We’ve long known that the journey to Mars could pose significant health risks for future astronauts. Now a new study says the risk of cancer on the journey there may be double what we thought.

The findings by Francis Cucinotta and Eliedonna Cacao from the University of Nevada, Las Vegas were published in Scientific Reports. Studying tumors in mice, they found that a chain reaction effect could put more cells in danger from galactic cosmic rays (GCRs).

Previous studies have mostly looked at how radiation can affect cells and DNA directly. This study, though, looked at non-targeted effects (NTEs) – where heavily damaged cells have a knock-on effect on healthier cells nearby.

“The NTE model is shown to predict a 2-fold or higher cancer risk compared to a TE [targeted effects] model for chronic GCR exposures,” the team write.

Cosmic rays can include things like iron and titanium atoms, which can heavily damage cells they pass through due to high rates of ionization. But previous studies only assumed cells needed a high dose; NTEs suggest a lower dose could also be a problem.

It’s not just cancer that could be an issue. Central nervous system effects, cataracts, circulatory diseases, and acute radiation syndromes could all affect Martian explorers. There are ways to mitigate the effects, but these have not been fully investigated.

So the researchers say it is crucial that more experimental data is gathered. This includes exposing tissues to low doses of radiation, particularly the lung, stomach, liver, breast, and colon.

"Galactic cosmic ray exposure can devastate a cell's nucleus and cause mutations that can result in cancers," Cucinotta said in a statement. "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."

Estimates for how long a Mars mission will take vary, but this paper assumes it will be 900 days or more. That’s perhaps a bit on the long side, as the travel time is about eight months each way, with a stay of a few weeks to months possible on the surface.

Mars and Earth align in such a way for a journey between the two every 26 months, so these researchers are probably assuming a Martian crew will stay on the surface for almost the full duration of this window.

Nonetheless, the health risks of such a mission are very present. Mars has a thin atmosphere, so on the surface astronauts will be subjected to a lot more cosmic radiation than on Earth, in addition to that experienced in transit. When we do send people to Mars, we’d better make sure we’re ready.