The aquanauts have been training at the Aquarius Reef Base, 19 metres below the surface since 2001 as the conditions there are comparable to those in space.

The name of the NASA Extreme Environment Mission Operations (NEEMO) project has a rich history in underwater exploration, from the fictitious submarine commander Captain Nemo to his talking clownfish namesake in the 2003 animation. But the NEEMO project is looking to the future, training astronauts of the US space agency for a possible mission to an asteroid.

NASA leases the Aquarius Reef Base, around 4.5 kilometres off Key Largo in Florida, from the National Oceanic and Atmospheric Administration, because conditions are comparable to those in space.

On the floor of the Atlantic Ocean, 19 metres below the surface, there is no air, and the buoyancy from the water is similar to weightlessness.

The aquanauts have been training here since 2001, and the latest intake last week donned their white suits, silver helmets and flippers for an underwater press conference via video link.

“It is a lot easier and cheaper to learn things on earth, than going on an asteroid and finding out it doesn’t work”, said Dorothy Metcalf-Lindenburger, commander of the 16th NEEMO mission. The 37-year-old astronaut, together with her US colleagues James Talacek, Justin Brown, Timothy Peake, Steven Squyres, and Kimiya Yui of Japan, spent each of the previous 12 days underwater simulating as best they could operations on an asteroid.

“We want to learn more about our solar system. So we ask, what can asteroids tell us about it? What additional resources could we find there?” Metcalf-Lindenburger said.

They are also threats, she said. “So the more we understand about asteroids, the better we can be prepared.” NASA wants to land on an asteroid by 2025. It would be the agency’s first step toward overcoming longer distances, to exploring deep space, and sending humans to Mars by mid-2030, as President Barack Obama put forward two years ago.

But the biggest obstacle to exploring an asteroid is not reaching one - indeed, some are close enough to threaten to hit Earth - but rather working with their very feeble gravitational pull.

Taking samples in near-zero gravity is a daunting prospect, as they can easily spin off into space if not properly contained.

So planning is necessary to avoid expensive, or even dangerous mistakes. Since NASA’s first mission 11 years ago, 45 astronauts have trained at the Aquarius facility.

This year’s course simulated an asteroid mission, and focussed on three areas: how to deal with delayed communications, how to secure and transport people, equipment and samples - known as restraint and translation techniques - and an evaluation of the optimum crew size for the cramped mission.

The team also carried out behavioural, physiological and psychological experiments.

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