Two years ago this week the Sun let off one of its periodic solar flares, and a new analysis of its force shows that human civilization had a very near miss indeed.
"If it had hit, we would still be picking up the pieces," said Daniel Baker of the University of Colorado this week.
On 23 July 2012, two coronal mass ejections (CME) burst out of the Sun's surface within 15 minutes of each other and headed out into space at more than 3,000km per second. If they had erupted nine days earlier Earth would have been directly in its path. Instead, NASA's Solar Terrestrial Relations Observatory (STEREO) satellite was perfectly positioned to record the blast.
For a paper [PDF] in the journal Space Weather, scientists analyzed the data from STEREO and found that the CMEs were the largest yet measured – and could even have exceeded the notorious 1859 Carrington event. Had they hit us, the resulting electromagnetic disturbance could have taken out most of the GPS network, communications satellites, electrical grids and some servers.
The Carrington Event, named after the British astronomer who spotted the CME, hit Earth in August 1859, back when electrical systems were in their infancy. It burnt out telegraph systems across Europe and the US - setting fire to some buildings - and extended the aurora borealis as far south as Cuba.
"In my view, the July 2012 storm was in all respects at least as strong as the 1859 Carrington event," said Baker. "The only difference is, it missed."
These days we are much more dependent on electronics that would be harmed by CMEs of this type. According to a study by the National Academy of Sciences, the total economic impact could exceed $2 trillion if a similar solar storm hit us today and the ensuing chaos would cost millions of lives as electrical grids failed.
"We believe that the 23 July 2012 solar storm was a 'shot across the bow' for policy makers and space weather professionals," the paper's authors conclude.
"The event gave a very clear and credible representation of how severe the solar space weather drivers can be; it gave direct observational values for solar wind forcing parameters with in situ STEREO data; and it did this when the Earth, other planets, and most space hardware were in a completely different sector of the heliosphere, so most assets were out of harm's way."
Of course, had NASA not bothered to place solar listening stations around the Sun, we never would have known quite how serious the CMEs were. But we may not be so lucky next time, and research published in February by physicist Pete Riley shows solar storms of this magnitude are more common than first thought.
Riley went through the last 50 years of solar data and calculated that the chances of a Carrington-class storm hitting Earth over a decade were 12 per cent.
"Initially, I was quite surprised that the odds were so high, but the statistics appear to be correct," said Riley. "It is a sobering figure." ®