NASA plans to subject a group of monkeys to radiation to study what might happen to humans on long-term space missions, such as trips to the moon and Mars.

The research is the first time in decades that NASA has performed tests on primates, though the agency famously relied on the close human relatives to make some of the first forays into space in the 1950s.

"The overall objective of the planned studies with the nonhuman primates is to help NASA predict neurobehavioral effects of space radiation, which are among the most poorly understood health risks for astronauts," NASA spokesman Bill Jeffs told SPACE.com.

The agency plans to aim high-energy gamma-ray radiation at 27 squirrel monkeys, then observe how they perform tasks to see how they're affected, Florida Today reported. The experiment is meant to simulate the type of radiation humans are exposed to in space. As NASA plans trips to farther destinations, people will be spending longer amounts of time outside the Earth's protective atmosphere, so more knowledge of the health effects is vital, NASA said.

"NASA-funded research has shown that simulated space radiation can affect nerve cells in culture and also the behavior of mice and rats, but these studies are limited in the extent they can be extrapolated to human behavior and performance," Jeffs said. "Studies in nonhuman primates are essential to be able to best predict neurobehavioral effects of radiation on humans."

But some animal welfare advocates object to the use of monkeys in research that could harm them.

A group of about 100,000 doctors, nurses and laypersons recently petitioned the space agency to put off the $1.75 million project, Florida Today reported, calling the experiment "one giant leap backward for NASA."

The doctor planning to carry out the study for NASA, Jack Bergman of Harvard Medical School's McLean Hospital in Boston, declined to comment for this story.

Other scientists who have worked with non-human primates defended the need for this type of research.

"Due to their phylogenetic proximity to human beings, non-human primates provide a research subject as close to humans as possible," said Christian R. Abee of the University of Texas M. D. Anderson Cancer Center, who is not involved in the current project, but consulted with NASA for earlier primate studies. "Therefore, they are the ultimate translational science subject for many studies."

The upcoming study intends to expose the test monkeys to a blast of radiation roughly equivalent to what astronauts would be exposed to on a three-year space voyage — the length of time it would take people to travel to Mars. Though the exposure will likely cause some cellular damage, it will not kill the monkeys, NASA told Florida Today.

NASA maintained that the experiments would provide important knowledge for designing future missions.

"Currently, there is no information regarding the effects of space radiation on neurobehavioral function in nonhuman primates," Jeffs said.