  Analysis of the radio tracking data from the Pioneer 10/11 spacecraft at
distances between about 20 - 70 AU from the Sun has indicated the presence of
an unmodeled, small, constant, Doppler blue shift which can be interpreted as a
constant acceleration of a_P= (8.74 \pm 1.33) \times 10^{-8} cm/s^2 directed
approximately towards the Sun. In addition, there is early (roughly modeled)
data from as close in as 5 AU which indicates there may have been an onset of
the anomaly near Saturn. We observe that the data now arriving from the New
Horizons mission to Pluto and the Kuiper Belt could allow a relatively easy,
direct experimental test of whether this onset is associated with distance from
the Sun (being, for example, an effect of drag on dark matter). We strongly
urge that this test be done.
