Could the Pioneer anomaly have a gravitational origin?

TL;DR

This paper investigates whether the Pioneer anomaly could have a gravitational origin by analyzing its effects on planetary motions, concluding that a gravitational explanation conflicts with planetary data, implying non-geodesic motion for the spacecraft.

Contribution

It provides a model-independent method to test gravitational origins of the Pioneer anomaly by analyzing planetary residuals within metric theories of gravity.

Findings

Gravitational perturbations consistent with the Pioneer anomaly conflict with Uranus and Pluto data.

The anomaly cannot be explained by a simple metric disturbance in a geodesic framework.

Pioneer spacecraft likely follow non-geodesic trajectories, ruling out certain gravitational models.

Abstract

If the Pioneer anomaly has a gravitational origin, it would, according to the equivalence principle, distort the motions of the planets in the Solar System. Since no anomalous motion of the planets has been detected, it is generally believed that the Pioneer anomaly can not originate from a gravitational source in the Solar System. However, this conclusion becomes less obvious when considering models that either imply modifications to gravity at long range or gravitational sources localized to the outer Solar System, given the uncertainty in the orbital parameters of the outer planets. Following the general assumption that the Pioneer spacecraft move geodesically in a spherically symmetric spacetime metric, we derive the metric disturbance that is needed in order to account for the Pioneer anomaly. We then analyze the residual effects on the astronomical observables of the three outer planets that would arise from this metric disturbance, given an arbitrary metric theory of gravity. Providing a method for comparing the computed residuals with actual residuals, our results imply that the presence of a perturbation to the gravitational field necessary to induce the Pioneer anomaly is in conflict with available data for the planets Uranus and Pluto, but not for Neptune. We therefore conclude that the motion of the Pioneer spacecraft must be non-geodesic. Since our results are model independent within the class of metric theories of gravity, they can be applied to rule out any model of the Pioneer anomaly that implies that the Pioneer spacecraft move geodesically in a perturbed spacetime metric, regardless of the origin of this metric disturbance.