By Dan Vergano, USA TODAY
From ancients who worshiped the Red Planet as a fierce war god in the sky, to scientists like Percival Lowell and fantasists like Ray Bradbury, Mars has taunted Earth's adventurers with the ultimate challenge.
Now that the fear of imaginary invaders has been replaced by a frustrating search for chemicals, a more fundamental question is emerging.
Could humans walk the sands of Mars, and actually live and work there?
That's what NASA's $2.5 billion Mars' Curiosity rover is trying to determine as it rolls heroically across the rugged but achingly desolate Martian landscape.
Placing men and women on a cold planet currently 173.5 million miles away is a question not just for poets and philosophers but for planners and policymakers.
When USA TODAY, celebrating its 30th anniversary with a look to the 21st-century future, spoke with NASA chief Charles Bolden, he emphasized "the critical importance of Curiosity, because it's not the first, but it's the most critical, the largest mission, that's a precursor for putting humans on Mars."
The first manned visits, he said, could happen around 2035 as an international endeavor. The first outposts on Mars could come after 2060.
"Mars has captured the human imagination certainly since people started gazing up at the sky," says NASA science chief John Grunsfeld, a former Hubble repair mission astronaut. "There are really very few places that humans could actually go and live some day. Mars is one of those."
Still, as the nation debates its goals for space exploration in a post-space shuttle era marked by an increasingly tight-fisted Congress and endless deficits, how improbable it is that the proposals even are being made.
Consider: John F. Kennedy's ambitious goal, announced 50 years ago this week, to put a man on the moon involved a trip of a mere 239,000 miles. Getting to Mars - even at its closest point to Earth - means a trip more than 140 times farther, lasting 200 days and with the possible bombardment by life-threatening radiation, a risk some scientists see as a deal-killer.
Even if all goes well, what would they find once they arrive?
Standing on the Red Planet
The day the first explorers set foot on Mars, a ruddy dust hanging like a curtain in the sky would tint their every view. If they stood where Curiosity is now, the sun would look less than half as large as it does from Earth. Temperatures would range from merely freezing during the day, to minus-103 degrees Fahrenheit at night.
Trying to take a breath would kill, the faint carbon dioxide atmosphere so thin it would count as a vacuum on Earth. Radiation rakes the surface in doses stronger than the astronauts receive aboard the International Space Station- the equivalent of at least 240 chest X-rays in a six-month stay.
The gravity on Mars is only 38% of Earth's. Walking - or bounding - could be Mars' first challenge, and an eventual Olympic sport.
Phoning home would be rare. The distance from Earth to Mars varies from 33 million to 249 million miles. It would take anywhere from three to 21 minutes to transmit "hello" even at the speed of light, making for stilted conversation. And for two weeks every two years, Mars and Earth are on opposite sides of the sun. Communications might be impossible.
Instead of the towering cities imagined by John Carter of Mars creator Edgar Rice Burroughs, lava caves lining the slopes of the largest volcano in the solar system, Olympus Mons,look safer as houses for future Martian tenants.
More likely, plans call for pre-fabricated Martian habitats to be launched two years before astronauts themselves begin their journey. Once secured on the surface, astronauts would spend their days on Mars brewing methane fuel and oxygen from water and carbon deposits that evidence suggests remain locked under the Martian soil.
For all that, NASA spacecraft have revealed a Red Planet that looks more hospitable - with caves, water and minerals - than the dusty picture painted by the 1970s Viking lander missions, which transmitted images of a sterile desert.
And as NASA's Curiosity rover makes its first tracks - exploring, sniffing and zapping the surface of Mars for signs that life once graced the Red Planet in the past - the panoramas it reveals are of a planet that looks not unlike the high deserts on Earth.
Surviving the rays of Mars
What Curiosity learns about the weather and radiation hazards of Mars will help determine how manned landings play out.
Just getting there would be a triumph. The trip could take 200 days, with Martian stayovers ranging from a month to 500 days.
Such long voyages might seem an eternity for astronauts trapped in a spacecraft. But in 2011, the European Space Agency's "Mars 500" project isolated six men for 520 days in a simulated Mars trip. And several cosmonauts have experienced long stays in space, one of them spending 14 months in microgravity.
"I think there will be good circumstantial evidence that people will be able to tolerate a Mars mission when the time comes," says space psychologist Nick Kanas of the University of California-San Francisco.
Radiation storms - which are known to affect telecommunications even on the atmosphere-shielded Earth - are the more immediate worry, especially in the barely-there atmosphere of Mars.
A radiation detector aboard the rover already has collected months of data about hazards astronauts would face from cosmic rays and solar storm radiation on their trip to Mars.
"Sitting down in the belly of the spacecraft, the rover had about as much radiation shielding as astronauts have on the International Space Station," says Donald Hassler of the Southwest Research Institute in Boulder, Colo. He heads the rover's radiation instrument team.
Astronauts could hide from solar storm radiation in shielded parts of a spacecraft or lander, Hassler says. Based on readings from Curiosity, "we are finding the radiation from solar events is significant," he concedes.
Even with shielding, Mars travel poses lifetime radiation risks for "cataracts, skin damage, central nervous system damage, and impaired immune systems," as well as fatal cancers, according to a 2008 National Research Council Report. Radiation sickness from solar storms might endanger a mission, the report added.
The rover's radiation measures also tell scientists how deeply microbes on Mars, and astronauts, might have to hide under the surface to escape those effects. Would people from Earth have to burrow below the Martian surface to survive?
Is Mars worth the risk?
"This would be the most exciting adventure I could possibly imagine," SpaceX chief Elon Musk told The Daily Show's Jon Stewart, who asked the space entrepreneur why he wanted to walk on Mars.
"It's not so crazy to talk about astronauts visiting Mars," scientist Hassler says. "It really is a question of when we decide to do it."
"It engages human fascination," says NASA's Grunsfeld.
Where rocket pioneer Werner von Braun once proposed sending fleets of silver-finned rocket ships to Mars on a trip piloted by 70 men, Musk today hopes to send people there on private rockets, eventually making the trip a $500,000 ticket.
A Dutch reality-television-show effort, Mars One, has announced a plan to send colonists on a one-way trip to Mars starting in 2023 (the group's website "frequently asked questions" list begins with, "Is this for real?" - to which it answers, "Yes, it is!").
Whether government-funded, privately financed or some undreamt reality-show adventure, "it is not an unreasonable question to ask whether 30 years from now, will people have been to Mars?" says space policy expert John Logsdon, author of John F. Kennedy and the Race to the Moon. "It is a possibility, verging on a probability," he says.
Others aren't so sure.
Congress has balked at sending astronauts to Mars for decades, most notably in 1989 after a $500 billion cost estimate killed off one ambitious NASA plan. For that reason, Bolden was emphatic in his remarks to USA TODAY's editorial board that any manned missions to Mars would be international ones that spread the costs around.
The Obama administration recently cut unmanned Martian exploration by $500 million. Republican candidate Mitt Romney has called for "clearer priorities" in space efforts.
Even Roger Launius, a former NASA historian who is curator at the Smithsonian Air and Space Museum in Washington, D.C., which is now offering a Mars exhibit, says he is "pessimistic" about Mars travel.
"We wanted to give people a glimpse of the future in the exhibit. I doubt we are going to see (Mars) any other way for the foreseeable future," he says.
"This is just not realistic, even spread out over many years, to think we will spend this kind of money," says Arizona State University historian Stephen Pyne, author of Voyager: Seeking Newer Worlds in the Third Great Age of Discovery. In an age of robotic rovers such as Curiosity, he says, "we don't need people to plant the flag anymore to be explorers."
The Martian dream
A century ago, astronomer Percival Lowell stirred generations with speculation that the deep scars in the planet's surface were canals created by Martians. Author H.G. Wells went a step further, writing that malevolent Martians watched Earth with "envious eyes." And author Bradbury predicted people from Earth would be the first real Martians.
In widely noted remarks in January, physicist Stephen Hawking called for human colonies on Mars in the next century as a safeguard against catastrophe on Earth. We'll need a colony of the Red Planet by the next century, Hawking suggested, given the way things are going on the Blue Planet.
In cosmic terms, Mars is achingly close and the technology seems to be there, at least judging by the remarkable success of rovers like Curiosity.
If and when the call to Mars comes, there will likely be plenty of volunteers, says Bolden, a former space shuttle pilot.
"I'd go myself," he says. "But my wife wouldn't let me."