Long-distance human spaceflight is, famously, a bust. So far, anyway—no doubt we'll figure it out someday. But the reason we haven't sent humans on five-year missions seeking out new life and new civilizations isn't because of cost, politics, or lack of warp drive. The real reason is that astronauts would probably be killed by radiation before they met their first gas giant.
They wouldn't be dead dead, of course. They might even make it back in time to die on Earth. Yet the outbound trip alone would be enough to send their risk for cancer shooting way beyond what NASA considers acceptable levels.
How do we know? Well, before they sent the Mars Curiosity rover to the red planet, scientists strapped on a sensor to measure the amount of radiation bombarding its ship. According to newly released data, for every day the vessel spent traveling to Mars, it recorded 1.8 milliSieverts of space radiation.
One thousand milliSieverts—or more simply, 1 Sievert—is enough to raise your risk for cancer by 5 percent.
With that, let's do some math. Given that Curiosity's interplanetary trip lasted 253 days, the rover accumulated 456 milliSieverts of radiation for the duration of the journey (which doesn't include time spent on the Mars surface). That's .46 Sieverts, or 46 percent of the 1 Sievert dosage linked to a 5-percent jump in cancer risk. By comparison, the average American absorbs an estimated 0.0036 Sieverts of radiation a year.
"The findings indicate radiation exposure for human explorers could exceed NASA's career limit for astronauts," NASA spokesperson Trent Perrotto wrote in an e-mail.
Even in Earth orbit, astronauts absorb radiation. Although we're pretty good at blocking dangerous particles emitted by the sun, our shielding technology isn't as good at blocking other types of space rays. NASA has, therefore, set a lifetime limit on the amount of time any person can spend up there. And that threshold is at the 3-percent-increased-risk-for-cancer mark.
The journey to Mars would therefore get you pretty close to your lifetime exposure cap, to say nothing of the radiation you'd suffer on the planet's surface or on the voyage home. It'd be 23 times the amount employees working for the Energy Department are allowed in a year, and 127 times what the ordinary person is exposed to annually.
We don't conclusively know what that level of exposure would mean for people on a mission to Mars—only that it would be very dangerous. Unless we develop better shielding (or faster-than-light travel), our first interplanetary travelers might never make a second trip.