Acid test is kidneys on a mission to Mars

The biggest threat to would-be Mars astronauts may not be the loneliness of space or the two-year trip or even stray meteors. The US space agency Nasa says if a crew member developed a kidney stone (typically about 4mm [0.15in] across) missions to Mars could be forced to turn back. But they may copy voyagers in the 18th century and use supplements from citrus fruits.

"Once renal stones start to move [from the kidneys] they can be excruciatingly painful," said Peggy Whitson, an astronaut and a biochemist at the Johnson Space Centre in Houston. Astronauts and cosmonauts who spent 100 days or more on the Mir space station had a considerably higher risk of developing the stones, formed by calcium salts crystallising in the urine in the kidneys.

The Nasa report, in New Scientist magazine today, says astronauts drink less fluid and excrete less urine than people on the ground. This, with the tendency to lose calcium from bones in low-gravity space, forms a potentially painful combination.

For ground-based sufferers, the best cures are to drink plenty of water, which reduces the risk of stones forming and helps to ease them out of the ureter. But that was not an option in space, Dr Whitson said. "Urinating in in-orbit toilets is time-consuming and crew members are very busy," she said. For those working outside a spacecraft, higher fluid intake was not possible.

But just as sailors in the 18th century were given lemons and limes to stop their scurvy (caused by lack of vitamin C), astronauts on long trips could use citrate supplements. A trial with 60 ground-based patients found taking citrate supplements reduced the risk of kidney stones by 85 per cent.