PARIS - If a human ever sets foot on Mars, will it be a giant step or an exhausted shuffle?

Long-term space flight so weakens fitness that an astronaut heading to the Red Planet may lose up to half the power in key muscles in the course of the mission, scientists have found.

The loss - equivalent to a crew member aged between 30 and 50 returning home with the muscles of an 80-year-old - would add a major danger to a trip already laden with peril, they said.