Astronauts aboard the International Space Station (ISS) had slower memory, attention and processing speed after six months, raising concerns about the impact of cognitive impairment on future space missions to Mars.

The extreme environment of space, with reduced gravity, harsh radiation and the lack of regular sunrises and sunsets, can have dramatic effects on astronaut health, from muscle loss to an increased risk of heart disease. However, the cognitive effects of long-term space travel are less well documented.

Now, Sheena Dev at NASA’s Johnson Space Center in Houston, Texas, and her colleagues have looked at the cognitive performance of 25 astronauts during their time on the ISS.

The team ran the astronauts through 10 tests, some of which were done on Earth, once before and twice after the mission, while others were done on the ISS, both early and later in the mission. These tests measured certain cognitive capacities, such as finding patterns on a grid to test abstract reasoning or choosing when to stop an inflating balloon before it pops to test risk-taking.

The researchers found that the astronauts took longer to complete tests measuring processing speed, working memory and attention on the ISS than on Earth, but they were just as accurate. While there was no overall cognitive impairment or lasting effect on the astronauts’ abilities, some of the measures, like processing speed, took longer to return to normal after they came back to Earth.

Having clear data on the cognitive effects of space travel will be crucial for future human spaceflight, says Elisa Raffaella Ferrè at Birkbeck, University of London, but it will be important to collect more data, both on Earth and in space, before we know the full picture.

“A mission to Mars is not only longer in terms of time, but also in terms of autonomy,” says Ferrè. “People there will have a completely different interaction with ground control because of distance and delays in communication, so they will need to be fully autonomous in taking decisions, so human performance is going to be key. You definitely don’t want to have astronauts on Mars with slow reaction time, in terms of attention-related tasks or memory or processing speed.”

It isn’t surprising that there were some specific decreases in cognitive performance given the unusual environment of space, says Jo Bower at the University of East Anglia in Norwich, UK. “It’s not necessarily a great cause for an alarm, but it’s something that’s useful to be aware of, especially so that you know your limits when you’re in these extreme environments,” she says.

That awareness could be especially helpful for astronauts on longer missions, adds Bower. “It’s not just how you do in those tests, but also what your perception of your ability is,” she says. “We know, for example, if you’re sleep deprived, that quite often your performance will decline, but you won’t realise your performance has declined.”