By Ker Than SPACE.com
Wednesday, May 24, 2006 Posted: 1424 GMT (2224 HKT)
The milestone, which comes about a year after Voyager 1's crossing, comes earlier than expected and suggests to scientists that the edge of the shock is about one billion miles closer to the sun in the southern region of the solar system than in the north.
This implies that the heliosphere, a spherical bubble of charged low-energy particles created by our sun's solar wind, is irregularly shaped, bulging in the northern hemisphere and pressed inward in the south.
Scientists determined that Voyager I was approaching the termination shock when it began detecting charged particles that were being pushed back toward the sun by charged particles coming from outside our solar system. This occurred when Voyager 1 was about 85 AU from the sun. (Full story (http://edition.cnn.com/2005/TECH/space/05/25/voyager.space/index.html))
One AU is the distance between the Earth and the sun, or 93 million miles.
In contrast, Voyager II began detecting returning particles while only 76 AU from the sun.
"This tells us that the shock down where Voyager II is must be closer the sun than where Voyager I is," said Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena.
The researchers think that the heliosphere's asymmetry might be due to a weak interstellar magnetic field pressing inward on the southern hemisphere.
"The [magnetic] field is only 1/100,000 of the field on the Earth's surface, but it's over such a large area and pushing on such a faint gas that it can actually push the shock about a billion miles in," Stone explained.
Both Voyager spacecrafts were launched from Cape Canaveral Air Force Station in Florida: Voyager II headed out on August 20, 1977, Voyager I on September 5, 1977.
Currently, Voyager I is about 8.7 billion miles from the sun and traveling at a speed of 3.6 AU per year while
Voyager II is about 6.5 billion miles away and moving at about 3.3 AU per year.