A wind of charged particles that stream constantly from the sun is at its lowest level ever recorded in the 50 years since spacecraft have made the measurement possible.
The Ulysses spacecraft observed the weak solar winds, the constant, high-speed stream of particles that races from the sun, during a quiet period in the sun's activity. The solar weather cycle affects Earth and other planets in the solar system.
"We know that the sun has been this cool before, this inactive before," said Nancy Crooker, a physicist at Boston University in Boston, Mass., during a NASA teleconference on Tuesday. "But that was prior to the Space Age, so we didn't have actual physical measurements until now."
The solar wind's charged particles blow out from the sun at a blistering 1 million mph, sweeping away background radiation and colliding with incoming galactic cosmic rays from distant stars. It effectively encloses our solar system in a protective bubble called the heliosphere.
For Ulysses, the finding is a swan song. The probe, which is dying, has spent the past 17 years watching the solar wind rise and fall during the sun's 11-year cycle of activity. Fast and steady solar winds came from the upper latitudes, while more unpredictable, slower solar winds blew from the sun's equator.
But during the sun's latest quiet period, the spacecraft found that the overall solar wind is 20 to 25 percent weaker, in terms of pressure and density, than during the previous solar minimum. Weaker solar winds mean a smaller and leakier heliosphere bubble, a protective sheath that surrounds the entire solar system. That means more background cosmic radiation gets through.
The danger to Earth from galactic cosmic rays remains rare in any case. However, future missions to the moon or Mars would probably try to plan the best times to travel outside of Earth's own protective magnetosphere bubble. Astronauts on the International Space Station and the space shuttle remain safely within Earth's magnetosphere.
When the sun is active, charged solar particles present risks to astronauts and even satellites.
"During a solar minimum there's relatively little solar radiation, so the primary risk is more from galactic cosmic rays," said Dave McComas, a Ulysses scientist at the Southwest Research Institute in San Antonion, Texas.
Current studies suggest that astronauts should undertake long-duration missions during solar maximums, when increased solar activity still poses a risk but also shields better against the constant threat of galactic cosmic rays, McComas added.
The solar wind's effects are also felt at the edge of the solar system, where the Voyager 1 and Voyager 2 probes have entered the heliosphere's outer envelope layer. Voyager 2 arrived at the solar system edge later than Voyager 1, yet found that that boundary to be almost a billion miles closer ? a possible sign of the shrinking protective bubble.
The wanderings of Ulysses are drawing to a close with its latest observations of the sun. The spacecraft can only look forward to an icy space tomb once its radioisotope thermoelectric generators fail to provide enough heat to keep the onboard hydrazine fuel from freezing.
However, scientists expressed nothing but satisfaction with the joint NASA-European Space Agency mission, which has lasted four times longer than the spacecraft's expected lifetime.
"For the first time we've got a spacecraft that flies over the poles of the sun," said Ed Smith, a Ulysses scientist at the Jet Propulsion Laboratory in Pasadena, Calif. "That allows us to see changes in the heliosphere in three dimensions, or four if you count time."
At least one lingering question still stands out whether the current weak solar winds represent an isolated phase or a longer term trend in future solar cycles.
- Video ? Ulysses RIP
- Video Player: Sun Storms
- The Best Images of the Sun