U.S. astronauts are now expected to return to the moon in 2025 instead of 2024, with NASA Administrator Bill Nelson describing the earlier time frame as unrealistic for technical reasons.

The National Aeronautics and Space Administration has been preparing to send two astronauts back to the lunar surface as part of its Artemis moon program, with the Trump administration previously setting a goal of landing them there by 2024.

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U.S. astronauts are now expected to return to the moon in 2025 instead of 2024, with NASA Administrator Bill Nelson describing the earlier time frame as unrealistic for technical reasons.

The National Aeronautics and Space Administration has been preparing to send two astronauts back to the lunar surface as part of its Artemis moon program, with the Trump administration previously setting a goal of landing them there by 2024.

At a briefing Tuesday, Mr. Nelson questioned whether the original time frame for returning people to the moon was ever viable. “Trump administration target of 2024 human landing was not grounded in technical feasibility,” he said.

Mr. Nelson said the agency also had lost time in part because of Covid-19-related delays and because of a lawsuit that Jeff Bezos ’ space company, Blue Origin, filed challenging NASA’s decision to award a single contract for a moon-lander system to rival SpaceX. During the suit and an earlier administrative protest by Blue Origin and another company, NASA paused its work on the lander, though Space Exploration Technologies Corp., the formal name for SpaceX, continued to work on the vehicle, officials said.

Last week, a federal judge dismissed Blue Origin’s suit, and NASA has made contact again with SpaceX, according to Mr. Nelson. Mr. Bezos said in a tweet his company would respect the judge’s decision.

NASA’s Artemis moon program is the agency’s effort to return astronauts to the lunar surface for the first time in decades as it faces new competition for moon-related activities from China’s space agency. NASA leaders have said they expect to appoint a woman and person of color as the first astronauts who will go back to the moon on the agency’s behalf, as it tries to pursue a broad agenda that includes scientific initiatives and exploration.

The space agency has faced other questions about whether it could deliver people to the moon in 2024. NASA’s inspector general said in an August report that spacesuits the agency has been developing likely wouldn’t be ready until 2025, making a lunar landing in 2024 unlikely.

In February, NASA expects to conduct an uncrewed test of the Space Launch System, a deep-space rocket meant to be used for lunar missions, which Boeing Co. is overseeing as the agency’s prime contractor. In 2024, astronauts are expected to board a spacecraft on top of the Space Launch System for a test flight, but not land on the moon’s surface, officials said Tuesday. Previously, the agency had hoped that flight could occur in the spring of 2023.

Write to Micah Maidenberg at micah.maidenberg@wsj.com