In 1859 the Sun erupted, and on Earth wires shot off sparks that shocked telegraph operators and set their paper on fire.
It was the biggest geomagnetic storm in recorded history. The Sun hurled billions of tons of electrons and protons whizzing toward Earth, and when those particles slammed into the planet's magnetic field they created spectacular auroras of red, green and purple in the night skies -- along with powerful currents of electricity that flowed out of the ground into the wires, overloading the circuits.
If such a storm struck in the 21st century, much more than paper and wires would be at risk. Some telecommunications satellites high above Earth would be disabled. GPS signals would be scrambled. And the surge of electricity from the ground would threaten electrical grids, perhaps plunging a continent or two into darkness.
Scientists say it is impossible to predict when the next monster solar storm will erupt -- and equally important, whether Earth will lie in its path. What they do know is that with more sunspots come more storms, and this fall the Sun is set to reach the crest of its 11-year sunspot cycle.
Sunspots are regions of turbulent magnetic fields where solar flares originate. Their ebb and flow have been observed for centuries, but only in the past few decades have solar scientists figured out that magnetic fields within the spots can unleash the bright bursts of light called solar flares and the giant eruptions of charged particles known as coronal mass ejections.
Experts are divided on the earthly consequences of a cataclysmic solar eruption, known as a Carrington event, for the British amateur astronomer who documented the 1859 storm.
A continentwide blackout would affect many millions of people, "but it's manageable," said John Moura of the North American Electric Reliability Corporation, a nonprofit group founded by utilities to help manage the power grid. Most of the grid could be brought back online within a week or so, he said.
Others are more pessimistic. They worry that a huge and well-aimed eruption from the Sun would cause not only the lights to go out, but would also damage transformers and other critical components of the grid.
Some places could be without power for months, and "chronic shortages for multiple years are possible," according to the National Research Council, the research arm of the National Academy of Sciences.
Still, this sunspot cycle has been quieter than most. And even if the Sun unleashes a huge burst, as it did last July, the odds are that it will head harmlessly in some other direction into the solar system. Only rarely does a giant solar blast fly directly at Earth.
Yet just as a hurricane-fueled surge hitting New York City at high tide during a full moon is rare, rare is not impossible.
"There's always the chance of a big storm, and the potential consequences of a big storm has everyone on the edge of their seats," said William Murtagh, program coordinator for the Space Weather Prediction Center, part of the National Oceanic and Atmospheric Administration.
A Huge Power Trip
The most studied, unambiguous example of the Sun's ability to snarl power grids occurred on March 13, 1989, in Quebec. In the early-morning hours, a solar storm generated currents in the transmission wires, tripping circuit breakers. Within minutes, a blackout stretched across the province, shutting down businesses, schools, airports and subways until power was restored later that day.
Canada was hit again a few months later, when another solar storm was blamed for computers shutting down at the Toronto Stock Exchange, halting trading.
Mr. Moura's organization put out a report last year saying that utilities would have enough warning to disconnect the grid and protect the transformers; a follow-up task force is taking a closer look to determine how vulnerable the transformers might be.
"There's a sense in the field that we don't have all the answers," said Antti Pulkkinen, a scientist at the NASA Goddard Space Flight Center in Greenbelt, Md.
The dangers will not go away after the so-called solar maximum -- the period of heaviest solar weather -- has passed. Even when quiet, with few sunspots, the Sun can still produce a giant eruption.
Solar flares, traveling at the speed of light, arrive at Earth in less than 8.5 minutes and can drown out some radio communications. But it is the coronal mass ejections -- in which billions of tons of electrons and protons are disgorged from the Sun and accelerate to more than a million miles per hour -- that cause more worry.
The particles, which generally take two or three days to travel the 93 million miles from the Sun to Earth, never hit the surface; the planet's magnetic field pushes them aside.
But then they are trapped in the field. The back-and-forth sloshing generates new magnetic fields, mostly over the night side, and they, in turn, induce electrical currents in the ground. Those currents surge out of the ground and into the electrical transmission lines.
"In a sense, we're playing Russian roulette with the Sun," said John Kappenman, an electrical engineer who owns Storm Analysis Consultants and who has been warning of potential catastrophe.
This solar cycle has so far defied easy understanding. It started late -- so late that some speculated that it was the beginning of an extended quiet period, like in the mid-1600s when almost no spots blemished the Sun for decades. It has been quieter than many experts expected, and so far it appears to have peaked early.
The two hemispheres of the Sun are out of sync. The northern hemisphere has been ahead of the curve, producing a large number of sunspots in late 2011 and has quieted since then, while the southern hemisphere has remained fairly quiet throughout.
Most solar scientists expect the southern hemisphere to perk up, and the number of sunspots to increase again, with the solar maximum arriving in the fall. Such double-peak patterns have appeared in some earlier solar cycles, including the last one.
"I believe I can say with strong confidence there will be a second peak in 2013," said Douglas Biesecker, a physicist at the Space Weather Prediction Center and the chairman of a panel that issued predictions about the solar cycle.
"Will the second peak be smaller or larger than the first peak?" he continued. "We're still in a waiting game with the southern hemisphere. The northern hemisphere has mostly played out."
If the second peak never occurs and it turns out that solar maximum has already come and gone, "then I would argue it would be very fair to call it an unusual cycle," Dr. Biesecker said.
Even with a quieter-than-average solar maximum, the Sun is still shooting off, on average, a few coronal mass ejections a day, including one on Friday that made a direct hit on Earth on Sunday, generating picturesque nighttime aurorae as far south as Colorado but causing no noticeable harm. In the past year, about 20 -- all minor or modest -- have reached Earth.
The Sun's huge eruption in July 2012 was aimed the wrong way, luckily for Earth, but it did cross one of NASA's Sun-watching craft, known as Stereo. The data from Stereo will help computer models predict what might happen to the power grid.
On the morning of Sept. 1, 1859, the British amateur astronomer Richard C. Carrington was sketching a large group of sunspots when he saw a blinding white flash engulf them: a solar flare. The magnetic currents that generated the flare also set off a coronal mass ejection, and when the particles arrived at Earth fewer than 18 hours later, they created an electrical current that overwhelmed telegraph circuits.
A telegraph operator in Washington reported that his forehead grazed a ground wire and "immediately, I received a very severe electric shock," and "an old man who was sitting facing me, and but a few feet distant, said that he saw a spark of fire jump from my forehead to the sounder."
Given that there has been no Carrington event since, scientists know such disruptions are rare. But they also know that it was not the only such storm to hit Earth in the planet's 4.5-billion-year history. Carrington-size solar storms "are 100 percent guaranteed to occur again," Mr. Kappenman said.
And when it happens, he continued, transformers and other key components of the electrical grid will suffer severe damage. These big transformers are expensive, and electric companies do not have many spares lying around. Some places could be powerless for months, he said, adding, "Think of Sandy magnified by a hundredfold."
In November, the federal agency overseeing the power grid proposed requiring electric companies to install devices that would block currents flowing from the ground and to take other measures to protect equipment. Industry groups have objected, arguing that existing systems would take the grid down automatically before transformers would be damaged.
NASA's Sun-watching spacecraft keep track of the sunspots, and they can provide some warning of which regions look likely to erupt.
While the craft can tell how large an eruption is, they cannot do one important measurement: which way the magnetic field is pointing within the swarm of particles. If the field is pointing north, Earth's magnetic field can absorb the shock fairly well. But if the field is facing south, in the opposite direction from Earth's field, the magnetic fields essentially snap and reconnect -- magnetic "short circuits" that release huge bursts of energy.
NASA does have one satellite, the Advanced Composition Explorer, or Ace, that can tell which way the field is pointing. But Ace is just 900,000 miles from Earth, at a spot where the gravitational pull between the Sun and Earth cancel. When it makes that crucial measurement, a giant, fast-moving coronal mass ejection could be just 10 minutes away. Utility companies would have to quickly make their final decisions -- and perhaps deliberately cause a continentwide blackout -- in order to protect the electrical grid from greater damage.
As scientists learn more about the Sun, they learn that a Carrington-size coronal mass ejection may not be a very rare event -- just rare that it hits Earth. As long as we remain lucky.
This article originally appeared in The New York Times.