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[[File:Dn11304-1 600.jpg|right|300px|thumb|Trajectories of the Pioneer and Voyager spacecraft.  (The Pioneer anomaly is much too small to be noticeable at this scale.)]]

[[File:Dn11304-1 600.jpg|right|300px|thumb|Trajectories of the Pioneer and Voyager spacecraft.  (The Pioneer anomaly is much too small to be noticeable at this scale.)]]

The '''Pioneer anomaly''' refers to deviations in projected courses for several spacecraft sent to the outer solar system.  The data sent back from both Pioneer spacecraft, Galileo, and Ulysses, represent one of the first meaningful tests of the precision of gravitation predictions over long distances.  The data contradict the [[theory of relativity]], and specifically [[general relativity]], as well as [[Law of Universal Gravitation|Newtonian mechanics]], indicating that both theories may be fundamentally flawed.

The '''Pioneer anomaly''' refers to deviations from projected courses for several spacecraft sent to the outer solar system.  The data sent back from both Pioneer spacecraft, Galileo, and Ulysses, represent one of the first meaningful tests of the precision of gravitation predictions over long distances.  The spacecraft have deviated from the courses which scientists predicted using [[general relativity]], as well as [[Law of Universal Gravitation|Newtonian mechanics]], indicating that both theories may be fundamentally flawed.

==Discovery==

==Discovery==

[[Pioneer 10]] and [[Pioneer 11]] were space probes sent to study the planets [[Jupiter]] and [[Saturn]].  After following a [[hyperbolic]] trajectory around these planets, they had reached [[escape velocity]] for the solar system and were flying out.  While their main mission was now ended, NASA stayed in radio contact with the craft to study the outskirts of the solar system<ref name="Nieto">Michael Martin Nieto and John D Anderson.  "[http://www.iop.org/EJ/abstract/0264-9381/22/24/008 Using Early Data to Illuminate the Pioneer Anomaly]".  ''Classical and Quantum Gravity'', 2005</ref>, including the hypothetical [[Oort cloud]].

[[Pioneer 10]] and [[Pioneer 11]] were space probes sent to study the planets [[Jupiter]] and [[Saturn]].  After following a [[hyperbolic]] trajectory around these planets, they had reached [[escape velocity]] for the solar system and were flying out.  While their main mission was now ended, NASA stayed in radio contact with the craft to study the outskirts of the solar system<ref name="Nieto">Michael Martin Nieto and John D Anderson.  "[http://www.iop.org/EJ/abstract/0264-9381/22/24/008 Using Early Data to Illuminate the Pioneer Anomaly]".  ''Classical and Quantum Gravity'', 2005</ref>.

Around the time of Pioneer 11's flyby of Saturn, it was found to be slightly off-course.  (Every spacecraft sent to the outer solar system is intended to follow a specific course, predicted by the theory of [[general relativity]].  Radio transmissions and radar are used to track spacecraft to ensure that they stay on course.)  While this in itself was within the range of error, astronomers continued tracking the craft to find that the anomalous sunward acceleration increased.  Currently, [[Pioneer 10]] and [[Pioneer 11]] are respectively over 30 and 70 [[Astronomical Unit|AU]] from the sun, the farthest any spacecraft has gone in near-free-fall.  By using [[Doppler effect|Doppler radar]], scientists have found that the courses for both the Pioneer spacecraft show a constant acceleration towards the sun of <math>8.74 \times 10^{-10} \frac{m}{s^2} \ </math> beyond theoretical preditions.<ref name="Nieto" />

Around the time of Pioneer 11's flyby of Saturn, it was found to be slightly off-course.  (Every spacecraft sent to the outer solar system is intended to follow a specific course, predicted by the theory of [[general relativity]].  Radio transmissions and radar are used to track spacecraft to ensure that they stay on course.)  While this in itself was within the range of error, astronomers continued tracking the craft to find that the anomalous sunward acceleration increased.  Currently, [[Pioneer 10]] and [[Pioneer 11]] are respectively over 30 and 70 [[Astronomical Unit|AU]] from the sun, the farthest any spacecraft has gone in near-free-fall.  By using [[Doppler effect|Doppler radar]], scientists have found that the courses for both the Pioneer spacecraft show a constant acceleration towards the sun of <math>8.74 \times 10^{-10} \frac{m}{s^2} \ </math> beyond theoretical preditions.<ref name="Nieto" />

Originally, scientists supposed that the Pioneer navigation code was in error.  However, the code was verified by an independent team.  After a rigorous search for all possible effects, the anomaly was determined to be real:  the course actually does diverge from models.<ref name="Nieto" />

Originally, scientists supposed that the Pioneer navigation code was in error.  However, the code was verified by an independent team.  After a rigorous search for all possible effects, the anomaly was determined to be real:  the course actually does diverge from models.<ref name="Nieto" />

Several possible explanations, some flawed, have been proposed this effect:

Several possible explanations, some flawed, have been proposed for this effect:

* 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.

* 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.  Dr. [[Russell Humphreys]] has proposed an explanation<ref name="Humphreys">Russell Humphreys.  "[http://creation.com/images/journal_of_creation/vol21/5181creationist.pdf Creationist cosmologies explain the anomalous acceleration of Pioneer spacecraft]."  ''Journal of Creation'', August 2007.</ref> of the Pioneer anomaly as part of [[Starlight_problem#Humphreys.27_model|his new cosmology]], as the pull from the center of mass of the universe, which he locates inside the [[solar system]].  However, he does not explain why this gravitational force does not influence the motion of the planets.

* There could be gravitational forces from other celestial bodies that have not been taken into account.  While the [[Kuiper Belt]] cannot provide the necessary gravitational force<ref>O Bertolami and P Vieira.  "[http://dx.doi.org/10.1088/0264-9381/23/14/005 Pioneer anomaly and the Kuiper Belt mass distribution]", ''Classical and Quantum Gravity'', 2006 </ref>, Dr. [[Russell Humphreys]] has proposed an explanation<ref name="Humphreys">Russell Humphreys.  "[http://creation.com/images/journal_of_creation/vol21/5181creationist.pdf Creationist cosmologies explain the anomalous acceleration of Pioneer spacecraft]."  ''Journal of Creation'', August 2007.</ref> of the Pioneer anomaly as part of [[Starlight_problem#Humphreys.27_model|his new cosmology]], as the pull from the center of mass of the universe, which he locates inside the [[solar system]].  However, he does not explain why this gravitational force does not influence the motion of the planets.

==References==

==References==