Changes

* The anomalous acceleration could be due to the spacecraft venting energy in certain directions.<ref>http://www.symmetrymagazine.org/breaking/2008/04/13/pioneer-spacecraft-a-step-closer-to-being-boring/</ref>  However, such effects would be expected to be more significant earlier on, when the power sources were less degraded.<ref name="Nieto" />  The opposite was actually the case.

* The anomalous acceleration could be due to the spacecraft venting energy in certain directions.<ref>http://www.symmetrymagazine.org/breaking/2008/04/13/pioneer-spacecraft-a-step-closer-to-being-boring/</ref>  However, such effects would be expected to be more significant earlier on, when the power sources were less degraded.<ref name="Nieto" />  The opposite was actually the case.

* Drag forces from space dust, analogous to air resistance, could be slowing the spacecraft down.  While the average concentration of interplanetary dust is not high enough to produce the observed acceleration, the Pioneer spacecraft could conceivably have picked up an electric charge which could be attracting dust<ref>William F. Hall.  "[http://dx.doi.org/10.1016/j.physletb.2007.01.006 Can charge drag explain the Pioneer anomaly?]"  ''Physics Letters B'', 1 March 2007</ref>. 

* Drag forces from space dust, analogous to air resistance, could be slowing the spacecraft down.  While the average concentration of interplanetary dust is not high enough to produce the observed acceleration, the Pioneer spacecraft could conceivably have picked up an electric charge which could be attracting dust.<ref>William F. Hall.  "[http://dx.doi.org/10.1016/j.physletb.2007.01.006 Can charge drag explain the Pioneer anomaly?]"  ''Physics Letters B'', 1 March 2007</ref>

* The theory of [[General Relativity]] and the [[Law of Universal Gravitation]] could be wrong; the gravitational force could be slightly stronger than predicted.  In this context, it is important to note that other spacecraft flying in hyperbolic trajectories around celestial bodies have also experienced anomalous gravitational effects.

* The theory of [[General Relativity]] and the [[Law of Universal Gravitation]] could be wrong; the gravitational force could be slightly stronger than predicted.  In this context, it is important to note that other spacecraft flying in hyperbolic trajectories around celestial bodies have also experienced anomalous gravitational effects.

* There could be gravitational forces from other celestial bodies that have not been taken into account.

* There could be gravitational forces from other celestial bodies that have not been taken into account.