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Voyager 1 sends messages from the edge of the solar system

NASA's Voyager 1 spacecraft has passed into a border region at the outer limits of the solar system and is sending back surprising information, says a New Mexico State University astronomer who has been following Voyager for decades.

New Mexico State University astronomer Bill Webber: "Voyager 1 has been one surprise after another." (NMSU photo by Darren Phillips)

At the edge of the solar system, the supersonic flow of energetic particles from the sun, known as the solar wind, abruptly slows as it encounters the interstellar medium that fills the vast areas of space between stars. This shock wave, the outer boundary of the sun's influence on the solar system, is called the "termination shock."

Scientists had expected Voyager would encounter a dramatic increase in the intensity level of high-energy particles as it passed through this boundary, followed by a drop in intensity as it entered the transition zone between the solar system and the interstellar medium of the galaxy, said NMSU astronomer Bill Webber.

"But we didn't see anything like what we expected," said Webber, who has been actively involved in Voyager experiments since the spacecraft was launched in 1977. "We expected high intensities at this boundary but didn't see them. Further out, beyond the termination shock, the intensity began to shoot up to amazingly high levels. The intensity has been going up steadily since the beginning of the year."

The spacecraft is enduring intensities 100 to 1,000 times what it encountered in the quiet regions between planets as it flew by Jupiter, Saturn, Uranus and Neptune, Webber said. The intense radiation could pose a threat to future space travelers, should they ever venture through that region, he noted.

Voyager 1 is the first spacecraft to explore the termination shock. Voyager 2, which took a different route in its tour of the solar system, is expected to reach the region in two or three years, Webber said.

"We will then get a three-dimensional view of what's going on out there," he said.

Based on data from Voyager 1, "the process must be more complex than the theorists thought," he said. But that is often the case when scientists explore new areas.

"When you explore new territory you always find new and interesting things," he said. "Voyager has been one surprise after another. That's what makes it so exciting."

Over the past couple of years, researchers debated whether Voyager 1 had encountered the boundary or not. But now they agree that the spacecraft crossed the termination shock in late 2004 and entered the transition zone known as the heliosheath, Webber said.

"What apparently has been happening over the last few years is that the termination shock has been going out right with the spacecraft," he said. "It's as if Voyager was surfing, like somebody riding the crest of a wave. Then the solar wind speed suddenly dropped because of decreased solar activity, the wave began to move back inward and Voyager crossed it in late 2004. It was decisive this time."

The spacecraft crossed the shock at a distance of about 94 astronomical units (au), or 14.1 billion kilometers from the sun. An au is the distance from the sun to Earth.

Voyager may be in the transition zone for five or 10 years before it gets completely outside the influence of the solar system, Webber said. The spacecraft should still be transmitting data at that time. Voyager 1 and 2, using nuclear power, are expected to continue operating until at least 2015.

"They are still doing beautifully," he said.

Webber was among the scientists who originally proposed the high-energy particle experiment for Voyager 33 years ago. At that time, he said, "none of us expected it to last more than five or 10 years. This has turned out to be one of man's most interesting voyages."

Webber is a co-author of a paper, "Voyager 1 Explores the Termination Shock Region and the Heliosheath Beyond," published in the Sept. 23 issue of the journal Science. The lead author is E.C. Stone of the California Institute of Technology. Other scientists on the research team are from the Institute of Science and Technology at the University of Maryland and NASA's Goddard Space Flight Center in Maryland.