Voyager 1 at Solar System Edge, Scientists Now Agree

Stefan Lovgren
for National Geographic News
June 2, 2005
After almost two years of debate, scientists agree that NASA's Voyager 1 spacecraft has entered the outer edge of our solar system.

Voyager 1 and its twin, Voyager 2, were launched in 1977 on a mission to explore Jupiter and Saturn. They've been traveling outward ever since.

Now astronomers believe Voyager 1 has passed through a boundary known as the termination shock and entered an area called the heliosheath, which envelops our solar system. (See diagram at lower right.)

In this vast and turbulent region, the solar winds dissipate and give way to magnetic interstellar medium—thin gases that float in the void between the stars.

"This is a totally new region of space," Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena, said in a telephone interview. "We have entered the last lap in our race to interstellar space."

Caltech manages NASA's Jet Propulsion Laboratory in Pasadena, which built and operates the Voyager spacecraft.

In or Out

The spacecrafts' visits to Jupiter in 1979 and Saturn in 1980 greatly expanded our knowledge of those outer planets. In addition, Voyager 2 is the only spacecraft yet to fly by Uranus and Neptune.

Voyager 1 is now 94 astronomical units from the sun. (One astronomical unit equals the distance between the Earth and the sun.)

In late 2003 scientists suggested that Voyager 1 had crossed through the termination shock and entered the heliosheath, perhaps as early as 2002 (see "Has Voyager 1 Space Probe Left Our Solar System?"). (The termination shock is where the solar wind, a stream of electrically charged particles blowing continuously outward from the sun, is slowed by pressure from gas floating between the stars.)

Other scientists, however, maintained the spacecraft was near the termination shock but had not yet crossed it.

Scientists can determine within yards how far the spacecraft has traveled. But it is difficult to determine when it crosses the termination shock and thereby enters the heliosheath. That's because the termination shock can expand, contract, and ripple, depending on changes in the speed and pressure of the solar wind.

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.

Interstellar Space

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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