Voyager Probes Send Surprises From Solar System's Edge

Richard A. Lovett
for National Geographic News
September 26, 2006

NASA's two Voyager spacecraft are sending back reports of magnetic potholes, sluggish solar wind, and unusual cosmic rays from the edge of the solar system, according to a leader of the Voyager mission.

Voyager 1 was the first of the twin probes launched for Jupiter, Saturn, and beyond in 1977. It is now 10 billion miles (16 billion kilometers) from Earth, at the outer edge of the solar wind's influence, in a region called the heliosheath.

There the solar wind streaming out from our sun begins to interact with particles of what can be thought of as the interstellar wind.

Nothing like the wind we know on Earth, the solar wind is made up of charged particles—a type of plasma.

Voyager 1 has been in the heliosheath since at least December 2004. (See "Voyager 1 at Solar System Edge, Scientists Now Agree" [June 2, 2005].)

The spacecraft is sending back a few surprises.

The first is that the solar wind doesn't simply slow down when it crosses into the heliosheath—it practically stops.

Closer to the sun, the solar wind moves at about 200 to 250 miles a second (300 to 400 kilometers a second).

"We expected the wind to slow to 100 kilometers [62 miles] per second or so," said Ed Stone, a physicist at the California Institute of Technology in Pasadena and former director of NASA's Jet Propulsion Laboratory.

"It turned out to be a lot slower than that," he added.

"Initially, in fact, it was only 17 kilometers per second [11 miles a second]."

Most likely, Stone says, that's because the sun is going though a natural cycle in which the solar wind is decreasing in strength.

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