Former moonwalker Buzz Aldrin has suggested Nasa should focus on sending astronauts to asteroids in deep space, rather than the agency's current plan to capture a meteor.
Nasa's Asteroid Redirect Mission (ARM), as part of the manned mission to Mars in the 2030s, consists of "lassoing" an asteroid and introducing it into the moon's orbit by 2015.
Yet Aldrin, who was the lunar module pilot in the Apollo 11 mission, believes that sending humans to an asteroid is better than using robotic technology to redirect the meteorite towards the moon.
At the Humans to Mars summit, he said: "I sure do not think bringing a rock back is better than what the president said - a human mission to an asteroid in 2015 - but send a robot there first so the two get there at the same time."
Under Aldrin's alternative proposal, Nasa would send an astronaut crew to an asteroid using the space agency's Space Launch System rocket and Orion space capsule.
He added, as reported in Space.com, that the astronaut crew would be able to operate the asteroid-exploring robot with no time delay.
Speaking at the same event, Charles Bolden, head of the US space programme, said the process of capturing an asteroid was a "stepping stone" on the path to the Red Planet.
Once the asteroid is in the moon's orbit, astronauts will visit the rock to collect samples in the 2020s, using the Orion capsule.
The planned mission is designed to help engineers test out ways of deflecting potentially harmful asteroids that could threaten Earth. If Nasa officials choose to launch a robotic mission that picks a boulder off a larger asteroid, they could also use the robot to try to move the large asteroid into a slightly different orbit.
Earlier this month, at a Senate hearing called From Here to Mars, William Gerstenmaier, an associate administrator at Nasa, explained: "To think we're moving a piece of the solar system for our use that will allow us to learn skill and techniques that we need to push the human presence into the solar system, that's pretty awe-inspiring."
Yet Aldrin said planetary defence was not a Nasa mission and instead, it should be "at the very highest level of the National Security Council".
The former astronaut added that he believed humans should establish a permanent, internationally co-operative base on Mars. He said an insightful leader could bring the pipedream to fruition, while he himself needed to convince others that a Mars base was not an "egghead idea".