Charles Redmond
Headquarters, Washington, D.C.                  February 23, 1990

Pete Waller
Ames Research Center, Moffett Field, Calif.


RELEASE:  90-29

PIONEER 11 PASSES NEPTUNE'S ORBIT, LEAVES SOLAR SYSTEM


     Pioneer 11 today will cross the orbit of Neptune and become 
the fourth spacecraft to leave the solar system, providing a coda 
to humanity's first major planetary explorations.  Pioneer 11 
will join Pioneer 10 and Voyagers 1 and 2 in searching for the 
heliopause, the point at which the Sun's electromagnetic 
influence gives way to the galaxy's influence.

     As it crosses Neptune's orbit, Pioneer 11 will be 2.8 
billion miles from the Earth.  Neptune's orbit currently marks 
one measure of the expanse of the solar system because, for the 
next 12 years, Pluto's eccentric orbit carries it inside 
Neptune's path.  Some scientists refer to the heliopause as the 
edge of the solar system.  By that definition, all four 
spacecraft are still within the solar system.

     Launched in 1973, Pioneer 11 provided scientists with their 
closest view of Jupiter, passing within 26,600 miles of the cloud 
tops in December 1974.  The close approach and the spacecraft's 
speed of 107,373 mph, by far the fastest speed ever reached by a 
man-made object, hurled Pioneer 1.5-billion miles across the 
solar system toward Saturn.

     Before reaching Saturn in 1979, Pioneer 11 reached an 
inclination of 17 degrees above the solar equatorial plane, high 
enough to illuminate the true character of the sun's magnetic 
field.  Now 780 million miles above the ecliptic plane where most 
of the planets orbit the sun, the spacecraft recently showed that 
many of the solar cosmic rays in the heliosphere originate 
outside the Sun's atmosphere in the interstellar gas, the space 
between the stars.

     Pioneer 11 flew to within 13,000 miles of Saturn and took 
the first close-up pictures of the planet.  Instruments located 
two previously undiscovered small moons and an additional ring, 
charted Saturn's magnetosphere and magnetic field and found its 
planet-size moon, Titan, to be too cold for life.

     Pioneer 11, which will traverse interstellar space in the 
same direction as the Sun moves, continues to return good data, 
but in 3 years, operating the radio transmitter and scientific 
instruments simultaneously will be difficult, says NASA Project 
Manager Richard Fimmel.  Technical adjustments may extend the 
craft's life through 1995.  Pioneer 10, with a stronger power 
supply, may return data through the year 2000, which would extend 
its original 30-month design life to 28 years.

     In June 1983, Pioneer 10 made history by becoming the first 
human artifact to leave the solar system, travelling in the 
direction opposite Pioneer 11's path.  Today, Pioneer 10 will be 
4.5 billion miles from Earth.  Returning data to Earth at the 
speed of light requires 6 hours, 36 minutes.  Pioneer 10 
continues to search for the heliopause for very long-wavelength 
gravity waves that would further understanding of Einstein's 
Theory of Relativity and for evidence of a 10th planet.

     The Pioneers are managed by the Ames Research Center, 
Mountain View, Calif., for NASA's Office of Space Science and 
Applications.  The spacecraft were built by TRW Space & 
Technology Group, Redondo Beach, Calif.