In the vast emptiness of space, two forms of radiation menace astronauts: Cosmic rays zip through the galaxy at near-light speeds, while solar activity produces a more subdued form of radiation. Both are a problem for space travelers, causing conditions ranging from impaired vision to cancer.
This radiation isn’t a problem here on Earth thanks to the planet's protective atmosphere, which blocks the worst of it. But engineers still don’t have effective methods to shield astronauts from these dangers, and that adds an extra level of risk to already risky plans to send humans to Mars on a three-year journey by the 2030s.
"There may be mission-level risks that literally put the mission at risk—the whole mission, not just the individual astronauts—if one or more crew members are incapacitated," says radiation expert Ron Turner, a senior science adviser at NASA's Institute for Advanced Concepts in Atlanta who studies risk management strategies for human space missions. "It's important that we get that data over the next ten years so we are able to make prudent planning for a future Mars mission."
The sun constantly sheds energetic particles through the solar wind. And levels of these particles rise and fall during the sun’s 22-year solar cycle. Solar storms also can hurl massive blobs of charged particles into space, with the 11-year peak producing the most activity. The powerful radiation can not only increase long-term cancer risks but also cause immediate issues such as vomiting, fatigue and vision problems.
Like solar activity, cosmic rays have the potential to cause cancer. These high-energy, high-velocity particles originate from outside the solar system and can severely damage human cells. Unlike radiation from the sun, however, cosmic rays could also spark long-term degenerative effects while still in space, including heart disease, reduced immune system effectiveness and neurological symptoms resembling Alzheimer's.
Without Earth's atmosphere to shield them, astronauts on board the International Space Station already have to deal with these radiation dangers. They can seek shelter in a more heavily shielded part of the ship when the sun releases a particularly high-powered burst of radiation. But avoiding the constant, steady assault of cosmic radiation presents a greater challenge. And no one on the ISS has yet to experience the full radiation dangers that would be seen on a three-year mission to Mars and back; the maximum amount of time anyone has spent on the space station is 14 months.
A thicker hull can help block lower-energy cosmic rays, but any high-powered rays can easily pass through, Turner notes. Plus, doubling the nominal thickness of a spaceship hull only reduces the threat to astronauts by about 10 percent, a number that depends on the nature of both the rays and the shielding. That extra shielding also adds weight to a spacecraft, limiting what can be devoted to supplies for science and survival.
Turner says the best way to mitigate the danger from cosmic rays won't come from shielding. Instead, he thinks the solution will come from reducing the time astronauts spend traveling to and from other worlds. Once humans touch down on Mars, the bulk of the planet will provide significant protection, effectively halving the amount of radiation that makes it through. While Mars’ thin atmosphere won't provide the same shield as Earth's thick layer of gases, it, too, will reduce the cosmic rays that reach explorers on the surface.
To understand how cosmic rays will affect human explorers, scientists will first need to measure the properties of the sun's magnetic field at a given time. "The better we know the galactic cosmic ray environment that we're sending our astronauts into, the better we can plan missions and understand the effect of a mission on the astronauts," says Turner. With that information, researchers might be able to forecast the effects of cosmic radiation a year or two before a mission launches, allowing better planning for specific space weather. That would be like knowing if an approaching storm on Earth were a hurricane or a thunderstorm; the information can help when tailoring protective measures.
Scientists are now gaining a better understanding of what cosmic rays look like outside of the sun’s protective shield by using data collected by the Voyager 1 spacecraft, which left the solar system in 2012. This should help them better understand how the changing solar activity affects the rays.
Voyager 1 "is the only instrument that humanity has made that has managed to get into the interstellar medium, the one part where we are outside of the influence of the solar magnetic field," says Ilias Cholis, a postdoctoral researcher at Johns Hopkins University in Maryland.
While Voyager 1 probes the cosmic radiation outside of the sun's reach, instruments such as the Russian satellite-based Payload for Antimatter Exploration and Light-nuclei Astrophysics (PAMELA) and the Alpha Magnetic Spectrometer (AMS) onboard the ISS sample it from inside the solar system. Comparing measurements from each of these sources is helping Cholis and other researchers to understand how the sun's activity has altered the dangerous radiation in the past, and how it could modify the radiation in future solar cycles. Together, these spacecraft and instruments are increasing the amount of information on cosmic rays, and this will only improve as time goes by.
Cholis and his colleagues, for instance, recently used new data from Voyager 1 to modify existing formulas that describe how the sun's magnetic field affects cosmic rays. Many cosmic rays come from supernovas—the explosion of a massive star that sends charged particles shooting outward. Unlike light from the explosion, the energetic material doesn't travel in a straight line but instead bounces off gas and dust in space in what Cholis described as "a very zigzag kind of path." That can make it difficult to determine where individual cosmic rays come from, especially once they pass into the solar system.
By stepping outside of the sun's influence, Cholis and his colleagues hoped to do a better job of identifying the source and properties of the rays. Not only will this help them learn more about where the energetic particles come from, but it can also improve understanding of their effects on humans, especially those traveling in space.
Radiation is “a risk we need to learn more about over the next decade so we can do the proper mitigation, so we can do the best we can for the astronauts who are going to be putting their lives at risk for a number of different threats," Turner says. But the optimum solution might be the one that, for now, seems difficult—going faster and avoiding as much radiation as possible. He says, "The best bang for the buck is advanced propulsion, not shielding."