On the Apollo missions, astronauts ventured about 400,000km (249,000 miles) from Earth and spent 8-12 days away from home. A trip to Mars would involve 56 million km travelling over eight or nine months – and a stay of about two years, to wait for the two planets to be close enough again to make the return journey over that shortest distance.
So, while the hardware challenge of making the return trip and surface stay is clearly daunting, the obstacles to successfully keeping a crew healthy are equally high.
As NASA chief scientist Ellen Stofan recently told a Royal Aeronautical Society audience, the International Space Station is a crucial laboratory. Weightlessness, she notes, has profound effects on the human body – which are not yet well understood.
Obvious effects of weightlessness include bone and muscle wastage – and these effects appear to be amplified by exposure to radiation, she says.
Also, according to NASA scientist James Thomas, who carried out a study of 12 astronauts for the US National Space Biomedical Research Institute, the heart becomes more spherical when exposed to long periods of microgravity in space – a change that could lead to cardiac problems. Part of the problem, says Thomas, is that “the heart doesn't work as hard in space, which can cause a loss of muscle mass”.
"That can have serious consequences after the return to Earth,” he says. “So knowing the amount and type of exercise astronauts need to perform to keep the heart healthy is going to be very important to guarantee their safety on a long flight like a mission to Mars.”
A probably less life-threatening issue – though potentially a big problem on a very long space mission – came to light in 2012, when it became evident that about 20% of men who make long-duration space flights of six months or more suffer permanent eyesight degradation. That is, after returning to Earth they are short-sighted and may need to wear glasses or undergo corrective laser surgery.
The theory being considered is that in microgravity, blood and fluid pressure in the legs is necessarily lower than on the ground, and that pressure is distributed elsewhere. Discussing the problem during another Royal Aeronautical Society presentation in 2013, UK astronaut Tim Peake noted that astronauts on the ISS tend to have puffy faces, and it may be the case that pressure on the retinas flattens them, in some cases permanently. Women are not affected, it appears.
Peake speculated that genetic screening might become a necessary part of astronaut selection.
Nutrition is another big question, even without considering the problems of providing food for a crew without resupply from Earth. Without normal gravity, says Stofan, astronauts experience an uncomfortable if not debilitating rise in intracranial pressure. It looks like the answer is to not eat too much, take very little salt and consume lots of fruit and vegetables. “It’s the same prescription” as that for healthy living on Earth, she says.