Buzz Aldrin says the first people to land on Mars should never return to Earth.
The second man to set foot on the moon was asked about the possibility of a permanent human presence on the red planet during an online question and answer session.
He was asked specifically about SpaceX founder Elon Musk, who believes people could visit Mars within a decade.
Aldrin said: "I have considered whether a landing on Mars could be done by the private sector.
"It conflicts with my very strong idea, concept, conviction, that the first human beings to land on Mars should not come back to Earth.
"They should be the beginning of a build-up of a colony or settlement. I call it a permanence."
He added: "To have an individual company, no matter how smart, send people to mars and bring them back, it is very, very expensive. It delays the obtaining of permanence."
He said that some people would call it a "suicide mission", but he disagreed saying: "Not me! Not at all."
There would be no shortage of volunteers for a one-way trip to Mars.
Mars One - a non-profit organisation hoping to create a settlement on Mars by 2023 - received 200,000 applications for volunteers to travel to the red planet and never come back.
The 84-year-old was answering questions on Reddit to mark the 45th anniversary of the Apollo 11 moon landing on July 20.
He also told how impressed he was with the movie Gravity, describing it as "thrilling", and said he "doesn't waste very much time" on conspiracy theorists who believe the moon landing was fakedhttp://news.sky.com/story/1297741/aldrin-first-man-on-mars-shouldnt-come-back