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hey were among the healthiest people in the country, but several Apollo astronauts succumbed early in life to heart disease — and deep space radiation may be to blame, according to a study published Thursday in Nature’s Scientific Reports.

Why it matters:

By 2030, NASA hopes to launch missions around the moon in preparation for sending people to Mars. The potential health effects are of particular concern.

Previous studies of the effects of space travel compared the health outcomes of astronauts to those of non-astronauts, who were “all over the place” in terms of health, said Michael Delp, dean of the Florida State University College of Human Sciences. “You get the hypertensive, diabetics, the obese. But astronauts are a different group from the general population. They are fitter than most, and highly educated, which is associated with better health.”

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So Delp’s study involved astronauts only. He compared Apollo astronauts who had flown into deep space with astronauts who had flown on low-Earth orbit missions — above airplane level but still within the Earth’s magnetic field. They were shielded from radioactive particles like those found in cosmic rays. There was also a control group of astronauts who had never gone into space.”

The nitty gritty:

Among the deceased Apollo astronauts, 43 percent died from cardiovascular problems. This rate was five times higher than in the other astronauts and significantly higher than the 27 percent cardiovascular disease death rate among the general public.

Interestingly, exposure to cosmic rays seemed to have no effect on their cancer death rates.

Delp and his team hypothesized that some combination of weightlessness and radiation exposure caused the cardiovascular problems. So they followed up by exposing mice to conditions mimicking those of deep space. The researchers simulated weightlessness through “hindlimb unloading” — lifting the mice by their tails until their back legs are off the ground and their bodies tilted, as well as head down, at a 30-degree angle, for 14 days. 

There were four groups of mice: one exposed to only weightlessness, and another to only radiation exposure. One group was exposed to both, and another, the control group, had no exposure to either.

What they’re saying:

The scientists noted similar outcomes in the mice as in the astronauts. Weightlessness and radiation somehow stopped the body’s production of nitric oxide, a substance that keeps blood vessels open.

“With radiation exposure and weightlessness, the endothelial cells that make nitric oxide become disabled. The blood vessels narrow and plaques form,” Delp said.

When they retested the mice six to seven months later, the equivalent of 20 human years, the radiation-exposed mice still had cardiovascular defects. Those that only had weightlessness simulation did not.

You’ll want to know:

Previous studies have looked at the impact of space travel on astronauts’ immune systems, sleep and vision.

This study adds to our knowledge, but has limitations: Only 24 people have gone into deep space, and all were male.

“We need to know if there’s an effect on female astronauts,” Delp said.

Another limitation is that the study of astronauts doesn’t show how deep space flight will affect ordinary people, said Richard Hughson, a vascular aging and brain health researcher at the University of Waterloo in Ontario.

“The only way we’re going to know is when we start sending humans to the moon and beyond,” said Hughson, who wasn’t involved in the study. “We need to test them before they go and while they’re there to determine whether the endothelial function is being impaired, perhaps by radiation.”

The bottom line:

Studies involving astronauts are only the beginning of understanding the effect of deep-space travel on humans’ long-term health.

Correction: A previous version of this story misstated the definition of low-Earth orbit.
What happens to your DNA when you spend a year in space? NASA astronaut Scott Kelly is working with scientists to find out. Matthew Orr/STAT
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