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.