The many, many reasons space travel is bad for the human body

Leaving Earth upends almost every system inside of us

After the astronaut Scott Kelly spent a year on the International Space Station, he returned to Earth shorter, more nearsighted, lighter and with new symptoms of heart disease that his identical twin brother did not share. (Mark Kelly, now a U.S. senator, also spent a brief time in space.)

Even their DNA diverged, as nearly 1,000 of Scott Kelly’s genes and chromosomes worked differently. (On the upside, he aged about 9 milliseconds less that year, thanks to how fast the space station circled the Earth.)

Most of these effects cleared up within a few months, but not all — underscoring the potential health hazards of space travel, many of which are unknown. These will ratchet up during ambitious future trips, such as NASA’s planned Artemis mission to the moon and later travel to Mars.

Even a partial list of the likely physical and emotional consequences of deep space travel is daunting.

Space motion sickness sets in almost immediately. The nausea, dizziness, headaches and confusion can linger for days.

“Puffy Face Bird Leg Phenomenon” develops, as blood and other bodily fluids rush to the upper body in low gravity and stay there, swelling heads and shrinking legs.

Astronauts’ appearances can change as their faces swell. The astronauts may feel congested, as though they have a constant head cold.

Muscles atrophy by as much as 1 percent every week in weightlessness, especially in the legs.

Blood volume drops — and with less blood to pump, the heart weakens and loses its signature heart shape, growing more rounded.

Like any other muscle, the heart doesn’t need to work as hard in microgravity and will begin to atrophy without rigorous exercise.

Doused with radiation, many immune cells die and immunity is lowered. There’s also DNA damage, potentially upping cancer risk.

Inflammation spikes throughout the body, possibly contributing to heart disease and other conditions.

Bones thin by about 1.5 percent a month. Spinal discs harden.

In the head, parts of the eyeball can flatten, causing sharper distance vision and dimmer near vision.

Fluids flood the skull, diminishing smell and hearing.

Gene activity changes, including in the brain. In mice, 54 different genes in the brain worked differently after weeks in space.

Brain cells can be affected by radiation, diminishing memory and thinking (in mice).

Circadian rhythms falter, making insomnia common.

Finally, months or years of solitude — or close confinement with fellow astronauts — can lead to lasting psychological stress.

“Space is just not very hospitable to the human body,” said Emmanuel Urquieta, chief medical officer at the Translational Research Institute for Space Health in Houston, which partners with NASA to study the effects of deep space exploration.

Humans evolved in conditions of plentiful gravity and relatively slight background radiation, he said. Space is the reverse and it upends the operations of almost every biological system inside of us. —

Most of the potential health risks of space travel can be mitigated to some extent, scientists point out. Exercise, for instance, “is quite effective” at helping astronauts maintain muscle mass and bone density, said Lori Ploutz-Snyder, the dean of the University of Michigan School of Kinesiology. She was previously a researcher at NASA, where she led studies of exercise and space travel.

Press Enter to skip to end of carousel
The New Space Age
As humanity looks once again to the heavens, this series examines the recent boom of the private space industry, the long-term ambitions of NASA and foreign space agencies, and the potentially profound implications of this moment for society.
End of carousel

On the space station, astronauts routinely work out for about an hour most days, she said, using specialized devices to run, cycle and lift weights, despite being weightless. But on lunar and Mars missions, which will involve smaller ships and possibly years-long durations, exercise equipment will need to be shrunk and astronauts’ willingness to keep up with the workouts enlarged.

[To counter the effect of sitting too much, try the astronaut workout]

The Earth’s magnetic field shields the relatively close-in space station as well from some of the worst deep-space radiation, but the lunar and Mars missions — higher and farther from Earth — will not enjoy that protection.

The moon and Mars journeys will demand advanced shielding, Urquieta said, together with drugs and supplements that might lessen some of the internal effects of the remaining — and inevitable — radiation. Antioxidants, such as vitamins C and E, could sop up a portion of the damaging molecules released after radiation exposure, while other protective drugs and nutrients are under investigation, he said.

Despite every available precaution and protection, deep space will remain a harsh, unwelcoming place for the human body. But it will also, and always, represent something else for the human imagination, Urquieta said — its endless sweep of sequined darkness sparking our ambitions, dreams and stories.

Which is why, even knowing better than most people the toll such a trip might take on him, he would go into space “in a heartbeat,” he said. “Absolutely. No question. It’s so inspiring. It’s space."

About this story

Additional design and development by Betty Chavarria. Editing by Kate Rabinowitz, Manuel Canales and Jeff Dooley. Copy editing by Wayne Lockwood.