Astronauts who took part in the Apollo space program are experiencing disproportionately high rates of cardiovascular problems, according to a paper published Thursday in Scientific Reports by exercise physiologist Michael Delp and colleagues at Florida State University. The likely culprit: exposure to deep space radiation levels encountered during lunar expeditions far exceeding those experienced by astronauts restricted to low Earth orbit (LEO) missions.
The study looked at the causes of death among seven Apollo astronauts versus those of 35 LEO astronauts and those of 35 astronauts who had not actually flown in space. Delp and co. note that, given the influence of Earth's geomagnetic field, it's generally been assumed that astronauts confined to LEO or even participating in short lunar trips are relatively safe from the cardiovascular effects of space radiation. This still seems to be the case.
The study found that 43 percent of Apollo astronauts (three of seven) died of some variety of cardiovascular problem
"The human experience with spaceflight has shown that space exploration comes with various health risks," the authors note. "As humans contemplate manned missions to Mars or prolonged habitation on the Moon, these health risks will rise as the duration of spaceflight increases and as travel goes beyond the Earth's protective magnetosphere. During such interplanetary travel, astronauts will be exposed to multiple sources of ionizing radiation, including galactic cosmic rays, solar particle events, and trapped radiation in the Van Allen belts."
The study found that 43 percent of Apollo astronauts (three of seven) died of some variety of cardiovascular problem, defined as one of the following: heart failure, myocardial infarction, stroke, brain aneurysm, or blood clots. This is a rate four to five times higher than that experienced by LEO astronauts or astronauts that never made it to space at all. However, the sample size of Apollo astronauts is tiny (necessarily), and the study itself notes that caution should be taken in drawing definitive conclusions about any implied health risks.
Presumably with this limitation in mind, Delp and his team looked to rodent models to investigate further. Mice were subjected to 14-day bouts of deep-space equivalent radiation, simulated weightlessness, or some combination of the two, and then examined six months later. This recovery delay, comprising about a quarter of a lab mouse's natural lifespan, is roughly equal to 20 human years.
The researchers found significant decreases in arterial functioning in the irradiated mice, but found no effects that could be attributed to weightlessness. In particular, the deep-space mice exhibited diminished vasodilation—relaxation of blood vessel walls—related to the blood vessel's innermost "endothelium" layer.
"As dysfunction of the vascular endothelium is central to the pathogenesis of vascular disease, such adverse arterial effects could lead to the development of occlusive arterial diseases, including myocardial infarction and stroke," Delp and co. write. "These findings suggest that in spite of the 'healthy worker' effect, short-duration deep space travel by this highly educated, trained and physically fit group results in a significantly elevated risk of death from CVD [cardiovascular disease]. The major environmental factor that would appear to underlie this phenomenon is deep space radiation."
Delp isn't the only one studying the blood vessels of astronauts. Researchers have found abnormal postflight increases in arterial stiffness in astronauts spending six months or longer in orbit (read: aboard the ISS) that are akin to the increases seen as part of the normal aging process. This is thought to be related to changes in arterial pressure and the relative lack of physical exercise experienced in a weightless environment. This is currently being investigated as part of the Canadian-run experiment Vascular Echo.