Abstract
The two Pioneer spacecraft are now the most distant man‐made objects in the solar system. They have unique advantages to observe and search, perhaps for the cause of persistent irregularities in the orbits of Uranus and Neptune that strongly suggest the possibilities of another unidentified planet far beyond the one that is the outermost planet. Because Pluto's orbit is greatly elongated, Pluto and Neptune take turns being the outermost planet, with Pluto currently inside Neptune's orbit for the next 17 years.Pioneer 10 is now between the orbits of Uranus and Neptune and will be beyond all the planets in July 1983. Pioneer 11 is between the orbits of Saturn and Uranus.
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