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 crossed the orbit of Neptune in 1990 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. When it crossed Neptune's orbit, Pioneer 11 was 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. Pioneer 11, which will traverse interstellar space in the same direction as the Sun moves, continues to return good data, but operating the radio transmitter and scientific instruments simultaneously will be difficult. Technical adjustments may extend the spacecraft's life through 1997. Pioneer 11 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. Pioneer 10 and 11 are managed by the Ames Research Center, Mountain View, Calif., for NASA's Office of Space Science and Applications.