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.

Speed Bumps

Voyager 1 has also found oddities in the heliosheath's magnetic field.

"There are 'potholes' where the magnetic field almost goes away, and 'speed bumps' where it's stronger," Stone said.

Nobody knows why. "This kind of variation is not found in the solar wind itself," he said.

Leonard Burlaga of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said, "The most recent surprise is that at times, trains of closely spaced and overlapping magnetic holes or magnetic humps are observed for days at a time, indicating a very 'rough road.'

"Neither the physical nature nor the 3-D geometry of these features is understood," Burlaga said by email.

That's not to say the heliosheath's "rough road" is actually shaking the probe.

"Only our sensitive instruments know that this is happening," Stone says.

Cosmic Rays

Another surprise is that the termination shock (the boundary between the solar system proper and the heliosheath) doesn't appear to be a great source of cosmic rays—which contradicts 30 years of scientific theory.

Most cosmic rays are extremely high-energy particles that bombard our solar system from interstellar space. The heliosheath deflects most of them harmlessly away from Earth.

But not all of the rays appear to come from outside our system.

The inner edge of the heliosheath seems to produce low-energy cosmic rays when it traps interstellar particles between magnetic boundaries and bounces them ever faster, back and forth—like a table tennis paddle hitting a bouncing Ping-Pong ball ever closer to the table.

Scientists had been expecting Voyager 1 to pass through the source of these particles when it first crossed the termination shock. But the spacecraft didn't encounter any more of them there than it had been seeing all along.

"So we didn't find the source," Stone said. "It's somewhere else."

Another surprise appears to be looming from Voyager 2, which is already seeing signs of the heliosheath, even though the craft is about 10 percent closer to the sun than Voyager 1 was when it started encountering the same signs.

This means that the heliosheath appears to press inward in the direction in which Voyager 2 is headed (the two probes are on slightly different paths).

"That's telling us something about what's outside [our solar system]," Stone said.

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