In the 1960s and 1970s, NASA astronauts on the Apollo missions have been to the moon, and were the sole humans flown farther than the Earth’s protective magnetic shielding. A latest study showed that deep-space radiation exposure could have taken a toll on their hearts.

The study compared the fates of the Apollo astronauts with the un-flown astronauts, who have never been into space, and the ones who lingered in low-Earth orbit. It noted an elevated level of life loss from cardiovascular issues in the deep-space adventurers. In a study conducted on mice, the researchers also discovered a long-term effect of radiation exposure on heart health.

The latest study’s main author Michael Delp, a researcher at Florida State University, said that in 2024, the International Space Station is scheduled to be decommissioned and countries (and private companies such as SpaceX) are looking forward to the moon and the Red Planet. Delp said that this would be accompanied by exposure to deep-space radiation, about which there is very slight information so far.

Speaking to Space.com, Delp said, “With deep-space radiation, really, the 24 Apollo astronauts that went to the moon are the only astronauts that have really been exposed to this, because all the others — cosmonauts and astronauts — have stayed in low-Earth orbit”.

Delp said though previously many studies have focused on the causes of death of astronauts, the most recent one was the first to conduct a comparison between astronauts and other astronauts rather than the general public. The control population of the study included astronauts assigned to Apollo, Gemini and space shuttle missions that got cancelled ultimately, and many others.

The astronauts were chosen for the mission, but they never made it to space, so they must have the same baseline health as the ones that did.

According to a report in Fox News by Rob Verger, "Researchers looked at the rate of death from cardiovascular disease among astronauts who never flew in space, went to space but stayed in low-Earth orbit, and the Apollo astronauts, who went further away from Earth than anyone else. Of the 24 Apollo astronauts who flew into deep space, eight have died and seven were included in the study."

The Earth’s magnetosphere helps to protect people on the ground and in orbit from cosmic radiation. "We know very little about the effects of deep space radiation on human health, particularly on the cardiovascular system," Michael Delp, the first author on the new study and a professor at Florida State University, said in a statement. "This gives us the first glimpse into its adverse effects on humans."

This new study comes at a time when NASA is eyeing a manned trip to Mars or an asteroid at some point in the future— and SpaceX turns its gaze to Mars too— and suggests that exposure to space radiation is an important factor to consider. As the study, which was published in the journal Scientific Reports, says, “These data suggest that human travel into deep space may be more hazardous to cardiovascular health than previously estimated.”

A report published in QZ revealed, "The Earth’s magnetic field offers protection for human space travelers moving through it, by deflecting charged particles that can enter the body. Astronauts who venture beyond the field are exposed to more deep-space radiation, said the study, which can damage cells and cause arterial clogging."

Several countries, including Russia and China have plans for manned missions to the moon, and the United States is planning a human mission to Mars. Some people are already thinking about how to govern the red planet.

“A lot of studies published both with humans and animals have very small numbers,” Delp told the Los Angeles Times. “It’s the norm rather than the exception, but it means you have to be very cautious in your interpretations.”