Voyager 11 to Cross Solar Wind "Termination Shock" Boundary

Using a computer model simulation, Haruichi Washimi, a physicist at University of California, Riverside, has predicted when the interplanetary spacecraft Voyager 2 will cross the “termination shock” the spherical shell around the solar system that marks where the solar wind slows down to subsonic speed at 7-8.5 billion miles from the sun in late 2007-early 2008.

Solar winds create geomagnetic storms that can knock out power grids on Earth, the Northern Lights, and the plasma tails of comet always pointing away from the sun.

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.

The point where the solar wind's strength is no longer great enough to push back the interstellar medium is known as the heliopause -considered to be the outer "border" of the solar system known to lie far outside the orbit of Pluto. Scientists hope to gain more perspective on the heliopause from data acquired through the Interstellar Boundary Explorer (IBEX) mission, to be launched in 2008.

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, the director of the Institute of Geophysics and Planetary Physics and 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.”

At the termination shock 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.

“This is the first time the termination-shock position has been forecast in this way,” said Washimi, the lead author of the research paper and a scientist at the Institute of Geophysics and Planetary Physics. “After it crosses this boundary, Voyager 2 will be in the outer heliosphere beyond which lies the interstellar medium and galactic space.

Simulations also show that the spacecraft will cross the termination shock again in the middle of 2008 because of the back and forth movement of the termination-shock boundary.The crossings will come to an end after the spacecraft escapes into galactic space.

Since its launch on Aug. 20, 1977, Voyager 2 visited four planets and their moons in the course of its journey into space.

Posted by Casey Kazan. Adapted from a UC Riverside release.

Links

http://en.wikipedia.org/wiki/Solar_wind
http://solarscience.msfc.nasa.gov/SolarWind.shtml