NASA's Apollo astronauts, the only humans to have travelled beyond Earth's protective magnetosphere, die disproportionately of heart and blood vessel diseases, researchers said Thursday, blaming radiation.

This raises health concerns for all humans with dreams of travelling to the Moon, Mars or beyond, as space agencies and private companies vie to expand humankind's extraterrestrial footprint.

"We know very little about the effects of deep space radiation on human health, particularly on the cardiovascular system," said Michael Delp of the Florida State University.

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

Of seven Apollo astronauts to have died to date, three (43 percent) succumbed to cardiovascular disease -- a group of ailments that includes heart attacks, brain aneurysms and strokes.

This was "four to five times higher" than for trained astronauts who never left Earth (nine percent), and those like the International Space Station crews who stayed closer to home in low-Earth orbit (11 percent).

The findings were published in the journal Scientific Reports.

"These data suggest that human travel into deep space may be more hazardous to cardiovascular health than previously estimated," the researchers wrote.

NASA's Apollo programme sent 11 manned flights into space between 1968 and 1972.

Of the 24 men who flew beyond Earth orbit into deep space, eight have died to date. The eighth, Edgar Mitchell, passed away this year, after the data had been analysed, and was not included in the study.

Beyond the magnetosphere, a magnetic "bubble" which shields Earth and its occupants, the Apollo astronauts were exposed to unprecedented levels of particle radiation, said the study.

The ISS, by comparison, orbits Earth within the magnetosphere.

- Mars colonies -

Energetic particles from galactic radiation can be dangerous to humans as they pass right through the skin and can damage cells or DNA, according to NASA.

The agency has plans for a manned trip to Mars in the 2030s, and has stated that radiation shielding "will be a crucial technology" for the voyage as well as for exploring the surface.

The study was the first to include non-flight astronauts as a comparison group in examining the long-term health of space travellers.

Comparing astronauts to the general population can yield confusing outcomes, as they are generally better educated and fitter, and have lifelong access to medical care -- all ingredients for better health.

In comparing different types of astronaut, the researchers were able to exclude any possible impact of zero-gravity on cardiovascular disease, as all three groups (Earth-bound, low-orbit and deep space) were exposed to it.

The researchers also found no difference between the three groups in terms of cancer or accidents as the cause of death.

To test a possible cause for lunar astronauts' higher incidence of cardiovascular disease, the team exposed mice to comparable radiation levels in the lab.

"After six months -- the equivalent of 20 human years -- the mice demonstrated an impairment of arteries that is known to lead to the development of atherosclerotic cardiovascular disease in humans," said a Florida State University statement.

"What the mouse data show is that deep space radiation is harmful to vascular health," added Delp.

US billionaire Elon Musk and Dutch company Mars One have mooted placing human colonies on the Red Planet.

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