for National Geographic News
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 mediumthin 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.
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