Some scientists have likened the termination shock to a weather front that could pass back and forth over Voyager 1. "Voyager's observations over the past few years show the termination shock is far more complicated than anybody thought," Eric Christian, a scientist for NASA's Sun-Solar Connection program in Washington, D.C., said in a statement.
Surfing the Shock
This time, however, scientists seem to agree that Voyager 1 has passed the termination shock.
In December 2004 Voyager 1's magnetometers observed the local magnetic field increasing by a factor of 2.5, which is expected when the solar wind slows down on the other side of the termination shock. The magnetic field has remained at these high levels since December.
"The missing thing last time was the evidence that the wind had gotten thicker," Stone said. "When the wind slows down and gets thicker, much like traffic on the freeway, the magnetic field compresses and gets more intense. We did not see that compression [starting] three years ago."
Instead, Stone suggests, the termination shock may have been moving outward from 2002 through 2004 at about the same speed that Voyager 1 was traveling.
"We were, in a sense, surfing the shock," he said.
Now Voyager 1 has entered uncharted territory: the heliosheath. The sheath also marks the transition between our solar system and the interstellar medium.
"There's a lot we don't know about this region," Stone said. "We are trying to learn how our sun creates this bubble around itself that shields us from cosmic rays which are out in the galaxy nearby."
Scientists don't know exactly how wide the heliosheath is, but they expect that it will take Voyager 1 another 10 years or so to travel through the region. The spacecraft is expected to keep going until it runs out of power, probably around 2020 at the earliest.
After traveling through the heliosheath, Voyager 1 will encounter the heliopause, a boundary region that may be as complicated as the termination shock. This is where the wind from our sun mixes with interstellar wind.
"Once it crosses the heliopause, Voyager will be the first human-made object to reach interstellar space," Stone said. "Once there, we can determine for the first time the strength of the interstellar magnetic field."
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