Scientists have debated the existence of life on Mars for more than a century. Mars boosters say it's time to settle the arguments by sending humans to the Red Planet. Humans would learn "whether we're … in a living universe, where life is common, or a dead universe," says Robert Zubrin, president of the Mars Society, a group dedicated to Mars exploration.
In all the vast universe, the Earth is the only place known to support life. Mars may be the next best place to nurture living things. It's not only the most Earth-like planet but also has stores of water, as confirmed by a NASA robot last year.
Two of Apollo 11's three crewmembers are Mars partisans. "As celestial bodies go, the moon is not a particularly interesting place, but Mars is," Apollo 11 astronaut Michael Collins said in a statement from NASA.
A return to the moon is "not very attractive," says Buzz Aldrin, who was the second man on the moon. "After 50 years, do we want to be known for returning to the moon?"
He favors human colonization of Mars. He envisions crews testing their skills and spaceships on Phobos, a Mars moon, before pushing on to the Red Planet.
A Mars trip may seem far-fetched, but such a mission was proposed by the first President Bush in 1989. The idea went nowhere then, but the younger President Bush's 2004 plan for NASA also included the goal of sending humans to Mars.
NASA took that directive so seriously that its engineers started work on a giant rocket so powerful it could launch spacecraft not just to the moon but also to Mars. Work on the rocket is on hold while the Obama administration sorts out its plans for space.
Even some strong supporters of space exploration say the best place to send America's astronauts would be nowhere at all.
Opponents of human spaceflight say robots can do the job just as well as astronauts, pose no safety worries and work cheaply. Sending humans into space isn't worth it, they say.
"The cost and risks are just too high," says physicist Robert Park of the University of Maryland, who wants NASA's manned program to be phased out.
Human space exploration also has run into trouble in Congress. In its spending bill for 2008, lawmakers ordered NASA not to spend any money to study sending humans to Mars.
"Manned space travel adds far more cost than is justified in terms of scientific return," says Rep. Barney Frank, D-Mass. Frank says he doesn't want to end the astronaut program but doesn't want to send humans to Mars or the moon. He'd restrict astronauts to tasks robots can't handle, such as the recent upgrade of the Hubble Space Telescope by a seven-astronaut team.
Opposition to NASA's astronaut program stretches across the political spectrum. Republican Newt Gingrich, former speaker of the House, wrote in Aviation Week & Space Technology last year that NASA should get out of the business of sending humans to space to make way for private space entrepreneurs.
For NASA, the most opposition may be from the people who pay the bills: the public. In a 2005 USA TODAY poll, 58% opposed spending money on a human mission to Mars.
Americans may support human spaceflight, but they don't make it a high priority, says historian Roger Launius of the Smithsonian Institution's National Air and Space Museum. Nor do political leaders, he says. "That leaves us in low-Earth orbit for the foreseeable future," Launius says "I hope it doesn't come to that, but I'm afraid it might."