Sending human travelers to Mars would require scientists and engineers to overcome a range of technological and safety obstacles. One of them is the serious risk posed by particle radiation（辐射） from the sun, distant stars and galaxies（星系）.
Answering two key questions would go a long way towards overcoming that difficulty: Would particle radiation pose too serious a threat to human life throughout a round trip to the red planet? And, could the very timing of a mission to Mars help shield astronauts and the spacecraft from the radiation?
In a new article published in the journal Space Weather, an international team of space scientists, including researchers from UCLA, answers those two questions with a "no" and a "yes". That is, humans should be able to safely travel to and from Mars, only if the spacecraft has enough protection and the round trip is shorter than approximately four years. And the timing of a human mission to Mars would indeed make a difference: The scientists determined that the best time for a flight to leave Earth would be when solar activity is at its peak（高峰期）, because, during this period, the most dangerous and energetic particles from distant galaxies are deflected（偏转） by the enhanced solar activity.
A trip of that length would be imaginable. The average flight to Mars takes about nine months, so depending on the timing of launch and available fuel, it is reasonable that a human mission could reach the planet and return to Earth in less than two years, according to Yuri Shprits, a UCLA research geophysicist and co-author of the paper.
"This study shows that while space radiation impose（强加） 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 possible," said Shprits, who also is head of space physics and space weather at GFZ Research Centre for Geosciences in Potsdam, Germany.
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