Washington, Nov 28: Physicists have predicted that the interplanetary spacecraft Voyager 2 will cross the 'termination shock' phase in late 2007 – early 2008.

The phrase 'Termination shock' is theorized to be a boundary marking one of the outer limits of the sun's influence. It also refers to the spherical shell around the solar system that marks where the solar wind slows down to subsonic speed.

The solar wind – a stream of charged particles ejected by the sun in all directions, travels at supersonic speeds when it leaves the sun, until it eventually encounters the interstellar medium made up of plasma, neutral gas and dust.

At the termination shock, located at 7-8.5 billion miles from the sun, the solar wind is decelerated to less than the speed of sound. The boundary of the termination shock is not fixed, however, but wobbly, fluctuating in both time and distance from the sun, depending on solar activity.

To make the forecast for the spacecraft crossing the particular area in the solar system, Haruichi Washimi, a physicist at UC (University of California) and his colleagues used data from Voyager 2 and performed a global "magneto-hydrodynamic simulation" – a method that allows for precise and quantitative predictions of geomagnetic disturbances caused by solar activities.

"This is the first time the termination-shock position has been forecast in this way," said Washimi. "After it crosses this boundary, Voyager 2 will be in the outer heliosphere beyond which lies the interstellar medium and galactic space," he added.

"Our simulations also show that the spacecraft will cross the termination shock again in the middle of 2008. This will happen because of the back and forth movement of the termination-shock boundary," said Washimi.

This means Voyager 2 will experience multiple crossings of the termination shock. These crossings will come to an end after the spacecraft escapes into galactic space.

Because Voyager 2's crossing of the shock is expected to be an abrupt and relatively brief event, scientists are working to ensure that the most is made of the opportunity. With an idea of when the spacecraft will cross the shock, they are better able to maximize coverage of the crossing.

"Washimi's model has predicted the location of a boundary that is approximately 90 times farther from the sun than is the Earth, to within a few percent," said Gary Zank, one of the coauthors of the research paper. "This is truly remarkable given the enormous complexity of the physics involved, the temporal and spatial scales involved, and the variability of the solar wind conditions," he added.

Voyager 2 was launched Aug. 20, 1977. It visited four planets and their moons in the course of its journey into space. (ANI)