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NMSU astronomer tracks Voyager's encounter with edge of solar system

NASA's plucky Voyager I is making for an active retirement for New Mexico State University astronomer Bill Webber.

Bill Webber, professor emeritus of astronomy at New Mexico State University, is a member of a team of scientists analyzing data from NASA's Voyager 1 as the spacecraft reaches the edge of the solar system. (NMSU photo by Darren Phillips)

an 25 years after leaving Earth, the spacecraft has reached the edge of the solar system, and Webber likens the event to the discovery of a new world.

"What's exciting and interesting about it is this is our first encounter with a real, live astronomical shock," said Webber, who has been directly involved in Voyager experiments since the spacecraft was launched in 1977. "It appears that Voyager has finally encountered perhaps the most important boundary of the sun's influence in the solar system -- the termination shock."

The termination shock is the point at which the solar wind -- streams of electrically charged gas blown from the sun -- collides with the interstellar medium that fills the vast areas of space between stars, creating a shock wave effect. It is about eight billion miles from Earth, or about three times farther out than the most distant planets.

Teams of scientists studying data from Voyager I disagree over whether the spacecraft has passed through the termination shock or a precursor of the shock, but it is clearly in the vicinity, said Webber, professor emeritus of astronomy at New Mexico State and a member of one of the teams that reported findings in the Nov. 6 issue of the science journal Nature.

Webber's team has been analyzing the cosmic rays and other particles from outside the solar system that are encountered by Voyager, watching for the dramatic acceleration of particles that would indicate the spacecraft had reached the termination shock.

"Most of the solar wind energy comes to a grinding halt at this shock, and this energy is transferred into a tremendous increase in the intensity of the energetic particles," he said.

Data sent back by Voyager from mid-2002 through early 2003 show the expected huge spikes in particle energy levels, but the picture is not entirely clear yet, he said.

Because the shock boundary expands and contracts, it is possible that the spacecraft passed through the shock only to have the shock pass over and move beyond the spacecraft, Webber said. "Or maybe the shock is more complicated than we thought -- it may be a double shock."

Webber was a scientist at the University of New Hampshire in the mid-1970s when he and colleagues from the Goddard Space Flight Center and Cal Tech proposed to NASA experiments using the Voyager 1 and 2 spacecraft to study cosmic rays. Webber helped design the instruments for the experiment. He joined New Mexico State University in 1990 and is technically retired now, but Voyager keeps him active as a researcher.

"When the Voyager spacecraft were launched in 1977, the main goal was to study the planets and the interplanetary region, but also at some point to get out of this region and explore the interstellar region," he said. "None of us were ever sure that the instruments would last this long. It's a tribute to the people who built it."

Being able to take direct measurements from the termination shock, and then to study interstellar medium beyond the boundary, "is like the discovery of a new world, like landing on a new planet," Webber said. "There is a gold mine of information for astronomers out there."

Voyager 1 already has the distinction of reaching farther into space than any other still-functioning spacecraft, and there's life in it yet. The spacecraft's nuclear power supply is expected to last until 2010 or 2012, Webber said.

Voyager 2, which took a different route in the interplanetary region, is expected to reach the termination shock in about five years, providing another look at the complex phenomenon, he said.

The lead author of the paper published in Nature by Webber's team is Frank B. McDonald of the University of Maryland. Other team members are Edward C. Stone and Alan C. Cummings of Cal Tech and Bryant Heikkila and Nand Lal of NASA's Goddard Space Flight Center.