerous.
Any plans for building a city beyond Earth will have to address these concerns. The Moon is certainly an easier target than Mars. Indeed, the crew of Artemis 2 is poised to travel around the Moon in early April. Yet, Musk’s claims that we can have a self-contained city there within ten years and a city on Mars just a decade or two later ignore the realities of biology. Solving the technological challenge of getting to either destination is only the first small step. Turning either place into a home for humanity is a giant leap we are not yet ready to take.
