NASA would end its participation in the International Space Station in 2025 under a Trump administration budget proposal released Monday, an idea that one influential senator immediately called a “non-starter.”

The $19.9 billion spending plan for 2019, up about $400 million from this year, seeks to refocus human exploration on the moon and shift responsibility for low Earth orbit missions to industry or international partners.

“In short, we are once again on a path to return to the moon with an eye toward Mars,” said Robert Lightfoot, NASA’s acting administrator.

The space agency next year would invest $150 million as an initial investment in commercial capabilities that by 2025 would provide a “seamless transition” from NASA’s existing ISS program.

It was not clear if NASA envisions partners taking over some or all of the ISS, or if the football field-length research complex would be dropped from orbit and replaced by commercial stations.

Some members of Congress oppose abandoning the space station so soon.

“The administration’s budget for NASA is a nonstarter,” said U.S. Sen. Bill Nelson, ranking member on the committee that oversees NASA. “Turning off the lights and walking away from our sole outpost in space at a time when we’re pushing the frontiers of exploration makes no sense.”

U.S. Sen. Ted Cruz, who leads the Senate Subcommittee on Space, Science and Competitiveness, also recently referred to supporters of exiting the ISS program as “numbskulls.”

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Congress has directed NASA to study the feasibility of extending ISS operations to 2028 or 2030, but the study is not yet complete.

Commercial space advocates welcomed the Trump administration’s goal, but even they worry the private sector may not be ready as soon as 2025 to replace NASA as primary underwriter for station operations.

“We don’t want to see a capability gap in low Earth orbit,” said Tommy Sanford, executive director of the Commercial Spaceflight Federation. “We want to ensure that there’s a smooth transition.”

The concern is that the nation could be left with nowhere to send astronauts and do science research and technology development in low Earth orbit, just as the nation is now in the midst of a nearly seven-year gap in its ability to launch astronauts from U.S. soil.

NASA spends about $3.5 billion annually on the ISS, including launches of cargo and astronauts.

Under the proposed plan, some portion of the savings from leaving the ISS that would be steered toward lunar exploration initiatives.

NASA in 2022 hopes to launch the first portion of a small station to be placed in orbit around the moon.

That would be the same mission as the first launch of NASA astronauts on the 322-foot Space Launch System rocket and Orion crew capsule from Kennedy Space Center.

No target date is set for a human mission to the lunar surface.

Another controversial budget proposal would cancel NASA's Wide Field Infrared Survey Telescope mission, or WFIRST, a top priority in the decadal survey for astrophysics.

The budget also would eliminate $100 million supporting NASA's Office of Education, repeating a proposal that failed a year ago.

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