NEW YORK • Only 24 people have ever gone to deep space, or to the area beyond the Earth's magnetic shield. These are the Apollo astronauts who flew to the moon, the last of whom did so in 1972.
Today, dreams of deep space exploration are surfacing again. Government space programmes and private corporations alike have their eyes set on returning to the moon for longer visits and venturing beyond, to Mars, in the coming decades.
Dr Michael Delp, a professor of human sciences at Florida State University, said researchers need to better understand and study the effects of deep space travel on the human body.
"There are an incredible number of unanswered questions," he said. "One of these is what deep space does to the cardiovascular system."
In a paper published last Thursday, a week after the 47th anniversary of the first moon landing, he and a team of researchers affiliated with the National Aeronautics and Space Administration (Nasa) examined how deep space travel may have affected the cardiovascular health of Apollo astronauts.
Comparing Apollo astronauts who have died with other astronauts who either never flew in orbital missions or flew only in low Earth orbit, he and his colleagues found a higher rate of cardiovascular deaths among Apollo astronauts. Based on further research in mice, they suggest that the cause of cardiovascular disease in these astronauts may have been deep space radiation.
Their study, published in the journal Scientific Reports, is the first to look specifically at the long-term health of Apollo lunar astronauts.
Researchers in the field agree that the question of how deep space affects cardiovascular health is an important one.
"Data from animal and cell biology experiments have suggested that there may be cause for concern," said Dr Dennis Kucik, a professor of pathology at the University of Alabama at Birmingham who studies radiation-induced cardiovascular disease for Nasa but was not involved in this study.
However, experts also have concerns about the scientific legitimacy of studying an extremely small number of astronauts. To date, only seven of the 24 Apollo lunar astronauts have died. Of those seven, three died of cardiovascular disease (Neil Armstrong, who flew on Apollo 11; James Irwin, who flew on Apollo 15, and Ronald Evans, who flew on Apollo 17).
The problem with a small sample size is that a one-person difference can drastically alter the statistics, said Dr Jay Buckey, a professor of medicine and engineering at Dartmouth College who did not participate in the study.
For instance, at least one of the three Apollo astronauts who died of cardiovascular disease, Irwin, may have had cardiovascular problems before he left Earth. During his flight on Apollo 15, he experienced a bout of heart arrhythmia, which was later noted in a Nasa report as being possibly related to "pre- existing, undetected coronary artery disease".
Unexplored risk factors could also explain why the other two Apollo astronauts died of cardiovascular disease, Dr Buckey said. "Were they smokers? Did they have a family history of heart disease? Did they have high cholesterol?
"Those are all factors that could lead to heart disease, without having to bring galactic cosmic radiation into the mix," he said.
Because this study relied on a small number of astronauts and did not consider such risk factors, "it is not possible to determine whether cosmic ray radiation affected the Apollo astronauts", Ms Tabatha Thompson, a representative for Nasa, wrote in an e-mail.
The epidemiology of Apollo astronauts is the novel portion of this study, but it is not the only one.
The research on mice supports previous studies that have shown similar results, said Dr Benjamin Levine, director of the Institute for Exercise and Environmental Medicine at Texas Health Resources and a professor at the University of Texas Southwestern Medical Center, who was not involved in the research.
In their study, Dr Delp and his colleagues exposed mice to simulated conditions of anti-gravity, cosmic radiation or both effects combined. After half a year (the equivalent of 20 human years), they found only the mice that had been exposed to radiation had sustained damage to their blood vessels. In particular, the researchers found damage to the lining of the blood vessels, which is typically the first indication of long-term heart disease leading to a heart attack or stroke.
The mouse studies provide some indication that radiation exposure may have affected the cardiovascular health of Apollo astronauts, Dr Delp said.
Astronauts who fly in low Earth orbit, including those on the International Space Station, have the protection of the Earth's so-called magnetosphere, which provides shielding from radiation.
Once the Apollo astronauts went beyond that magnetic bubble, their only protection against radiation came from the shielding of their spacecraft and spacesuits.
Dr Delp said his team felt that it was important to release their findings, even with shortcomings.
"What it does is point to the fact that we need a lot more study on this," he said. "One of the things that Nasa and others have been most concerned about with space radiation is cancer. The cardiovascular system has barely gotten mentioned."
Understanding how cosmic radiation affects blood vessels and the heart - and then investigating possible solutions, such as antioxidants, exercise habits and better shielding on spacecraft - may be relevant sooner rather than later.
With federal funding for the International Space Station set to expire in 2024, Nasa wants to shift its focus to deep space, aiming to fly astronauts to an asteroid by 2025 and to Mars by the 2030s.
China, Russia and the European Space Agency are all looking into launching people to Mars, as well as establishing long-term bases on the moon. Private organisations have joined the fold as well, with Mars One and SpaceX both expressing a wish to land people on Mars by the mid-2020s.
"With only 10 years before people are planning to send people back to deep space, we thought, 'We can sit on this study or we can try to get it out there,'" Dr Delp said. "We felt it was important enough to get it out."
NEW YORK TIMES