Image copyright SCIENCE PHOTO LIBRARY Image caption Artist's impression: When will the first astronaut set foot on Mars?

President Barack Obama says the US will work with private companies on its plan to send humans to Mars in the 2030s.

President Obama announced his proposals for a crewed mission to the Red Planet in 2010.

But Nasa's plan to realise this presidential vision has been broadly criticised, particularly by Congress.

In an article, President Obama pledged to work with private companies to "to build new habitats that can sustain and transport astronauts".

"We have set a clear goal vital to the next chapter of America's story in space: sending humans to Mars by the 2030s and returning them safely to Earth, with the ultimate ambition to one day remain there for an extended time," Obama said in an opinion piece for CNN.

Uncertainty ahead for US 'Mars ship'

The comments are not entirely surprising: Nasa is already working closely with the private sector to resupply the International Space Station. And many private space firms - particularly SpaceX, whose Dragon capsule delivers cargo to the ISS - have made no secret of their ambitions to explore Mars.

Last month, Elon Musk, who founded SpaceX, outlined his proposal for a permanent base on Earth's smaller, colder neighbour.

Reaction to this plan was mixed: some space experts criticised the plan as unrealistic, while others praised Mr Musk for outlining a detailed - and audacious - architecture for getting to Mars.

Media playback is unsupported on your device Media caption Entrepreneur Elon Musk has outlined his vision for getting to Mars.

The absence of detail is something that has cropped up as a recurrent criticism of Nasa's own initiative, named "Journey to Mars" - a response to the Red Planet vision announced by President Obama six years ago.

Earlier this year, members of Congress and expert witnesses claimed the US space agency lacked a clear road map for Mars and that the plan needed achievable milestones to succeed.

However, this has apparently not dented President Obama's enthusiasm for expeditions to the Red Planet.

In his opinion piece, he said: "We have set a clear goal vital to the next chapter of America's story in space: sending humans to Mars by the 2030s and returning them safely to Earth, with the ultimate ambition to one day remain there for an extended time.

"Getting to Mars will require continued cooperation between government and private innovators, and we're already well on our way."

He said that he would be convening leading scientists, engineers and innovators in Pittsburgh this week to "dream up ways to build on our progress and find the next frontiers".