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 the particles in space, analogous to air resistance, could be slowing the spacecraft down.  While the average concentration of the particles is not high enough to produce the observed acceleration, the Pioneer spacecraft could conceivably have picked up an electric charge which could be attracting the particles.<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 the particles in space, analogous to air resistance, could be slowing the spacecraft down.  While the average concentration of the particles is not high enough to produce the observed acceleration, the Pioneer spacecraft could conceivably have picked up an electric charge which could be attracting the particles.<ref>William F. Hall.  "[https://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.

* A Sept. 2011 paper claims that the anomaly has been shrinking, but is still a mystery.[http://www.sciencemag.org/content/333/6047/1208.short]

* A Sept. 2011 paper claims that the anomaly has been shrinking, but is still a mystery.[http://www.sciencemag.org/content/333/6047/1208.short]

The generally accepted explanation, published in March 2011, is that the anomaly is caused by the reflection of the radiation from the power source off of the back of the antenna dish.<ref>http://www.technologyreview.com/blog/arxiv/26589/</ref>  The solution is sometimes described as an application of "Phong shading", a technique of computer graphics that is now considered imprecise.  But Phong shading itself is not what is important.  The "ray tracing" computer graphics technique that underlies Phong shading was what inspired the scientists to take reflection into account.<ref>http://en.wikipedia.org/wiki/Pioneer_anomaly</ref>

The generally accepted explanation, published in March 2011, is that the anomaly is caused by the reflection of the radiation from the power source off of the back of the antenna dish.<ref>http://www.technologyreview.com/blog/arxiv/26589/</ref>  The solution is sometimes described as an application of "Phong shading", a technique of computer graphics that is now considered imprecise.  But Phong shading itself is not what is important.  The "ray tracing" computer graphics technique that underlies Phong shading was what inspired the scientists to take reflection into account.<ref>https://en.wikipedia.org/wiki/Pioneer_anomaly</ref>

The most detailed analysis to date, by some of the original investigators, explicitly looks at two methods of estimating thermal forces, then states "We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains."<ref name="thermal">{{cite journal|doi=10.1103/PhysRevLett.108.241101|title=Support for the Thermal Origin of the Pioneer Anomaly|year=2012|last1=Turyshev|first1=Slava G.|last2=Toth|first2=Viktor T.|last3=Kinsella|first3=Gary|last4=Lee|first4=Siu-Chun|last5=Lok|first5=Shing M.|last6=Ellis|first6=Jordan|journal=Physical Review Letters|volume=108|issue=24|arxiv = 1204.2507 |bibcode = 2012PhRvL.108x1101T|pmid=23004253|pages=241101}}</ref>

The most detailed analysis to date, by some of the original investigators, explicitly looks at two methods of estimating thermal forces, then states "We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains."<ref name="thermal">{{cite journal|doi=10.1103/PhysRevLett.108.241101|title=Support for the Thermal Origin of the Pioneer Anomaly|year=2012|last1=Turyshev|first1=Slava G.|last2=Toth|first2=Viktor T.|last3=Kinsella|first3=Gary|last4=Lee|first4=Siu-Chun|last5=Lok|first5=Shing M.|last6=Ellis|first6=Jordan|journal=Physical Review Letters|volume=108|issue=24|arxiv = 1204.2507 |bibcode = 2012PhRvL.108x1101T|pmid=23004253|pages=241101}}</ref>