One of the first two men to walk on the moon wants President Barack Obama to skip the moon and aim for a new destination: Mars.
The other isn't so sure.
On Monday, all three Apollo 11 crewmembers were to meet with Obama at the White House. The night before, they spoke at the Smithsonian Institution's National Air and Space Museum in Washington, D.C..
Two of them, Buzz Aldrin and Michael Collins, expressed concerns about NASA getting bogged down on the moon. Neil Armstrong, the first human to walk on another world, was more reserved.
Aldrin, who set foot on the lunar surface about 15 minutes after Armstrong, began his speech Sunday at the Smithsonian Institution's National Air and Space Museum with a call for action.
He said the best way to honor the Apollo astronauts "is to follow in our footsteps; to boldly go again on a new mission of exploration."
"Four decades have passed since Neil, Mike and I passed across the blackness of space to win a race," he said. "This time, instead of a moon race, we can try to make the moon a true stepping stone to more exciting and habitable destinations ... If we persevere, we can reach Mars itself before 2035."
NASA's current plans involve setting up a permanent base on the moon by 2020 before sending astronauts on to Mars — a scenario that's drawn criticism from several Apollo 11 veterans.
Apollo 11 command module pilot Michael Collins, who circled the moon alone while Armstrong and Aldrin walked on it, told the packed house — 7,000 people had entered a lottery for 485 seats — that the moon was not interesting, but Mars is.
"Sometimes I think I flew to the wrong place. Mars was always my favorite as a kid and it still is today," Collins said. "I'd like to see Mars become the focus, just as John F. Kennedy focused on the moon."
Armstrong, who makes few public appearances or statements, mostly sidestepped the Mars-vs.-moon issue.
He delivered a professorial lecture entitled "Goddard, Governance and Geophysics," which went over the inventions and discoveries that led to his historic "small step for a man" on July 20, 1969.
"History is a sequence of random events and unpredictable choices, which is why the future is so difficult to foresee," he said, adding with a wry smile: "But you can try."
The man who founded and directed Mission Control in Houston, Christopher Kraft Jr., also jumped on the go-somewhere-new, do-something-different bandwagon.
"What we need is new technology; we have not had that since Apollo," Kraft said as part of the lecture at the Smithsonian. "I say to Mr. Obama: Let's get on with it. Let's invest in the future."
Other notables at the lecture included new NASA Administrator Charles Bolden, himself a former astronaut and retired Marine major-general, and the crew of space shuttle mission STS-125, which repaired the Hubble Space Telescope in May.
NASA's current plan is based on building new rockets that the former NASA administrator called "Apollo on steroids," with an alternative — a derivative of the space shuttle — floating through the space agency.
Although they didn't directly criticize NASA's current plans, Aldrin and Collins said the moon is old hat. Collins said he is afraid that NASA's exploration plans would be bogged down by a return visit to the moon.
Aldrin presented an elaborate slide detailing how to make a quick visit to the moon a stepping stone to visits to the Martian moon Phobos, Mars itself, and even some asteroids like Apophis that may someday hit Earth.
Aldrin said he and Armstrong landed on the moon 66 years after the Wright brothers first flew an airplane. What he would like would be for humanity to land on Mars 66 years after his flight. That would be 2035.
And even though Armstrong didn't talk about the future in his 19-minute discourse, Aldrin dragged his commander onto the Mars bandwagon anyway.
"It was a great personal honor to walk on the moon, but as Neil once observed, there are still places to go beyond belief," he said. "Isn't it time to continue our journey outward, past the moon?"
The Times of London and the Associated Press contributed to this report.