Only 24 astronauts have ventured past Earth’s protective magnetic field. These were the moon-bound astronauts of the Apollo missions. However, one group of scientists has brought attention to a possible side-effect of these deep-space missions: Heart disease.

A recent study, published in the journal Scientific Reports, reported on the troubling relationship. The title says it all: Apollo Lunar Astronauts Show Higher Cardiovascular Disease Mortality: Possible Deep Space Radiation Effects on the Vascular Endothelium. Yes, some Apollo astronauts are dying of heart disease at a higher rate than would be expected, according to the recent study. If true, this health risk could have implications for future deep-space missions to Mars.

The study looked at three distinct groups: seven of the Apollo astronauts (of the 24, 16 are still alive), 35 trained astronauts that have never flown to space, and 35 astronauts who have only flown in low-Earth orbit. The results show a clear difference in heart disease fatalities. “[T]he [cardiovascular] mortality rate among Apollo lunar astronauts (43%) was 4–5 times higher than in non-flight and [low-Earth orbit] astronauts,” the report says.

It should be noted that this study has an extremely small sample size, a fact which lead author Michael Delp at Florida State University is very-much aware. "Certainly, that small sample size is something to keep in mind, but in almost all space studies with humans you're dealing with a small number of individuals," Delp told Popular Mechanics. However, this part of the study has fallen under a fair amount of criticism.

It's important to note that of the eight astronauts who have passed away, only three are known to have died of heart disease: Neil Armstrong; James Irwin, and Ronald Evans. (Edgar Mitchell died in February 2016 in hospice care, but the cause of his death has not been released.)

Still, Delp and his team believe it was the astronauts' exposure to cosmic radiation, which has caused the asserted high mortality rate from heart disease. Why these symptoms only manifest years later? Delp cannot say—no one can really say.

There are known risks going out into deep space. One major issue NASA will have to contend with is getting Mars astronauts safely to the Red Planet without melting their brains. Space does strange things to the human body, like thinning the skin. It’s not entirely certain that we can add heart disease to the list. It will be interesting to see what follow-up studies discover.

Below NASA engineer discusses how the they plan to send astronauts deeper into space, safely: