NASA remains committed to landing a man on Mars - but needs help to do it.
The American space agency wants to enlist other countries in its mission to get to the Red Planet by 2035.
"In the near term, Mars remains our primary focus," said Nasa chief scientist EllenStofan at the Royal Institution in London. "This is not something any one nation can do on their own. It's something that humanity is going to do together."
Nasa, whose Apollo missions to the Moon were driven by a patriotic zeal to beat the Soviet Union, estimates that a return trip to Mars would take three years.
It currently has a rover exploring Mars and Dr Stofan said the ultimate aim was for manned missions to create a base to which humans could return.
"I don't think that first group will necessarily stay there, but we need to think of this as establishing an outpost," she said. "We want it to be possible for those people to come back if they want to."
That is more than some organisations are promising. Mars One, a private not-for-profit organisation, claims it will have people on the planet by 2025 but currently has no option to bring them home.
Its mission is to establish a permanent settlement with crews of four departing every two years, starting in 2024.
