"There are no signs to announce the edge of the solar system, but when the venerable Voyager 2 spacecraft approached this final frontier last Aug. 31 it was in for quite a shock. So were the scientists who analyzed the data that the craft radioed back to Earth, along with related observations by NASAÕs twin Earth-orbiting STEREO spacecraft.

The signals reveal that at a distance of 83.7 astronomical units (1 AU is the average Earth-sun separation), Voyager 2 had at least five encounters with a turbulent region known as the termination shock, the researchers report in the July 3 Nature. ThatÕs the place where the solar wind Ñ the sunÕs hot supersonic wind of protons and other charged particles, which carves the heliosphere, a bubble in space extending well beyond the orbit of Pluto Ñ slams into cold interstellar space and abruptly slows.

Analyzing the encounter is critical for understanding how the bubble interacts with surrounding space, and how the bubbles carved by other stars affect their surroundings, notes Voyager lead investigator Ed Stone of the California Institute of Technology in Pasadena, Calif.

Researchers had expected that Voyager 2 would have only one encounter with the shock. The multiple crossings indicate that Òthe shock is not the steady structure that is predicted by the simplest theory,Ó says Len Burlaga of NASAÕs Goddard Space Flight Center in Greenbelt, Md. ÒIt is like a wave approaching a beach, that grows, breaks, dissipates, and then re-forms closer to shore.Ó


ON THE EDGELocation of the two Voyager spacecraft at the fringes of the solar system and the STEREO spacecraft. New data form the Voyager 2 craft and STEREO are providing fresh insight about the structure of the edge of the solar system. L. Wang/UC Berkeley
Gusts in the solar wind may cause the shock to Òcome and go, re-forming itself and decaying,Ó Stone suggests." (Science News)