This story was updated at 4:13 p.m. EDT.
It has been four decades since people from Earth first landed on the moon, but it has also been nearly that long since humans stopped going.
In the three years that followed NASA?s historic July 20, 1969 moon landing by Apollo 11 astronauts Neil Armstrong and Buzz Aldrin, five more missions successfully touched down on the lunar surface. In all, 12 men walked on the moon during those flights, the last of them in 1972 during Apollo 17. Today, NASA is on a path to return to the moon by 2020, but some believe the goal of human spaceflight should reach much farther - especially in the 40 years to come.
Among them is Aldrin, who believes that humanity can surpass the moon and reach the next stepping stone - Mars - by 2031, before the next four decades are up, but only through an international effort by many countries.
An international endeavor
The moon, Aldrin said, can be a springboard for places beyond and a robust space presence in low Earth orbit. By working together to form an international base on the moon, individual spacefaring nations like the United States can free up more time for other projects like exploring asteroids, comets, the moons of Mars, and then the red planet itself.
?It?s leading toward global space leadership,? Aldrin said of an international push to the moon, Mars and orbital stations. ?It promotes change and enlightened cooperation.?
That cooperation will be key, especially in 40 years when the amount of spacecraft and junk orbiting Earth will have most assuredly increased along with the number of humans and countries in space, Aldrin explained.
?We can disagree about human relations, piracy and secrecy, or land grabs below 100 kilometers,? Aldrin told SPACE.com. ?But up above, it?s a different story. I think we can really challenge our diplomacy and our relations with the rest of the world in that region.?
Private enterprise in space
Peter Diamandis, whose company Zero Gravity, Corp., offers weightless joyrides aboard a modified jet, believes that the nature of spaceflight could change in the upcoming decades, but only through true innovation.
"First of all, I think that we need to be looking at trying to change how spaceflight is implemented, because today I think it's ridiculous that spaceflight is as expensive or more expensive than it was 40 years ago,? said Diamandis, who founded the X Prize Foundation that is offering a $30 million Google Lunar X Prize to the first privately funded team to put a rover on the moon. ?We're not trying new approaches. We're using effectively the same technology. Where is true, really risky breakthrough work being done? I don't see it happening at NASA and I don't see it being tested.
?So I'd love to see some true research and development in propulsion taking place," Diamandis said. "I'd love to see private companies supported to the maximum extent possible, for example, the COTS D program [a NASA program that can encourage commercial development of manned spacecraft for station flights ], where for me I think it's a no-brainer. For the cost, it is probably the most logical and most prudent move that NASA could make to have a backup to programs which may ultimately not work or may ultimately get cancelled. Why not?
?I think that we need to create real markets," he said. "We do live in a capitalist society, we have a free economy, and NASA should be practicing capitalism and market economics to the maximum extent possible and leading the way."
Pushing beyond Mars
For planetary scientist Alan Stern, a former NASA associate administrator for science, the year 2049 should bring routine commercial space travel, manned missions to near-Earth asteroids and even Mars. By then, astronauts should be marking the Apollo 11 moon landing anniversary from inside their own lunar research outposts, Stern said.
But bases on Mars can be the precursor to grander flights since the technology required for the endeavor can be reapplied to asteroids or more remote targets, he said.
?There aren't serious technical barriers to go to Mars. That's not to say it's not hard, but we're already pioneering long duration human spaceflight with the space station, which has now been inhabited for the better part of 10 years, which is much longer than a Mars journey,? Stern told SPACE.com. ?We're understanding what the logistical and repair needs are. We've had people in space a year at a time, and then when they land, they're out walking around the streets a day or two later. They're not feeling entirely back 100 percent for a long time, for weeks, but that shows you can send people on a year-long journey to Mars. And if there are any health concerns, you can always the spacecraft if you have the money to do it and create artificial gravity.
?Radiation is a hazard, but you can, if you have the financial wherewithal, build a shelter inside the spacecraft for the occasional bad day event where you have to hide out," Stern said. ?So all these things are tractable if someone wanted to cut the check, and invest, as a private investor, to send us to Mars on a 10-year program, it's entirely within our technology today. Entirely.?
Permanent Martian settlement
Cornell University astronomer Steve Squyres is already on Mars, at least robotically via NASA?s Spirit and Opportunity rovers, which he has been overseeing as principal rover scientist. But the next 40 years, he said, carry the promise of a permanent human settlement on a completely different planet.
?Where I hope and think we'll be is that we will have gone back to the moon in order to prove out the technologies and techniques that we need to venture further out in the solar system,? Squyres said in an interview. ?We will have conducted the first human missions to near-Earth asteroids, and we'll be in the process of using the first permanent, scientific base on the surface of Mars. That's what I'd like to see and expect to see."
That is not to say that there are hurdles. Squyres said there is still a barrier of will that humanity must overcome before reaching the red planet in person.
?There are new launch vehicles that have to be created and built and tested,? he said.? There are new spacecraft that have to be designed, but there are no technical miracles standing in our way. All we've got to do is decide we want to do it."
Good citizens of the galaxy
Writer-producer Ann Druyan, widow of the famed astronomer Carl Sagan, may be in a minority among those pressing for humanity?s expansion out into the cosmos. The people of Earth, Druyan believes, may need the next four decades to mature before setting up shop on other worlds.
?Well, I'm not a big fan of human colonization,? Druyan told SPACE.com. ?Just in terms of the kinds of horrendous crimes that we've committed in the past. And so part of me feels that before we start actually moving into some of these other worlds, we should be working on our citizenship skills. And our ability to feed each other, and some of the other things we have to figure out how to do on this planet.
?But I would love an exploratory mission to Titan, which to me has one of the most alluring planetary surfaces that I've yet to see," she said. "Of course there will be all kinds of revelations coming from Kepler, and us being able to get up to speed on visiting some of these other worlds is a big thing I'd love to happen.
?We do not know definitively what the case is on Mars. Really, what the situation is. There's been tantalizing evidence here and there, and I think that certainly bears much further investigation as soon as possible," Druyan said.
?These missions are the things that people like to say, well what about hunger? I just don't see them as necessarily being antithetical to each other," she said. "In fact I think they feed us in different ways. But also, the money necessary to do this kind of exploration, when compared with the defense budget, is really not even a significant fraction. So I think getting our house in order has to go hand in hand with a much more exciting and vigorous program of exploration.?
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SPACE.com Senior Editor Tariq Malik, Senior Writer Andrea Thompson and Staff Writer Clara Moskowitz contributed to this report from New York City. This story was updated to reflect the correct date of the Apollo 11 moon landing.