Astronauts scampered to shielded areas of the international space station and space shuttle Discovery Tuesday night to protect themselves from possibly being exposed to high levels of radiation from an unusually large solar flare, NASA said.

Activity aboard Discovery and the space station was interrupted when the solar flare erupted late Tuesday, as two astronauts were finishing the first spacewalk of the current shuttle mission.

Space.com categorized it as an X-3 flare, in the most dangerous category. Such storms are fairly common when the Sun is at its most active, but they are rare during the current low point in the 11-year cycle of solar activity.

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NASA spokesman Bill Jeffs told FOXNews.com that crew members slept overnight in "heavily shielded areas" of their respective craft — such as airlocks and the Destiny science lab aboard the space station — as a precautionary measure.

"That move was made to avoid having to wake the crew during their sleep period," NASA spokesman John Ira Petty told Space.com. "It was never a danger to the crew."

Jeffs said the X-ray part of the solar flare, which travels at the speed of light, hit the conjoined space shuttle and space station at about 9:25 p.m. EST.

The more dangerous wave, consisting of slower-moving accelerated protons, hit at just about 10 p.m. EST, just before NASA astronaut Robert Curbeam and European Space Agency astronaut Christer Fuglesang re-entered the space station after installing a two-ton truss between solar panel arrays.

Jeffs said NASA estimated the astronauts were exposed to no more than an extra day in space's worth of radiation. Discovery is on a 12-day mission.

The solar flare was ejected from a large sunspot, numbered 930, that has been moving across the face of the Sun for several days. Last week it spewed out what astronomers call a "solar tsunami," enhancing auroral activity near Earth's poles.

NASA was keeping an eye on the sunspot, according to Jeffs, and was considering rescheduling the two remaining mission spacewalks, currently set for Thursday and Saturday, so that both would take place when the international space station was on the other side of the Earth from the Sun.

Later Wednesday, the astronauts began retracting via remote control a 115-foot solar panel on the space station, likening the tricky task to folding a road map back up and stuffing it in the glove compartment.

The electricity-generating solar array served as a temporary power source aboard the orbiting outpost.

NASA needed to move it out of the way so that a new, permanent pair of solar wings could rotate in the direction of the sun.

The folding-up began shortly before 1:30 p.m. EST and was expected to take about five hours.

A crease developed when the array was about a quarter of the way retracted, forcing controllers and astronauts to stop the folding for about an hour.

They eventually decided to release the array slightly, which fixed the crease, astronaut Sunita "Suni" Williams reported from the space station.

"That's good news. You've got a bunch of guys about to turn blue here," Mission Control radioed back, echoing words said by a flight controller after Neil Armstrong and Buzz Aldrin landed on the moon in 1969.

Because it had been six years since the array was last folded up, flight controllers and astronauts were not sure how easy it would be.

"It's kind of like folding a map up," space shuttle Discovery commander Mark Polansky radioed Mission Control after the crease appeared. "You start folding it and the folding goes the wrong way. ... There's nothing you can do to it other than pop it back in place or unfold it and try again."

NASA needed the old solar array to be retracted at least 40 percent to provide enough clearance for a pair of giant solar wings that were delivered by space shuttle Atlantis in September. The old array will be moved to another spot during a later shuttle mission.

The space agency hoped to fit the old array into a 21-inch-high box. If it didn't fold up properly, NASA had the option of using spacewalkers to manually retract it at another time.

Flight controllers also watched to see whether the silicone coating on the 32,800 solar cells flaked off as the array was folded up. It would look like a "small, little snowstorm" but would be no reason for concern, said Joel Montalbano, a space station flight director.

During the two remaining scheduled spacewalks, astronauts will rewire connectors from the old solar array to the new solar wings.

Reconfiguring the power system will enable the station to provide electricity to laboratories that will be added to the structure over the next few years.

The Associated Press contributed to this report.