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.
