Solar storms that can injure or kill astronauts are more likely to occur at certain times of the solar cycle, a fact that should be considered during ongoing planning for humans to return to the moon, say researchers.
The sun goes through a cycle that lasts approximately 11 years during which there is a spike and then lull in solar activity. Previously, it was thought that the largest solar storms occur randomly and were unconnected to this cycle, says Mathew Owens at the University of Reading, UK.
This is largely because of a lack of data. We have 150 years of daily geomagnetic readings, which measure how much Earth’s magnetic field is disturbed by solar storms, and only six major solar storms have occurred in that time.
But statistical analysis techniques called the Monte Carlo method have now shed light on the matter. Owens and his colleagues created one simulation of the sun where extreme weather occurs randomly and another where it is more likely to occur at the peak of the solar cycle.
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They sampled data from each model hundreds of thousands of times and checked to see how often the results correlated with the small amount of data that we do have. The team was able to work out with 99 per cent confidence that these solar storms were more likely to occur at the peak of the solar cycle.
Furthermore, the analysis suggests that extreme space weather events are more likely to occur early in even-numbered solar cycles and late in odd-numbered ones, such as cycle 25 which began in December 2019.
With all this in mind, Owens says that the space weather conditions are likely to be better in the first half of this decade than the second half for anyone planning a crewed mission to the moon.
“These big events can always surprise us, they’re rare occurrences that can crop up whenever, but they’re more likely at some times than others,” he says. “If you want to protect the health of your astronauts, there are definitely better times to go and worse times to go. There are stormy seasons and quiet seasons, but you don’t know on a particular day whether you’re going to get a storm or not.”
Astronauts have had narrow escapes from this problem in the past. Between the Apollo 16 and 17 crewed missions to the moon in 1972, there was a large solar storm. It had the potential to kill any astronaut completing a spacewalk at the time, and also to increase the risk of cancer for any astronaut even inside a craft.
The findings could have implications for the NASA-led Artemis programme that plans to return humans to the moon in 2024. If, as some suggest, it is delayed by several years, then the risk of extreme solar activity could be at its peak when the mission does take place.
The discovery could also help to plan protection against solar flares on Earth, which can disrupt electrical grids and satellite communications including GPS.
Journal reference: Solar Physics, DOI: 10.1007/s11207-021-01831-3