For NASA and its astronauts, the moon is no farther away in terms of distance, but it is slipping further into the future.

Officials at the space agency announced on Tuesday that Artemis II, the first American mission to send astronauts close to the moon in more than 50 years, will not take place late this year, as had been scheduled.

They set a target date of September 2025 for the mission, which will swing around the moon without landing there.

The delay in Artemis II also pushes back the subsequent mission, Artemis III, which is to land two astronauts on the moon near its south pole. That will now occur no earlier than September 2026.

Artemis II will be the first mission to send astronauts to space using NASA’s huge Space Launch System rocket and Orion capsule, and NASA officials want to fix potential problems that could endanger the crew.

“We don’t fly until it’s ready,” Bill Nelson, the NASA administrator, said during a news conference on Tuesday afternoon. “Safety is paramount.”

For the mission’s delay, the officials cited a slew of technical issues including concerns about electronics in the life support system that will keep the astronauts alive inside Orion, continuing analysis of wear and tear of the capsule’s heat shield during an earlier uncrewed mission and repairs to the launch tower.

Unlike the Apollo missions, Artemis II will not enter orbit around the moon; the Orion capsule will swing around the moon, using lunar gravity to sling it back to Earth for a Pacific Ocean splashdown. The entire trip should take around 10 days.

The crew will consist of three NASA astronauts — Reid Wiseman, Victor Glover and Christina Koch — and one Canadian astronaut, Jeremy Hansen.

Amit Kshatriya, the deputy associate administrator in charge of the Moon to Mars program at NASA, said the discovery of problems with the valves in the Orion capsule’s life support system was the main cause for the Artemis II delay.

Valves that were destined for the Orion capsule for Artemis III failed in tests. “That gave us pause to stop and look at that circuit in a more detailed way,” Mr. Kshatriya said.

The valve components for Artemis II had passed tests and had been installed, but “it became very clear to us that it was unacceptable to accept that hardware, and we need to replace it in order to guarantee the safety of the crew,” Mr. Kshatriya said.

He said NASA also discovered a potential deficiency in Orion’s batteries if the spacecraft needed to separate quickly from the rocket in case of an emergency.

The current moon program started in December 2017 when President Trump directed NASA to send astronauts back to the moon, a place where they have not stepped since the Apollo 17 mission in December 1972. That was a shift from President Obama, who had wanted NASA to focus on sending astronauts to new destinations like an asteroid and eventually Mars.

An enormous orange rocket lifts off from the launchpad at night.
The uncrewed Artemis I rocket lifting off from Cape Canaveral, Fla., in 2022.Credit…Bill Ingalls/NASA
The Starship spacecraft in the first moments of a launch, rockets firing from the launch stand at early morning.
A Starship test flight in November. NASA hired SpaceX to build the Artemis mission’s lander, using a version of the giant Starship rocket it is developing, to take NASA astronauts from lunar orbit to the surface of the moon.Credit…Joe Skipper/Reuters

Even with the change of destination, the major pieces of NASA’s human spaceflight plans — the S.L.S. rocket and the Orion capsule — had already been under development for years and remained unchanged.

Initially, the pace of the lunar return was languid, with astronauts not scheduled to land until at least 2028. Then in 2019, Vice President Mike Pence, who chaired the National Space Council, announced a sudden acceleration, and said that American astronauts would walk on the moon again by the end of 2024 “by any means necessary.”

Mr. Pence and other critics said NASA was not moving with urgency, pointing out that only eight years elapsed between President John F. Kennedy’s famous announcement in 1961 of a plan to reach the moon and the landing of Apollo 11.

Mr. Pence also raised the specter of China, which had just put a robotic lander on the far side of the moon and was aiming to land astronauts on the moon by 2030.

The moon program was given a name: Artemis, who in Greek mythology is the twin sister of Apollo.

In 2021, NASA hired Elon Musk’s company SpaceX to build the lander for Artemis III. The company is adapting the giant Starship rocket to be able to take two NASA astronauts from lunar orbit to the moon’s surface.

NASA’s accelerated schedule started slipping. Artemis I, a test launch of the S.L.S. rocket that sent an uncrewed Orion capsule on a weekslong test flight around the moon, was scheduled for late 2020 but did not launch until November 2022.

Artemis I was largely successful, and NASA officials were hopeful that Artemis II could follow two years later.

Although NASA’s budget has received large increases in recent years, it is still a much smaller slice of the federal budget than at the height of the Apollo program in the 1960s.

A view from the Orion spacecraft in the darkness of space with the moon and Earth seen in the distance, with the moon closer and larger than Earth.
A view from the Orion spacecraft during the Artemis I mission around the moon in 2022.Credit…NASA

In December, the Government Accountability Office said the December 2025 target for the Artemis III moon landing was unlikely, pointing to overly optimistic schedules for the development of the Starship lunar lander and the spacesuits that astronauts would need for walking on the moon.

Two test launches of Starship last year failed to reach orbit, although SpaceX said both provided bounties of data to make improvements. The accountability office said that if Starship took as long as the average major project at NASA, it would not be ready until 2027.

The delay allows more time for SpaceX to tackle the challenges of developing Starship, including a full-scale uncrewed lunar landing of the vehicle, now scheduled for 2025.

James Free, NASA’s associate administrator, said that the revised Artemis schedule was not overly optimistic, although he admitted that additional delays could still occur.

“We’ve tried to address the unknown unknowns and set a realistic plan in place,” Mr. Free said.

Other parts of NASA’s moon program have also not gone as planned.

On Monday, Peregrine, a commercial robotic lunar lander carrying five NASA experiments, was successfully sent on a trajectory toward the moon after a launch, but then its propulsion system suffered a crippling failure soon afterward. While that is a setback for NASA lunar studies, it is unlikely to add to the Artemis delays.

In its latest update on Tuesday afternoon, Astrobotic Technology of Pittsburgh, which built the spacecraft, said it might run out of propellant for its maneuvering thrusters within about 40 hours.

“Given the propellant leak, there is, unfortunately, no chance of a soft landing on the moon,” the company said. The engineers are continuing to look for ways to extend the spacecraft’s lifetime and gather data that could help future missions.

NASA already has additional experiments booked on other commercial landers, part of an effort to conduct scientific research on the moon at a lower cost. NASA officials have said they expect some of these commercial missions to fail.

However, NASA might also be leery about proceeding with Astrobotic’s second mission, to take a $433.5 million rover called VIPER to the south polar region where it would explore for water ice and other resources. That flight would use a larger lander named Griffin.

The rover is by far the most complex and expensive payload NASA has planned for the commercial lunar missions.

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