Sometime in the next decade, NASA envisions being able to send a spacecraft to snag a small asteroid passing nearby and guide it into orbit around the moon, where astronauts could fly up to study it and return samples to Earth. Agency officials say it’s a way to gain experience and develop some of the technologies it would need to send humans to Mars in the 2030s.

“This experience in human spaceflight beyond low earth orbit will help NASA test new systems and capabilities, such as solar electric propulsion, we’ll need to support human missions to Mars,” NASA Administrator Charles Bolden told an audience last week at a conference hosted by the group Explore Mars Inc.

For such a mission to be possible, the space agency would need the backing of Congress. Right now, some key lawmakers — mainly Republicans — don’t like the idea, and others haven’t decided.

“I think they could find better things to spend the taxpayers’ money on,” said Sen. Richard C. Shelby of Alabama, ranking Republican on the Senate Appropriations Committee and the subcommittee that approves NASA’s budget.

Shelby said he didn’t see the connection between capturing an asteroid and a mission to Mars, adding that maybe NASA could explain.

Virginia Republican Frank R. Wolf, who chairs the House Appropriations subcommittee that deals with NASA, was just as skeptical of the asteroid idea. “I don’t agree with what they’re doing,” Wolf said. “There’s no vision.”

Wolf said at an Appropriations hearing in early April that the asteroid mission “does not seem to have captured imaginations among Congress … or the American public.”

Wolf would rather see astronauts return to the moon. In fact, he wrote to President Barack Obama late last year calling for a return to the moon instead, citing competition from China, public and international interest in returning to the moon and the importance of U.S. leadership in space.

At a hearing in March, the chairman of the House Science, Space and Technology Committee, Republican Lamar Smith of Texas, said the asteroid mission was one that lacked a “realistic budget, without a destination and without a certain launch date.”

He said the committee had heard concerns about the mission and had heard about “promising alternatives” like a Mars and Venus flyby in 2021.

The flyby had challenges, Smith said, but “it is intriguing and would catch the public’s imagination.”

A NASA authorization bill that Smith’s committee marked up last summer would have blocked the mission, but a new bill the committee marked up this week did not. It only directed NASA to draft a blueprint for getting to Mars.

Democrats were more supportive of the asteroid idea, or at least noncommittal. “Before I formulate any opinions, I really want to hold my NASA hearing,” said Maryland Democrat Barbara A. Mikulski, who chairs the Senate Appropriations subcommittee with jurisdiction over NASA. Sen. Bill Nelson, the Florida Democrat who chairs the Commerce, Science and Transportation Subcommittee on Science and Space, is on board with the project.

Cowboys in Space

The idea of sending astronauts to an asteroid goes back to science fiction at the beginning of the 20th century, said Howard McCurdy, a public policy professor at American University. And a 1986 National Commission on Space report discussed asteroids and asteroid mining.

In 2010, after Obama proposed cancelling the Constellation space exploration program, he talked about sending astronauts to an asteroid as the first destination beyond the moon.

But it turned out that a mission like that couldn’t be done with the current generation of equipment — it would take as many as three to five months of flight time and rockets bigger than the Space Launch System being built right now to reach an asteroid to land on, said Louis Friedman, co-founder of the Planetary Society who co-authored a 2012 study by the Keck Institute for Space Studies that hatched the proposal for retrieving a smaller rock.

The idea would be for NASA to send an unmanned spacecraft that would use a large sack to enfold an asteroid around 30 feet across or smaller and coax it into lunar orbit. Alternatively, they might take a chunk of a larger asteroid. Using the Orion crew capsule that’s expected to be test flown this year and a heavy rocket that NASA is currently building, astronauts would dock with the asteroid and bring some of it home.

William H. Gerstenmaier, head of NASA’s human exploration directorate, has said such a mission could benefit from work the agency is already doing — locating asteroids, developing solar electric propulsion, a new launch system and the Orion capsule. For the upcoming fiscal year, NASA is asking Congress for $133 million for the preliminary stage of what it calls an Asteroid Redirect Mission.

NASA initially made the proposal in its fiscal 2014 budget request and says it has $78 million for this fiscal year. Appropriators neither explicitly endorsed nor blocked funding.

But lawmakers wrote in their joint report on the fiscal 2014 appropriations that while the mission was still an emerging concept, the agency hadn’t given “satisfactory justification materials,” such as detailed cost estimates.

Money is still the major obstacle to the mission. As Leroy Chiao, a former astronaut and advisor at the Space Foundation, sees it, NASA has been told to go beyond low earth orbit, where the International Space Station circulates, but without enough money.

“This is the best they can do with what they’ve been given,” he said.

In Chiao’s opinion, the project isn’t “optimal” but it does advance capabilities and experiences to go to Mars, though he thinks going to the moon would be a better option.