NASA wants to send humans to Mars as early as the 2030s , but astronauts will face particle radiation from the Sun, distant stars, and galaxies, as they travel on their nine-month-long journey to the Red Planet.
In a new article published in the peer-reviewed journal Space Weather, an international team of space scientists, including researchers from UCLA, determined two important conclusions about the risks of radiation: A human-led Mars mission is viable depending on spacecraft shielding and flight timing.
Astronauts on their way to Mars will face two types of dangerous radiation. Radiation originating from the Sun, known as solar energetic particles (SEP), could be very damaging if the travelers are not protected by the spacecraft. Even one solar eruption can create a critical radiation dose.
The second type of radiation, galactic cosmic rays (GCR), originates outside of our solar system. The intensity of this radiation is in general not as high as that of SEP, but the radiation can accumulate to harmful levels over time.
The scientists’ calculations demonstrate that it would be possible to shield a Mars-bound spacecraft from energetic particles during a period known as the "solar maximum," when the Sun is at its highest level of activity. The most dangerous and energetic particles from distant galaxies are deflected by the enhanced solar behavior in this period of time.
Additionally, the researchers recommend a mission not longer than four years because a longer journey would expose astronauts to a dangerously high amount of radiation during the round trip.
A launch to Mars and back is possible within half that time, according to Yuri Shprits, a UCLA research geophysicist, a co-author of the paper, and also a professor in Germany. The average flight to Mars takes about nine months.
“This study shows that while space radiation imposes strict limitations on how heavy the spacecraft can be and the time of launch, and it presents technological difficulties for human missions to Mars, such a mission is viable,” said Shprits, who also is head of space physics and space weather at GFZ German Research Centre for Geosciences in Potsdam, Germany, in a recent news release .
Shprits and colleagues from UCLA, MIT, Moscow’s Skolkovo Institute of Science and Technology and GFZ Potsdam (University of Potsdam) combined geophysical models of particle radiation for a solar cycle with models for how radiation would affect both human passengers — including its varying effects on different bodily organs — and a spacecraft. The modeling determined that having a spacecraft’s shell built out of a relatively thick material — aluminum measuring 30 g/cm2 — could help protect astronauts from radiation.
In a short Q&A with Tech Briefs below, Shprits explains more about the spacecraft material, and other ways to protect Mars-bound astronauts from radiation.
Tech Briefs: What has been the assumption (before your study) about the radiation on Mars, specifically in its impact on astronauts, and did your study change any of these assumptions?
Yuri Shprits: Often past studies looked separately at the effects of galactic cosmic rays and the effects of solar energetic particles.
Three were separate studies focusing on modeling the evolution of radiation, biological effects, and models of how radiation propagates through spacecraft but the combination of all these studies that would answer whether such mission is possible, how long should be such mission, and what should be the optimal shielding was missing.
We combined a realistic model of GCRs with actual observations of SEPs and performed calculations for a human phantom in a simplified spacecraft for different launch times, different durations of the mission, and different thickness of spacecraft. That all allowed us to combine all this knowledge and make realistic predictions for the duration of the mission.
Tech Briefs: What data sources are you pulling from to achieve a kind of simulation of the Mars environment, so that you can draw these conclusions?
Yuri Shprits: We use models of the GCRs based on long term historical and ground observations, and also historical observations of SEP from space.
Tech Briefs: What did you want to use this model to figure out exactly?
Yuri Shprits: Our goal was to make an estimate of how long approximately such a trip may be. There are still lots of uncertainties.
The biological effects of different types of radiation are not fully understood. The limit of 1Sv [units of sievert] may be revised in the future. At the same time the use of hydrogen-rich composite materials may provide a bit better shielding. While our calculations should not be interpreted as exact evaluation, they give a ballpark estimate and show that the mission would be difficult but viable.
Tech Briefs: What is the solar maximum, and why is it so important to launch a trip during that time window? Also, do we know exactly when this solar maximum will be?
Yuri Shprits: Usually, solar maximum is determined as a time when the highest number of sunspots is observed on the Sun during the 11-year solar cycle. During that time there is the highest number of coronal mass injections, and also the solar magnetic field is most distorted. It is difficult to know when the solar maximum would occur or if we are now at the maximum or not. However, it roughly repeats every 11 years, which could give a somewhat reasonable estimate.
Tech Briefs: Given the conclusions of your study, what should a spacecraft shell be made out of?
Yuri Shprits: We made estimates for aluminum. There are suggestions to use hydrogen compound materials. That would help to decrease the radiation and allow for a slight increase in the duration of the mission. To save on mass it's also possible to keep the fuel in the shielding of the spacecraft. It remains to be seen if such suggestions can be technologically possible.
In this initial study we considered only aluminum shielding which is the most commonly used material for the spacecraft. The estimate of the maximum mission duration should be approximately correct no matter what other materials one can use. Some adjustments can be made by using innovative solutions and that should be a subject of future research.
The main conclusion is that there is an optimal shielding, which for aluminum is 30 g/cm2. Thicker shielding would not help and would make things worse, as secondary radiation — particles produced from the interaction of the primary radiation with a spacecraft — would significantly increase. So, hydrogen-containing materials would help but would not dramatically change our conclusions.
At the end we gave an accurate ballpark estimate of maximum mission duration of 4 years. To reach this limit one would need to provide sufficient shielding to the spacecraft that would be comparable to the shielding on the International Space Station. If that's difficult, one would need to reduce the flight duration or fly faster, which requires more fuel.
Tech Briefs: Why 4 years? What starts to happen to human explorers after 4 years of exposure to the elements on Mars?
Yuri Shprits: At least currently the most accepted limit for the lifetime dose of radiation is 1 Sv. This limit will be reached in approximately 4 years no matter how well you shield the spacecraft.
Tech Briefs: Do the rewards of Mars exploration outweigh the potential risks from radiation?
Yuri Shprits: Well, that's more of a philosophical question. What we can say is that with using the state-of-the-art models and current knowledge of the radiation effects, it looks like a safe mission to Mars would be extremely difficult but still possible. Humans have always explored new places and pushed the frontiers of our reach further. I am sure we will do that in the future; that’s in our human nature.
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