Exploring the Weak Limit of Gravity at Solar System Scales

Abstract

Precision tracking of multiple spacecraft in the outer Solar System has shown an unmodelledperturbation, consisting of a small, constant, radial acceleration directed towards theSun. Since its detection, a great deal of work has been devoted to explaining this Pioneereffect, both in terms of spacecraft-generated systematics and external physical causes. Itscontinuing importance is found in the fact that it has been impossible to explain away theeffect through conventional means. This leaves open the possibility, however unlikely, thatnew physics is represented in the effect. This new physics, in turn, would be connectedintimately to gravity with huge implications across astrophysics and beyond.With this as motivation, this dissertation investigates two areas related to the Pioneereffect. The rst goal is to investigate the use of planets, comets, and asteroids to determinethe reality of the Pioneer effect through precision astrometry.Here, we showed that asteroids can be used to evaluate the gravitational field in theouter Solar System. The observations can be conducted with modest allocations of telescopetime, and would provide a definitive answer to the question within the next 20 years.In assessing current knowledge of Pluto's orbit, we determined that it is not known wellenough at present to preclude the existence of the Pioneer effect. We also showed thatcomets are not ideal candidates for measuring gravity in the outer Solar System, althoughsome present intriguing observational targets for related reasons. Finally, we showed thatPan-STARRS and LSST are likely to lead to a capability to test gravity in the outer SolarSystem in the near future.The second goal of the dissertation involved exploring two general mechanisms for explainingthe Pioneer effect. The First approach involved investigating the effective massdensity that would be produced in the Solar System as a result of the capture of elementaryparticle dark matter by means of a hypothetical weak interaction between the dark matterparticles and the matter of the Sun. The second approach involved three body capture ofdark matter from the Galactic halo into Solar orbit. The three bodies interacting are theGalactic barycenter, the Sun, and the dark matter particle.In this phase of the dissertation, we showed that capture of Galactic dark matter intoSolar orbit by a weak interaction with Solar matter does not accumulate dark matter inthe region where the Pioneer effect manifests itself. It is possible that it does accumulateat smaller distances, however. Similarly, we showed that three body gravitational captureis not feasible as a cause of the Pioneer effect either. Dark matter captured by this mechanismwould occur generally at distances far greater than that needed to cause the Pioneeranomaly. Thus, neither mechanism for capture of dark matter into Solar orbit sufficed toexplain the Pioneer effect.Finally, we discuss a number of future research areas that became apparent during thecourse of the research.