This undated NASA image obtained on February 5, 2016 shows Astronaut Edgar D. Mitchell, Apollo 14 lunar module pilot standing by the deployed U.S. flag on the lunar surface during the early moments of the mission's first spacewalk. AFP PHOTO / NASA / Handout

WASHINGTON, July 28 (Xinhua) -- U.S. astronauts who flew the Apollo moon-landing missions in the 1960s and 1970s are experiencing higher rates of heart problems that are thought to be caused by their exposure to deep space radiation, new research said Thursday.

Apollo astronauts are the only humans that have traveled beyond Earth's protective geomagnetic field, while other manned spaceflight has taken place exclusively in low Earth orbit, where such protection against space radiation is still present.

In the new study, researchers from the Florida State University compared the cause of death for seven Apollo lunar astronauts to 35 astronauts who flew only in low Earth orbit and 35 astronauts who never flew orbital missions in space.

They found that 43 percent of deceased Apollo astronauts died from a cardiovascular problem. That is four to five times higher than non-flight astronauts and astronauts who have traveled in low Earth orbit.

"We know very little about the effects of deep space radiation on human health, particularly on the cardiovascular system," Professor Michael Delp of the Florida State University, who led the study, said in a statement.

"This gives us the first glimpse into its adverse effects on humans," Delp said.

To test a possible mechanistic basis for these findings, the researchers exposed mice to the type of radiation that Apollo astronauts would have experienced.

After six months -- the equivalent of 20 human years -- the mice demonstrated an impairment of arteries that is known to lead to the development of atherosclerotic cardiovascular disease in humans.

"What the mouse data show is that deep space radiation is harmful to vascular health," Delp said.

The findings were published in the British journal Scientific Reports.

The Apollo program, most notable for landing men on the moon, ran from 1961 to 1972. The program included 11 manned flights between 1968 and 1972, nine of which flew beyond Earth's orbit into deep space.

Of the 24 men who flew into deep space on the Apollo lunar missions, eight have died and seven were included in the study. The eighth, Edgar Mitchell, died after the data analysis had been completed.