Pioneer Anomaly - Previously Proposed Causes

Previously Proposed Causes

Before the thermal recoil explanation became accepted, other proposed explanations fell into two classes — "mundane causes" or "new physics". Mundane causes include conventional effects that were overlooked or mis-modeled in the initial analysis, such as measurement error, thrust from gas leakage, or uneven heat radiation. The "new physics" explanations proposed revision of our understanding of gravitational physics. Recent developments point towards the mundane cause of thermal radiation pressure forces inherent in the spacecraft.

If the Pioneer anomaly had been a gravitational effect due to some long-range modifications of the known laws of gravity, it did not affect the orbital motions of the major natural bodies in the same way (in particular those moving in the regions in which the Pioneer anomaly manifested itself in its presently known form). Hence a gravitational explanation would need to violate the equivalence principle, which states that all objects are affected the same way by gravity. It is therefore argued that increasingly accurate measurements and modelling of the motions of the outer planets and their satellites undermine the possibility that the Pioneer anomaly is a phenomenon of gravitational origin. Others believe that our knowledge of the motions of the outer planets and dwarf planet Pluto, is still insufficient to disprove the gravitational nature of the Pioneer anomaly. The same authors ruled out the existence of a gravitational Pioneer-type extra-acceleration in the outskirts of the Solar System by using a sample of Trans-Neptunian objects.

The magnitude of the Pioneer effect is numerically quite close to the product of the speed of light and the Hubble constant, hinting at a cosmological connection, but the significance of this, if any, is unknown. In fact the latest Jet Propulsion Laboratory review (2010) undertaken by Turyshev and Toth claims to rule out the cosmological connection by considering rather conventional sources while other scientists provided a disproof based on the physical implications of cosmological models themselves.

Gravitationally bound objects such as the solar system, or even the galaxy, are not supposed to partake of the expansion of the universe—this is known both from conventional theory and by direct measurement. This does not necessarily interfere with paths new physics can take with drag effects from planetary secular accelerations of possible cosmological origin.