Image: X-ray photo of the sun Aug. 1, 2010
NASA
NASA's Solar Dynamics Observatory snapped this X-ray photo of the sun early in the morning of Sunday, Aug. 1. The dark arc near the top right edge of the image is a filament of plasma blasting off the surface — part of the coronal mass ejection. The bright region is an unassociated solar flare.
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updated 8/6/2010 1:05:46 PM ET 2010-08-06T17:05:46

A solar flare from the sun this week poses no threat to astronauts on the International Space Station or plans for vital spacewalk repairs on the orbiting lab, NASA officials say.

The solar flare occurred Sunday, Aug. 1 and unleashed a huge plasma eruption known as a coronal mass ejection that sent charged particles streaking toward Earth at more than 2.2 million mph. [Video of the sun eruption.]

But those particles only created stunning aurora displays as they interacted with Earth's magnetic field. They posed no radiation threat to the six astronauts living aboard the space station, or their weekend spacewalk to begin replacing a faulty cooling system pump.

"There has been some solar activity in the last few days and the teams have been watching that closely," NASA spokesperson Kyle Herring said Wednesday during mission commentary. "There appear to be no effects with relation to radiation associated with the solar flare during the upcoming spacewalk."

Saturday's spacewalk will send NASA astronauts Douglas Wheelock and Tracy Caldwell Dyson outside the International Space Station in spacesuits to begin repairing the outpost's cooling system. An ammonia pump failed on July 31, knocking out half of the space station's U.S. cooling system and forcing astronauts to shut down many systems and leave others without backups to keep the space station overheating.

Eruption on the sun
The Aug. 1 sun eruption, while powerful, stemmed from a Class C3 solar flare and began striking Earth's magnetic field on Tuesday. It was not strong enough to do more than create bright auroras and spark a strong geomagnetic storm in Earth's magnetic field that lasted about 12 hours, NASA officials have said.

If a solar flare or eruption is strong enough, astronauts can seek shelter in a more shielded part of the International Space Station to wait out the sun storm, NASA spokesperson William Jeffs told SPACE.com. That wasn't the case for this week's solar weather events.

"There was no need to relocate the ISS crew during this recent event," Jeffs said in an e-mail.

Space radiation is a chief concern for astronauts in space because large doses can damage human tissue and cause sickness, increase susceptibility to cancer or in extreme cases lead to death.

The risk near Earth, however, is limited. Astronauts in low-Earth orbit do not receive the full brunt of space radiation because the space station flies within Earth's protective magnetic field, which extends farther out.

"Based on the NASA Space Radiation Programs analysis of events back to the 15th century, we estimate it would be almost impossible for a solar storm to occur that would be so large to lead to a recommendation to evacuate the ISS," Jeffs said.

Astronauts on a spacewalk, like the one planned for Saturday, can receive higher doses of space radiation, so they are timed to avoid high radiation events, NASA officials said.

Space radiation risk
Radiation exposure is measured in milliSeiverts. On Earth, the average human receives a radiation dose of about 2 mSv a year from background radiation. One mSv of space radiation is about the equivalent of receiving three chest X-rays, according to a NASA description.

Space station astronauts on six-month space missions typically receive about 80 mSv of space radiation during solar maximum about half the amount they receive when the sun's weather is at a solar minimum.

Jeffs said that most solar weather events tend to lead to lower space radiation exposure for astronauts because increased solar activity can result in lower doses of galactic cosmic rays.

Those stronger cosmic rays space radiation that does not come from the sun are a larger risk for station astronauts and future explorers to Mars or elsewhere beyond Earth orbit, he added.

NASA will broadcast the International Space Station spacewalk repairs live from space on NASA TV, with the first spacewalk slated to begin Saturday at 7 a.m. EDT.

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