Travelling to the moon, Mars or beyond could dramatically increase an astronaut’s risk of dying from cardiovascular disease, the first research into the long-term health of Apollo spacemen has revealed.

Nasa’s Apollo programme sent nine manned missions and 24 astronauts beyond low Earth orbit during the 1960s and early 1970s, including Apollo 11, which delivered Neil Armstrong and Buzz Aldrin to the moon. But it seems such missions might have taken their toll.

A team of researchers looking into the fate of the Apollo astronauts has discovered that their rate of death from cardiovascular disease is four to five times higher than that seen for astronauts of the same era who only flew in low Earth orbits, or who never flew on an orbital mission at all.

That, the researchers say, suggests that venturing beyond the Earth’s protective magnetic field could cause long-lasting damage to the cardiovascular system, potentially as a result of exposure to deep space radiation.

The findings come as a number of space agencies and commercial enterprises are looking to venture to the moon and beyond, with Nasa planning to send humans to Mars in the 2030s.

“With all of these plans for going into deep space we have only had this one really small group [of Apollo astronauts] that has actually done it,” said Michael Delp, first author of the study from Florida State University. “Previous to this study nobody had ever looked at their long-term health consequences - really pulling out the Apollo astronauts out of the group of other astronauts.”

Jeff Hoffmann, former Nasa astronaut and director of the Man Vehicle Lab at the Massachusetts Institute of Technology, agreed, adding that when it comes to travelling to the moon and beyond, the findings suggest speed might be of the essence.

“We can’t shield against high energy cosmic radiation, not with our current mass-limiting capabilities, but it does re-emphasise the importance of getting to Mars as quickly as possible,” he said. “The less time you spend in deep space the less exposed you are getting to the radiation.”

Writing in the journal Scientific Reports, a team of researchers including scientists from Johnson Space Centre and Nasa Ames Research Centre describe how they compared the causes of death for the seven Apollo astronauts who had died up to that point with those of 35 low Earth orbit astronauts from the same era, and 35 non-flight astronauts.

While the sample size is tiny - a clear limitation to the study - the results reveal that 45% of the Apollo astronauts died from cardiovascular disease, compared to only 9% of non-flight astronauts and 11% of low Earth orbit astronauts.

While no difference was seen when the rate was compared to that of the general population, Delp believes that is most likely because astronauts are very fit and healthy, whereas the members of the public suffer from a wide range of conditions that could increase their risk of cardiovascular disease.

“Part of the reason why this cardiovascular effect has never been uncovered is because you are always looking and comparing [astronauts with] a general population,” he said.

To probe whether deep space radiation might be behind their findings, the researchers exposed mice to conditions that simulated weightlessness as well as the radiation found in deep space.

With previous studies showing that both radiation and weightlessness can, in the short term, damage the blood vessels of the mice, the researchers waited 6-7 months before investigating the effects - a period equivalent to around 20 human years.

The results revealed that while weightlessness did not cause any lasting cardiovascular harm, the radiation did. Mice exposed to radiation were found to have sustained damage to the cells lining their blood vessels - damage that could increase the risk of cardiovascular disease.

The fact that, by contrast, weightlessness caused no lasting damage makes sense, says Delp. “That corresponds with what we saw with the low Earth orbit [astronauts] - they may have problems immediately when they come back, which we know they do, but they recover.”

Richard Hughson, of the University of Waterloo in Canada, who has studied the effect space travel on the cardiovascular systems of astronauts, described the new research as “fascinating”.

While Hughson notes that the sample size for the Apollo lunar astronauts is very small, he believes the study puts forward a plausible suggestion for why lunar astronauts appear to be succumbing cardiovascular disease at a higher than expected rate.

“I think it is really important that Nasa and the European Space Agency, the Canadian Space Agency, and so on, recognise now that the cardiovascular system should be an important focus, one they look at, when they send astronauts beyond low Earth orbit,” he said.

With only an ultrasound device and a blood-pressure test needed to check for damage to the lining of the blood vessels, Hughson believes astronauts could monitor the effects of the radiation during their time in deep space, while the results also flag the importance of checking up on the cardiovascular health of astronauts once their have returned to Earth. “We just did this within the last month on Tim Peake when he came back from space,” he said.

Hoffman agrees that such check-ups are important. “I go down to Nasa every year for a thorough physical examination and if they know that cardiovascular fitness is important, that is something they can monitor you for every year and hopefully reduce the risk of a premature death from heart problems,” he said.

Delp admits that a deeper look into the medical history of the Apollo astronauts is needed to make sure that the findings are not the result of other factors - including their somewhat eclectic post-mission lifestyles. But, he adds, the new research also raises further conundrums, from the radiation doses needed to damage the cardiovascular system to whether the effects differ between the sexes and whether the problem can be lessened.

“All of these are future questions that really I think are important to address before we start sending humans into deep space again,” he said.