It's been almost half a century since we first sent human beings to the Moon and yet we're still sorting out the effects of this wild journey.

A new study published today in Scientific Reports found that, compared to astronauts travelling to the International Space Station (ISS), deep-space exploring astronauts might be more likely to suffer heart problems later in life.

Of the 24 Apollo astronauts who ventured into deep space, eight have died.

This study only included seven of the deceased astronauts because the eighth died after the data analysis had been completed. Still, it found that 43 percent of these Apollo deaths were due to cardiovascular problems.

This rate is about four to five times higher than in other astronauts, and nearly double the 27 percent cardiovascular disease death rate of the general public.

Because the study was based on such a small sample size, we should be cautious about drawing definitive medical conclusions from the data, Michael Delp, lead author of the study, told Discover.

Still, it's definitely something that scientists should think about, especially as they plan out future manned missions to Mars or the Moon.

"We thought it was important to publish these results because it is also difficult to ignore the possibility that deep space travel … may be having a much more adverse effect on cardiovascular health than previously estimated," Delp said.

The scientists believe that these effects are because of high-energy radiation like cosmic rays and charged particles from solar flares.

Here on Earth and on the ISS (which is in low Earth orbit), Earth's magnetic field deflects most of this radiation. But in deep space, these high-energy particles run rampant. So, the astronauts on the Moon were largely unprotected from it.

These particles have so much energy that they strip electrons from atoms as they pass by.

This can damage the cells lining the walls of blood vessels, which are the same cells that prevent plaque from forming inside the walls of blood vessels. Plaque build-up can lead to blood clots, coronary heart disease, heart attacks, or strokes.

Delp and his team exposed mice to similar radiation to what the Apollo astronauts were exposed to. And, after six months (which is equal to about 20 human years for mice), they found that the radiation had negatively affected the vascular health of the mice.

"We know very little about the effects of deep space radiation on human health, particularly on the cardiovascular system," Delp said. "[But] this gives us the first glimpse into its adverse effects on humans."

This kind of research is especially important as we continue to push the boundaries of human space exploration further into the great unknown of space.

This article was originally published by Business Insider.

More from Business Insider: