NEW YORK — Six members of the advocacy group People for the Ethical Treatment of Animals (PETA) held a protest Nov. 19 outside NASA headquarters in Washington in response to radiation experiments the U.S. space agency is planning to conduct on monkeys.

The radiation experiments will be performed on squirrel monkeys to study the potential health effects of long-term space missions, such as a trip to Mars. They were proposed by researcher Jack Bergman of McLean Hospital in Boston.

The agency plans to aim high-energy gamma-ray radiation at 28 squirrel monkeys, then observe how they perform on various tasks.

The PETA protest, consisting of people wearing monkey masks while locked in small cages and carrying signs that read, “No Tax $ for Animal Abuse” and “Stop Radiation Tests on Monkeys,” followed on the heels of a petition from an unrelated group asking NASA Administrator Charles Bolden to suspend the planned radiation experiments.

The Physicians Committee for Responsible Medicine, a group of 100,000 physicians, scientists and laypersons, sent Bolden a seven-page “petition for administrative action” Nov. 5 saying that the proposed experiments would violate NASA’s ethics guidelines while promising to yield dubious scientific results given the genetic, physiological and anatomical differences between humans and monkeys.

“Interplanetary travel is, at best, a highly speculative aim for the foreseeable future,” the physicians group wrote. “To put animals through radiation tests now in anticipation of such an enterprise is in no way justified.”

NASA spokesman Morrie Goodman said that the agency considers the research necessary to its mission and its responsibility to ensure the health of its astronauts. For possible future long-term missions, “space radiation is one of our most important problems,” he said.

Goodman said the research animals would be cared for in accordance with appropriate guidelines. “Our welfare concerns are for both animals and humans,” he said.