Being pelted with high amounts of radiation can lead to radiation sickness - a loss of appetite, vomiting and diarrhea. Higher doses of radiation can damage to cells and tissue, leading to cancer or cataracts.

If the sun is acting up before the scheduled spacewalks, astronauts Scott Parazynski and Chris Hadfield would be required to stay inside the shuttle until the danger has passed.

NASA officials are monitoring solar activity.

"(Flight surgeons) will tell us when we have a high-radiation environment that we will not be able to send a crew out on," said Jeff Patrick, the mission's top spacewalk official.

As it has in the past, NASA could delay or cut short a spacewalk if necessary, said Jeff Jones, medical operations lead flight surgeon for the station.

Some of the sun's larger sunspots unleashed several violent storms in the past weeks.

Sunspot region 9393, which is 13 times the size of Earth, released possibly the biggest solar flare on record April 2. Most of it was not directed toward Earth. The event produced aurora borealis, or Northern Lights, as far south as Mexico, NASA reported.

Another set of sunspots, in region number 9415, is five times the size of Earth and has been very active the last few weeks:

It released a massive solar flare April 3. No geomagnetic storms were reported.

The same set of sunspots released another more massive flare April 6, causing a strong geomagnetic storm when particles slammed into Earth's magnetosphere.

A medium-sized flare from the sunspot sent radiation spewing toward Earth on Monday.

Two severe solar flares Tuesday and Thursday each caused hour-long radio blackouts across much of the Eastern hemisphere.

The region is moving to the sun's far side, so if it provides more explosions, they likely won't be directed toward Earth and its orbiting astronauts, officials said.

Space station Alpha has radiation monitors posted throughout the station, and if levels start to climb, an alarm goes off.

Former Alpha residents Bill Shepherd, Yuri Gidzenko and Sergei Krikalev absorbed about 30 to 40 millirems of radiation a day. That's similar to receiving three or four chest X-rays every day, said Gautam Badhwar, head scientist on the Phantom Torso radiation experiment being sent up on the Endeavour mission.

The crew received a larger than normal dose in November after a major solar flare. During the dangerous parts of their orbits, they took shelter in the rear section of the Russian service module.

"There should be really no impact for the length of missions we are talking about - 90 to 120 days," Badhwar said. "The thing we have to watch is these solar flares and coronal mass ejections from the sun."

Still, the astronauts take precautions. Their sleeping areas are protected with a special material made of high-density polyethylene to help reduce their radiation exposure at night, Jones said.

Since astronaut Susan Helms does not a have a permanent sleep station, she has had to surround herself in the laboratory with a temporary shield and extra bags of water, which provides good protection since it is hydrogen rich.

For astronauts on their way to Mars, solar radiation becomes a bigger concern.

They no longer will be protected by the magnetosphere, the magnetic field surrounding Earth that deflects a lot of the harmful material.

Badhwar has an experiment on the recently launched Odyssey spacecraft that will take measurements of solar radiation en route to Mars and then orbiting the Red Planet. The instrument is to be activated April 23.

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