The notion of landing astronauts on Mars has long been more fantasy than reality. The planet is, on average, 225 million km from Earth, and its atmosphere is not 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 that 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 will 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,” said NASA Administrator Charles Bolden.

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, are to meet Monday at a conference at George Washington University.

Buzz Aldrin, who in 1969 became the second person to walk on the moon, 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 wrote in “Mission to Mars,” which was 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 Plymouth or Jamestown 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 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.

Musk got into the space business with the ambition of sending many people to Mars. The first of these missions is “further off than I would like,” he said, “but far closer than many expect.”

The successful landing of Curiosity — at 1 ton, by far the largest vehicle ever flown to Mars — is put forward as one reason a human mission is increasingly conceivable. There is 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.

Radiation measurements made by an instrument on Curiosity have found high — but not prohibitively high — levels of high-energy cosmic and solar rays both en route to Mars and on the surface of the planet. Extensive shielding of astronauts will be needed, scientists have found, but the risk of later illness due to radiation would not be significantly higher in going to Mars than in a long-term stay on the space station, according to Bent Ehresmann, a member of the Curiosity radiation monitoring team.

Long-term isolation is also a significant issue, and astronauts will be staying longer at the ISS in the years ahead to study that concern. And, of course, NASA will have to design spacesuits that can withstand the bitter cold of Mars as well as its thin atmosphere, which is made up largely of carbon dioxide.

The biggest impediment, though, may be money. President Barack 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.

John Grunsfeld, NASA’s director of the Science Mission Directorate, said that sending humans to Mars will be the ultimate expression of the agency’s long-term goal of more intimately combining exploration and science. Having flown on the space shuttle to the Hubble Space Telescope three times to fix and upgrade it, Grunsfeld has firsthand experience with the capabilities that only astronauts can bring.

The same will be true on Mars, he said. “In a matter of a week, astronauts could probably complete the entire (two-year) Curiosity mission.”

The upcoming 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.

About 75 percent of respondents either “agreed “or “strongly agreed” with the statement that it is worthwhile to increase NASA funding to 1 percent of the federal budget (in other words, to double NASA’s share) in order to fund a mission to Mars. More than 84 percent said that if Curiosity finds evidence of past or current life on Mars, NASA should send a human crew to try to verify the finding.

Nonetheless, getting past the question of why the United States should some day send astronauts to Mars will not be easy.

The country’s support for the Apollo moon program was driven in significant part by the Cold War competition with the Soviet Union, and there are no similar rivals now. Some people are also 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?”