Exposure to space radiation may increase astronauts' risk of a dangerous heart condition, a new study in rats hints.

The research, published Sept. 30 in the journal PLOS One, found that rats exposed to levels of radiation similar to what an astronaut might experience on a long-term trip to Mars developed more scar tissue around the small blood vessels of their hearts than rats that were not exposed to this radiation. This scar tissue is indicative of a condition known as cardiac fibrosis, which is known to increase the risk of heart failure.

This scar tissue was observed nearly a year after the rats were first exposed to the radiation.

But since this study was in rats, it cannot definitively say whether this effect will play out in humans. For example, given that humans have a longer life expectancy than rats (80 years versus 2 to 4 years), the effects seen in rats may take around 30 years to show up in humans. However, the study authors say the research highlights the importance of following astronauts' health long after they return to Earth.

"The message that we try to convey … is that the follow-up time is really important," study co-author John Baker, a professor of surgery in the Radiation Bioscience Laboratory at the Medical College of Wisconsin, told Live Science. "When astronauts come back to the Earth, they expect ‪—‬ and we expect ‪—‬ that they're going to live for a long time. I think we have to be wary that they could well develop at least some type of cardiac disease."

How space radiation affects the heart

The link between exposure to radiation and cardiac issues has been well documented. However, "no astronaut has ever been exposed to galactic cosmic radiation for long periods of time," Baker said, which makes it difficult to determine exactly how space radiation affects the heart.

Although Baker and his team previously pinpointed cardiac issues among rats exposed to radiation, this new study utilized NASA's Simulated Galactic Cosmic Ray Simulation (SimGCRSim). This new simulator provides a more precise comparison of the levels of space radiation an astronaut might experience while exploring deep space for more than a year.

During the study, the researchers exposed rats to several minutes of radiation proportionate to their size, similar to what they might experience on a trip to Mars and back (a roughly 18-month round trip, with current technology). Some of the rats' hind legs were suspended, to "simulate conditions of weightlessness," Baker said.

After 360 days, the rats that were exposed to cosmic radiation developed twice as much scar tissue around the heart's small blood vessels than unexposed rats. This buildup of scar tissue means "the heart can't relax to the point when it needs to," thereby increasing the risk of heart failure, Baker said.

This effect was not seen in the rats who were exposed only to simulated weightlessness, confirming "it was only the radiation that produced the damaging effect on the heart," Baker said. However, long-term testing revealed that this group of rats had lower levels of key immune cells, suggesting that microgravity also may affect aspects of long-term health. (Numerous studies have shown various short-term health impacts related to microgravity.)

This study "is a very valuable contribution to space biology," Rupak Pathak, an associate professor of pharmaceutical science at the University of Arkansas for Medical Sciences who was not involved in the study but holds a doctorate in radiation biology, told Live Science. While rats and humans have clear differences, "they share more than 90% of genetic information," he said, as well as very similar cardiac systems.

Given that the study authors were able to expose the rats to radiation only once, Pathak would like to see further experiments exploring whether chronic radiation exposure affects the function of the heart and other vital organs.

Baker and his colleagues hope that future experiments can be used to help NASA implement safeguards for astronauts on long-term missions, such as providing astronauts heart medications or improving shielding for the spacecraft.

"The more we know before we start, then the better NASA is going to be at working out what types of countermeasures can be developed," said Baker.