One of the first men to orbit the Moon has told BBC Radio 5 Live that it's "stupid" to plan human missions to Mars.
Bill Anders, lunar module pilot of Apollo 8, the first human spaceflight to leave Earth's orbit, said sending crews to Mars was "almost ridiculous".
Nasa is currently planning new human missions to the Moon.
It wants to learn the skills and develop the technology to enable a future human landing on Mars.
Anders, 85, said he's a "big supporter" of the "remarkable" unmanned programmes, "mainly because they're much cheaper". But he says the public support simply isn't there to fund vastly more expensive human missions.
"What's the imperative? What's pushing us to go to Mars?" he said, adding "I don't think the public is that interested".
Meanwhile, robotic probes are still exploring Mars. Last month, the InSight lander, which will sample the planet's interior, successfully touched down at Elysium Planitia.
In a statement, Nasa said it was "leading a sustainable return to the Moon, which will help prepare us to send astronauts to Mars".
"That also includes commercial and international partners to expand human presence in space and bring back new knowledge and opportunities."
In December 1968, Anders, along with crewmates Frank Borman and Jim Lovell, lifted off from Cape Canaveral in Florida atop a Saturn V, before completing 10 orbits around the Moon.
The crew of Apollo 8 spent 20 hours in orbit, before returning to Earth.
They splashed down in the Pacific on 27 December, landing just 5,000 yards (4,500 metres) from their target point. They were picked up by the aircraft carrier USS Yorktown.
It was the furthest humans had ever been from their home planet at that point - and a vital stepping stone on the road to Apollo 11's historic moon landing just seven months later.
But the former astronaut is scathing about how Nasa has evolved since the heady days of President John F Kennedy's pledge to land a man on the Moon by the end of the 1960s.
"Nasa couldn't get to the Moon today. They're so ossified... Nasa has turned into a jobs programme... many of the centres are mainly interested in keeping busy and you don't see the public support other than they get the workers their pay and their congressmen get re-elected."
Anders is also critical of the decision to focus on near-Earth orbit exploration after the completion of the Apollo programme in the 1970s. "I think the space shuttle was a serious error. It hardly did anything except have an exciting launch, but it never lived up to its promise," he said.
"The space station is only there because you had a shuttle, and vice-versa. Nasa really mismanaged the manned programme since the late lunar landings."
It's a view that might seem surprising from a proud patriot and servant of the US military, who still remembers his own mission to space with great fondness. It's also a view that Anders accepts doesn't sit too well with some in the space community.
"I think Nasa's lucky to have what they've got - which is still hard, in my mind, to justify. I'm not a very popular guy at Nasa for saying that, but that's what I think," he explained.
His former crewmate, Frank Borman, who commanded the Apollo 8 mission and also spent two weeks in Earth orbit during the Gemini programme, is slightly more enthusiastic.
"I'm not as critical of Nasa as Bill is," he told 5 Live. "I firmly believe that we need robust exploration of our Solar System and I think man is part of that."
But asked about the the plans of Space X founder Elon Musk and Amazon boss Jeff Bezos - who have both talked of launching private missions to Mars, Borman is less complimentary.
"I do think there's a lot of hype about Mars that is nonsense. Musk and Bezos, they're talking about putting colonies on Mars, that's nonsense."
Reflecting on their own historic mission to the Moon, Borman described Apollo 8 as a "great endeavour" and agreed that it had won the space race.
Anders said he felt that the lasting legacy of the mission would be "Earthrise" a photo taken by the crew showing humanity's home planet hanging in the blackness of space above the lunar horizon.
Speaking to BBC Radio 4's PM, their crewmate Jim Lovell also reflected on the Earthrise moment: "When I looked at the Earth itself... I started to wonder why I was here, what's my purpose here… it sort of dawned me," he said.
"And my perspective is that God has given mankind a stage on which to perform. How the play turns out, is up to us."
Frank Borman and Bill Anders were speaking to BBC Radio 5 Live as part of a special documentary on the 50th anniversary of their historic lunar mission. Apollo 8: Christmas On the Far Side of the Moon will be broadcast on 5 Live on 24 December 2018 at 20:00 GMT.