A public letter from NASA employees on Monday urges leaders of the space agency not to carry out deep cuts sought by the Trump administration.

“We are compelled to speak up when our leadership prioritizes political momentum over human safety, scientific advancement and efficient use of public resources,” the employees wrote in the letter. It is addressed to Sean Duffy, the secretary of transportation, whom President Trump appointed this month as acting NASA administrator.

Cuts to NASA programs have been arbitrary and in defiance of priorities set by Congress, the NASA employees said. “The consequences for the agency and the country alike are dire,” they wrote.

NASA did not immediately respond to a request for comment.

The NASA letter follows similar letters of criticism by federal employees at the National Institutes of Health and the Environmental Protection Agency. Dr. Jay Bhattacharya, the N.I.H. director, said he welcomed respectful dissent, but the E.P.A. placed 144 employees who signed that agency’s letter on leave.

“We’re all scared that we’re going to get laid off,” said Monica Gorman, an operations research analyst at the NASA Goddard Space Flight Center in Greenbelt, Md. “We’re scared of retaliation. We huddle in the bathroom. We go to the bathroom to talk to each other, and look under the stalls to make sure that no one else is there before we talk.”

Ms. Gorman is one of 287 current and former NASA employees who signed the letter, although more than half did so anonymously. More than 15,000 people work at the space agency. Prominent scientists outside of NASA, including 20 Nobel Prize winners, also offered their names in support.

The N.I.H. letter inspired some people at NASA to put together a similar effort. The N.I.H. letter writers had called their dissent the “Bethesda Declaration” — the agency is in Bethesda, Md. — and the NASA letter writers called theirs the “Voyager Declaration,” in honor of Voyager 1 and Voyager 2, the two NASA space probes launched in the 1970s that continue to function as they fly through interstellar space.

“One of the messages that NASA management has been passing down to every employee is that no one is coming to save us, including Congress,” said one of the organizers of the letter, who spoke on the condition of anonymity out of fear for retaliation. “So the Voyager Declaration is one effort to raise our voices and speak out to save ourselves,” the person added.

The letter is framed as a “formal dissent” — an official process at NASA for registering disagreements that managers may not want to hear. It was part of changes instituted at NASA after the losses of the Columbia and Challenger space shuttles when concerns of some engineers were brushed aside.

Stand Up for Science, a nonprofit organization that has organized opposition to the Trump administration’s cutbacks of science research, has helped coordinate the letters to the three agencies.

In its budget request to Congress in June, the Trump administration proposed slashing NASA’s budget by almost 25 percent. NASA’s science mission directorate, which includes earth science, solar system missions and astrophysics, would face a cut of 47 percent, to $3.9 billion from $7.3 billion. Nineteen currently operating science missions, including the Chandra X-ray Observatory, the Juno mission at Jupiter and the two Orbiting Carbon Observatories, would be turned off and discarded.

Congress does not appear to agree with such draconian cuts. In its budget proposal for NASA, a Senate subcommittee would provide $7.3 billion for NASA’s science mission directorate for fiscal year 2026, the same amount as the current year. Its counterpart in the House of Representatives was less generous, proposing $6 billion for the science missions.

While NASA has so far avoided widespread layoffs, thousands of employees have left or are planning to leave under early retirement and buyout offers.

“Some I know have left because they want to make space for other younger people to stay,” Ms. Gorman said. “Like my supervisor has retired, and she said that she wanted to try to make it more likely that the rest of us wouldn’t get laid off.”

One complaint of the letter writers is that NASA leaders have made decisions based on the president’s proposed cuts, without waiting to see what Congress will provide.

Another concern is that even if Congress provides money for science missions, the administration could refuse to spend it.

Last week, in a letter to Mr. Duffy, two Democratic members of the House Committee on Science, Space and Technology — Zoe Lofgren of California and Valerie Foushee of North Carolina — accused NASA of illegally implementing Mr. Trump’s priorities.

“Congress directs NASA to spend money on certain missions, programs and priorities; and Congress sets the policies that NASA must implement,” Ms. Lofgren and Ms. Foushee wrote. “These congressional actions are not friendly requests. They are the law.”

Ms. Gorman said her department, which provides independent assessment on the cost and schedule of projects at the NASA research center where she works, had been informed that it would not be financed next year.

“I really want the general public to be aware that things are being disrupted and cut off now,” she said. “Things are already not OK right now, and once senior scientists and senior engineers and people leave, that’s it. That knowledge is gone, and we’re all going to suffer from that brain drain for years and years.”

Ms. Gorman, who is a lead organizer for a union that represents 1,700 scientists, engineers and technicians at Goddard, has been outspoken.

“I’m already at risk of losing my job, and I’d rather speak out and try to save something at NASA, rather than just hide under my desk until I get laid off,” she said. “But I am scared.”