IT may have been a giant leap for mankind but travelling to the Moon was a step backwards for the health of astronauts who made the journey, research has shown.

The Apollo missions in the late 1960s and early 1970s exposed crews to levels of radiation which heightened their risk of fatal heart disease, a study suggests.

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Scientists compared causes of death for seven lunar astronauts and 35 who never got further than Earth orbit.

They found that 43 per cent of the Apollo astronauts had died from a cardiovascular problem compared with 11 per cent of the low Earth orbit astronauts – a four-fold difference.

Another group of 35 astronauts who had not flown a mission at all had a heart disease death rate of nine per cent.

Laboratory tests indicated that radiation rather than the effects of weightlessness was responsible for the high proportion of Apollo astronauts suffering life-threatening heart problems.

The findings, published in journal Scientific Reports, have implications for planned long-duration space missions to the moon and Mars.

Lead scientist Professor Michael Delp, from Florida State University, US, said: “We know very little about the effects of deep space radiation on human health, particularly on the cardiovascular system.

“This gives us the first glimpse into its adverse effects on humans.”

The Apollo programme launched 11 manned flights into space between 1968 and 1972. These included Apollo 11 in 1969, during which Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon.

Nine of these manned flights flew astronauts beyond Earth orbit into deep space.

Neil Armstrong and Buzz Aldrin on the Apollo 11 mission made history when they landed their lunar module on the surface of the moon on July 20 1969.

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Six further missions to land men on the moon followed. One, Apollo 13, was aborted after the spacecraft was crippled by an explosion. The story of how the crew nursed their stricken craft back to Earth and survived was turned into an award-winning film starring Tom Hanks.

To date the Apollo astronauts have been the only space travellers to have strayed outside the Earth’s magnetosphere – the “umbrella” that shields the planet from dangerous high energy particles.

Of the 24 men who flew into deep space on the Apollo missions, eight have died and all but one of them were included in the study.

To investigate the likely cause of the heart deaths, Prof Delp’s team exposed mice to simulated weightlessness and the radiation the Apollo astronauts would have encountered.

The irradiated mice later showed changes in cell lining of blood vessels that are known to lead to heart disease in humans.

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Simulated weightlessness did not appear to have made any long-term contribution to the effect.