What space does to your body: Swollen heads, shrunken legs, round hearts

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.