The Pioneer 10 and 11 spacecraft have been precisely tracked for over thirty
years as they have crossed and then departed the solar system. When they passed
a distance of 20 AU from the sun, both probes exhibited a systematic error in
their trajectories that can be interpreted as a constant acceleration of
8.74e-8 cm/sec/sec towards the sun. This anomalous acceleration has come to be
referred to as the Pioneer Effect, and although spacecraft systematics are its
most likely explanation, there have been no convincing arguments that that is
indeed the case. The alternative, that the Pioneer Effect represents a real
phenomenon, is very appealing for many reasons. What is lacking is a means of
measuring the effect, its variation, its potential anisotropies, and its region
of influence. The present paper shows that minor planets provide an
observational vehicle for investigating the gravitational field in the outer
solar system, and thus provide a means of measuring the Pioneer Effect and
potentially to either support or refute its existence as a real phenomenon.
Minor planets can be used for this purpose because they have a large mass and
are large and bright enough to be observed for useful intervals. Thus, even if
the Pioneer Effect does not represent a new physical phenomenon, minor planets
can be used to probe the gravitational field in the outer solar system. Since
there are very few intermediate range tests of gravity at the multiple AU
distance scale, this is a worthwhile endeavor in its own right. However,
depending upon the characteristics of the measurements, it might even be
possible to differentiate between the predictions of alternative explanations
for the Pioneer Effect.
