WASHINGTON - The notion of landing astronauts on Mars has long been more fantasy than reality: The planet is, on average, 140 million miles from Earth, and its atmosphere isn't hospitable to human life.
But a human voyage to the planet is now, for the first time, within the realm of possibility, according to space advocates inside and outside government. As a result, plans for a mission around the planet, and ultimately for lengthier stays, have been sprouting like springtime flowers.
The new momentum, some space experts say, comes from the successful landing of the large rover Curiosity in a Martian crater last year, the growing eagerness of space entrepreneurs to mount and fund missions to Mars, and encouraging new data about the radiation risks of such an expedition.
NASA says it hopes to land astronauts on the planet within the next two decades, and the agency is developing a heavy-lift rocket and a new space capsule to achieve this goal. It has even established an optimal time frame for this event - in the early 2030s, when the very different orbits of the two planets bring them closest to each other.
The challenges of space technology - including how to keep astronauts alive en route and on the planet - as well as government support and funding remain daunting, but the goal of landing humans on Mars is seeming less and less like a pipe dream.
"A human mission to Mars is a priority, and our entire exploration program is aligned to support this goal," NASA Administrator Charles Bolden said.
NASA has "overcome the technical challenges of landing and operating spacecraft on Mars" robotically, Bolden said. "We're developing today the technologies needed to send humans to Mars in the 2030s."
With both the promise and the obstacles in mind, Bolden and other top NASA planners, rocket developers, and scientists, as well as leaders from the commercial space industry and organizations and agencies abroad will meet Monday at a conference at George Washington University. Buzz Aldrin, the second man to walk on the moon in 1969, will be a keynote speaker at the three-day gathering. He has just written a book that he refers to as a manifesto laying out the reasons humans can and should set their sights on not only landing on Mars but also setting up a permanent settlement there.
It is "human destiny" to explore space and settle on other planets, he writes in Mission to Mars, which is being released this week. He has his own step-by-step plans on how to accomplish a Mars campaign, but he makes room for others as well.
"Our world isn't just Earth anymore, and we need to get much more serious about that," Aldrin said in an interview, adding that the leaders who take us to Mars and the pioneers who inhabit it "will go down in human history as heroes and be honored for thousands and thousands of years."
A colony on Mars is by all accounts a distant goal, but the timetable for sending humans there for a quick orbit and return to Earth, or even a landing on one of its moons, could be considerably faster.
Investment adviser Dennis Tito, who paid $20 million to go to the International Space Station in 2001, recently announced plans to send two astronauts to Mars for a 2018 flyby; a Dutch group called Mars One is raising funds for a landing in the 2020s. Elon Musk of the rocket and capsule company SpaceX says he will unveil his company's Mars exploration plans in the months ahead.
Unlike the others, Musk has a significant spaceflight track record. His Dragon spacecraft has docked three times at the International Space Station during NASA-funded cargo runs.
The successful landing of Curiosity - at one ton, by far the largest vehicle ever flown to Mars - is put forward as one reason a human mission is increasingly conceivable. There's still a long way to go in terms of landing technology, said Michael Gazarik, associate administrator of the NASA's Space Technology Mission Directorate, because a Mars descent with humans would require a capsule weighing something like 40 tons. Nonetheless, Gazarik said the technology is being developed, and having a Mars lander by the 2030s is "plausible."
Another reason is that the health risks associated with radiation in space and on Mars appear to be somewhat lower than previously believed.
The biggest impediment may be money. President Obama has challenged NASA to send astronauts to Mars in the 2030s, but NASA's budget is now a small fraction of what it was in the years after President John F. Kennedy set a precise timetable for landing on the moon. The agency gets less than 0.5 percent of the federal budget; at the peak of the Apollo program, it was 4 percent. Also, Obama will have left office long before the big decisions about a 2030s mission are made, and his successors might have different priorities.
The funding problem is one reason private companies and space agencies from other nations are expected to play a significant role in any human mission to Mars.
But NASA remains the indispensable player or partner for any human landing on the surface.
The Mars conference - cosponsored by the nonprofit group Explore Mars and the GWU's Space Policy Institute - is designed to examine the feasibility and rationale for a human mission to Mars and to highlight the public's seeming embrace of the idea.
A recent poll commissioned by Explore Mars and Boeing questioned 1,101 people about sending humans to Mars; the public's views were overwhelmingly positive.
Still, some people are wary of a human mission to Mars because of the inherent risk and others because they believe robotic missions can answer the important scientific questions at a much lower price.
But NASA has a unique connection with Mars: All seven of the vehicles that have landed on the planet and succeeded in their missions have been sent by the United States.
That will, no doubt, change in the decades ahead as Europe, Russia, India, and China expand their Mars programs, with landing a human team as the ultimate goal.
"It will be done, regardless of U.S. leadership," Aldrin said of an eventual manned Mars landing. "The real question is: How long does the exceptionalism of the United States last?"