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     Nearly 15 years after they left home, the Voyager 1 and 2 spacecraft have
discovered the first direct evidence of the long- sought-after heliopause --
the boundary that separates Earth's solar system from interstellar space.

     "This discovery is an exciting indication that still more discoveries and
surprises lie ahead for the Voyagers as they continue their journey to the
outer reaches of our solar system," said Dr. Edward C. Stone, Director of the
Jet Propulsion Laboratory (JPL), Pasadena, Calif., and Voyager Project
Scientist.

     Since August 1992, the radio antennas on the spacecraft, called the plasma
wave subsystem, have been recording intense low- frequency radio emissions
coming from beyond the solar system.  For months the source of these radio
emissions remained a mystery.

     "Our interpretation now is that these radio signals are created as a cloud
of electrically charged gas, called a plasma, expands from the sun and
interacts with the cold interstellar gas beyond the heliopause," said Dr. Don
Gurnett, Principal Investigator of the Voyager plasma wave subsystem and a
professor at the University of Iowa.

     The sun is the center of our solar system.  The solar wind is a stream of
electrically charged particles that flows steadily away from the sun.  As the
solar wind moves out into space, it creates a magnetized bubble of hot plasma
around the sun, called the heliosphere.  Eventually, the expanding solar wind
encounters the charged particles and magnetic field in the interstellar gas.
The boundary created between the solar wind and interstellar gas is the
heliopause.

     "These radio emissions are probably the most powerful radio source in our
solar system," said Gurnett. "We've estimated the total power radiated by the
signals to be more than 10 trillion watts.  However, these radio signals are at
such low frequencies, only 2 to 3 kilohertz, that they can't be detected from
Earth."

     In May and June 1992, the sun experienced a period of intense solar
activity which emitted a cloud of rapidly moving charged particles.  When this
cloud of plasma arrived at the heliopause, the particles interacted violently
with the interstellar plasma and produced the radio emissions, according to
Gurnett.

     "We've seen the frequency of these radio emissions rise over time.  Our
assumption that this is the heliopause is based on the fact that there is no
other known structure out there that could be causing these signals," Gurnett
continued.

     Because of the Voyagers' unique positions in space, they serendipitously
detected and recorded the radio emissions. "Earth-bound scientists would not
know this phenomenon was occurring if it weren't for the Voyager spacecraft,"
Gurnett added.

     Exactly where the heliopause is remains one of the great unanswered
questions in space physics.

     "It's this Voyager radio data combined with the plasma measurements taken
at the spacecraft that give us a better guess about where the heliopause is.
Based on the solar wind speed, the time that has elapsed since the mid-1992
solar event and the strength of the radio emissions, my best guess for the
upper limit of the heliopause currently is about 90 to 120 astronomical units
(AU) from the sun," said Dr. Ralph McNutt, a co-investigator on the Voyager
plasma science experiment and a researcher at the Johns Hopkins University
Applied Physics Laboratory in Laurel, Md. (One AU is equal to 93 million miles
(150 million kilometers) or the mean distance from the Earth to the sun.)

     Voyager 1 currently is at 52 AU (4.9 billion miles or 7.8 billion
kilometers from the sun), and Voyager 2 is at 40 AU (3.7 billion miles or 6
billion kilometers) from the sun.

     Voyager 1 was launched on Sept. 5, 1977 and completed flyby exploration of
both Jupiter and Saturn. The spacecraft now is rising above the ecliptic plane
-- the plane in which most of the planets orbit the sun -- at an angle of about
35 degrees at a rate of about 320 million miles (about 520 million kilometers)
a year.

     Voyager 2 was launched on Aug. 20, 1977 and also completed visits to
Jupiter and Saturn and then went on to explore Uranus and Neptune, completing
the reconnaissance of the giant outer planets.  The spacecraft is now diving
below the ecliptic plane at an angle of about 48 degrees and a rate of about
290 million miles (about 470 million kilometers) a year.

     Gurnett presented his findings today at a meeting of the American
Geophysical Union in Baltimore.

     The Voyager Interstellar Mission is managed by JPL for NASA's Office of
Space Science, Washington, D.C.
     
Source:NASA Spacelink    Modem:205-895-0028  Internet:192.149.89.61

Paula Cleggett-Haleim
Headquarters, Washington, D.C.                        
May 26, 1993

Mary A. Hardin
Jet Propulsion Laboratory, Pasadena, Calif.

RELEASE:  93-099