The sun's solar winds which shield the earth against cosmic rays outside the solar system are at a record low.  That's according to data gathered by the Ulysses spacecraft, a space probe jointly operated by the United States and the European Space Agency.  As VOA's Jessica Berman reports, experts say the reduced solar winds could affect human activity.

For the past 18 years, the Ulysses spacecraft has made nearly three orbits around the sun's northern and southern poles, taking measurements of its solar winds, a stream of charged particles ejected from the sun's upper atmosphere.  

The solar winds blow a protective bubble, or heliosphere, around the solar system that for the most part protects planets, including earth, from harmful cosmic rays from outside the galaxy.  

But officials say the latest data, gathered in 2007, show the solar winds are at their lowest levels of activity in a half century.

Dave McComas, Ulysses solar wind principal investigator in San Antonio, Texas, likens the solar winds to air that are pumped into a tire.

"There's a million mile per hour wind blowing out from the sun in all directions in space all time, and it pushes outwards, and it pushes the material from the local part of the galaxy outwards, and it inflates a kind of bubble in space, sort this kind of tire analogy," said Dave McComas. "And the harder it blows the bigger this bubble is and the less hard it blows the small the bubble is."

Scientists found solar wind activity decreased by 20 percent compared to measurements taken by Ulysses during the last solar cycle.

Scientists say a shrunken heliosphere could make space travel a little riskier.  They say there could be an increase in the intensity of cosmic ray bursts, rare explosions of high-energy radiation of unknown origin, that penetrate  the solar bubble.  

Nancy Crooker a research professor at Boston University in Massachusetts.

"And the second effect of reduced of solar activity is that it leads to the cooling of earth's upper atmosphere," said Nancy Crooker. "And if earth's upper atmosphere is cooler, then there's less drag on satellites up there, and this means we are left with more debris up there, which is also something astronauts have to look out for."

Scientists say the reduction in solar wind activity does not influence global warming, which is a different phenomenon.

Ulysses was launched in 1990 aboard the space shuttle Discovery and the mission is soon set to expire.  

A space probe, dubbed the Interstellar Boundary Explorer, is due to be launched next month that will explore the outer boundaries of the solar system's interactions with weaker solar winds.