NASA is dead-set on sending astronauts to Mars within the next 15 to 20 years. China has said it hopes to send people there between 2020 and 2030, and even Russia is floating plans to put boots on the red planet.

Meanwhile, SpaceX founder Elon Musk is trying to cut the cost of spaceflight enough to start establishing a permanent Martian colony of 1 million people as soon as possible.

But if a study of radiation exposure in mice has any bearing on humans, going to Mars may be much more dangerous than anyone expected.

The root problem is cosmic rays, as detailed in a Nature study and highlighted by a recent Business Insider video.

The danger of cosmic rays

Cosmic rays are high-energy atomic and subatomic particles that get blasted out from exploding stars, black holes, and other powerful sources in space.

The rays can damage DNA, increase the risk of cancer, lead to vision-impairing cataracts, cause nervous system damage, and give rise to blood circulation issues, among other health effects in astronauts.

Researchers know that astronauts receive much higher radiation exposure than those of us who remain on Earth, since the planet's atmosphere absorbs a lot of that harmful energy.

Earth's magnetic field also diverts and deflects a lot of space radiation, which helps protect astronauts on the International Space Station - which orbits just 250 miles (402 km) above the planet.

On a trip to Mars, however, it's open season for cosmic rays. Health scientist Frank Cucinotta and Eliedonna Cacaoat the University of Nevada Las Vegas investigated this problem by reexamining the results of four previous studies of tumours in mice.

In addition of looking for the effects of a cosmic ray's direct hit to cells, which could coax them to develop into cancer, the researchers also looked at how secondary or "non-targeted effects" might play a role.

What they found is a risk of cancer in deep space (at least for mice) that's about two times higher than previous estimates.

Why deep-space travel may be more dangerous than expected

The researchers think this elevated cancer risk comes down to how damaged DNA spreads throughout the body.

When a cell is struck by a cosmic ray, it doesn't simply keep the change to itself. It can give off chemical signals to other cells, which might trigger nearby healthy cells to also mutate into cancer.

Previous models hadn't really accounted for this domino effect.

Even more worrisome, the type of radiation responsible for causing the effect was "only modestly decreased by radiation shielding," Cucinotta and Cacao wrote in their study.

Human exploration of Mars need not stop before it starts, though.

Space agencies and private companies are actively working to mitigate space radiation.

An Israeli startup is developing a body vest designed to more fully absorb radiation, for example, and one NASA scientist recently pitched the idea of deploying a satellite that would serve as an artificial magnetic shield to divert harmful radiation around Mars.

And as the researchers noted in their study, "significant differences" exist between mouse-model cancer rates and those actually seen in people.

"These differences could limit the applicability of the predictions described in this paper," they wrote.

But the scientists add that this knowledge gap is precisely why future deep-space explorers and their respective agencies should exercise caution.

"[S]tudies … are urgently needed prior to long-term space missions outside the protection of the Earth's geomagnetic sphere," they said.

This article was originally published by Business Insider.

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