Deep-space travel takes a toll on the body—and it’s apparently something you can’t moon-walk off.

Apollo astronauts who have ventured out of the protective magnetosphere of mother Earth appear to be dying of cardiovascular disease at a far higher rate than their counterparts—both those that have stayed grounded and those that only flew in the shielding embrace of low-Earth orbit. Though the data is slim—based on only 77 astronauts total—researchers speculate that potent ionizing radiation in deep space may be to blame. That hypothesis was backed up in follow-up mouse studies that provided evidence that similar radiation exposure led to long-lasting damage to the rodents’ blood vessels. All of the data was published Thursday in the journal Scientific Reports.

The study, while not definitive, may add an extra note of caution to the potential hazards of future attempts to fly to Mars and elsewhere in the cosmos.

“These data suggest that human travel into deep space may be more hazardous to cardiovascular health than previously estimated,” concluded the study authors, led by health researcher Michael Delp at Florida State University.

While NASA tries to minimize astronauts’ exposure to space radiation, it acknowledged that exposure was basically an unavoidable part of the job. “Radiation-exposure risks to crewmen were assessed and balanced against mission gain,” NASA experts wrote in a 1973 report (PDF) on the Apollo experience.

Since then, other studies have tried assessing health consequences of space travel. In many cases, studies have come up relatively empty-handed. However, Delp and his colleagues say some of those studies may be off base because they tried comparing health stats of astronauts to those of the general public. This would likely mask health effects, Delp and his colleagues argue, because astronauts are generally healthier and more health-conscious to begin with than the general public. Astronauts are physically fit with high levels of education and consistent access to medical care. In America, those are not necessarily norms.

In the new study, Delp and coauthors compared health data on 42 astronauts that had traveled into space—seven of which got past the magnetosphere and to the Moon—to the medical records of 35 astronauts that were grounded for their careers. The death rate from cardiovascular disease among the Apollo lunar astronauts was a whopping 43 percent, which is around four to five times the rate seen in the non-fliers and low-fliers (nine and 11 percent, respectively).

To figure out if deep-space ionizing radiation or, perhaps, weightlessness might explain the apparent jump in cardiovascular disease deaths, the researchers turned to a mouse model. Mice were either exposed to a single dose of radiation, had their hind limbs elevated to prevent weight-bearing for two weeks, or received both treatments. The researchers then let the faux-astronaut mice recover for six to seven months, which in human terms would be about 20 years.

The researchers found that the mice exposed to radiation, or both radiation and simulated weightlessness, had sustained damage to their blood vessels. Namely, the mice had impaired vasodilation, or problems expanding their blood vessels to adjust for blood pressure. This can be a precursor to heart attacks and stroke. The mice that just experience simulated weightlessness, on the other hand, seemed normal.

While the rodent data complement the findings in real astronauts, the authors were clear about the limitations of the study. “Caution must be used in drawing definitive conclusions regarding specific health risks,” they concluded. The astronaut numbers are very small for an epidemiological study, there may be other factors in the space environment that could explain the possible health effects, and the type of radiation given to the mice wasn’t exactly the same as the type astronauts experience.

Delp and his colleagues are working with NASA on follow-up studies of astronauts’ health.

Scientific Reports, 2016. DOI: 10.1038/srep29901 (About DOIs).