By Joey Roulette and David Shepardson

WASHINGTON (Reuters) -President Donald Trump’s pick to lead NASA will tell senators on Wednesday that the agency will prioritize an astronaut mission to Mars, while four astronauts in the agency’s moon program are set to make a rare appearance at the hearing, sources said.

"We will prioritize sending American astronauts to Mars. Along the way, we will inevitably have the capabilities to return to the moon and determine the scientific, economic, and national security benefits of maintaining a presence on the lunar surface," said Jared Isaacman, a 42-year-old billionaire entrepreneur, in written testimony for his U.S. Senate Committee on Commerce, Science, & Transportation confirmation hearing on Wednesday.

Reuters reported on Monday that Isaacman told Senate staff last week that returning humans to the moon before China sends its own astronauts there is a national imperative. His remarks allayed some concerns that NASA’s multibillion-dollar moon effort could be upended by Trump and SpaceX CEO Elon Musk’s focus on Mars as a top destination for U.S. astronauts.

Isaacman’s varying views on the moon program and Trump’s fixation on Mars in recent months have set off panic and political jockeying within NASA and among lawmakers overseeing the agency’s many centers across the country, setting the stage for what could be a tense hearing on Wednesday.

Symbolizing the stakes of Isaacman’s views, NASA is planning to send to the hearing its crew of astronauts who are assigned to the agency’s Artemis 2 mission, according to two people familiar with the plans - a rare use of the U.S. astronaut corps in a political showdown.

Planned for 2026, the Artemis 2 mission will be the program’s first crewed test flight to journey around the moon and back before a subsequent moon landing mission.

The U.S. space agency’s Artemis moon program was spawned by Trump in his first presidential term as an effort to speed up the U.S. return to the moon, which would be used as a proving ground for eventual, farther-off missions to Mars.

After his assurances to lawmakers about the moon strategy, Isaacman’s written testimony raises questions on whether his prioritization of Mars would affect NASA’s existing moon missions or represent a rebranding of the program to give Mars a more central emphasis in the space agency’s strategy.

The billionaire nominee’s stance on the program is expected to be a major topic during his Senate hearing on Wednesday.

The agency has committed billions of dollars to its moon program, involving U.S. allies and leaning heavily on dozens of private companies - including Musk’s SpaceX - that have set their sights on a future lunar marketplace.

But in his second term, Trump has fixated on Mars in public remarks, while Musk, who spent $250 million in support of Trump’s presidential campaign and pushed for Isaacman’s nomination, openly considers the moon a distraction from his ultimate goal to send crews to the Red Planet.

Isaacman in his testimony said delayed and over-budget NASA programs are "discouraging."

"This is discouraging because people look up at the stars and wonder what is out there today, not decades down the road," his testimony said.