NASA is aiming to send astronauts back to the moon, because that is what President Trump set as the destination of the agency’s human spaceflight program during his first term.

On Dec. 11, 2017, Mr. Trump signed what the White House called Space Policy Directive 1, which stated that “the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations.”

The people attending Mr. Trump’s signing of the directive included Buzz Aldrin, the second person to step on the moon during the Apollo 11 mission after Neil Armstrong, and Harrison Schmitt, an astronaut on Apollo 17, the last time people were on the moon.

Three reasons are often offered for NASA to send astronauts back to the moon:

• Much knowledge about the solar system can be gleaned from the moon, especially in the eternally shadowed craters in the polar regions that appear to hold frozen water.
• The moon is the logical next stop for astronauts.
• The United States needs to get back to the moon before China gets there.

For a couple of decades after the Apollo moon landings, not many people paid much attention to the moon. The moon was a “been there, done that” place, a lifeless, airless world of rocks and dust and not much else. Scientists and NASA looked to explore Mars and other planets.

The discovery of frozen water in the craters rekindled interest in the moon.

Moon water could simplify astronaut missions going there. Humans need to drink water to survive, and water molecules can be split into hydrogen and oxygen. Oxygen provides air to breathe, and hydrogen and oxygen can also be used as rocket propellants to return home to Earth, or to travel elsewhere in the solar system.

Layers of ice in the craters could also provide a history of the solar system, much as ice cores from Greenland and Antarctica provide a record of Earth’s climate.

Spacecraft like Athena, the Intuitive Machines mission whose fate was uncertain after a moon landing attempt on Thursday, aim to advance those goals.

The rapid rise of China’s space program has also set the stage for a new space race. At the Commercial Space Conference in Washington in February, Bruce Babin, a Texas congressman who is chairman of the House Committee on Science, Space and Technology, raised the specter of “No trespassing” signs on the moon written in Chinese.

“My top priority is ensuring that U.S. astronauts return to the lunar surface and that we do so before the C.C.P.,” Mr. Babin said, using the initials for Chinese Communist Party.

Whether Mr. Trump still wants to go to the moon is less certain.

During his campaign, his inaugural address in January and his address to Congress this week, Mr. Trump has mentioned planting a U.S. flag on Mars. He almost never talks about the moon anymore.

Elon Musk, the chief executive of SpaceX and a key adviser to Mr. Trump, says the moon is a distraction, even though SpaceX has a multibillion-dollar NASA contract to build the landers that are to take astronauts to the moon’s surface as part of the Artemis program.