Apollo astronauts suffer from their space flights
NASA'S Apollo astronauts, who travelled beyond Earth's protective magnetosphere, die disproportionately of heart and blood vessel diseases, researchers said last week, blaming radiation.
This raises health concerns for humans with dreams of travelling to the Moon, Mars or beyond, as space agencies and private companies compete 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 Florida State University.
"This gives us the first glimpse into its adverse effects."
Of seven Apollo astronauts to have died to date, three (43 per cent) succumbed to cardiovascular disease - ailments that include heart attacks, brain aneurysms and strokes.
This was "four to five times higher" than for trained astronauts who never left Earth (9 per cent) and those like the International Space Station crews who stayed closer to home in low-Earth orbit (11 per cent).
"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, died 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 astronauts were exposed to unprecedented levels of particle radiation, said the study.
The ISS orbits Earth within the magnetosphere.
Energetic particles from galactic radiation can be dangerous to humans as they pass through the skin and can damage cells or DNA, said Nasa.