The Voyager 2 spacecraft, which launched from Earth 30 years ago and visited four planets and their moons, is coming to another milestone, scientists say, as it approaches a mysterious region of space near the edge of the solar system.

According to computer models, the space probe will cross a threshold in the solar system called the "termination shock" sometime in late 2007 or early 2008, scientists at the University of California Riverside say in new research.

The termination shock is 13.5 billion kilometres from the sun, or 90 times more distant from the sun than the Earth is. It marks the beginning of a region of space where the charged particles from the sun — called the solar wind — collide with a mix of particles that act as a buffer zone between our solar system and high-energy particles emanating from other stars in the galaxy.

It's an important scientific threshold, study co-author and UC-Riverside physics professor Gary Zank said, since it will give astronomers a window into the stuff that makes up a significant portion of the universe.

"Most of our knowledge of the interstellar medium, whether it's neutral gases or cosmic rays, comes from telescope experiments, and so the picture they provide is extremely coarse," Zank said.

"Voyager will give us an opportunity to get real 'in situ' data of what makes up the universe outside of the sun's influence."

Cosmic rays, for example, are particles travelling at close to light speed for which scientists have yet to determine a point of origin. But only a tiny fraction of cosmic rays reach the Earth from outside the solar system because the particles collide with, and are redirected by, the solar wind.

That effect is lessened as the solar wind spreads out in a spherical pattern from the sun, losing density and thus force in much the same way water from a garden hose exerts more "push" at the nozzle than it does as it fans out, Zank said.

Theoretically, the termination shock is the point where the hot solar wind finally breaks against the cold, thin gas of interstellar space, and in practice it manifests itself in a rapid decline in speed of the solar wind particles to subsonic velocities.

Voyager 1 crossed this barrier several years ago, but its sensors were not sending signals when this occurred, Zank said.

The study, to be published in the Dec. 1 issue of the Astrophysical Journal Letters, and led by UC-Riverside physicist Haruichi Washimi, used a computer simulation to try to predict when the space probe would reach the threshold; the aim was to help prepare Voyager scientists to be ready to make the most of the opportunity to study the event. Because the threshold oscillates, the probe is expected to cross the termination shock again in mid-2008.

Voyager 2 launched in 1977 and flew by Jupiter, Saturn, Uranus and Neptune between 1979 and 1989. Like Voyager 1, it carries with it a gold record containing pictures, sounds and information from Earth.

The nuclear power sources on the two spacecraft should last until 2020.