Paul McCartney has joined other celebrities such as comedian Bill Maher in urging NASA not to conduct cruel radiation experiments on monkeys. 

"I believe NASA has the ingenuity to investigate the health effects of space travel without confining and experimenting on animals as was done in the old days," he writes in a letter to agency administrator Charles Bolden. "It would be terribly disappointing if in our zeal to explore new frontiers and to learn about the fascinating universe where we live, we began to regress in our treatment of the animals with whom we share this planet."

McCartney wrote the letter on behalf of a campaign by PETA spotlighting the research. 

Unlike the European Space Agency, which states that it "declines any interest in monkey research", NASA is prepping to begin tests on 18 to 28 squirrel monkeys. The experiments are meant to study the effects of low-dose radiation that astronauts traveling to Mars can expect to encounter. 

Critics say the tests are unnecessary and lack scientific merit.

"There are better, more humane ways of understanding the potential dangers of interplanetary travel to humans," the group Physicians Committee for Responsible Medicine said last fall. "Progress can only proceed with a strong ethical foundation.” 

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

Paul McCartney blasts NASA for planned monkey experiments
Former Beatle urges space agency to back down on plan to use animals in radiation exposure tests.