GALVESTON, Texas -- NASA astronauts training for a possible mission to Mars have been practicing water manoeuvrs in a mock-up Orion space capsule in the Gulf of Mexico.

Several astronauts trained Thursday in the open water, about 6.5 kilometres from Galveston, Texas.

An Orion spacecraft is being prepared at Johnson Space Center in nearby Houston. It is intended for flight with NASA's new SLS or Space Launch System, the most powerful rocket the agency says it has ever built.

The astronauts wore bright orange training uniforms as they trained for a possible water landing, jumping into the water, using flotation devices and deploying a life raft.

Coast Guard and other NASA and military safety personnel were positioned nearby.