Mars, the closest planet to the Earth, has always fascinated humans. From H.G Wells 1898 novel The War of the Worlds, to Orson Welles’ infamous 1938 radio broadcast about a Martian invasion, to Gustav Holst’s 1914 classical music piece Mars, the Bringer of War, to the 2015 Academy Award nominated film, The Martian mankind has long dreamed about Mars. Now NASA and private companies such as SpaceX, are beginning to plan trips to our nearest neighbor. Mars is 140 million miles from the Earth and the trip would take about seven months. A typical mission would likely be two and a half years. For comparison, the moon is 239,000 miles away and it takes about three days to reach it. There will be many obstacles and many risks to be overcome on a Mars mission. One of the issues is the health risk to the astronauts on a prolonged interplanetary journey. How would a trip to Mars affect an astronaut’s heart? What lessons does that have for those here on Earth?
The biggest health hazard of a prolonged, multiyear space trip is radiation. In space, the major cause of damaging radiation is high-energy galactic rays. These rays come from deep in space and are caused by supernova explosions and other phenomena. Radiation can penetrate the skin and travel through the body causing damage to DNA and to the mechanisms used to repair cells. These effects can lead to cancer. Astronauts endure higher exposure to radiation in space than they do on Earth. On the space station, astronauts received ten times the radiation they would have been exposed to if they stayed on the ground. On a two-year Mars mission, it is estimated they may have 100 times the exposure. Unfortunately, engineers still haven’t found a way to protect astronauts from galactic rays. On Earth, humans are protected from galactic rays by the Earth’s magnetic field and by the atmosphere, which deflects the high-energy radiation. Under the Earth’s protective umbrella, people are exposed to a variety of sources of radiation. These include ultraviolet radiation from the sun (which is lower in energy than the deep galactic rays), radon, occupational exposure, small amounts of radiation from TVs and cell phones and radiation from medical procedures (the second most common source).
Excess radiation exposure, whether on Earth or in space, increases the risk for cancer, it can damage the nervous system and produce cataracts. In addition, radiation can cause heart disease and can affect all three layers of the heart. The effects of radiation on the heart have been studied in patients who have received high dose radiation therapy to the left side of the chest for various cancers (such as Hodgkin’s Lymphoma and breast cancer). Many years after their cancer is cured, these patients manifest heart disease. When the outer protective layer of the heart, the pericardium, is damaged by radiation, it can become inflamed causing chest pain (pericarditis). Long-term radiation to the pericardium causes it to stiffen and compress the beating heart. Radiation can affect the inner layer of the heart producing damage to the heart valves. When damaged by radiation, the valves may not open as widely or may leak severely. With significant valve damage, open-heart surgery and valve replacement may be needed. Lastly, radiation can affect the heart arteries, causing them narrow, reducing the blood flow and resulting in chest pain. Unlike atherosclerosis, which causes a discrete blockage in a heart artery, radiation damages long segments of the artery, making it more difficult to place stents or relieve chest pain with bypass surgery. Do galactic rays cause heart disease? The answer is unknown, but there is a clue when looking at astronauts’ health over time. As a rule, astronauts are healthier than the general population as they are more fit and have access to high quality healthcare throughout their lives. However, the Apollo astronauts, the only humans who have been beyond the Earth’s protective magnetic field, have a higher rate of dying from heart disease compared to astronauts who have never flown in space.
Since most people won’t have to worry about galactic rays and one of the major causes of radiation on Earth is medical testing, what can be done to reduce a patient’s lifetime exposure to radiation? Recently there has been a big push in the medical community to educate doctors about radiation risk. Campaigns touting the ALARA principle (As Low As Reasonably Achievable) and Appropriate Use criteria encourage doctors to choose wisely when ordering tests. Patients should discuss the risks and benefits of radiologic procedures (for example, chest X Rays, CT scans) with their doctor and ask about alternatives without radiation risk such as ultrasound or MRI. Lastly, it might be prudent to forgo that trip to Mars.