Radiation damages cells' DNA, which can lead to cell death or permanent changes that may result in cancer. However, "there's no convincing human evidence for excess abnormalities in offspring of radiation-exposed adults," Semones said.
While orbiting the Earth, astronauts get exposed to greater concentrations of cosmic background radiation than here on Earth in addition to charged particles trapped in the upper atmosphere and from the sun, said Robert J. Reynolds, epidemiologist at the University of Texas Health Science Center.
As a spacecraft moves into deep space, the people on board would be exposed to even more cosmic radiation and solar particles, which is "fairly dangerous," Reynolds said.
Interestingly, according to Reynolds, astronauts' risk of dying of cancer is lower than that of the general public because they tend to be in shape, eat well, don't smoke and receive careful monitoring from doctors. Of course, none of them have been to Mars.
Semones emphasized that NASA does not study the health effects of Mars colonization and that it's focusing on shorter recognition missions of the surface of Mars. "We're not looking at colonization of Mars or anything. We're not focusing our research on those kinds of questions."
Can it be done?
Mars One isn't the only group hoping to make history by sending people to the red planet. The Inspiration Mars Foundation wants to launch two people -- a man and a woman -- on a 501-day, round-trip journey to Mars and back in 2018 without ever touching down.
At this time there is no technology that can protect astronauts from an excess of space radiation. "The maximum number of days to stay with our standards is on the order of 500 days. So any mission that would exceed 500 days would not be doable," Semones said.
Reynolds agreed: "At this point it's completely infeasible to try to send someone to Mars unless we can get there faster or we develop better shielding for a spacecraft."
NASA is working on engines intended to cut the travel time to Mars by the 2030s, but those systems won't be ready for many years, Chris Moore, NASA's deputy director of advanced exploration systems, told CNN this year. In the meantime, Moore said engineers could try to limit travelers' exposures by designing a spacecraft in such a way that it provides more protection.
But Mars One founder Lansdorp insisted his group will get people landing on Mars by 2023.
"The risks of space travel in general are already very high, so radiation is really not our biggest concern," he said.
If that all sounds good, you can still sign up.
But remember: You can never go home again.