Some of the pet hazard scenarios that science-fiction movies toss out are about the crew losing it. Or, spacewalks going awry. Or, starships blowing up. Or, on-board computers turning nefarious. And then, of course, there’s the all-time favorite of alien attacks.
Dramatic as these events are, they don’t touch on an invisible peril that men and women encounter each time they leave Earth. Overcoming it one of the biggest obstacles to going far out from home.
The cosmos isn’t a benign, inky void, dotted with twinkling stars, translucent rocks, and strikingly beautiful gaseous balls with glittering Hula-hoops around them. It’s also bristling with harmful radiation—deadly because of its penetrative power.
It packs so much energy that it can rip through metal, flesh, and bones, and smash any atom it strikes, knocking electrons out of their orbits. It can pierce a spacesuit, puncture the skin, enter cells, and tear apart a DNA strand, irreparably damaging it.
One source of such radiation is “galactic cosmic rays,” which originate outside the solar system, and hurtle through interstellar space at speeds very close to that of light. They’re the bare nuclei of atoms of nearly every element, and can range in size anywhere between a single proton (hydrogen) to a platoon of 92 protons and 143 neutrons (uranium.)
Within the solar system, the Sun is a wellspring of various emissions. At its default setting, it gives off a low-energy spray of charged subatomic particles that are present everywhere in the heliosphere. This is called the “solar wind.”
From time to time, that’s compounded by explosions on the Sun. “Flares” are giant bursts of x-ray (an electromagnetic wave, emitted by electrons) that shoot up from its photosphere. They travel at the speed of light in all directions, and take eight minutes to reach Earth.
The Sun’s upper atmosphere, the corona, also belches what’s called “coronal mass ejections.” They’re huge clouds of plasma (a soup of ions and free electrons), which lift off its surface, and drift away more slowly, making the journey to Earth in 24 to 72 hours.
A watered-down version of this radiation mix reaches us all the time, but it’s mostly blocked out by Earth’s robust and far-ranging magnetic field, generated by its molten iron core. On top of that, the atmosphere acts like a 15-feet-wide concrete shell, shielding us from the lethal billows.
But once outside this protective cocoon, humans will be exposed to its full fury. A radiation-proof vessel hasn’t been built yet.
According to the Nuclear Regulatory Commission, on average, a U.S. resident is exposed to an annual doze of 620 millirem (or 0.62 rem or 6.2 millisieverts) of radiation, which comes from all around us: from the radon in the air we breathe to the bananas we eat.
But that’s nowhere near as pernicious as what we’re exposed to in space.
A study published in Science, in May, 2013, found that during a one-way trip to Mars, the crew will be exposed to 38,600 millirem (or 38.6 rem or 386 millisieverts) of radiation. During their excursions on Martian soil, they’ll be bombarded by another 1,110 millirem (or 1.1 rem or 11 millisieverts) per year.
This means that the settlers will be able to spend about 60 years on Mars before reaching an astronauts career limit of anywhere between 60,000 millirem (or 60 rem or 600 millisieverts) and 120,000 millirem (or 120 rem or 1,200 millisieverts) under NASA recommendations.