WASHINGTON — NASA is losing four key senior officials close to its flagship moon program, according to people familiar with the changes, adding more uncertainty over the agency's space exploration trajectory as US President Donald Trump and his associate Elon Musk promote missions to Mars.

Jim Free, NASA's associate administrator who has been a central voice defending the agency's Artemis moon program, will retire on Saturday, the agency said in a statement on Wednesday.

In Huntsville, Alabama, three key officials at NASA's Marshall Space Flight Center — one of the agency's 10 field centers and the epicenter of its Artemis moon program — had their retirements announced internally on Tuesday, according to a person familiar with the announcement.

The leadership shake-up adds more uncertainty over NASA's direction in space as Musk, the CEO of SpaceX who has long envisioned crewed missions to Mars, oversees a sweeping review of NASA records as a "special employee" of the Trump administration seeking cuts to staff and programs.

Musk's SpaceX has $15 billion worth of contracts with NASA, including a contract to land humans on the moon with its Starship rocket.

Program criticized

Some agency officials expected Free's eventual departure as many Trump advisers criticize elements of NASA's moon program, such as its Space Launch System, an over-budget but operational moon rocket.

The Trump administration earlier sidelined Free and put Janet Petro, who was the director of NASA's Kennedy Space Center, in the acting role — a move seen by many officials in the agency as an attempt to prevent Free from potentially shielding the moon program from future changes.

Musk and Trump in recent months have touted potential missions to Mars as a possible alternative to the moon, the much closer celestial body that Trump in his first presidential term had set as NASA's core space exploration target, with long-term moon bases functioning as a proving ground for far-off Mars missions.

But with Musk's roughly quarter-billion-dollar support for Trump and his influential new role in the White House, talk of prioritizing new, more difficult missions to the Red Planet has threatened to upend the agency.

Petro has said hundreds of NASA employees have accepted buyout offers from the government. The shake-ups and looming strategic shift heap more uncertainty for the agency's almost 18,000 employees who have been whipsawed this month between shifting expectations of indiscriminate layoffs.

Agencies Via Xinhua