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For years, critics have been taking shots at NASA's plans to corral a near-Earth asteroid before moving on to Mars — and now NASA's chief has a message for those critics: "Get over it, to be blunt."

NASA Administrator Charles Bolden defended the space agency's 20-year timeline for sending astronauts to the Red Planet on Tuesday, during the opening session of this year's Humans 2 Mars Summit at George Washington University in the nation's capital.

That timeline calls for NASA to develop a new Orion crew capsule and a heavy-lift rocket called the Space Launch System while continuing research on the International Space Station. By the mid-2020s, astronauts would travel to a near-Earth asteroid that was brought to the vicinity of the moon. That'd set the stage for trips to Mars and its moons sometime in the 2030s.

NASA Administrator Charles Bolden discusses his agency's plan to get astronauts to Mars during a session at the Humans 2 Mars Summit in Washington on Tuesday. Win McNamee / Getty Images

Some members of Congress want NASA to forget about the asteroid and go directly to Mars or the moon's surface instead. But Bolden said NASA needed the asteroid mission as a "proving ground" for the farther-out missions to Mars.

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"We don't think we can just go," the former astronaut and Marine general said.

Bolden said missions to Mars would be important not only to learn whether life once existed beyond Earth, but also to set the stage for interplanetary settlement. That would serve as an insurance policy against any potentially planet-destroying catastrophe on Earth's.

"Only multiplanet species survive for long periods of time," Bolden said, echoing echoing a call for outer-space colonization that has been made by luminaries ranging from physicist Stephen Hawking to SpaceX's billionaire founder, Elon Musk.

Bolden said getting astronauts to Mars by the 2030s would require "modest increases" in NASA's budget. Musk has said he could do it sooner, perhaps in 10 years if enough money was available. But Bolden said an Apollo-scale push to Mars isn't in the cards.

A NASA chart lays out the agency's step-by-step plan for human exploration of Mars. NASA

"Reality is the budget," he said, "and we are not going to get 4 percent of the federal budget to go to Mars or any other place."

NASA's current annual budget of $17.65 billion represents about 0.5 percent of the federal spending plan, as opposed to the 4 percent level at the peak of Project Apollo. The current level of space spending has been putting a lot of pressure on NASA to balance its human spaceflight program with its robotic exploration program, which has its own big ambitions for missions to Mars, Europa and elsewhere.

Even though he ruled out an Apollo-level budget, Bolden voiced hope that NASA would someday get a bigger share of the budgetary pie. "One percent would be a gold mine," he said.

And even though he told critics to get over their objections to NASA's exploration plan, Bolden signaled that he was still open to constructive criticism. "This is the path we have chosen," he said. "Help us tweak it."

Bolden also addressed more immediate topics:

He was "cautiously optimistic" that operations on the International Space Station would not be affected by the downturn in U.S.-Russian relations that was sparked by conflicts in Ukraine. He pointed out that the space station project was unaffected by an earlier crisis focusing on the former Soviet republic of Georgia. NASA recently curtailed contacts with Russian officials not having to do with the space station.

Bolden called on Congress to provide the full $848 million sought for NASA's commercial crew program during the upcoming fiscal year. That level of funding would make it possible to start transporting astronauts to and from the station on U.S.-made commercial spaceships in the 2017 time frame, he said. He said he was willing to "get down on hands and knees" to seek full funding.

The Humans 2 Mars Summit runs through Thursday, and video from the summit sessions is being streamed live via Livestream.