Add to List of Health Hazards: Space Travel

astronauts en route to a medical tent after landing in remote Kazakhstan in 2012.

In space, heads swell.

A typical human being is about 60 percent water, and in the free fall of space, the body’s fluids float upward, into the chest and the head. Legs atrophy, faces puff, and pressure inside the skull rises.

“Your head actually feels bloated,” said Mark E. Kelly, a retired NASA astronaut who flew on four space shuttle missions. “It kind of feels like you would feel if you hung upside down for a couple of minutes.”

The human body did not evolve to live in space. And how that alien environment changes the body is not a simple problem, nor is it easily solved.

Some problems, like the brittling of bone, may have been overcome already. Others have been identified — for example, astronauts have trouble eating and sleeping enough — and NASA is working to understand and solve them.

Then there are the health problems that still elude doctors more than 50 years after the first spaceflight. In a finding just five years ago, the eyeballs of at least some astronauts became somewhat squashed.

The biggest hurdle remains radiation. Without the protective cocoon of Earth’s magnetic field and atmosphere, astronauts receive substantially higher doses of radiation, heightening the chances that they will die of cancer. How much of a cancer risk later in life is acceptable?

At the Johnson Space Center here, the home base for NASA’s human spaceflight program, scientists probably have until the 2030s to dissect these problems before the agency sends astronauts to Mars — a mission that would take about 2.5 years, or nearly six times the current standard tour of duty on the space station.

The longest any human has been off Earth is almost 438 days, by Dr. Valery Polyakov on the Russian space station Mir in 1994 and 1995.

In 2009, during his six-month stay on the International Space Station, Dr. Michael R. Barratt, a NASA astronaut who is also a physician, noticed he was having some trouble seeing things close up, as did another member of the six-member crew, Dr. Robert B. Thirsk, a Canadian astronaut who is also a doctor. So the two performed eye exams on each other, confirming the vision shift toward farsightedness.

They also saw hints of swelling in their optic nerves and blemishes on their retinas.

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