A severe solar radiation storm this week prompted NASA to order astronauts aboard the International Space Station to take shelter in a more heavily shielded Russian-built section of the orbiting outpost.
The three crewmen remained in the aft end of the Zvezda module for about 12 hours before the warning was lifted.
During that time, Russian scientists estimated, the crew received the equivalent of a week's normal radiation exposure.
"It wasn't life-threatening, and they weren't projected to receive any exposures that would put them even close to their limits," said Michael Golightly, chief of space science at the Johnson Space Center in Houston. "But we don't like them to get any extra exposure if we can help it."
Major proton event
The instructions to take shelter followed the eruption Wednesday evening of solar flare activity described as the fourth-largest solar radiation storm since 1976.
Rated an S-4 event by the federal Space Environment Center, in Boulder, Colo., the solar flare launched a torrent of high-energy protons toward Earth at rates 100,000 times greater than normal.
"This is a very large proton event," said Terrance Onsager, a plasma physicist at the SEC.
An S-4 radiation event is defined as posing unavoidable radiation hazards to astronauts caught outside their spacecraft on spacewalks.
Passengers and crew on commercial airliners flying over the arctic can receive exposures equivalent to 10 chest X-rays.
The radiation can also disrupt computer memories, star-tracking devices and solar panels on satellites, and black out high-frequency radio communications in polar regions.
An average of three such events are expected during each of the sun's 11-year cycles of activity. The current solar cycle is at its peak this year and next.
Chris Balch, a duty forecaster at the Space Environment Center yesterday, said this week's radiation storm began at 6:55 p.m. Wednesday.
The flow, or "flux" of high-energy protons - powerful enough to penetrate spacecraft - quickly began to rise.
'Really got our attention'
It peaked just before midnight, measured at 347 particle flux units per second, per square centimeter.
"That's a very unusual event," Balch said. "The threshold when we wake people up about this is at one flux unit. It's rare that we even go above 100. "That's why it really got our attention."
Such solar radiation storms do not pose a health risk to people on the ground, where Earth's magnetic field and its atmosphere provide an ample shield.
The magnetic field also protects manned spacecraft in low Earth orbits, except when their orbits carry them near Earth's magnetic poles.
In those regions - over Canada and the ocean southwest of Australia - the solar radiation is captured and funneled close to the surface.
NASA officials calculated that the International Space Station's orbits would carry it into the danger zones during seven or eight orbits between 5 a.m. and 5 p.m. Thursday.
As a precaution, U.S. and Rus- sian flight controllers told station Commander Bill Shepherd, and his Russian crewmates Yuri Gidzenko and Sergei Krikalev, to set up radiation detection monitors in the station's Zvezda living quarters.
The devices would sound an alarm if radiation reached higher-than-expected levels.
In addition, NASA flight surgeons asked the crew to spend their workday in the aft section of the Zvezda module.
"They didn't have to stay there the whole time," Golightly said. There were two 15- or 20-minute windows during each 90-minute orbit when they were free to move about the cabin.
He said Zvezda is built heavily enough to reduce the crew's radiation exposure by 60 percent. NASA is planning to provide station crews with additional radiation shielding in the U.S. Habitation module, scheduled for launch in 2005.
By late Thursday, the space station's orbits had again moved out of the danger zones.
By midday yesterday, the flow of the most worrisome high-energy protons had subsided to one or two flux units per second, Balch said, and warnings were expected to be canceled.
The flow of less-energetic protons remained 30 to 40 times higher than the level at which NASA is normally alerted - high enough to cancel spacewalks had any been planned, Balch said.
Warnings for these were to remain in effect until Monday.