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Last month (April 8-11), scientists, government officials, emergency planners and others converged on Boulder, Colorado, for NOAA's Space Weather Workshop—an annual gathering to discuss the perils and probabilities of solar storms.

The current solar cycle is weaker than usual, so you might expect a correspondingly low-key meeting. On the contrary, the halls and meeting rooms were abuzz with excitement about an intense solar storm that narrowly missed Earth.

"If it had hit, we would still be picking up the pieces," says Daniel Baker of the University of Colorado, who presented a talk entitled The Major Solar Eruptive Event in July 2012: Defining Extreme Space Weather Scenarios.

The close shave happened almost two years ago. On July 23, 2012, a plasma cloud or "CME" rocketed away from the sun as fast as 3000 km/s, more than four times faster than a typical eruption. The storm tore through Earth orbit, but fortunately Earth wasn't there. Instead it hit the STEREO-A spacecraft. Researchers have been analyzing the data ever since, and they have concluded that the storm was one of the strongest in recorded history. "It might have been stronger than the Carrington Event itself," says Baker.

The Carrington Event of Sept. 1859 was a series of powerful CMEs that hit Earth head-on, sparking Northern Lights as far south as Tahiti. Intense geomagnetic storms caused global telegraph lines to spark, setting fire to some telegraph offices and disabling the 'Victorian Internet." A similar storm today could have a catastrophic effect on modern power grids and telecommunication networks. According to a study by the National Academy of Sciences, the total economic impact could exceed $2 trillion or 20 times greater than the costs of a Hurricane Katrina. Multi-ton transformers fried by such a storm could take years to repair and impact national security.

A recent paper in Nature Communications authored by UC Berkeley space physicist Janet G. Luhmann and former postdoc Ying D. Liu describes what gave the July 2012 storm Carrington-like potency. For one thing, the CME was actually two CMEs separated by only 10 to 15 minutes. This double storm cloud traveled through a region of space that had been cleared out by another CME only four days earlier. As a result, the CMEs were not decelerated as much as usual by their transit through the interplanetary medium.

Had the eruption occurred just one week earlier, the blast site would have been facing Earth, rather than off to the side, so it was a relatively narrow escape.

"In my view the July 2012 storm was in all respects at least as strong as the 1859 Carrington event," says Baker. "The only difference is, it missed."

In February 2014, physicist Pete Riley of Predictive Science Inc. published a paper in Space Weather entitled "On the probability of occurrence of extreme space weather events." In it, he analyzed records of solar storms going back 50+ years. By extrapolating the frequency of ordinary storms to the extreme, he calculated the odds that a Carrington-class storm would hit Earth in the next ten years.

The answer: 12%.

"Initially, I was quite surprised that the odds were so high, but the statistics appear to be correct," says Riley. "It is a sobering figure."

In their Dec. 2013 paper, Baker et al. estimated Dst for the July 2012 storm. "If that CME had hit Earth, the resulting geomagnetic storm would have registered a Dst of -1200, comparable to the Carrington Event and twice as bad as the March 1989 Quebec blackout."

The reason researchers know so much about the July 2012 storm is because, out of all the spacecraft in the solar system it could have hit, it did hit a solar observatory. STEREO-A is almost ideally equipped to measure the parameters of such an event.

"The rich data set obtained by STEREO far exceeded the relatively meagre observations that Carrington was able to make in the 19th century," notes Riley. "Thanks to STEREO-A we know a lot of about the magnetic structure of the CME, the kind of shock waves and energetic particles it produced, and perhaps most importantly of all, the number of CMEs that preceded it."