A NASA spacecraft headed out of the solar system at a speed
that would streak from New York to Los Angeles in less than four
minutes could reach the first main feature of the boundary
between our solar system and interstellar space within three

The Voyager 1 spacecraft, the farthest human-made object
from Earth, may reach the beginning of his boundary region
between early next year and the end of 2003, said Dr. Edward
Stone, Voyager project scientist and director of NASA’s Jet
Propulsion Laboratory, Pasadena, Calif.

The feature, called the termination shock, is not quite
where the Sun’s influence ends. It is where a pressure wave backs
up from an even farther-out boundary, the heliopause, where the
region of space dominated by particles streaming from the Sun
ends and interstellar space begins.

But the termination shock won’t stand still. Expected solar
activity will likely crank up the solar wind of outbound
particles and begin pushing back the heliopause in about three
years, Stone said. The shock region could move outward even
faster than Voyager 1.

“If we don’t encounter it in the next three years, we may
not catch up with it for several more years,” Stone said. “On the
other hand, it would be wonderful if we got out past it, then it
overtook us so we could have a second look at it.”

Stone estimated the distance to the shock and the heliopause
during a Monday session of a five-day conference of the American
Geophysical Union in San Francisco.

Voyager 1, built, flown and managed by JPL, left Earth in
1977 and made a string of discoveries while flying past Jupiter
in 1979 and Saturn in 1980. It is now about twice as far from the
Sun as Pluto’s orbit. Its twin, Voyager 2, made a grand tour of
four outer planets and is currently about 80 percent as distant
as Voyager 1. NASA’s current mission for both craft is to learn
more about the edge of the solar system and what is just outside
of it.

As the Sun moves through the galaxy, it hauls with it a
surrounding bubble, the heliosphere, filled with particles of the
solar wind. The solar wind carries an average of only about a
half-dozen ions per cubic centimeter (about 100 per cubic inch)
at Earth’s orbit and even fewer farther from the Sun, but it
exerts an outward pressure and extends the magnetic influence of
the Sun.

Somewhere roughly 100 or more times farther from the Sun
than Earth is, the pressure of the solar wind is counterbalanced
by a pressure outside of the heliosphere. That’s the pressure of
the interstellar wind, a faint current of particles flowing
through the galaxy. The interstellar wind surrounds each star’s
sphere of influence like a river around rocks whose shapes could
change like elastic balloons.

“We’re hoping to learn how big a bubble the Sun creates for
itself and, for the first time, how much pressure there is in
interstellar space,” Stone said.

One method of estimating the distance to the bubble’s edge
makes use of radio emissions that are believed to originate from
the boundary between interstellar space and the heliosphere in
response to bursts of particle ejections by the Sun. Measuring
the time lag between when a solar burst occurs and when the
response echoes back to the spacecraft allows a calculation of
how far the particles traveled to reach the heliopause.

Another method uses the difference between the rate at which
cosmic rays of a certain type are reaching Voyager 1 and the rate
at which they are reaching Voyager 2.

Those methods and others suggest the termination shock may
be 80 to 90 times as far from the Sun as Earth is, Stone said.
That’s farther than anticipated earlier in the Voyager mission.

The shock is expected to have particle-density and magnetic-
field characteristics that will make its detection unambiguous
when Voyager 1 reaches it, providing an accuracy check for the
ideas used in predicting its location.

“Once we know where the termination shock is, we’ll have a
better idea how much farther it is to the heliopause,” Stone

Additional information about the Voyager Interstellar
Mission is available at http://vraptor.jpl.nasa.gov . JPL, a
division of the California Institute of Technology in Pasadena,
manages the mission for NASA’s Office of Space Science,
Washington, D.C.