Image: Voyager probe
The Voyager 2 probe, shown in this artist's conception, is on course to pass beyond the outermost layer of our solar system earlier than expected, scientists say.
updated 5/23/2006 6:00:50 PM ET 2006-05-23T22:00:50

Voyager 2 could pass beyond the outermost layer of our solar system, called the "termination shock," sometime within the next year, NASA scientists announced at a media teleconference Tuesday.

The milestone, which comes about a year after Voyager 1's crossing, would be earlier than expected and suggests to scientists that the edge of the shock is about 1 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 1 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. One AU, or Astronomical Unit, is the distance between Earth and the sun, or 93 million miles (150 million kilometers).

In contrast, Voyager 2 began detecting returning particles while only 76 AU from the sun.

"This tells us that the shock down where Voyager 2 is must be closer the sun than where Voyager 1 is," said Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena.

Image: Solar system boundaries
Voyager 2 is approaching the termination shock, an area where charged particles flowing out from the sun meet up with particles coming in from outside the solar system.

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 one-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 of the Voyager spacecraft were launched from Cape Canaveral Air Force Station in Florida: Voyager 2 headed out on Aug. 20, 1977, Voyager 1 on Sept. 5, 1977.

Currently, Voyager 1 is about 8.7 billion miles from the sun and traveling at a speed of 3.6 AU per year, while Voyager 2 is about 6.5 billion miles away and moving at about 3.3 AU per year.

NASA is offering a podcast and a slide show about Voyager's "cosmic potholes."

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