Mars Astronauts Face Double the Cancer Risk as Previously Estimated, Says Study

The radiation risk for a manned Mars mission would already be dangerously high, but damage to cells that are next to heavily damaged cells may double the estimated risk.

The cancer risk for a human mission to Mars may be double what was previously expected, says a new study that is partly based on studying tumors in mice.

Astronauts are exposed to radiation from galactic cosmic rays when they travel outside the protective shield of Earth’s magnetic field. These cosmic rays — which originate from the shockwaves of star explosions, or supernovas — can disrupt the bonds inside human cells, leading to DNA damage and mutation.

“Exploring Mars will require missions of 900 days or longer and includes more than one year in deep space, where exposures to all energies of galactic cosmic ray heavy ions are unavoidable,” said lead author Francis Cucinotta, a professor of radiation and space physics at the University of Nevada, Las Vegas, in a statement. His study was recently published in Scientific Reports.

NASA is hoping to send humans to Mars in the 2030s. Later today, the agency will announce the new class of astronaut candidates. If the agency’s policy holds for the next decade or so, it’s possible that these people would be among the first to explore Mars — as long as the space agency can mitigate the health risks that they will face.

Risks from galactic cosmic ray exposure can include cancer, cataracts, circulatory problems, or acute radiation, among other damaging effects. Worse, added Cucinotta, current levels of spacecraft radiation shielding would only moderately decrease the exposure risk.

The calculation of a doubled risk comes from comparisons between a “targeted effect” model, which looks at effects within or close to the DNA of cells that are exposed to high doses of radiation during a space mission, and a “non-targeted effect” (NTE) model, which assumes bystander cells (those that are next to heavily damaged cells) are also susceptible to cancer.

“The scarcity of data with animal models for tissues that dominate human radiation cancer risk, including lung, colon, breast, liver, and stomach, suggest that studies of NTEs in other tissues are urgently needed prior to long-term space missions outside the protection of the Earth’s geomagnetic sphere,” the study says.

NASA’s Space Radiation Program Element web page says the agency researches both effects as it seeks to better understand space radiation.

How high is the risk? It depends on what non-targeted effect model you use and how thick the shielding would be on the Martian-bound spacecraft. The new study bases its work on previous studies of mice that developed Harderian gland tumors after exposure to radiation. (Harderian glands are common in animals with “third eyelids”; humans have a rudimentary version of this gland.)

Based on these mice studies, the baseline targeted effect model assumes an 8 to 9 percent tumor prevalence after one year of exposure to galactic cosmic rays, depending on the thickness of aluminum shielding used on the spacecraft.

But for the non-targeted effect model — the one that assumes damaged cells increase the risk to adjacent cells — the paper shows three scenarios, all predicting increased risk for astronauts. The first scenario has a 12 to 13 percent prevalence of tumors, a second has a 16 to 18 percent tumor prevalence, and a third a 20 to 25 percent prevalence.

“Tumor prevalence” refers to the number of mice with tumors, Cucinotta’s paper explained, divided by the number of total number of mice in the experimental groups surveyed in past research. The paper further cautions that there are only two Harderian glands per mouse — one for each eye — but the number of animals with these tumors was the measure used in past research.

“Galactic cosmic ray exposure can devastate a cell’s nucleus and cause mutations that can result in cancers,” Cucinotta added. “We learned the damaged cells send signals to the surrounding, unaffected cells and likely modify the tissues’ microenvironments. Those signals seem to inspire the healthy cells to mutate, thereby causing additional tumors or cancers.”

Cucinotta called for more studies to look at cosmic ray exposures on tissues that are susceptible to human cancer, and said these studies should commence before astronauts regularly venture outside of Earth’s magnetic field.

NASA is already conducting radiation-related studies on its Mars rover mission. In 2013, for example, the agency released results from the Curiosity rover’s Radiation Assessment Detector. Based on measurements from the detector, the agency modeled a 180-day trip to Mars, a 180-day trip back to Earth, and 500 days on the surface.

NASA said this mission scenario would give astronauts a cumulative radiation dose of roughly 1 sievert. This is about the same as current European Space Agency lifetime limits for its astronauts that visit the International Space Station. With 1 sievert of exposure, lifetime fatal cancer risk in humans would increase by about 5 percent, which goes beyond NASA’s parameters of a 3-percent limit.

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