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.