During a conference this week in Washington D.C., enthusiasts are attempting to rouse support for a manned mission to Mars sometime in the next two decades. NASA is there, as are many key players in the spaceflight community. But there continue to be major obstacles to manned Mars missions.

A new study highlights one of the big problems with extended space travel: galactic cosmic ray radiation. According to the report, astronauts on the International Space Station would receive doses that exceed their lifetime limits after just 18 months for women and two years for men. A Mars mission crew would be spending at least this long in the harsh radiation of deep space.

Cosmic rays are a unique type of radiation in that they are difficult to shield against. And the new research points out that the cancer an astronaut could develop after too much cosmic ray radiation is bound to be very dangerous.

“The type of tumors that cosmic ray ions make are more aggressive than what we get from other radiation,” said Francis Cucinotta a radiation expert at the University of Nevada, Las Vegas, and author of the new report published Apr. 23 in PLoS One.

The news isn’t all doom and gloom. During certain periods in the solar cycle, galactic cosmic rays are reduced. Biologists are also working out exactly what kinds of medicine, from antioxidants to aspirin, could help deal with cosmic ray damage in the body. But more than anything, Cucinotta and other radiation experts recommend that NASA gather far more data about the health risks their astronauts are exposed to.

Approximately 41 percent of people in the U.S. will be diagnosed with some type of cancer at some point in their lives. Certain types of work—like cleaning up after nuclear disasters or flying in space—will increase your risk of developing it. NASA’s guidelines prohibit its astronauts from increasing their probability of dying from cancer by more than 3 percent. If you’ve flown out in space too many times and accumulated too much radiation, that’s it, you’re grounded.

The problem is that nobody knows exactly how much is too much. We have information about your odds of getting cancer after a nuclear explosion but that’s because we have historical data on large populations that have been exposed. Just over 500 people have flown in space, a sample size too small for epidemiological studies. And the most harmful type of radiation in space is very different from that which people are exposed to after an atomic bomb. Astronauts are exposed to galactic cosmic rays, the nuclei of atoms careening through space with incredible speed and energy. If they hit an important cellular structure, like DNA, they can generate mutations.

Because they move so fast, galactic cosmic rays aren’t stopped much by shielding. And there are a lot of them. Out in space, it is estimated that it would take about three days for every single one of your trillions of body cells to be hit by a high-energy proton (the lightest and most common galactic cosmic ray). Over the course of a year, each of your cells would likely have encountered at least one heavy and damaging iron nuclei. Other types of radiation are relatively weak and diffuse, sort of like a BB pellet, making a galactic cosmic ray a cannonball – large, weighty, and packing a punch.

One way to reduce astronauts’ exposure to galactic cosmic rays could be to send them to space only during the peak of the sun’s natural 11-year solar cycle. During solar maximum, the sun’s radiation blows counteractively against the cosmic rays streaming in to our solar system, reducing an astronaut’s exposure. Of course, being in space during this time also means the sun could unleash a potentially deadly solar flare, frying astronauts in their spaceship.

What kind of extra exposure are astronauts normally dealing with? People living in the U.S. are exposed to about 3 millisieverts of radiation from natural background sources each year (millisieverts are units of radiation exposure in the human body). A nuclear accident, like Fukushima, might raise this by about 1 millisievert. An astronaut on a round-trip, two-and-a-half-year Mars mission, by contrast, can expect to receive around a sievert of cosmic ray radiation, nearly 1,000 times more.

If 41 percent of people in the U.S. can expect to be diagnosed with cancer that means, out of 100 people, on average 41 of them will get cancer. If you exposed 100 people to the 1 sievert of cosmic ray radiation that a Mars astronaut would get, there would now be 61 total incidents of cancer, an increase of 20, according to reports from the U.S. National Academy of Sciences (.pdf) and United Nations Scientific Committee on Atomic Radiation (UNSCEAR). About half of those tumors would result in death.

Certain types of cancer, including lung, breast, and colorectal cancer, are the most likely to appear from cosmic ray radiation and tend to be more aggressive than normal. Cucinotta estimates that an astronaut’s lifespan after exposure to radiation on a Mars trip would be shortened between 15 and 24 years from the average.

So the concerns from radiation are very real. But there could be a number of biological ways to mitigate cancer for long-duration spaceflight, said oncologist and cell biologist Mary Helen Barcellos-Hoff of NYU, who was not involved in the recent study. Radiation doesn’t just damage DNA; it also seems to change the ways that cells signal to one another. Tumor cells are helped along, for instance, by impairment in the immune system’s macrophages, which seek out and destroy defective cells. If macrophages aren't doing their job, it can promote the tumor’s growth, allowing it to invade and metastasize in the body.

These kinds of immune response changes are similar to those in chronic inflammatory diseases, which produce oxidants that mess up intercellular signaling. It’s possible that antioxidants and non-steroidal anti-inflammatory drugs, like aspirin, taken during spaceflight could help hold back some of the worst effects of cancer, said Barcellos-Hoff. There will always be a higher chance of developing tumors during a deep-space mission, “but getting into a rocket and shooting to Mars is not exactly a risk-free activity,” she said.

NASA is planning one-year stays on the ISS (astronauts currently take six-month shifts) and mulling over the idea of sending humans to an asteroid or beyond. A recent report from the Institute of Medicine stated that NASA should develop an ethical framework for exposing its astronauts to the health hazards of long-term spaceflight.

One of the major problems in our understanding of cancer risks for astronauts is the high amount of uncertainty. Cucinotta’s data is based on studies where mice here on Earth were exposed to heavy ions from particle accelerators. These mice tend to be engineered to be more susceptible to certain tumors and are often inbred, which most humans are not.

These are all complicating factors in making the most recent cancer risk estimates for astronauts. The most important thing for him would be to simply get more and better data. NASA has not done a great deal of studies on the biology of radiation. Some were conducted decades ago. But our knowledge of cancer is constantly being renewed and studies from more than ten years ago tend to be out of date.

Before humans are sent to the Red Planet, NASA should do a focused decade-long study on all the potential health problems that radiation could cause. Hopefully, this would bring up new ways to combat some of the worst effects.

“Once we know better, we can find the true answer, and it could lower our risk estimate,” said Cucinotta.