A large electromagnetic "storm" has broken out on the face of the sun, sending masses of charged subatomic particles through the space that surrounds planet Earth.
Images from the SOHO space probe -- short for Solar and Heliospheric Observatory -- showed a bright flare near the sun's equator on Wednesday, and another was reported by ground-based observatories today.
Several of SOHO's sensors were temporarily overwhelmed by the amount of radiation, engineers said.
Such flares, known as coronal mass ejections, are actually fairly common, scientists say. Earth is well protected by both its atmosphere and its magnetic field.
The 10 least protected human beings are those currently in orbit -- the crews of the Space Shuttle Discovery and the International Space Station.
The flare had one surprising effect: It temporarily knocked out the gyroscopes that keep the space station oriented in orbit.
Apparently, NASA said, the energy from the flare caused the outer layers of Earth's atmosphere to thicken slightly.
Even at the station's altitude of 220 miles, there's a tiny bit of air -- and there was more on Thursday.
The gyroscopes had been turned off this week, while astronauts Robert Curbeam and Christer Fuglesang did two spacewalks to update the station's wiring.
When the gyros were turned back on, onboard computers sensed that they were working harder than they ought to -- and shut the gyros off again.
NASA said the astronauts were in no danger. The shuttle's steering jets will keep the docked ships stable until the gyro problem is solved.
Overnight the astronauts were instructed to sleep in sections of their ships that were the best shielded from radiation. NASA said the shuttle mission schedule would not be changed because of the solar storm.
Earth on the Lookout
Here on Earth, the most visible effect of the flare was the aurora borealis -- the so-called Northern Lights, often seen over Alaska and Canada at night.
Late Thursday and early today, aurora displays were reported as far south as Michigan and Minnesota.
Many technology companies were also on alert.
In an increasingly electronic age, solar storms sometimes have surprising effects.
In 1998, a communications satellite broke down because of the radiation striking it, cutting off pager signals, long-distance calls, and some radio transmissions.
In 1989, there was a blackout in Quebec, and scientists decided afterward that it was the result of a power surge directly related to a solar flare.
Long-haul airliners on routes over the Arctic are sometimes diverted south, because Earth's magnetism offers less protection near the poles.
"It is a rare occurrence to have a strong event like this so late in the solar cycle," said Larry Combs, a forecaster at NOAA's Space Environment Center in Boulder, Colo.
The frequency of solar flares fluctuates in an 11-year pattern, and the last peak was in 2002.
But NOAA said the magnetic pulse was so far not of a type that was likely to cause major problems.
On a scale used by scientists, this storm was a "G2," meaning it had moderate strength.
"So far, yes, there's been a magnetic disturbance," said Joe Kunches, chief of the Forecast and Analysis Branch at the Space Environment Center. "And yes, it's coming from the sun, but the reality is that it's been less than severe -- so far."
Kunches' staff reported that more radiation was likely to pass Earth over the weekend.